Structural Change in Ni-Fe-Ga Magnetic Shape Memory Alloys after Severe Plastic Deformation

Severe plastic deformation (SPD) is widely considered to be the most efficient process in obtaining ultrafine-grained bulk materials. The aim of this study is to examine the effects of the SPD process on Ni-Fe-Ga ferromagnetic shape memory alloys (FSMA). High-speed high-pressure torsion (HSHPT) was applied in the as-cast state. The exerted key parameters of deformation are described. Microstructural changes, including morphology that were the result of processing, were investigated by optical and scanning electron microscopy. Energy-dispersive X-ray spectroscopy was used to study the two-phase microstructure of the alloys. The influence of deformation on microstructural features, such as martensitic plates, intragranular γ phase precipitates, and grain boundaries’ dependence of the extent of deformation is disclosed by transmission electron microscopy. Moreover, the work brings to light the influence of deformation on the characteristics of martensitic transformation (MT). Vickers hardness measurements were carried out on disks obtained by SPD so as to correlate the hardness with the microstructure. The method represents a feasible alternative to obtain ultrafine-grained bulk Ni-Fe-Ga alloys.


Introduction
Shape memory alloys (SMA) are functional materials which are predominantly used as actuators [1]. On the other hand, magnetic shape memory alloys (MSMA) may be used both as microactuators and displacement/force sensors or dampers [2,3]. In these alloys responsible for the shape memory effect is martensitic transformation (MT) in addition to the transition of magnetic order-disorder [4,5]. In fact, the shape memory effect and superelasticity underlie the fast response, reversible, and repeatable operation. Some of the most promising MSMA are Heusler-type ferromagnetic SMAs [6,7]. Heusler alloys are intermetallic compounds with the stoichiometry X 2 YZ, where X and Y represent transition metals and Z elements are from groups III, IV, or V [8]. Current industrial appliances are based on the reversible martensitic transformation between austenite (B2 or L21 structure) and martensite (tetragonal structure L10 unmodulated or modulated by seven or five atomic periods). MT has the effect of modifying the electronic structure and magneto-structural interactions (spin-phonon, electron-magnon). Thus the functionality of these alloys is linked. When the MT takes place at temperatures lower than the magnetic ordering temperature (Curie temperature, Tc), the alloy exhibits a ferromagnetic shape memory effect. Ni-Fe-Ga MSMA have close magnetic and martensitic transformation temperatures suitable for transformation temperatures for the martensitic transformation. The samples were characterized for their magnetic properties (M (T)) in small and large magnetic fields, using a SQUID magnetometer (San Diego, CA, USA) in the RSO mode (for T <350K).

Optical Microstructure Analysis
Buttons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3Ga23Al2 in the as-cast state were successfully severely plastically deformed. After HSHPT, the disks present two phase zones in the microstructure, results that are in agreement with published work on the same system alloy deformed by hot-rolling [10]. It is well-known that in Ni-Fe-Ga-based alloys with Ga ≤ 27 at%, the composition has a typical β + ɤ two phase zone. In addition, the ɤ (FCC) second phase is considered responsible for the improved ductility of these alloys [7,10]. The effect of deformation of the matrix and phase precipitates is clearly manifested in the microstructure. Figure 1 illustrates the morphologies of severely plastically deformed Ni57Fe18Ga25-based alloy and the alloys with Co and Al substitution observed with the optical microscope. The micrographs of all alloys severely plastically deformed up to a logarithmic strain of 2 show quite similar dual-phase features: a matrix phase with a fine structure and an orderly network of second phase (Figure 1a-c). Such a dual-phase structure is consistent with that from an earlier work on Co-Ni-Ga alloy system [15]. The two phases were indicated as lamellar martensite and the ɤ second phase. Further TEM examinations showed that the microstructure of the severely deformed Ni-Fe-Ga alloy possessed a martensitic matrix.
The alloys under study manifest the apparent deformation more evident by increasing the level of deformation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an orderly network of second phase expanded in radial and circular directions (Figure 1a). The ɤ consists of globular and elongated grains. In the case of the Ni52Fe20Co2Ga26 alloy that has undergone the same logarithmic strain of 0.95 as the base Ni-Fe-Ga alloy the ɤ grains have a globular morphology and a certain orientation along with shear direction (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elongated and fragmented second phase precipitates (Figure 1c). The curved morphology is associated with HPT processes. The technological application advantage of this alloys over the other MSMA is related to their improved ductility. This is actually linked to the low volume fraction of the secondary ɤ phase. The alloying with some additional elements (e.g., Co, Al) are suitable to promote the precipitation of ɤ phase [7]. In the alloy with Co and Al substitution that was processed to give rise to a logarithmic strain of 2.52 the individual ɤ grains or grain boundaries were not in a range detectable by standard optical microscopy observations (Figure 1d).
Combining high hydrostatic compression of the order of 1 GPa and high rotational speed (900 rpm or 1795 rpm) the HSHPT technology leads to large grain refinement in the studied alloys. The compressive force applied concomitant with torsion effort produces the refinement by grains shearing. Also the HSHPT processing technique combines a very efficient grain refining with the capability of keeping shape memory properties due to dynamic recrystallization as our research shows on other metallic alloys [12][13][14]. The technology leads to heat generation by intense friction between the anvils and the sample. The time span of processing (up to 7 s) control the prevalence of fine structure. The heat transferred by conduction from the sample to the tools helps achieve an ultrafine structure. In addition, the heat developed during processing by HSHPT method causes post deformation annealing (PDA) that is required after classical HPT to regain shape memory properties. two phase zone. In addition, the Materials 2019, 12, x FOR PEER REVIEW transformation temperatures for the martensitic transformation. The samples w their magnetic properties (M (T)) in small and large magnetic fields, using a S (San Diego, CA, USA) in the RSO mode (for T <350K).

Optical Microstructure Analysis
Buttons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3 state were successfully severely plastically deformed. After HSHPT, the disk zones in the microstructure, results that are in agreement with published work alloy deformed by hot-rolling [10]. It is well-known that in Ni-Fe-Ga-based allo the composition has a typical β + ɤ two phase zone. In addition, the ɤ (F considered responsible for the improved ductility of these alloys [7,10]. The eff the matrix and phase precipitates is clearly manifested in the microstructure. F morphologies of severely plastically deformed Ni57Fe18Ga25-based alloy and th Al substitution observed with the optical microscope. The micrographs o plastically deformed up to a logarithmic strain of 2 show quite similar dual-ph phase with a fine structure and an orderly network of second phase (Figure 1astructure is consistent with that from an earlier work on Co-Ni-Ga alloy system were indicated as lamellar martensite and the ɤ second phase. Further TEM e that the microstructure of the severely deformed Ni-Fe-Ga alloy possessed a m The alloys under study manifest the apparent deformation more evident b of deformation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an order phase expanded in radial and circular directions (Figure 1a). The ɤ consists of gl grains. In the case of the Ni52Fe20Co2Ga26 alloy that has undergone the same loga as the base Ni-Fe-Ga alloy the ɤ grains have a globular morphology and a cert with shear direction (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elongated an phase precipitates (Figure 1c). The curved morphology is associated with technological application advantage of this alloys over the other MSMA is rela ductility. This is actually linked to the low volume fraction of the secondary with some additional elements (e.g., Co, Al) are suitable to promote the precipit the alloy with Co and Al substitution that was processed to give rise to a loga the individual ɤ grains or grain boundaries were not in a range detectable microscopy observations (Figure 1d).
Combining high hydrostatic compression of the order of 1 GPa and high rpm or 1795 rpm) the HSHPT technology leads to large grain refinement in th compressive force applied concomitant with torsion effort produces the r shearing. Also the HSHPT processing technique combines a very efficient gr capability of keeping shape memory properties due to dynamic recrystalliza shows on other metallic alloys [12][13][14]. The technology leads to heat generatio between the anvils and the sample. The time span of processing (up to 7 s) con fine structure. The heat transferred by conduction from the sample to the to ultrafine structure. In addition, the heat developed during processing by HSHP deformation annealing (PDA) that is required after classical HPT to regain shap (FCC) second phase is considered responsible for the improved ductility of these alloys [7,10]. The effect of deformation of the matrix and phase precipitates is clearly manifested in the microstructure. Figure 1 illustrates the morphologies of severely plastically deformed Ni 57 Fe 18 Ga 25 -based alloy and the alloys with Co and Al substitution observed with the optical microscope. The micrographs of all alloys severely plastically deformed up to a logarithmic strain of 2 show quite similar dual-phase features: a matrix phase with a fine structure and an orderly network of second phase (Figure 1a-c). Such a dual-phase structure is consistent with that from an earlier work on Co-Ni-Ga alloy system [15]. The two phases were indicated as lamellar martensite and the transformation temperatures for the martensitic transformation. The samples were characterized for their magnetic properties (M (T)) in small and large magnetic fields, using a SQUID magnetometer (San Diego, CA, USA) in the RSO mode (for T <350K).

Optical Microstructure Analysis
Buttons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3Ga23Al2 in the as-cast state were successfully severely plastically deformed. After HSHPT, the disks present two phase zones in the microstructure, results that are in agreement with published work on the same system alloy deformed by hot-rolling [10]. It is well-known that in Ni-Fe-Ga-based alloys with Ga ≤ 27 at%, the composition has a typical β + ɤ two phase zone. In addition, the ɤ (FCC) second phase is considered responsible for the improved ductility of these alloys [7,10]. The effect of deformation of the matrix and phase precipitates is clearly manifested in the microstructure. Figure 1 illustrates the morphologies of severely plastically deformed Ni57Fe18Ga25-based alloy and the alloys with Co and Al substitution observed with the optical microscope. The micrographs of all alloys severely plastically deformed up to a logarithmic strain of 2 show quite similar dual-phase features: a matrix phase with a fine structure and an orderly network of second phase (Figure 1a-c). Such a dual-phase structure is consistent with that from an earlier work on Co-Ni-Ga alloy system [15]. The two phases were indicated as lamellar martensite and the ɤ second phase. Further TEM examinations showed that the microstructure of the severely deformed Ni-Fe-Ga alloy possessed a martensitic matrix.
The alloys under study manifest the apparent deformation more evident by increasing the level of deformation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an orderly network of second phase expanded in radial and circular directions ( Figure 1a). The ɤ consists of globular and elongated grains. In the case of the Ni52Fe20Co2Ga26 alloy that has undergone the same logarithmic strain of 0.95 as the base Ni-Fe-Ga alloy the ɤ grains have a globular morphology and a certain orientation along with shear direction (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elongated and fragmented second phase precipitates (Figure 1c). The curved morphology is associated with HPT processes. The technological application advantage of this alloys over the other MSMA is related to their improved ductility. This is actually linked to the low volume fraction of the secondary ɤ phase. The alloying with some additional elements (e.g., Co, Al) are suitable to promote the precipitation of ɤ phase [7]. In the alloy with Co and Al substitution that was processed to give rise to a logarithmic strain of 2.52 the individual ɤ grains or grain boundaries were not in a range detectable by standard optical microscopy observations (Figure 1d).
Combining high hydrostatic compression of the order of 1 GPa and high rotational speed (900 rpm or 1795 rpm) the HSHPT technology leads to large grain refinement in the studied alloys. The compressive force applied concomitant with torsion effort produces the refinement by grains shearing. Also the HSHPT processing technique combines a very efficient grain refining with the capability of keeping shape memory properties due to dynamic recrystallization as our research shows on other metallic alloys [12][13][14]. The technology leads to heat generation by intense friction between the anvils and the sample. The time span of processing (up to 7 s) control the prevalence of fine structure. The heat transferred by conduction from the sample to the tools helps achieve an ultrafine structure. In addition, the heat developed during processing by HSHPT method causes post deformation annealing (PDA) that is required after classical HPT to regain shape memory properties.

SEM Analysis
To understand the effect of the severe plastic deformation imparted by HSHPT on Ni-Fe-Ga magnetic SMA an investigation was performed by SEM-EDX. As expected, a significant fragmentation of ɤ -phase precipitates was observed in the severely plastically deformed microstructure ( Figure 2). An important finding is the large refinement of martensite phase after severe plastic  The alloys under study manifest the apparent deformation more evident by increasing the level of deformation from 0.95 up to 2.52. The Ni 57 Fe 18 Ga 25 specimen shows an orderly network of second phase expanded in radial and circular directions (Figure 1a). The

Optical Microstructure Analysis
Buttons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3Ga23Al2 in t state were successfully severely plastically deformed. After HSHPT, the disks present t zones in the microstructure, results that are in agreement with published work on the sam alloy deformed by hot-rolling [10]. It is well-known that in Ni-Fe-Ga-based alloys with Ga the composition has a typical β + ɤ two phase zone. In addition, the ɤ (FCC) second considered responsible for the improved ductility of these alloys [7,10]. The effect of defor the matrix and phase precipitates is clearly manifested in the microstructure. Figure 1 illu morphologies of severely plastically deformed Ni57Fe18Ga25-based alloy and the alloys wi Al substitution observed with the optical microscope. The micrographs of all alloy plastically deformed up to a logarithmic strain of 2 show quite similar dual-phase feature phase with a fine structure and an orderly network of second phase (Figure 1a-c). Such a d structure is consistent with that from an earlier work on Co-Ni-Ga alloy system [15]. The t were indicated as lamellar martensite and the ɤ second phase. Further TEM examination that the microstructure of the severely deformed Ni-Fe-Ga alloy possessed a martensitic m The alloys under study manifest the apparent deformation more evident by increasin of deformation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an orderly network phase expanded in radial and circular directions (Figure 1a). The ɤ consists of globular and grains. In the case of the Ni52Fe20Co2Ga26 alloy that has undergone the same logarithmic str as the base Ni-Fe-Ga alloy the ɤ grains have a globular morphology and a certain orienta with shear direction (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elongated and fragment phase precipitates (Figure 1c). The curved morphology is associated with HPT proc technological application advantage of this alloys over the other MSMA is related to their ductility. This is actually linked to the low volume fraction of the secondary ɤ phase. Th with some additional elements (e.g., Co, Al) are suitable to promote the precipitation of ɤ p the alloy with Co and Al substitution that was processed to give rise to a logarithmic str the individual ɤ grains or grain boundaries were not in a range detectable by standa microscopy observations (Figure 1d).
Combining high hydrostatic compression of the order of 1 GPa and high rotational s rpm or 1795 rpm) the HSHPT technology leads to large grain refinement in the studied a compressive force applied concomitant with torsion effort produces the refinement shearing. Also the HSHPT processing technique combines a very efficient grain refining capability of keeping shape memory properties due to dynamic recrystallization as ou shows on other metallic alloys [12][13][14]. The technology leads to heat generation by inten between the anvils and the sample. The time span of processing (up to 7 s) control the pre fine structure. The heat transferred by conduction from the sample to the tools helps a ultrafine structure. In addition, the heat developed during processing by HSHPT method c deformation annealing (PDA) that is required after classical HPT to regain shape memory p consists of globular and elongated grains. In the case of the Ni 52 Fe 20 Co 2 Ga 26 alloy that has undergone the same logarithmic strain of 0.95 as the base Ni-Fe-Ga alloy the their magnetic properties (M (T)) in small and large magnetic fields, using a SQUID magnetometer (San Diego, CA, USA) in the RSO mode (for T <350K).

Optical Microstructure Analysis
Buttons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3Ga23Al2 in the as-cast state were successfully severely plastically deformed. After HSHPT, the disks present two phase zones in the microstructure, results that are in agreement with published work on the same system alloy deformed by hot-rolling [10]. It is well-known that in Ni-Fe-Ga-based alloys with Ga ≤ 27 at%, the composition has a typical β + ɤ two phase zone. In addition, the ɤ (FCC) second phase is considered responsible for the improved ductility of these alloys [7,10]. The effect of deformation of the matrix and phase precipitates is clearly manifested in the microstructure. Figure 1 illustrates the morphologies of severely plastically deformed Ni57Fe18Ga25-based alloy and the alloys with Co and Al substitution observed with the optical microscope. The micrographs of all alloys severely plastically deformed up to a logarithmic strain of 2 show quite similar dual-phase features: a matrix phase with a fine structure and an orderly network of second phase (Figure 1a-c). Such a dual-phase structure is consistent with that from an earlier work on Co-Ni-Ga alloy system [15]. The two phases were indicated as lamellar martensite and the ɤ second phase. Further TEM examinations showed that the microstructure of the severely deformed Ni-Fe-Ga alloy possessed a martensitic matrix.
The alloys under study manifest the apparent deformation more evident by increasing the level of deformation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an orderly network of second phase expanded in radial and circular directions ( Figure 1a). The ɤ consists of globular and elongated grains. In the case of the Ni52Fe20Co2Ga26 alloy that has undergone the same logarithmic strain of 0.95 as the base Ni-Fe-Ga alloy the ɤ grains have a globular morphology and a certain orientation along with shear direction (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elongated and fragmented second phase precipitates (Figure 1c). The curved morphology is associated with HPT processes. The technological application advantage of this alloys over the other MSMA is related to their improved ductility. This is actually linked to the low volume fraction of the secondary ɤ phase. The alloying with some additional elements (e.g., Co, Al) are suitable to promote the precipitation of ɤ phase [7]. In the alloy with Co and Al substitution that was processed to give rise to a logarithmic strain of 2.52 the individual ɤ grains or grain boundaries were not in a range detectable by standard optical microscopy observations (Figure 1d).
Combining high hydrostatic compression of the order of 1 GPa and high rotational speed (900 rpm or 1795 rpm) the HSHPT technology leads to large grain refinement in the studied alloys. The compressive force applied concomitant with torsion effort produces the refinement by grains shearing. Also the HSHPT processing technique combines a very efficient grain refining with the capability of keeping shape memory properties due to dynamic recrystallization as our research shows on other metallic alloys [12][13][14]. The technology leads to heat generation by intense friction between the anvils and the sample. The time span of processing (up to 7 s) control the prevalence of fine structure. The heat transferred by conduction from the sample to the tools helps achieve an ultrafine structure. In addition, the heat developed during processing by HSHPT method causes post deformation annealing (PDA) that is required after classical HPT to regain shape memory properties.
grains have a globular morphology and a certain orientation along with shear direction (Figure 1b). The Ni 50 Fe 22 Ga 25 Co 3 alloy shows elongated and fragmented second phase precipitates (Figure 1c). The curved morphology is associated with HPT processes. The technological application advantage of this alloys over the other MSMA is related to their improved ductility. This is actually linked to the low volume fraction of the secondary Materials 2019, 12, x FOR PEER REVIEW transformation temperatures for the martensitic transformation. The samples we their magnetic properties (M (T)) in small and large magnetic fields, using a SQ (San Diego, CA, USA) in the RSO mode (for T <350K).

Optical Microstructure Analysis
Buttons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3G state were successfully severely plastically deformed. After HSHPT, the disks zones in the microstructure, results that are in agreement with published work alloy deformed by hot-rolling [10]. It is well-known that in Ni-Fe-Ga-based alloy the composition has a typical β + ɤ two phase zone. In addition, the ɤ (FC considered responsible for the improved ductility of these alloys [7,10]. The effe the matrix and phase precipitates is clearly manifested in the microstructure. The alloys under study manifest the apparent deformation more evident by of deformation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an orderly phase expanded in radial and circular directions (Figure 1a). The ɤ consists of glo grains. In the case of the Ni52Fe20Co2Ga26 alloy that has undergone the same logar as the base Ni-Fe-Ga alloy the ɤ grains have a globular morphology and a certa with shear direction (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elongated and phase precipitates (Figure 1c). The curved morphology is associated with H technological application advantage of this alloys over the other MSMA is relate ductility. This is actually linked to the low volume fraction of the secondary ɤ with some additional elements (e.g., Co, Al) are suitable to promote the precipita the alloy with Co and Al substitution that was processed to give rise to a logar the individual ɤ grains or grain boundaries were not in a range detectable b microscopy observations (Figure 1d).
Combining high hydrostatic compression of the order of 1 GPa and high ro rpm or 1795 rpm) the HSHPT technology leads to large grain refinement in the compressive force applied concomitant with torsion effort produces the ref shearing. Also the HSHPT processing technique combines a very efficient grai capability of keeping shape memory properties due to dynamic recrystallizat shows on other metallic alloys [12][13][14]. The technology leads to heat generation between the anvils and the sample. The time span of processing (up to 7 s) contr fine structure. The heat transferred by conduction from the sample to the too ultrafine structure. In addition, the heat developed during processing by HSHPT deformation annealing (PDA) that is required after classical HPT to regain shape phase. The alloying with some additional elements (e.g., Co, Al) are suitable to promote the precipitation of Materials 2019, 12, x FOR PEER REVIEW transformation temperatures for the martensitic transformation. The sam their magnetic properties (M (T)) in small and large magnetic fields, usi (San Diego, CA, USA) in the RSO mode (for T <350K).

Optical Microstructure Analysis
Buttons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52F state were successfully severely plastically deformed. After HSHPT, th zones in the microstructure, results that are in agreement with publishe alloy deformed by hot-rolling [10]. It is well-known that in Ni-Fe-Ga-ba the composition has a typical β + ɤ two phase zone. In addition, th considered responsible for the improved ductility of these alloys [7,10]. the matrix and phase precipitates is clearly manifested in the microstruc morphologies of severely plastically deformed Ni57Fe18Ga25-based alloy Al substitution observed with the optical microscope. The microgra plastically deformed up to a logarithmic strain of 2 show quite similar d phase with a fine structure and an orderly network of second phase (Figu structure is consistent with that from an earlier work on Co-Ni-Ga alloy were indicated as lamellar martensite and the ɤ second phase. Further that the microstructure of the severely deformed Ni-Fe-Ga alloy possess The alloys under study manifest the apparent deformation more ev of deformation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an phase expanded in radial and circular directions (Figure 1a). The ɤ consis grains. In the case of the Ni52Fe20Co2Ga26 alloy that has undergone the sam as the base Ni-Fe-Ga alloy the ɤ grains have a globular morphology and with shear direction (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elonga phase precipitates (Figure 1c). The curved morphology is associated technological application advantage of this alloys over the other MSMA ductility. This is actually linked to the low volume fraction of the secon with some additional elements (e.g., Co, Al) are suitable to promote the p the alloy with Co and Al substitution that was processed to give rise to the individual ɤ grains or grain boundaries were not in a range dete microscopy observations (Figure 1d).
Combining high hydrostatic compression of the order of 1 GPa and rpm or 1795 rpm) the HSHPT technology leads to large grain refinemen compressive force applied concomitant with torsion effort produces shearing. Also the HSHPT processing technique combines a very effic capability of keeping shape memory properties due to dynamic recry shows on other metallic alloys [12][13][14]. The technology leads to heat ge between the anvils and the sample. The time span of processing (up to 7 fine structure. The heat transferred by conduction from the sample to ultrafine structure. In addition, the heat developed during processing by deformation annealing (PDA) that is required after classical HPT to regai phase [7]. In the alloy with Co and Al substitution that was processed to give rise to a logarithmic strain of 2.52 the individual 019, 12, x FOR PEER REVIEW 3 of 9 ation temperatures for the martensitic transformation. The samples were characterized for gnetic properties (M (T)) in small and large magnetic fields, using a SQUID magnetometer go, CA, USA) in the RSO mode (for T <350K).

s and Discussion
al Microstructure Analysis ons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3Ga23Al2 in the as-cast re successfully severely plastically deformed. After HSHPT, the disks present two phase the microstructure, results that are in agreement with published work on the same system ormed by hot-rolling [10]. It is well-known that in Ni-Fe-Ga-based alloys with Ga ≤ 27 at%, position has a typical β + ɤ two phase zone. In addition, the ɤ (FCC) second phase is ed responsible for the improved ductility of these alloys [7,10]. The effect of deformation of ix and phase precipitates is clearly manifested in the microstructure. Figure 1 illustrates the ogies of severely plastically deformed Ni57Fe18Ga25-based alloy and the alloys with Co and itution observed with the optical microscope. The micrographs of all alloys severely y deformed up to a logarithmic strain of 2 show quite similar dual-phase features: a matrix th a fine structure and an orderly network of second phase (Figure 1a-c). Such a dual-phase is consistent with that from an earlier work on Co-Ni-Ga alloy system [15]. The two phases icated as lamellar martensite and the ɤ second phase. Further TEM examinations showed icrostructure of the severely deformed Ni-Fe-Ga alloy possessed a martensitic matrix. alloys under study manifest the apparent deformation more evident by increasing the level ation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an orderly network of second panded in radial and circular directions (Figure 1a). The ɤ consists of globular and elongated the case of the Ni52Fe20Co2Ga26 alloy that has undergone the same logarithmic strain of 0.95 se Ni-Fe-Ga alloy the ɤ grains have a globular morphology and a certain orientation along ar direction (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elongated and fragmented second recipitates (Figure 1c). The curved morphology is associated with HPT processes. The gical application advantage of this alloys over the other MSMA is related to their improved . This is actually linked to the low volume fraction of the secondary ɤ phase. The alloying e additional elements (e.g., Co, Al) are suitable to promote the precipitation of ɤ phase [7]. In with Co and Al substitution that was processed to give rise to a logarithmic strain of 2.52 idual ɤ grains or grain boundaries were not in a range detectable by standard optical py observations (Figure 1d). bining high hydrostatic compression of the order of 1 GPa and high rotational speed (900 795 rpm) the HSHPT technology leads to large grain refinement in the studied alloys. The sive force applied concomitant with torsion effort produces the refinement by grains . Also the HSHPT processing technique combines a very efficient grain refining with the y of keeping shape memory properties due to dynamic recrystallization as our research n other metallic alloys [12][13][14]. The technology leads to heat generation by intense friction the anvils and the sample. The time span of processing (up to 7 s) control the prevalence of cture. The heat transferred by conduction from the sample to the tools helps achieve an structure. In addition, the heat developed during processing by HSHPT method causes post tion annealing (PDA) that is required after classical HPT to regain shape memory properties.
grains or grain boundaries were not in a range detectable by standard optical microscopy observations (Figure 1d).
Combining high hydrostatic compression of the order of 1 GPa and high rotational speed (900 rpm or 1795 rpm) the HSHPT technology leads to large grain refinement in the studied alloys. The compressive force applied concomitant with torsion effort produces the refinement by grains shearing. Also the HSHPT processing technique combines a very efficient grain refining with the capability of keeping shape memory properties due to dynamic recrystallization as our research shows on other metallic alloys [12][13][14]. The technology leads to heat generation by intense friction between the anvils and the sample. The time span of processing (up to 7 s) control the prevalence of fine structure. The heat transferred by conduction from the sample to the tools helps achieve an ultrafine structure. In addition, the heat developed during processing by HSHPT method causes post deformation annealing (PDA) that is required after classical HPT to regain shape memory properties.

SEM Analysis
To understand the effect of the severe plastic deformation imparted by HSHPT on Ni-Fe-Ga magnetic SMA an investigation was performed by SEM-EDX. As expected, a significant fragmentation of OR PEER REVIEW 3 of 9 mperatures for the martensitic transformation. The samples were characterized for operties (M (T)) in small and large magnetic fields, using a SQUID magnetometer SA) in the RSO mode (for T <350K).
iscussion structure Analysis i57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3Ga23Al2 in the as-cast ssfully severely plastically deformed. After HSHPT, the disks present two phase ostructure, results that are in agreement with published work on the same system y hot-rolling [10]. It is well-known that in Ni-Fe-Ga-based alloys with Ga ≤ 27 at%, has a typical β + ɤ two phase zone. In addition, the ɤ (FCC) second phase is nsible for the improved ductility of these alloys [7,10]. The effect of deformation of hase precipitates is clearly manifested in the microstructure. Figure 1 illustrates the severely plastically deformed Ni57Fe18Ga25-based alloy and the alloys with Co and observed with the optical microscope. The micrographs of all alloys severely ed up to a logarithmic strain of 2 show quite similar dual-phase features: a matrix structure and an orderly network of second phase (Figure 1a-c). Such a dual-phase stent with that from an earlier work on Co-Ni-Ga alloy system [15]. The two phases s lamellar martensite and the ɤ second phase. Further TEM examinations showed cture of the severely deformed Ni-Fe-Ga alloy possessed a martensitic matrix. nder study manifest the apparent deformation more evident by increasing the level om 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an orderly network of second in radial and circular directions (Figure 1a). The ɤ consists of globular and elongated e of the Ni52Fe20Co2Ga26 alloy that has undergone the same logarithmic strain of 0.95 -Ga alloy the ɤ grains have a globular morphology and a certain orientation along ion (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elongated and fragmented second es (Figure 1c). The curved morphology is associated with HPT processes. The lication advantage of this alloys over the other MSMA is related to their improved actually linked to the low volume fraction of the secondary ɤ phase. The alloying onal elements (e.g., Co, Al) are suitable to promote the precipitation of ɤ phase [7]. In and Al substitution that was processed to give rise to a logarithmic strain of 2.52 grains or grain boundaries were not in a range detectable by standard optical rvations (Figure 1d). high hydrostatic compression of the order of 1 GPa and high rotational speed (900 ) the HSHPT technology leads to large grain refinement in the studied alloys. The ce applied concomitant with torsion effort produces the refinement by grains e HSHPT processing technique combines a very efficient grain refining with the ping shape memory properties due to dynamic recrystallization as our research -phase precipitates was observed in the severely plastically deformed microstructure ( Figure 2). An important finding is the large refinement of martensite phase after severe plastic deformation by HSHPT. The dual-phase features (Figure 2a

Optical Microstructure Analysis
Buttons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3Ga23Al2 in the as-cast state were successfully severely plastically deformed. After HSHPT, the disks present two phase zones in the microstructure, results that are in agreement with published work on the same system alloy deformed by hot-rolling [10]. It is well-known that in Ni-Fe-Ga-based alloys with Ga ≤ 27 at%, the composition has a typical β + ɤ two phase zone. In addition, the ɤ (FCC) second phase is considered responsible for the improved ductility of these alloys [7,10]. The effect of deformation of the matrix and phase precipitates is clearly manifested in the microstructure. Figure 1 illustrates the morphologies of severely plastically deformed Ni57Fe18Ga25-based alloy and the alloys with Co and Al substitution observed with the optical microscope. The micrographs of all alloys severely plastically deformed up to a logarithmic strain of 2 show quite similar dual-phase features: a matrix phase with a fine structure and an orderly network of second phase (Figure 1a-c). Such a dual-phase structure is consistent with that from an earlier work on Co-Ni-Ga alloy system [15]. The two phases were indicated as lamellar martensite and the ɤ second phase. Further TEM examinations showed that the microstructure of the severely deformed Ni-Fe-Ga alloy possessed a martensitic matrix.
The alloys under study manifest the apparent deformation more evident by increasing the level of deformation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an orderly network of second phase expanded in radial and circular directions (Figure 1a). The ɤ consists of globular and elongated grains. In the case of the Ni52Fe20Co2Ga26 alloy that has undergone the same logarithmic strain of 0.95 as the base Ni-Fe-Ga alloy the ɤ grains have a globular morphology and a certain orientation along with shear direction (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elongated and fragmented second phase precipitates (Figure 1c). The curved morphology is associated with HPT processes. The technological application advantage of this alloys over the other MSMA is related to their improved ductility. This is actually linked to the low volume fraction of the secondary ɤ phase. The alloying with some additional elements (e.g., Co, Al) are suitable to promote the precipitation of ɤ phase [7]. In the alloy with Co and Al substitution that was processed to give rise to a logarithmic strain of 2.52 the individual ɤ grains or grain boundaries were not in a range detectable by standard optical microscopy observations (Figure 1d).
Combining high hydrostatic compression of the order of 1 GPa and high rotational speed (900 second phase is present. Increasing the level of deformation to ε = 1.45 in the Ni 52 Fe 20 Co 2 Ga 26 sample leads to the generalized sliding path for

Optical Microstructure Analysis
Buttons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3Ga23Al2 i state were successfully severely plastically deformed. After HSHPT, the disks presen zones in the microstructure, results that are in agreement with published work on the alloy deformed by hot-rolling [10]. It is well-known that in Ni-Fe-Ga-based alloys with the composition has a typical β + ɤ two phase zone. In addition, the ɤ (FCC) seco considered responsible for the improved ductility of these alloys [7,10]. The effect of de the matrix and phase precipitates is clearly manifested in the microstructure. Figure 1 i morphologies of severely plastically deformed Ni57Fe18Ga25-based alloy and the alloys Al substitution observed with the optical microscope. The micrographs of all all plastically deformed up to a logarithmic strain of 2 show quite similar dual-phase featu phase with a fine structure and an orderly network of second phase (Figure 1a-c). Such structure is consistent with that from an earlier work on Co-Ni-Ga alloy system [15]. Th were indicated as lamellar martensite and the ɤ second phase. Further TEM examinat that the microstructure of the severely deformed Ni-Fe-Ga alloy possessed a martensitic The alloys under study manifest the apparent deformation more evident by increas of deformation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an orderly netwo phase expanded in radial and circular directions (Figure 1a). The ɤ consists of globular an grains. In the case of the Ni52Fe20Co2Ga26 alloy that has undergone the same logarithmic as the base Ni-Fe-Ga alloy the ɤ grains have a globular morphology and a certain orien with shear direction (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elongated and fragme phase precipitates (Figure 1c). The curved morphology is associated with HPT pr technological application advantage of this alloys over the other MSMA is related to the ductility. This is actually linked to the low volume fraction of the secondary ɤ phase. with some additional elements (e.g., Co, Al) are suitable to promote the precipitation of the alloy with Co and Al substitution that was processed to give rise to a logarithmic the individual ɤ grains or grain boundaries were not in a range detectable by stan microscopy observations (Figure 1d).
-phase precipitates, as illustrated in Figure 2b. The Ni 50 Fe 22 Ga 25 Co 3 alloy subjected to a low logarithmic strain level of 0.81 exhibited dual-phase features likewise the other three Ni-F-Ga alloy under study. The large grains of second phase were dispersed in the martensite matrix, as seen in Figure 2c. However, the increased degree of deformation (ε = 1.89) in the alloy without substitution reveals a highly deformed microstructure (Figure 2d). The feature pertaining to the martensite matrix could not be observed as they were outside the range of detection by SEM. Some precipitate particles were found while grain boundaries in the sample that were subjected to plastic deformation to the extent 1.89 could not be observed.

TEM Analysis
In order to study in detail the microstructural characteristics of the Ni57Fe18Ga25 magnetic SMA achieved by HSHPT processing, TEM images and were carried out. Bright field TEM image ( Figure  3a) highlights different morphologies of martensite distorted by severe deformation. Some martensite variants are twinned and well self-accommodated.

TEM Analysis
In order to study in detail the microstructural characteristics of the Ni 57 Fe 18 Ga 25 magnetic SMA achieved by HSHPT processing, TEM images and were carried out. Bright field TEM image (Figure 3a) highlights different morphologies of martensite distorted by severe deformation. Some martensite variants are twinned and well self-accommodated.

TEM Analysis
In order to study in detail the microstructural characteristics of the Ni57Fe18Ga25 magnetic SMA achieved by HSHPT processing, TEM images and were carried out. Bright field TEM image ( Figure  3a) highlights different morphologies of martensite distorted by severe deformation. Some martensite variants are twinned and well self-accommodated.  Small grains of about 100 nm of fine martensitic lamellae are clearly identifiable on the right side of the micrograph. Inside ultrafine grains are nucleated high-density inner microtwins. The central area of the image shows serrated stress fields with a dark contrast. The stress field is typical of severe plastic deformation obtained via HSHPT [16]. Additionally, fine and ordered martensite lamellar plates can be observed on the left side of the image.
The twinned structure of martensite provide microstructural processes underlying the unique deformation propensity [17]. The MSMA alloys hold particular attribute of deformation recovery in addition to thermomechanical stimuli via applied magnetic field. The applied magnetic stimuli generate the driving force necessary for twin boundary movement. The differences between the martensitic variants (regarding magnetic domain) became very sensitive at applying magnetization field.
The zig-zag shaped of twin boundaries, which make-up martensitic variants of investigated alloy is highlighted in Figure 3b. It is known that ordered structure of the SMA lattice constitute one of significant factor for improved MSMA alloys.

Optical Microstructure Analysis
Buttons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and N state were successfully severely plastically deformed. After HSHPT zones in the microstructure, results that are in agreement with publis alloy deformed by hot-rolling [10]. It is well-known that in Ni-Fe-Gathe composition has a typical β + ɤ two phase zone. In addition, considered responsible for the improved ductility of these alloys [7,10 the matrix and phase precipitates is clearly manifested in the microstr morphologies of severely plastically deformed Ni57Fe18Ga25-based allo Al substitution observed with the optical microscope. The micro plastically deformed up to a logarithmic strain of 2 show quite simila phase with a fine structure and an orderly network of second phase (F structure is consistent with that from an earlier work on Co-Ni-Ga allo were indicated as lamellar martensite and the ɤ second phase. Furth that the microstructure of the severely deformed Ni-Fe-Ga alloy posse The alloys under study manifest the apparent deformation more of deformation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows phase expanded in radial and circular directions (Figure 1a). The ɤ con grains. In the case of the Ni52Fe20Co2Ga26 alloy that has undergone the as the base Ni-Fe-Ga alloy the ɤ grains have a globular morphology a with shear direction (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elon phase precipitates (Figure 1c). The curved morphology is associat technological application advantage of this alloys over the other MSM ductility. This is actually linked to the low volume fraction of the se with some additional elements (e.g., Co, Al) are suitable to promote th the alloy with Co and Al substitution that was processed to give rise the individual ɤ grains or grain boundaries were not in a range d microscopy observations (Figure 1d).
Combining high hydrostatic compression of the order of 1 GPa rpm or 1795 rpm) the HSHPT technology leads to large grain refinem compressive force applied concomitant with torsion effort produ shearing. Also the HSHPT processing technique combines a very ef capability of keeping shape memory properties due to dynamic rec shows on other metallic alloys [12][13][14]. The technology leads to heat between the anvils and the sample. The time span of processing (up t fine structure. The heat transferred by conduction from the sample ultrafine structure. In addition, the heat developed during processing deformation annealing (PDA) that is required after classical HPT to reg ); and point 2: martensitic matrix. The martensitic matrix in this ternary alloy is found to be richer in Ga but leaner in Fe, as compared to the second-phase precipitates. The Ni content is about the same in all locations on the surface. Small grains of about 100 nm of fine martensitic lamellae are clearly identifiable on the right side of the micrograph. Inside ultrafine grains are nucleated high-density inner microtwins. The central area of the image shows serrated stress fields with a dark contrast. The stress field is typical of severe plastic deformation obtained via HSHPT [16]. Additionally, fine and ordered martensite lamellar plates can be observed on the left side of the image.
The twinned structure of martensite provide microstructural processes underlying the unique deformation propensity [17]. The MSMA alloys hold particular attribute of deformation recovery in addition to thermomechanical stimuli via applied magnetic field. The applied magnetic stimuli generate the driving force necessary for twin boundary movement. The differences between the martensitic variants (regarding magnetic domain) became very sensitive at applying magnetization field.
The zig-zag shaped of twin boundaries, which make-up martensitic variants of investigated alloy is highlighted in Figure 3b. It is known that ordered structure of the SMA lattice constitute one of significant factor for improved MSMA alloys.   Figure 5 shows the variations in the Ni, Fe, Ga, and Co contents along the line across full thickness of severely plastically deformed Ni52Fe20Co2Ga26 disk. The corresponding EDX mapping analysis is illustrated, too.

EDX Analysis
The appearance of ɤ phase is accompanied by a sudden rise in the peak intensity corresponding to Fe and Co elements profile and drop in Ga content.

Microhardness Considerations
The results from the microhardness measurement are in good agreement with the microstructure of the studied alloys after processing by HSHPT. The hardness was enhanced with increasing the level of deformation. This is strictly correlated to the decrease in the size of the grains. The strengthening effect during plastic deformation is generated by the increase in the amount of grain boundaries which act as a strong obstacle for dislocation mobility through the material. Discussion crostructure Analysis f Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3Ga23Al2 in the as-cast cessfully severely plastically deformed. After HSHPT, the disks present two phase icrostructure, results that are in agreement with published work on the same system d by hot-rolling [10]. It is well-known that in Ni-Fe-Ga-based alloys with Ga ≤ 27 at%, on has a typical β + ɤ two phase zone. In addition, the ɤ (FCC) second phase is ponsible for the improved ductility of these alloys [7,10]. The effect of deformation of phase precipitates is clearly manifested in the microstructure. Figure 1 illustrates the of severely plastically deformed Ni57Fe18Ga25-based alloy and the alloys with Co and n observed with the optical microscope. The micrographs of all alloys severely ormed up to a logarithmic strain of 2 show quite similar dual-phase features: a matrix ine structure and an orderly network of second phase (Figure 1a-c). Such a dual-phase nsistent with that from an earlier work on Co-Ni-Ga alloy system [15]. The two phases as lamellar martensite and the ɤ second phase. Further TEM examinations showed structure of the severely deformed Ni-Fe-Ga alloy possessed a martensitic matrix. s under study manifest the apparent deformation more evident by increasing the level from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an orderly network of second ed in radial and circular directions (Figure 1a). The ɤ consists of globular and elongated ase of the Ni52Fe20Co2Ga26 alloy that has undergone the same logarithmic strain of 0.95 -Fe-Ga alloy the ɤ grains have a globular morphology and a certain orientation along ection (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elongated and fragmented second tates (Figure 1c). The curved morphology is associated with HPT processes. The application advantage of this alloys over the other MSMA is related to their improved is actually linked to the low volume fraction of the secondary ɤ phase. The alloying ) and (2) martensitic matrix. Figure 5 shows the variations in the Ni, Fe, Ga, and Co contents along the line across full thickness of severely plastically deformed Ni 52 Fe 20 Co 2 Ga 26 disk. The corresponding EDX mapping analysis is illustrated, too.
The appearance of transformation temperatures for the martensitic transformation. The samples were characterized for their magnetic properties (M (T)) in small and large magnetic fields, using a SQUID magnetometer (San Diego, CA, USA) in the RSO mode (for T <350K).

Optical Microstructure Analysis
Buttons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3Ga23Al2 in the as-cast state were successfully severely plastically deformed. After HSHPT, the disks present two phase zones in the microstructure, results that are in agreement with published work on the same system alloy deformed by hot-rolling [10]. It is well-known that in Ni-Fe-Ga-based alloys with Ga ≤ 27 at%, the composition has a typical β + ɤ two phase zone. In addition, the ɤ (FCC) second phase is considered responsible for the improved ductility of these alloys [7,10]. The effect of deformation of the matrix and phase precipitates is clearly manifested in the microstructure. Figure 1 illustrates the morphologies of severely plastically deformed Ni57Fe18Ga25-based alloy and the alloys with Co and Al substitution observed with the optical microscope. The micrographs of all alloys severely plastically deformed up to a logarithmic strain of 2 show quite similar dual-phase features: a matrix phase with a fine structure and an orderly network of second phase (Figure 1a-c). Such a dual-phase structure is consistent with that from an earlier work on Co-Ni-Ga alloy system [15]. The two phases were indicated as lamellar martensite and the ɤ second phase. Further TEM examinations showed that the microstructure of the severely deformed Ni-Fe-Ga alloy possessed a martensitic matrix.
The alloys under study manifest the apparent deformation more evident by increasing the level of deformation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an orderly network of second phase expanded in radial and circular directions (Figure 1a). The ɤ consists of globular and elongated grains. In the case of the Ni52Fe20Co2Ga26 alloy that has undergone the same logarithmic strain of 0.95 as the base Ni-Fe-Ga alloy the ɤ grains have a globular morphology and a certain orientation along phase is accompanied by a sudden rise in the peak intensity corresponding to Fe and Co elements profile and drop in Ga content.
The phase-specific microhardness test reveals that martensitic matrix is harder (339 to 377 HV0.02) than the second phase (296 to 352 HV0.02) having relatively small differences between the Ni-Fe-Ga studied samples. Results were valid for all samples deformed at low degrees of deformation. The specimens severely plastically deformed to the extent of true strain of 2 showed a reversal of this trend. Unexpectedly, the ɤ phase became harder than the martensite. For example in the case of martensite in Ni57Fe18Ga25 MSMA the hardness value was 377 as against 383 for the ɤ phase.  Figure 6 presents significant results consisting of thermomagnetic curves for Ni52Fe20Co2Ga26 and Ni52Fe20Co3Ga23Al2 samples before and after severe deformation. Table 1 synthesized the characteristic MT temperatures and the magnetic ordering temperature (Tc) for the initial and severely deformed specimens.

Thermo-Magnetic Data
As shown in Figure 6a, the deformed sample Ni52Fe20Co2Ga26 at 0.95 true strain presents the thermal hysteresis associated with reversible martensitic transformation.  Ni52Fe20Co2Ga26 _undeformed  243  218  250  258  328  Ni52Fe20Co2Ga26_s1  224  207  230  238  351  Ni52Fe20Co2Ga26_s2  ----182  Ni52Fe20Co3Ga23Al2_undeformed  236  210  245  260  355  Ni52Fe20Co3Ga23Al2_s2  240  145  238 267 >400 The magnitude of the order-shift magnitude transitions and the way the structural transformations occur are reflected by magnetic properties. Martensitic transition is shifted to lower temperatures compared to those for the undeformed sample. The Curie temperature value increases to 351 K from 328 K for the undeformed sample. The magnetization of the deformed sample decreases. Marked changes occur in the severely deformed Ni52Fe20Co2Ga26 alloy at a very high degree of deformation: MT disappears, while the magnetic ordering temperature drops to 182 K (curve s1 in Figure 6a). This may be the result of the decrease in the size of the grains, reaching nanometric dimensions. The MT disappearing in the alloy calls for further investigation in the future. It can also be noted that the magnetization of all samples from Figure 6a does not drop to zero as the temperature increases, indicating the presence of a magnetic phase with a Tc temperature above 380 HSHPT'ed Ni 52 Fe 20 Co 2 Ga 26 sample: EDX line across full thickness and EDX elemental mapping.

Microhardness Considerations
The results from the microhardness measurement are in good agreement with the microstructure of the studied alloys after processing by HSHPT. The hardness was enhanced with increasing the level of deformation. This is strictly correlated to the decrease in the size of the grains. The strengthening effect during plastic deformation is generated by the increase in the amount of grain boundaries which act as a strong obstacle for dislocation mobility through the material.
The phase-specific microhardness test reveals that martensitic matrix is harder (339 to 377 HV 0.02 ) than the second phase (296 to 352 HV 0.02 ) having relatively small differences between the Ni-Fe-Ga studied samples. Results were valid for all samples deformed at low degrees of deformation. The specimens severely plastically deformed to the extent of true strain of 2 showed a reversal of this trend. Unexpectedly, the transformation temperatures for the martensitic transformation. The samples were characterized for their magnetic properties (M (T)) in small and large magnetic fields, using a SQUID magnetometer (San Diego, CA, USA) in the RSO mode (for T <350K).

Optical Microstructure Analysis
Buttons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3Ga23Al2 in the as-cast state were successfully severely plastically deformed. After HSHPT, the disks present two phase zones in the microstructure, results that are in agreement with published work on the same system alloy deformed by hot-rolling [10]. It is well-known that in Ni-Fe-Ga-based alloys with Ga ≤ 27 at%, the composition has a typical β + ɤ two phase zone. In addition, the ɤ (FCC) second phase is considered responsible for the improved ductility of these alloys [7,10]. The effect of deformation of the matrix and phase precipitates is clearly manifested in the microstructure. Figure 1 illustrates the morphologies of severely plastically deformed Ni57Fe18Ga25-based alloy and the alloys with Co and Al substitution observed with the optical microscope. The micrographs of all alloys severely plastically deformed up to a logarithmic strain of 2 show quite similar dual-phase features: a matrix phase with a fine structure and an orderly network of second phase (Figure 1a-c). Such a dual-phase structure is consistent with that from an earlier work on Co-Ni-Ga alloy system [15]. The two phases were indicated as lamellar martensite and the ɤ second phase. Further TEM examinations showed that the microstructure of the severely deformed Ni-Fe-Ga alloy possessed a martensitic matrix.
The alloys under study manifest the apparent deformation more evident by increasing the level of deformation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows an orderly network of second phase expanded in radial and circular directions (Figure 1a). The ɤ consists of globular and elongated grains. In the case of the Ni52Fe20Co2Ga26 alloy that has undergone the same logarithmic strain of 0.95 as the base Ni-Fe-Ga alloy the ɤ grains have a globular morphology and a certain orientation along with shear direction (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elongated and fragmented second phase precipitates (Figure 1c). The curved morphology is associated with HPT processes. The technological application advantage of this alloys over the other MSMA is related to their improved ductility. This is actually linked to the low volume fraction of the secondary ɤ phase. The alloying with some additional elements (e.g., Co, Al) are suitable to promote the precipitation of ɤ phase [7]. In the alloy with Co and Al substitution that was processed to give rise to a logarithmic strain of 2.52 the individual ɤ grains or grain boundaries were not in a range detectable by standard optical microscopy observations (Figure 1d).
Combining high hydrostatic compression of the order of 1 GPa and high rotational speed (900 phase became harder than the martensite. For example in the case of martensite in Ni 57 Fe 18 Ga 25 MSMA the hardness value was 377 as against 383 for the

Optical Microstructure Analysis
Buttons of Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni state were successfully severely plastically deformed. After HSHPT zones in the microstructure, results that are in agreement with publis alloy deformed by hot-rolling [10]. It is well-known that in Ni-Fe-Gathe composition has a typical β + ɤ two phase zone. In addition, considered responsible for the improved ductility of these alloys [7,10 the matrix and phase precipitates is clearly manifested in the microstr morphologies of severely plastically deformed Ni57Fe18Ga25-based allo Al substitution observed with the optical microscope. The micro plastically deformed up to a logarithmic strain of 2 show quite similar phase with a fine structure and an orderly network of second phase (F structure is consistent with that from an earlier work on Co-Ni-Ga allo were indicated as lamellar martensite and the ɤ second phase. Furth that the microstructure of the severely deformed Ni-Fe-Ga alloy posse The alloys under study manifest the apparent deformation more of deformation from 0.95 up to 2.52. The Ni57Fe18Ga25 specimen shows phase expanded in radial and circular directions (Figure 1a). The ɤ con grains. In the case of the Ni52Fe20Co2Ga26 alloy that has undergone the as the base Ni-Fe-Ga alloy the ɤ grains have a globular morphology a with shear direction (Figure 1b). The Ni50Fe22Ga25Co3 alloy shows elon phase precipitates (Figure 1c). The curved morphology is associat technological application advantage of this alloys over the other MSM ductility. This is actually linked to the low volume fraction of the se with some additional elements (e.g., Co, Al) are suitable to promote th the alloy with Co and Al substitution that was processed to give rise the individual ɤ grains or grain boundaries were not in a range d microscopy observations (Figure 1d). phase.  Table 1 synthesized the characteristic MT temperatures and the magnetic ordering temperature (Tc) for the initial and severely deformed specimens.

Thermo-Magnetic Data
As shown in Figure 6a, the deformed sample Ni 52 Fe 20 Co 2 Ga 26 at 0.95 true strain presents the thermal hysteresis associated with reversible martensitic transformation. K. This is the ɤ phase which depletes the austenitic matrix in 3D-elements and is found in higher amount in the Ni52Fe20Co2Ga26 sample with ɛ = 2.2. The undeformed Ni52Fe20Co3Ga23Al2 sample ( Figure 6b) describes a MT with a very narrow hysteresis. After a higher severe deformation the Ni52Fe20Co3Ga23Al2 alloy sample shows a very wide thermal hysteresis, which does not close after the specific cooling-heating MT cycle. The heat test does not reach the initial austenitic state, indicating unstable austenite.

Conclusions
The primary findings of the current study can be summarized as follows: 1. For the first time, buttons of Ni-Fe-Ga (with and without Co and Al substitution) in the as-cast condition were successfully severely plastically deformed by HSHPT at room temperature. 2. The microstructure of the two-phase Heusler Ni-Fe-Ga FSM alloys Ni57Fe18Ga25, Ni50Fe22Ga25Co3, Ni52Fe20Co2Ga26, and Ni52Fe20Co3Ga23Al2 after SPD was explored after SPD with an optical microscope, SEM-EDX, as well as TEM. 3. Martensitic transformation that takes place in severely deformed Ni-Fe-Ga alloys with Co and Al substitutions has been highlighted by magnetic measurements. 4. In the temperature range over which the martensitic transformation occurs a microstructural change take place producing discontinuities in the thermal dependence of magnetization. 5. The severe deformation at 0.95 logarithmic degree induces a decrease in MT temperatures, an increase in Tc and a decrease in magnetization, while a 2.2 degree of deformation induces a loss of the shape memory effect in the Ni52Fe20Co2Ga26 alloy, and unstable austenite in the Ni52Fe20Co3Ga23Al2 alloy.