Microstructure and Mechanical Behaviour of Shape Memory Alloys

1. Introduction

The attractive physical, mechanical, and operational characteristics of the structural metal materials most widely used in the world economy can be achieved through diffusion-free phase martensitic transformations (MT) in combination with their atomic ordering and decomposition of the supersaturated solid solutions [1,2]. The “intelligent” or smart alloys and steels with highly reversible thermoelastic MTs (TMTs) are of particular interest to the practice due to the diverse and practically important physical phenomena and the thermally, mechanically, and magnetically induced shape memory (SM), superelasticity (SE), and active or passive high damping (HD) effects inherent to these materials [3,4,5,6,7,8,9,10].

The multicomponent smart alloys, based on the ordered intermetallic non-ferrous compounds B2–TiNi, L2₁–Ni₂MnGa, B2– and D0₃–Cu–Me (Me = Al, Ni, Zn) and disordered Ti-based alloys and steels, which represent a special important class of materials, attract innovation regarding their design and subsequent employment in the capacity of thermomechanical active or passive SM and SE systems in various branches of science engineering, medicine, and the social sphere [3,4,5,6,7,8,9,10,11].

It should be noted that in metastable non-ferrous alloys with qualitative agreement between the observed pre-martensitic anomalous, the softening of the elastic constants phenomenon is realized mainly by the Ziner channel C′ on the shear system {110}<$1\stackrel{\mathrm{-}}{1}0$> [4,5,10,11]. Their changes are essentially anisotropic (A = C₄₄/C′ >> 1), which correlates with the possibility of shuffling shear displacements of atoms and a noticeable softening of the phonon modes of the TA₂ branch, also called martensite precursor effects. For example, for the copper-based alloys, A = 12–13, and for the ferromagnetic intermetallic compound Ni₂MnGa, A = 23.

In the Ti–Ni–Cu-based alloys, the parameter of anisotropy also increases almost twice that of a binary TiNi compound. A unique key feature of the B2-austenite of titanium nickelide alloys in the pre-martensitic state is the total isotropic softening of all the elastic constants Cij, modules E and G, discovered and first discussed in [10,12]. In this case, according to inelastic neutron scattering, there is a significant softening of the acoustic transversely polarized phonon modes TA₂ <ξξ0> k <$1\stackrel{\mathrm{-}}{1}0$> e, especially in the vicinity of the wave vectors k at ξ = 1/3 and ξ = 1/2, which show progress when approaching the start temperature M_s of TMTs [13,14,15,16]. Anisotropic diffuse scattering of the X-rays, electrons, and the striation of tweed image contrast are also found in the transmission electron microscopy (TEM) [10].

The martensite temperatures M_s, M_f, A_s, A_f, and their dependence on alloy composition are of the utmost importance in shape memory technology. In recent years, a number of papers have widely discussed the important role of the weighted average number of valence electrons per atom e_v/a and their concentration c_v = e_v/et (where et = Z is the total number of electrons) in the temperature–concentration behavior of temperatures of TMTs as the factors responsible not only for elastic properties, but also for the overall stability of the atomic crystal lattice of the austenite in relation to the TMTs [10,17,18].

However, it is well-known a number of unresolved problems that considerably restrict the development and wide practical application of smart alloys remain. They are connected, first, by tough—sometimes mutually exclusive—requirements regarding the physico-mechanical properties of such materials, and second, by their—often restricted—resource capabilities. Among the latter, in the first place, are low strength-related and plastic properties, insufficient levels of SM parameters, and their instability upon thermal, mechanical, or thermo- and mechano-cycling external actions that are accompanied by degradation of the properties, structure, and phase composition of alloys. So, for instance, atomically ordered copper-based alloys, such as Cu-Zn, Cu-Zn-Al, Cu-Al-Ni, and others, in a single-crystal state for certain crystallographic orientations, exhibit pronounced, valuable SM and SE effects. However, when in an ordinary, polycrystalline state, they lose them to a considerable extent, first of all, because of a drop in the ability to deform (in plasticity), and then, because of an increase in brittleness and instability upon thermal or mechanical cycling as a result of aging and fatigue-related and long-term strength.

Also, the very attractive thermo-deformation and magnetically controlled SM effects of some cast single crystals of ferromagnetic intermetallic Heusler alloys of Ni₂MnMe (Me = Ga, Pb, Sn, and others) type can be considered. However, because of chemical liquation and grain boundary brittleness, SM effects in cast polycrystal, usually coarse-grained (CG, <d> > 100 μm), alloys of the same chemical compositions are largely problematic.

While deforming and changing temperature, the plastic flow and evolvement of TMT as a combined phenomenon in smart materials is rigorously restricted in the limits of separate grains, leading to the concentration of strains and defects in the vicinity of grain boundaries, to their weakening, and, as a consequence, to intercrystallite damage.

The majority of the problems listed can be resolved by purposeful micro and macroalloying, as well as refinement of the grain structure of these materials up to micro (MG, 10–100 μm), submicro (SMG, 0.1–10 μm), and nanocrystalline (NG, less 100 nm) characteristic levels. The specific role of fine-grain structure in such materials has already been discussed in earlier works [3,4,5,6] and in more recent works on studying the conditions of obtaining and influencing NG and SMG states in ultrafine-grained (UFG) alloys [10,11].

Note that upon refinement of the structure up to the NG and SMG states, the size of the martensite plates that are formed in the course of transformations is limited by the small sizes of crystallites. The decrease in sizes of the martensite plates leads correspondingly to a decrease in the deformations and concentration of stresses at the front of plate motion and to a decrease in the concentration of defects in the local region in the vicinity of boundaries of dispersed UFGs. From this, one can expect that defects, such as phase-transformation-induced dislocations, generated during TMTs, are distributed in UFG materials more uniformly over the volume of material and boundaries of grains of large specific area, and the critical concentration of defects in the vicinity of grain boundaries, whose presence there is the course of embrittlement of coarse-grained materials, will take no place in UFG materials.

Achieving a UFG state in brittle materials is possible owing to the application of severe plastic deformation (SPD) methods at high quasi-hydrostatic pressures, following special schemes and schedules that allow the samples to remain intact and prevent their failure [10,11]. As a consequence of low plasticity, this cannot be implemented in cast intermetallic alloys merely via the employment of traditional deformation offset schemes, nor via stretching or rolling.

The purpose of this Special Issue is to gather original research and review articles on recent achievements in the study of thermoelastic martensitic transformations in titanium nickelide-based alloys [Contributions 1–4], Ni₂MnGa [Contribution 5], copper [Contribution 6], Ni-free Ti-based alloys [Contribution 7], and steels [Contribution 8], their microstructure (including electron microscopy studies with atomic-level resolution), and their physical and mechanical properties. We paid special attention to studying the effect of doping with various chemical additives, temperature, pressure, external deformation, and magnetic field on the structure and thermoelastic martensitic transformations, as well as their relationship with various properties of shape memory alloys.

2. Overview of the Published Articles

Among the structural and multifunctional materials experiencing thermoelastic martensitic transformations (TMT), the TiNi based alloys are distinguished by possessing the most attractive mechanical behavior: global isotropic softening of moduli, SMEs significant in magnitude and reproducibility, and high reliability and durability (of mechanothermal, mechanocyclic, and thermocyclic characteristics) during exploitation. Kuranova et al. [Contribution 1] presented a brief overview of the structural and phase transformations and mechanical properties of bulk binary TiNi shape memory alloys, which demonstrate attractive commercial potential. The main goal of this work was to create a favorable microstructure of bulk alloys using both traditional and new alternative methods of thermal and thermomechanical processing. They found that the implementation of an ultrafine-grained structure by different methods determined an unusual combination of strength, ductility, reversible deformation, reactive resistance of these alloys to subsequent tensile or torsion tests at room temperature, and, as a consequence, highly reversible effects of shape memory and superelasticity. They have shown that the alloys Ti_49.8Ni_50.2 and Ti_49.4Ni_50.6 are incapable of aging, and, after being subjected to ECAP, are characterized by their high strength (σ_u up to 1200 MPa) and ductility (δ up to 60–70%). A combined treatment of multi-pass rolling and HT of the Ti_49.5Ni_50.5 and Ti₄₉Ni₅₁ alloys prone to aging provides even greater strength (σ_u up to 1400–1500 MPa) with slightly lower ductility (25–30%). Thus, the unique UFG bulk states synthesis technologies used in the work in metastable low-modulus titanium nickelide alloys make it possible to significantly reduce and control the optimal grain size within narrow limits for both 3D volumetric and long-dimensional small semi-finished products of Ti-Ni alloys.

The effect of the surface oxide layer on the shape memory effect (SME) and superelasticity (SE) after marforming (deformation in the martensitic state, followed by annealing at 713 K for 0.5 h in an inert helium gas and in dry air) on Ti-50.1Ni (at.%) single crystals, oriented along [011]-direction, under compression was investigated by Chumlyakov et al. [Contribution 2]. Quenched [011]-oriented crystals of the Ti-50.1Ni alloy experience a one-stage B2–B19’ martensitic transformation (MT) without SE under compression. Marforming leads to a two-stage B2-R-B19’ MT and creates conditions for SE. A thin TiO₂ oxide layer of 170 nm thick was formed on the sample surface upon annealing at 713 K for 0.5 h in dry air. In [011]-oriented crystals with and without an oxide layer, the SE value reached a maximum of 4%, and the SME was 2.4 and 2.6%, respectively. Appearance of an oxide layer upon annealing in dry air (i) reduces the stresses of the B2-phase by 50 MPa from M_d to 473 K, (ii) decreases Θ = dσ/dε from 6.5 GPa in crystals without an oxide layer to 2.0 GPa with an oxide layer, and (iii) does not affect the SME and SE values.

The structure, phase transformations, and deformation behavior of a nanocrystalline Ti-50.9 at.% Ni alloy with a grain–subgrain structure after aging at various temperatures were studied by Poletika et al. [Contribution 3]. They established that with an increase in the aging temperature, the size and spatial distribution of Ti₃Ni₄ particles change from being located on dislocations at an aging temperature of 300 °C to precipitation at sub-boundaries at an aging temperature of 400 °C, 450 °C. Correspondingly, the morphology of the R phase changes from nanodomain-like to a lamellar self-accommodation structure. Studies have shown that the morphology of the R phase, in turn, affects the deformation response of the material. In the case of lamellar self-accommodation morphology, localized R transformation develops in a Lüders-like manner. This knowledge can be used to fine tune the interval of martensitic transformations in NC Ni-rich TiNi alloy in the process of medical product manufacturing.

A well-known methods of severe plastic deformation, which makes it possible to obtain an ultrafine-grained structure in metals and alloys, is the forging method with a change in the deformation axis in three mutually orthogonal directions, which is called “multi-axial forging” or “abc-pressing”. The mechanical properties of Ti_49.8Ni_50.2 (at%) alloy under tension at room temperature in dependence on the true strain (e = 1.84–9.55) specified during isothermal multi-axial forging (abc-pressing) were investigated by Lotkov et al. [Contribution 4]. They found that the stress at the beginning of the pseudoyield plateau does not depend on the value of the true abc-strain. They observed that after abs-pressing, already at a true strain e = 1.84, the yield stress σ_y was 900 ± 25 MPa, which is more than twice as high compared to σ_y in the initial state of the specimens. With a further increase in the abc-strain, the yield stress continues to increase slightly and reaches 1000 ± 25 MPa at e = 9.55. In this case, the ultimate tensile strength of the samples increases by about 15%. The strain-hardening coefficient θ = dσ/dε at the III (linear) stage of the σ(ε) curve has a similar dependence on e. They have shown that after abc-pressing with e from 1.84 to 9.55, the yield stress and ultimate tensile increase linearly with increasing d^−1/2 in accordance with the Hall–Petch relation, where d is the average grain–subgrain size. The formation of the submicrocrystalline structure of Ti_49.8Ni_50.2 alloy during abc-pressing at 573 K does not affect the subsequent reorientation of initial domains of martensitic phases and development of stress-induced R⟶B19’ martensitic transformation under tension at room temperature.

Among alloys with thermoelastic martensitic phase transformations, of course, are stand-out, atomically ordered L2₁ Heusler alloys. Heusler alloys are the subject of considerable interest because they exhibit a martensitic transformation (MT), a shape memory effect, and a giant magnetocaloric effect. As it is commonly believed, the pronounced magnetoelastic coupling plays a crucial role; however, the effect of alloy composition on MT is still under discussion. To describe the features of MT in Ni_0.75−xMn_xGa_0.25 Heusler alloys, Razumov and Gornostyrev [Contribution 5] have developed the Landau-type phenomenological model that consistently considers the magnetic and lattice degrees of freedom and their mutual interplay. The magnetic entropy contribution was estimated within the framework of the microscopic approach. The proposed model allows us to describe the dependence of the martensitic transformation start temperature M_s(x) on the Mn concentration x in reasonable agreement with the experiment. They have shown that (i) the proposed model allows you to correctly describe the change in the slope of the curves M_s(x) and T₀(x) upon the passage to the ferromagnetic state of alloy and (ii) to explain an anomalous behavior (plateau) of M_s(x) and T₀(x) curves in the region of intermediate concentrations of Mn (0.19 < x < 0.20), the formation of modulated martensite with lower energy must be taken into account.

Another class of economically promising “smart” materials consists of copper and low-modulus alloys with thermoelastic martensitic transformations and the shape memory effect of the Cu-Al-Ni system. Svirid et al. [Contribution 6] investigated the structural features of the polycrystalline shape memory eutectoid Cu-Al-Ni-(B) alloys doped by aluminum (of 10 and 14 wt% Al in total amount), nickel (of 3, 4, and 4.5 wt% Ni), and boron (0.02–0.3 wt% B) in various compositions. They found that microalloying with boron provides grain size refinement up to 500–100 microns in the cast α + β and β Cu-Al-Ni-B alloys, whereas in the corresponding cast alloys without boron, grain sizes attain even a 1.5 mm size. The localization of Al-B and Al-Ni-B particles along grain boundaries was established in the structure. This was investigated, and an effect of grain growth inhibition in the (α + β) and β Cu-Al-Ni-B alloys was established, both in the cast state of the alloys considered and after their heat treatment. Boron-doped alloys have also demonstrated, during uniaxial tensile mechanical tests, the effect of strengthening and an increase in plasticity.

Sheremetyev et al. [Contribution 7] presented the results of a study of a biomedical Ni-free Ti-18Zr-15Nb (at%) shape memory alloy subjected to a low-temperature equal channel angular pressing (ECAP) at 200 °C for three passes and post-deformation annealing (PDA) in the 400–650 °C temperature range for 1 to 60 min. They observed that ECAP led to the formation of an inhomogeneous, highly dislocated substructure of β-phase with a large number of differently oriented deformation bands containing nanograined and nanosubgrained areas. In this state, the alloy strength increased significantly, as compared to the undeformed state, but its ductility and superelasticity deteriorated appreciably. As a result of a short-term (5 min) PDA at 550–600 °C, the processes of polygonization of an entire volume of the material and recrystallization inside the deformation bands were observed. After PDA at 600 °C for 5 min, the alloy manifested a relatively high strength (UTS = 650 MPa), a satisfactory ductility (d = 15%), and a superior superelastic behavior with a maximum superelastic recovery strain of ε_r^se_max = 3.4%.

It is already known that metastable austenitic Fe-Mn-Si-based steels alloyed with a large amount of manganese (up to 30 wt%) and silicon (up to 6 wt%) are increasingly used among alloys exhibiting the shape memory effect (SME). The structure and mechanical properties of new dispersion-hardened Mn-(Cr)-Si-V-C steels with an SME, which undergo strengthening due to the precipitation of VC carbides in a steel matrix, were investigated by Sagaradze et al. [Contribution 8]. As a result of carbide aging, there is also a significant increase in the yield strength in SME steels (up to 642–750 MPa) while maintaining their satisfactory ductility characteristics (a relative elongation of 14–39%). The recovery of the shape of the samples was carried out as a result of both the γ⟶ε transition in the course of heating after preliminary deformation in the initial austenitic state (i.e., due to the transformation of the γ to the ε phase) and the shear re-twinning of martensite in the martensitic ε phase (i.e., due to the occurrence of transformation of the ε to the twinned ε_tw phase).

3. Summary and Outlook

The studies collected in this Special Issue the Microstructure and Mechanical Behavior of Shape Memory Alloys report the latest scientific achievements on the study of the thermoelastic deformation-induced martensitic transformations, microstructure, behavior of physical and mechanical properties in various metallic alloys and compounds with shape memory effects. They are valuable for scientists and engineers engaged in the research and development of shape memory alloys for their application to industrial and medical purposes.