The Effect of Fe Content on the Shape Memory Effect of Ni-Mn-Ga-Fe Shape Memory Alloy Microwires after Ordering Heat Treatment

Abstract

The shape memory capabilities of Heusler alloy microwires with two different contents of Fe element instead of Ga element following step-by-step ordering heat treatment were explored based on the stoichiometric ratio of Ni₂MnGa. The melt-drawing technique was used to create the polycrystalline microwires, and the two microwires had Fe atomic contents of 4.7 at.% and 5.5 at.%, respectively. The field emission scanning electron microscope was used to analyze the microwire’s surface condition as well as the microscopic tensile fracture morphology. Using an X-ray diffractometer, the microwires’ crystal structure was identified for phase analysis. Differential scanning calorimetry was used to examine the microwires’ behavior during martensitic transformation. Using a dynamic mechanical stretcher, the elongation and recovery rate of microwires’ one- and two-way shape memory behavior were examined. The findings demonstrated that the microwire phase structure, martensitic transformation behavior, and shape memory capabilities all displayed good properties after the heat treatment was ordered.

1. Introduction

Shape memory alloy is a kind of intelligent metal material [1] that may deform in response to changes in temperature or magnetic field. Ferromagnetic form memory alloys can react to temperature changes as well as variations in the external magnetic field, in contrast to traditional shape memory alloys, which only react to temperature changes. Due to its broad application range and high response frequency [2], FSMAs have emerged as a current research hotspot [3]. As a new functional material, memory alloy is widely used in industrial and medical fields, mainly for thermal drives, artificial joints, orthodontic wire, connections, and fasteners. In the single crystal alloy Ni₂MnGa, Ullakko et al. discovered a significant magnetically generated strain and a recoverable strain of 0.2% [4]. Since then, this recoverable single-crystal substance has been the main focus of FSMA research. There is no grain boundary, the single crystal lattice is complete, and a greater strain can be achieved. Many related studies have shown that some Ni-rich Ni-Mn-Ga alloys have a high phase-change temperature, good shape-memory performance, high super-elastic energy, and excellent thermal cycle stability. Compared with other high-temperature shape memory alloys (such as Ti-Ni-Hf, Cu-Al Ni-Pd alloys, and Ti-Ni-Mn-Ga alloys), they have greater potential for development. Nevertheless, the low stability, low martensitic transformation temperature (M_s), expensive cost, challenging production method, and high intrinsic brittleness of single crystal alloys limit their practical use. Researchers have been working to enhance FSMA performance in the last few years. Among them, the addition of a fourth component is seen to be a successful method for creating a polycrystalline alloy [5].

The extant literature provides evidence that the incorporation of a fourth component into a Ni-Mn-Ga alloy to form a polycrystalline alloy can alter the alloy’s mechanical properties and martensitic transformation temperature, all the while maintaining its Heusler-type structure post-doping. Fe [6] and Cu [7] transition metal elements are frequently added, and rare earth elements like Gd [8] can also be added. Merely a minimal quantity of element doping might influence the alloy’s surface appearance and microstructure throughout the addition process. It is discovered that the element Cu takes the place of Ga. Cu element content ranging from 3 to 10 at%. The Curie temperature (T_c) remains constant while the martensitic transition temperature rises from 318 K to 674 K [7]. The alloy exhibits improved shape memory. The Ga element is replaced with the Gd element, which has a content of 0.1 at.%. T_c stays constant while the martensitic transformation temperature rises, the pre-strain is 4.6%, and the shape memory recovery rate is 86%, both of which are greater than that of the undoped alloy [8]. In this experiment, the fourth component selects the Fe element instead of the Ga element. The elements Fe and Cu belong to the same period, and the elements Gd and Fe belong to the same transition group. Because of this swap, the alloy continues to have an L2₁-type structure. Fe is a ferromagnetic element and a cheap substance at the same time. The radius of an atom of Fe is 1.72 Å, whereas the radius of an atom of Ga is 1.81 Å. Compared to the Ga atom (4s²4p¹), the Fe atom (3d⁶4s²) has a higher valence electron number. It is anticipated that doping will raise the valence electron concentration and decrease grain size, increasing the alloy’s M_s. The tensile strength and shape memory recovery ability of the alloy are improved after the Fe atoms solid solution.

The alloy’s shape memory attributes are directly correlated with the material’s size. Currently, the majority of FSMAs are derived from polycrystalline research, which has a maximum strain of 6% to 8% at room temperature and was the main focus of early experimental research of FSMAs [9]. To create a film with a thickness ranging from several microns to tens of microns, the grains are dispersed throughout the microstructure. Under a microscope, there is less restriction on the twin variations’ orientation perpendicular to the film surface, and the strain can range from 2.0% to 8.7% [10]. Melt drawing was used to prepare the material into microwires. The three-dimensional space alloy material is converted into a one-dimensional microwire while maintaining the alloy’s qualities. This lowers the cross-sectional number of grains and raises the microwire’s high aspect ratio. In addition to enhancing the stability and cycle life of the alloy microwire’s shape memory effect, the greater surface area and volume ratio also help to activate the material’s shape memory performance by limiting lattice deformation and grain boundary movement. The microwire can attain a strain of 6% to 10% once the alloy has been formed as a microwire and stress is applied [11]. This demonstrates that the material’s strain is significantly impacted by the reduction in material size. Grain size, grain boundary, and surface effect are all noticeably improved as SMA becomes very small. Therefore, in recent years, FSMAs have been mainly studied using small-sized microwires.

Based on the aforementioned study, the Ni-Mn-Ga alloy microwire, which was made using the melt drawing process, served as the experiment’s matrix. The association between an increase in M_s temperature and a larger shape memory recovery strain of small-size polycrystalline microwires was investigated, as was the influence of varying Fe content on the shape memory effect of Heusler alloy microwire following ordered heat treatment.

2. Experimentation

As raw materials for this experiment, high purity mass fraction > 99.98% Ni, Mn, Ga, and Fe were chosen, and the materials were then added to a pre-vacuum 6.0 × 10⁻³ Pa arc melting furnace in accordance with a predetermined proportion [6]. Rods made of Ni-Mn-Ga-Fe alloy of approximately 10 cm in length and 8 mm in diameter were manufactured while enclosed in an Ar atmosphere. Microwires obtained from the microwire casting rods of an alloy with an alloy pulling device [6] of the melting matrix are shown in Figure 1a. The melt-pulling apparatus comprises a metal roller, induction heating coil, alloy rod, molten metal liquid, hollow ceramic cylinder, and linear stepping motor. The alloy rods that were ready were fastened within a hollow ceramic cylinder. After prevacuuming with Ar, the switch was activated, causing the metal roller to rotate at a predetermined speed of 30 m/s. Under the action of a heating circuit of approximately 20 kW, the matrix alloy casting bar completely melted, formed by the surface tension of the liquid under the effect of the bouncing melting dungeon at a rate of 3 × 10⁻⁵ m/s into the mother alloy. The wire-grinding device was activated at 106 K/s. The reverse rotation of the clock wheel, combined with high-speed rotation, facilitated the contact and cutting of the microwire. The microwire, with a diameter of 10~50 μm, was obtained through natural cooling. Because the crystal cell solidifies quickly during the preparation process, there is residual tension within it, and the components do not occupy the precise lattice position. For this reason, a step-by-step ordering heat treatment was used. As illustrated in Figure 1b, internal tension and flaws brought on by fast solidification can be removed by annealing stress relief. In general, it is possible to increase composition homogeneity and the degree of order of atoms. The microwire was heated at a rate of 7 K/min and cooled to room temperature at 2 h, 10 h, and 20 h at 993 K, 953 K, and 723 K, respectively.

After the ordering heat treatment of the microwire, the phase structure of the microwire was determined by X-ray diffraction (XRD) using a Japanese (Rigaku, Tokyo, Japan) D/MAX-γB rotating anode X-ray diffractometer. The basic parameters are Cu-K α-ray as the X-ray source (characteristic wavelength of 0.15418 nm), using a graphite monochromator filter, acceleration voltage of ~40 kV, power of ~12 kW, diffraction angle (2θ) of 20–100, and scanning angular velocity of ~5/min. The structure of the microwire phase was labeled at room temperature by X-ray diffraction (XRD) with a Rigaku D/MAX-γB rotating anode for phase analysis. With the use of scanning electron microscopy (SEM) on a Helios Nanolab 600i manufactured by the American (Washington, DC, USA) business FEI, the surface condition and tensile fracture of the microwire were examined [12]. The EDS energy spectrum was used to test the microwire composition. The American (Washington, DC, USA) TA company’s TA DSC Q200 differential scanning calorimeter (DSC) was utilized to ascertain the microwire’s martensitic transformation behavior [13]. Five K/min was the pace at which the microwires were heated and cooled. The One-way shape memory effect (OWSME) and two-way shape memory effect (TWSME) of the microwires were tested using the TA company’s Q800 Dynamic Mechanical Analyzer (DMA). The OWSME test involves loading the microwire to the pre-straightening state, loading and deforming it into a non-self-cooperative martensitic state in the martensitic state, unloading the microwire to zero, raising the temperature to cause the microwire to transition from the martensitic state to the austenitic state, and then annealing to release the stress and record the microwire recovery. There is a 0.02 N/min loading rate and a 0.04 N/min emptying rate. The TWSME test was conducted using a continuous cycle of heating and cooling stress, as outlined in the method above. The rates of heating and cooling are 5 K/min. The two microwires’ tensile stress values indicate how much of a load there is.

3. Experimental Results

3.1. Microwire Microstructure

The microwire’s SEM scanning diagram following an ordered heat treatment is shown in Figure 2. Figure 2a demonstrates a chosen microwire diameter of 40 to 50 μm and length of 5 to 10 cm. The texture of the microwires is homogeneous, and their surface is smooth. Two types of microwire cross sections with varying Fe concentrations are shown in Figure 2b,c. The graphic shows that the microwire’s cross-section has a “D” form, which is made up of two circular surfaces, B and A, rather than a regular circle. A portion of the molten metal preferentially touches the metal copper wheel during the microwire preparation process, condensing quickly to form a flat surface A, while the other portion freely solidifies to form a circular surface B. During the process of solidification, Figure 2b) in part I, the grains grow around following heat treatment because of the preferred contact with the disk to produce a nuclear point. The microwire’s microscopic cross-section shows that, following the ordered heat treatment, the microwire’s grains longitudinally stretch into part II, and the fine grains on part III’s surface transform into huge flake grains [14]. It can decrease the grain boundary energy and area and increase the microwire’s mechanical stability. In order to improve the degree of microwire ordering, ordering heat treatment usually entirely diffuses the microwire since the quick condensation of Mn and Ga atoms during preparation does not occupy the correct lattice position. The cross-sections of two microwires with varying Fe contents following ordered heat treatment are shown in Figure 2b,c, respectively. The Figure 2c diagram’s Fe concentration is higher than the Figure 2b diagram’s. It is discovered that the grain size is smaller in the cross-section of the Figure 2c diagram than it is in the Figure 2b diagram. The material’s surface energy, grain boundary area, and grain size are all improved by the doping of Fe content. Furthermore, as the grain size is reduced and refined, the material’s strength increases as internal stress rises and grain slip decreases. Testing of the mechanical qualities is required afterward. Figure 2d,e show the free solidification grains on the surface of the microwire after ordered heat treatment, respectively. At this time, the grains are approximately elliptical, not round, which indicates that the grain may be ‘grown’ during the heat treatment process, thus causing the grain to be elongated.

3.2. Microwire Phase Structure

The EDS energy spectrum was used to determine the two microwires’ atomic percentage after heat treatment was ordered. Ni_49.3Mn_24.9Ga_21.1Fe_4.7 and Ni_49.9Mn_24.7Ga_19.9Fe_5.5, respectively, are the two microwires listed in

Table 1. EDS energy spectrum analysis of ordered heat-treated microwire composition.

Ordering Heat-Treated Ni-Mn-Ga-Fe Alloy Microwires
Element Atomic %				e/a	Element Atomic %				e/a
Ni	Mn	Ga	Fe		Ni	Mn	Ga	Fe
49.3	24.9	21.1	4.7	7.68	49.9	24.7	19.9	5.5	7.76

. Using the electron energy band theory as a guide, the valence electron concentration (e/a) of ordered heat-treated Ni-Mn-Ga-Fe alloy microwires was determined [15].

$$e/a={\displaystyle \frac{(\mathrm{Ni})\mathrm{at}.\%\times 10+(\mathrm{Mn})\mathrm{at}.\%\times 7+(\mathrm{Ga})\mathrm{at}.\%\times 3+(\mathrm{Fe})\mathrm{at}.\%\times 8}{100}}$$

(1)

Following heat treatment, the electron concentration of Ni_49.3Mn_24.9Ga_21.1Fe_4.7 alloy microwire is around 7.68, 7.60 < e/a < 7.70, and for Ni_49.9Mn_24.7Ga_19.9Fe_5.5 alloy microwire, it is approximately 7.76, e/a > 7.70, according to calculations. The relevant data [14,15] indicate that after ordering heat treatment, the Ni_49.3Mn_24.9Ga_21.1Fe_4.7 alloy microwire adheres to the 5 M martensite structure. During the ordering heat treatment, the Ni_49.9Mn_24.7Ga_19.9Fe_5.5 alloy microwire adapts to the 7 M martensite structure.

An X-ray diffractometer was used to mark the diffraction peaks of the two microwires. The presence of strain or several distinct crystal orientations within the lattice is indicated by the splitting of the diffraction peak. The major peak M (202) of the Ni_49.3Mn_24.9Ga_21.1Fe_4.7 alloy microwire breaks into two peaks, M (220) and M (022), respectively, following undergoing heat treatment, as illustrated in Figure 3. The diffraction peak with the primary peak of A (440) is found in the picture, indicating that lattice distortion or new phase change occurs in the alloy. This is a mixed phase consisting of 5 M martensite and a small amount of austenite [16,17,18]. The 5 M martensite often exhibits a lesser lattice distortion and a lower phase transition temperature, suggesting that the martensitic transformation is not yet complete. The phase determines how the Ni_49.9Mn_24.7Ga_19.9Fe_5.5 alloy microwire is designated. After ordering heat treatment, the major peak M (220) splits into three peaks, which are M (220), M (202), and M (022), in that order. After ordering heat treatment, the parent phase A (422) splits into four peaks, which are M (214), M (422), M (242), and M (224), in that order. The properties of 7 M martensite are compatible with this splitting mode. Larger lattice distortion and a greater phase transition temperature are typically linked to 7 M martensite. According to the XRD results, the ordering heat treatment resulted in notable microstructural changes in the two alloy microwires, which are associated with the alloy’s phase transformation properties.

Following the ordering heat treatment, the microwire’s atomic percentage of Fe increases, and it becomes more martensite-like, ranging from 5 M to 7 M. This raises the temperature at which the martensitic transition occurs. Reduced lattice distortion can be obtained by heat-treating the alloy to encourage atom rearrangement. But, 7 M martensite’s layered structure necessitates a more substantial rearranging of atoms, leading to a larger local lattice distortion, which alters the location and strength of diffraction peaks. This structure agrees with the findings of the valence electron concentration calculations for the two microwires.

After the heat treatment,

Table 2. Microwire phase structure, phase structure, and unit cell volume with different Fe content after ordering heat treatment.

Fe Content (at.%)	Phase Structure	Crystal Lattice Parameters				Unit Cell Volume (Å3)
		a (Å)	b (Å)	c (Å)	c/a
4.7	5 M	5.941	5.941	5.603	0.9431	197.8
5.5	7 M	6.095	5.812	5.564	0.9129	197.1

displays the unit cell volume and lattice constant of the microwires with atomic percentages of Fe element of 5.5 at.% and 4.7 at.%. The lattice constant of Ni_49.3Mn_24.9Ga_21.1Fe_4.7 alloy microwire is found to satisfy the 5 M martensitic structure of a = b ≠ c with an increase in Fe element concentration of around 0.8% [19,20]. The lattice constant of the Ni_49.9Mn_24.7Ga_19.9Fe_5.5 alloy microwire is in agreement with the findings of the preceding XRD investigation and satisfies the 7 M martensitic structure of a > b > c [19]. The unit cell volume declines, and the microwire’s tetragonality c/a decreases as the fraction of Fe atoms increases. This is due to both the volume shrinkage brought on by the decrease in atomic radius size and the amount of Fe atoms replacing Ga atoms occupying the unit cell’s lattice location. The crystal exhibits a progressive decrease in the distance between neighboring atoms and an increase in the contact force between them. The grains of various phases multiply and become finer, improving the microwire’s mechanical characteristics.

The valence electron concentration and unit cell volume of the microwires alters when the proportion of Fe element increases, as can be observed from the above, indicating that the phase structures of the two microwires after the heat treatment are different since Ga atoms are replaced by Fe ones. The valence electron numbers of the Fe and Ga atoms are 3d⁶4s²,4s²4p¹, respectively, according to the energy band theory [20]. As a result, the electrons of the Fe atoms occupying the Fermi surface are readily excited to create a large number of free electrons. In accordance with Pauli’s principle of incompatibility [21], these electrons are incapable of being absorbed and instead transition to higher energy levels, causing a complete change in the martensitic phase. At the same time, as the Fe atom (1.722 Å) replaces the Ga atom (1.811 Å), the lattice volume shrinks, the grains become somewhat more refined, the grain boundary area increases, and the dislocation storage energy of the microwire increases, improving the microwire’s tensile strength and deformation recovery ability. Theoretically, Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire has better mechanical properties than Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire; however, more testing is required to confirm this.

3.3. Martensitic Transformation

The DSC images of the two microwires with the prescribed heat treatment are displayed in Figure 4. The tangent technique is used to indicate the phase transition temperature (M_s, M_f, M_p, A_s, A_f, and A_p), and

Table 3. Two kinds of microwire phase transition temperature.

	As (K)	Af (K)	Ap (K)	Ms (K)	Mf (K)	Mp (K)	Ap − Mp (K)	T0 (K)	T0′ (K)	∆HCooling (J/g)	∆HHeating (J/g)	∆SCooling (J/g·K)	∆SHeating (J/g·K)
Ordered heat-treated Ni49.3Mn24.9Ga21.1Fe4.7	305.0	310.9	308.0	303.8	296.8	300.8	7.2	307.35	300.9	6.33	5.80	2.11	1.93
Ordered heat-treated Ni49.9Mn24.7Ga19.9Fe5.5	314.1	324.6	316.2	317.1	283.9	309.6	6.6	320.85	299	8.63	7.89	1.15	3.76

records the temperature change that represents the phase transition hysteresis (A_p − M_p). Figure 4 shows that Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire and Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire have a single endothermic peak and a single exothermic peak, respectively. This suggests that when martensitic transformation takes place, both microwires are one-step thermoelastic MT.

The martensitic transition temperature is directly influenced by the material’s microstructure. Figure 4 shows the phase-transition temperature determined using the tangent approach. Table 3 lists the experimental results. The Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire’s martensitic transition temperature (MT) is 303.8 K. There are very few austenite phases and a mixed phase of martensite phase at room temperature, indicating that the martensitic transformation was not fully completed [22]. Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire’s MT temperature at 317.1 K is significantly higher than its room temperature, indicating a single martensitic state. This is in line with Figure 3’s XRD study results. Table 3 illustrates that the two microwires’ differences in Fe content are barely 0.8%. There is a 14.6% rise in the M_s temperature of the two microwires and a 16.6% increase in the phase transition temperature. Based on pertinent data [23], the variation in Fe content is 5.1%, and the rate of increase in phase transition temperature is 28.1 K/at.%. This indicates that the temperature-dependent composition of the martensitic phase transition is changing, and the M_s temperature rises as the Fe content increases, corresponding with the microwire’s valence electron concentration. The most stable alloy L2₁ structure in the Heusler alloy occurs when the Fermi surface is in contact with the Brillouin zone (110). Table 1 shows that as the Fe content rises, the electron concentration rises from 7.68 to 7.76. The density close to the Fermi surface rises as the concentration of valence electrons increases, covering and filling the Brillouin zone [17] and creating a new Brillouin zone. The number of free electrons in Fe atoms is more than in Ga atoms when Fe partially replaces Ga atoms. In accordance with earlier experimental findings, the excess electrons cause the crystal’s energy to rise sharply, alter the martensitic phase entirely, and raise the phase transition temperature. This transforms the 5 M martensitic state of the Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire into the 7 M martensitic state of the Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire. Furthermore, the MT temperature was affected by the reduction in particle size. There are r_Fe < r_Ga, and when the Fe content rises, more Ga atoms are substituted, resulting in smaller grains and reduced volume. The aforementioned reasons are usually thought to be the cause of the rise in MT temperature.

The critical temperatures of microwires at cooling and heating are roughly as follows, according to Wayman C.M. [24]:

$$T_0=(M_{\mathrm{s}}+A_{\mathrm{f}})/2$$

(2)

$${T_0}^{\prime }=(A_{\mathrm{s}}+M_{\mathrm{f}})/2$$

(3)

Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire has a critical cooling temperature (T₀) of 307.35 K, and a critical heating temperature (T₀′) of 300.9 K. Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire has a critical cooling temperature (T₀) of 320.85 K and a critical heating temperature (T₀′) of 299 K. The experimental data indicate that M_s > T₀′ for both microwires. The phase transition temperatures of the two microwires follow the A_f > T₀ > M_s > A_s > T₀ > M_f rule, as indicated by the pertinent data [25]. This indicates that both microwires represent the second kind of martensitic transition from a thermodynamic perspective. Compared to the first type of martensitic transformation (direct transformation from austenite to martensite), this type of transformation has different thermodynamic and kinetic properties, which could result in more intricate mechanisms of energy absorption and release during the phase transformation process.

The temperature difference, which drops from 7.2 K to 6.6 K, or around 0.6 K, after ordering heat treatment, indicates the phase transition hysteresis |A_p − M_p| of the two microwires. The energy lost during the MT process is connected to the phase transition hysteresis. As the twin interface travels to overcome the friction work and interface dislocation, the elastic strain energy accumulated in the phase transition process is dissipated in the microwire’s positive and inverse phase transitions. Phase transition hysteresis rises during this process. Following the microwire’s heat treatment, the atoms’ ordering degree rises, and the amount of residual strain falls. As internal stress and flaws reduce, so does the energy loss of the two microwire components. The recoverable strain of the microwire grows in proportion to the elastic energy stored during the phase shift process. The Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire has lower thermal hysteresis compared to the Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire. Additionally, the microwire exhibits superior elastic performance during the phase transition process and better deformation ability [26]. ∆H_Cooling and ∆H_Heating are the enthalpy values of the martensitic transformation and the reverse transformation, respectively. DSC was used to integrate the peak. An increase in the Fe element content indicated a corresponding rise in the microwire’s phase transformation enthalpy. ∆H_Cooling increased from 6.33 J/g to 8.63 J/g, about 2.30 J/g, and ∆H_Heating increased from 5.80 J/g to 7.89 J/g, about 2.09 J/g. The enthalpy of phase change is the primary force behind phase transition, and the rule of thermodynamics states that the magnitude of enthalpy change is correlated with temperature change. Based on prior experimental findings, an increase in Fe content causes the microwire to transition from 5 M martensite to 7 M martensite, enhancing molecular thermal motion, increasing kinetic energy, and raising the phase transition temperature—all of which result in an increase in enthalpy ΔH. The driving force of the microwire phase transformation increases, the phase transformation is more complete, and the martensite state is more stable when the transformation enthalpy increases.

3.4. Shape Memory Effect

3.4.1. One-Way Shape Memory Effect

The mechanical characteristics of the microwires were also investigated. The one-way shape memory effect of the ordered heat-treated microwires (OWSME) is depicted in Figure 5. Stress loading below the martensitic transformation temperature (M_s) resulted in the deformation of the shape memory alloy microwires. The microwires were heated to a temperature higher than the parent phase reverse transformation temperature, As, after being unloaded to zero, thereby restoring their original shape. This is a one-way shape memory phenomenon, meaning that it only deforms when heated and cooled. The microwires were then subjected to stress. The buildup of stress–strain causes the martensite to grow preferentially in some directions, forming detwinned martensite as a result of the distinct deformation and strain directions of the martensite. It has been discovered that the macro is currently under much strain. The one-way shape memory tensile curve of the heat-treated material is shown in Figure 5a for Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire, and the one-way shape memory tensile curve of the heat-treated microwire is shown in Figure 5b for Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire.

The OWSME curve of the Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire at 298 K is broken down into the following sections in Figure 5a: The microwire is unloaded to zero, the residual strain ε_r is 0.45%, the elastic recoverable strain ε_sme is 0.69%, and the A-B segment is loaded into the pre-strain ε of 1.14% in the martensitic condition. The residual strain of the microwire instantly recovers to 311 K, the strain recovers to 0.16%, and the D-F section continues to heat to 336 K while the C-D segment keeps heating the microwire. Internal tension is released during annealing, and 100% of the shape is recovered.

The OWSME curve of the Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire at 303 K is broken down into the following sections in Figure 5b: The microwire is loaded to pre-strain 1.16% in the A-B segment, and to zero in the B-C portion. The residual strain is 0.24%, and the elastic recoverable strain is 0.92%. The C-D segment is heated to the microwire, which is recovered instantly between 303 K and 327 K. At 327 K, the microwire’s residual recovery is 0.21%. The microwire has nearly recovered at this point; the D-E section shows that it is currently in the 327–336 K range on the curve. It is discovered that the microwire is under increasing strain, and the martensite within the microwire may be thought of as preferentially orientated. After heating to 373 K annealing, the internal stress in the D-F section is released, the recovery strain is fully recovered, and the shape recovery rate is 100%. In conclusion, the experimental findings show that the Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire is ultimately heated to 336 K at 298 K, at which point the dislocation-induced residual strain of 0.45% is fully restored and the strain recovery rate is 100%. After heating the Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire to 373 K at 303 K, all residual strain of 0.24% is fully restored, and the strain recovery ratio is 100%. After heat treatment, both microwires’ strain can be increased to 100%. Table 3 and

Table 4. One-way shape recovery of ordered heat-treated microwires.

	ε (%)	εr (%)	εsme (%)	ερ (%)	E (GPa)
Ordered heat-treated Ni49.3Mn24.9Ga21.1Fe4.7	1.14	0.45	0.69	0.16	29.40
Ordered heat-treated Ni49.9Mn24.7Ga19.9Fe5.5	1.16	0.24	0.92	0.21	17.20

show that Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire has a M_s temperature that is 15 K higher than Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire and an A_s temperature that is 8.8 K higher than Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire. The coexistence phase of martensite and a trace amount of austenite is converted into a single martensite phase, the atomic structure is more ordered, and the twin boundary moves more freely with an increase in Fe content. The residual strain of the two microwires drops from 0.45% to 0.24% when the microwire is unloaded to 0% (the tensile strain of the two microwires is just 0.02%, insignificant). There is an increase in elastic recoverable strain from 0.69% to 0.92%. Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire strain is 0.68%, while Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire strain is 1.6%.

3.4.2. Two-Way Shape Memory Effect

The “two-way shape memory effect” (TWSME) describes how a material’s shape changes from its room-temperature martensite phase to its high-temperature austenite phase. The material undergoes a reversible shift in shape and phase from the austenite to the martensite when the temperature falls below a specified critical point. The microwire changes from the high-temperature austenite state to the low-temperature martensite state when the material is heated over the critical point once more, restoring the parent phase austenite phase’s shape. The Ni-Mn-Ga-Fe alloy has shape-controllable properties due to this two-way shape memory effect, which offers special benefits in a variety of applications [27]. The microwire displays one-step thermoelastic martensite during the martensitic transformation, as seen in Figure 4. The austenite phase and the martensite phase have a critical temperature, M_s. The crystal structure alters at or below this critical temperature, causing macroscopic deformation visible to the microwire. Because there is a directed tension inside the microwire, this explains why the microwire can bend and why Ni-Mn-Ga-Fe microwires have a two-way shape memory ability. During phase change, martensite grows more selectively. According to Figure 2, when MT happens, the macroscopic structure of the microwire surface remains unchanged, while the alloy microwire’s atomic structure arrangement changes based on XRD.

The two microwires’ two-way tensile and recovery curves under constant stress are displayed in Figure 6, respectively. The graphic indicates the phase transition temperature. Two traits of the martensitic transformation—reversibility and thermal hysteresis—are displayed in the tensile curve. The two-way tensile curve of Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire under constant stress and with ordered heat treatment in the temperature range of 240–355 K is shown in Figure 6a.

Table 5. Two-way shape recovery of ordered heat-treated microwires.

		σ	80 MPa	120 MPa	160 MPa	200 MPa	240 MPa
	εsme
Ordered heat-treated Ni49.3Mn24.9Ga21.1Fe4.7			0.357%	0.449%	0.537%	0.628%	0.677%
		σ	85 MPa	113 MPa	142 MPa	170 MPa	198 MPa
	εsme
Ordered heat-treated Ni49.9Mn24.7Ga19.9Fe5.5			0.344%	0.488%	0.582%	0.693%	0.837%

presents the experimental data. The strain recovery ratio is 100%, the recoverable strain is between 0.357% and 0.677%, and the stress ranges from 80 MPa to 240 MPa. The two-way tensile curve of Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire under constant stress and ordered heat treatment in the 280–375 K temperature range is shown in Figure 6b. The recoverable strain is between 0.334% and 0.837%, the strain recovery ratio is 100%, and the stress ranges from 85 MPa to 198 MPa. Compared with the previous literature [28], it was found that the incorporation of Fe resulted in higher stretchable stress and better mechanical properties for the microwires; after doping with Fe, the total strain that it can achieve is reduced owing to the fining of grains; at the same stress level of 162 MPa, the strain achieved by Ni-Mn-Ga microwire is 2.6%, the strain achieved by Ni_49.3Mn_24.9Ga_21.1Fe_4.7 is 0.82%, the strain achieved by Ni_49.9Mn_24.7Ga_19.9Fe_5.5 is 1.04%. It can be seen from this that after the addition of the Fe element, as the Fe content increases, the strain of the microwire also increases, but it is lower than the strain of the Ni-Mn-Ga microwire, which is also due to the refinement of grain size. The melt-extracted Ni-Mn-Ga microwire sustains 2.7% strain without breaking and recovers 92% of its 1.55% plastic deformation strain upon heating. The advantage is that the strain recovery rate reaches 100%.

In conclusion, it is possible to recover the recovery strain of ordered heat-treated Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire from 0.357% of 85 MPa to 0.877% of 240 MPa, and from 0.334% of 85 MPa to 0.837% of 198 MPa for ordered heat-treated Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire. In line with the microwire’s one-way shape memory effect, the microwire’s shape memory effect improves as the Fe concentration rises. The curve also demonstrates how, during the thermo-mechanical cycle, stress and M_s both rise. This suggests that the microwire’s phase transition temperature shifts in the direction of high temperatures and that once the microwire reaches the critical temperature, it deforms quickly. The long-axis direction of martensite progressively accumulates in the continuous stress cycle. The ordered heat treatment Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire has a higher retention strain than the ordered heat treatment Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire after the stress is released. As a result, the Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire needs more energy during the inverse phase transition, and the twin variant’s reorientation is the martensite deformation mode. Stress causes the distorted variation to move. The crystal variant travels more smoothly, and the internal flaws and dislocation density in the parent phase diminish with increasing temperature. The number of movable twins grows under the same stress in the continuous stress thermal cycle, and this raises the likelihood of stress-induced martensitic variations [29] in orientation, which modifies the material’s internal stress and amplifies the shape memory effect [30].

The microwires’ phase transition temperatures are indicated in Figure 6 using the tangent approach, and their stress–temperature diagram is depicted in Figure 7. The microwires in the heat engine are subjected to continuous tension in order to circulate heat [31]. The M_s of Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire increases from 296 K to 307 K and further increases from 317 K to 328 K as stress increases. The As rises from 329 K to 335 K, whereas the M_s of the Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire increases from 303 K to 307 K. The findings demonstrate that the microwire phase transition temperature steadily rises with increasing stress, which is in line with the DSC findings. According to Clausius–Clapeyron [32], the experimental data demonstrate that the martensitic transition temperature not only moves like a high temperature but also exhibits a linear relationship between stress and temperature.

$${\displaystyle \frac{\mathrm{d}\sigma }{\mathrm{d}T}}={\displaystyle \frac{\mathsf{\Delta }H\rho }{\epsilon T_0}}$$

(4)

For the heat-treated Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire, the relationship between stress and temperature can be described by the following formula by linear fitting:

$${\sigma }_{M\mathrm{s}}=14.5\times (T-288)$$

(5)

$${\sigma }_{M\mathrm{f}}=16.3\times (T-292)$$

(6)

$${\sigma }_{A\mathrm{s}}=11.9\times (T-311)$$

(7)

$${\sigma }_{A\mathrm{f}}=10.8\times (T-314)$$

(8)

The formula for the heat-treated Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire is as follows:

$${\sigma }_{M\mathrm{s}}=25.7\times (T-296)$$

(9)

$${\sigma }_{M\mathrm{f}}=30.2\times (T-299)$$

(10)

$${\sigma }_{A\mathrm{s}}=18.6\times (T-326)$$

(11)

$${\sigma }_{A\mathrm{f}}=14.4\times (T-331)$$

(12)

The microwire’s slope is roughly but not exactly parallel, and the crossing point of the diagram is absent, based on the linear fitting results. However, the microwire stretches under increased tension, causing the junction point to appear in the other direction. The heat-treated Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire has a larger strain rate than the heat-treated Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire, based on the fitting findings of the two microwires. The elastic strain energy is the main reason for the increased susceptibility of the Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire to deformation during martensitic transformation. This is in line with the microwires’ DSC analysis results. The temperature difference |A_s − M_f| represents the microwire’s phase transition hysteresis. Figure 7 shows that the phase transition hysteresis of the microwire increases with increasing stress. This suggests that the driving force needed for the microwire’s shape memory grows with increasing stress but at a very low rate. The displacement of the crystal interface and the interfacial friction loss during the microwire’s martensitic transformation are the causes of the phase transition hysteresis. This might also be related to the microwire’s internal dislocation. The phase transition hysteresis is exacerbated by these losses, but the elastic strain energy within the microwire also rises in proportion to the increase in stress. This portion of the elevated elastic strain energy influenced the driving force of the inverse phase transition of the microwire. It may be concluded that the phase transition hysteresis of the microwire remains constant under their combined operation. The phase transition hysteresis of the Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire increases from 23.6 K to 26.2 K, with an increase of 2.6 K, and the phase transition hysteresis of the Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire increases from 29.9 K to 32.5 K, with an increase of 2.6 K, as indicated by the data in the analysis diagram. In contrast, it is discovered that the phase transition width of the microwire is |M_s − M_f|, |A_f − A_s|, and that the amount of the element Fe increases by 0.8 at.%. It is discovered that both microwires’ phase transition widths grow as stress increases. The phase transition breadth widens, and the microwire’s reverse phase fully shifts when the martensitic reverse phase transition takes place due to the elastic strain energy acting as a resistance. In conclusion, the mechanical properties of Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire are greater than those of Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire when the two-way shape memory effect is present. By comparison with Table 3, the Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwire martensite phase transition temperature is at 292 K. Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwire Martensite phase transition temperature at 299 K. The difference between the two is found in 11.8 K and 18.1 K, respectively. This is quite close to the extrema obtained by the tangent method of DSC.

4. Conclusions

Following the heat treatment, the two microwires were ordered when the Fe content was increased by 0.8%. The stability was enhanced, the martensitic phase transition shifted from 5 M to 7 M, the lattice constant of the two microwires changed from a = b ≠ c to a > b > c, and the martensitic phase changed entirely. Concurrently, the tetragonal concentration of the microwire decreased, its electron concentration increased, its electron free energy increased, its unit cell volume shrank, its grains became more refined, its crystal interface grew, the elastic potential energy stored in MT increased, and its tensile strength increased. The phase transition enthalpies for cooling and heating increased by 2.3 J/g and 2.09 J/g, respectively, whereas the Ms increased by around 15 K, and the phase transition hysteresis decreased by about 0.9 K. The microwire demonstrated a 100% shape recovery ratio in the shape memory effect ability test, with the residual strain of the one-way shape memory effect decreasing from 0.45% to 0.24% or approximately 0.21%. After heat treatment, the two-way shape memory recoverable strain of the Ni_49.3Mn_24.9Ga_21.1Fe_4.7 microwires ranged from 0.357% at 80 MPa to 0.677% at 240 MPa. Ni_49.9Mn_24.7Ga_19.9Fe_5.5 microwires have a two-way shape memory recoverable strain ranging from 0.837% of 198 MPa to 0.334% of 85 MPa. The shape recovery ratio of both microwires is 100%. It can be seen that after the ordering heat treatment, with an increase in the Fe content, the mechanical properties of the microwires improved. In short, it improves the possibility of practical application of the microwire and lays the foundation for the exploration of new functional materials. The microwire is expected to be used in microdriven devices with constant stress output.