Compound Twins Research Articles

Martensitic transformation induced by cooling NiTi wire under various tensile stresses: Martensite variant microstructures, textures, recoverable strains and plastic strains

Whenever the forward and/or reverse B2 cubic to B19’ monoclinic martensitic transformation in NiTi shape memory alloy wire proceeds under external stress above certain threshold, it generates incremental plastic strains which accumulate during thermomechanical cyclic loading and lead to functional fatigue preventing many promising engineering applications from realization. In this work, unique thermomechanical loading experiments were performed on NiTi shape memory wire with the aim to reveal the mechanism by which the forward martensitic transformation upon cooling under external stress generates plastic strain. Recoverable transformation strains and plastic strains generated by the forward transformation on cooling under various tensile stresses were evaluated, martensite variant microstructures in grains were reconstructed by nanoscale orientation mapping in TEM, martensite textures after cooling at room temperature were evaluated by in-situ synchrotron x-ray diffraction and permanent dislocation defects in martensite were analyzed by TEM. Based on the obtained experimental evidence, it is proposed that the forward martensitic transformation on cooling under stress proceeds via habit plane interfaces between austenite and second order laminate of (001) compound twins in martensite. Depending on the magnitude of the applied stress, the induced martensite reorients, partially detwins and deforms plastically via [100](001) dislocation slip in martensite in extent permitted by the requirement for compatible deformation of grains in nanocrystalline NiTi wire via a single deformation system. During the forward MT upon cooling under highest stresses 600 MPa, the martensite deforms via kwinking deformation enabling generation of large plastic strains 8 %.

Stress induced martensitic transformation in NiTi at elevated temperatures: Martensite variant microstructures, recoverable strains and plastic strains

To shed light on the origin of the loss of functional properties of NiTi with temperature increasing above 100 °C, we have investigated stress induced martensitic transformations in nanocrystalline NiTi shape memory wire by thermomechanical tensile testing supplemented with post-mortem reconstruction of martensite variant microstructures in grains by nanoscale orientation mapping in TEM. The stress induced martensitic transformation generating recoverable transformation strain as well as plastic strain is not completed at the end of the upper stress plateau. The higher is the test temperature, the larger is the volume fraction of retained austenite as well as the plastic strain. The martensite variant microstructures in NiTi wire deformed up to the end of the stress plateau at 120 °C contain partially detwinned single domains of (001) compound twin laminate filling entire grains. It is proposed that the stress induced martensitic transformation proceeds via habit plane interface between austenite and second order laminate of (001) compound twins and that the martensite promptly reorients and deforms plastically by dislocation glide in the [100](001) slip system. When the wire is loaded further beyond the end of the stress plateau, the stress induced martensitic transformation continues and the martensite deforms plastically. It is concluded that the observed gradual loss of superelastic functionality of NiTi with increasing temperature does not originate from the plastic deformation of austenite, as widely assumed in the literature, but that it derives from the loss of resistance of the stress induced martensite to the plastic deformation under increasing stress.

Role of grain size in the evolution of stress-induced martensite and the tensile deformation behavior of Ti-7333 alloy

The evolution of martensite and the tensile deformation behavior of Ti-7333 alloy with different β grain size was studied. It is found that the β grain size (45–260 μm) significantly affects the triggering stress of stress-induced α″ martensite (SIMα″), and then affects the distribution and morphology of α″ martensite. At the initial stage of deformation, the larger β grain can inhibit the stress-induced secondary α″ martensitic transformation and the formation of α″ twinning. While under the same strain, the stress-induced martensite and martensite twinning can be activated simultaneously in the alloy with the smaller β grain, including {111}α″ type Ⅰ twinning, {130}<310>α″ compound twinning, <211>α″ type Ⅱ twinning, {021}<512>α″ type Ⅱ twinning, which can effectively accommodate localized strains and improve the work hardening rate of Ti-7333 alloy. When the β grain size varies in the range of 45–84 μm, there is a higher work hardening plateau in the deformation stage II of Ti-7333 alloy. The yield strength and β grain size are in accordance with the Hall-Petch relation. As the grain size increases to 128 even to 260 μm, the work hardening rate continues to decrease and the yield strength inversely increases. It is demonstrated that the triggering stress for SIMα″ exhibits U-shaped fluctuations as β grain size increases, and the inflection point of Ti-7333 alloy is between 84 μm and 260 μm.

Enhanced thermal cycle stability and shape memory effect in NiTiHf shape memory alloys fabricated by laser powder bed fusion

We report on the fabrication of NiTiHf high temperature shape memory alloys (SMAs) via laser powder bed fusion (LPBF), which are almost free of micro-pores and display (Ti, Hf)2Ni nanoprecipitates with variable size and volume fraction. The effect of variable laser powers of 50 W, 60 W and 70 W on the martensite variant microstructures and mechanical properties of the LPBFed SMAs was investigated. Interestingly, all LPBFed NiTiHf samples display enhanced thermal cycle stability of transformation temperatures with a temperature eviation of −3°C, which is attributed to the lack of dislocation slip originated from the habit plane configuration characteristic with multidomain (001) martensite compound twins. Meanwhile, all LPBFed NiTiHf samples exhibit trained one-way and two-way shape memory effects due to the dislocation slip upon compression deformation at room temperature.

Reorientation induced plasticity in full αʺ martensite Ti-9V-1Fe-4Al alloy

The microstructure and deformation mechanism of a full orthorhombic αʺ martensite Ti-9V-1Fe-4Al (wt.%) alloy were investigated by XRD, TEM, EBSD analyses and tensile tests. The results show that the quenched alloy has a twinned-martensite microstructure and {111}αʺ type I internal twins are formed as the lattice invariant shear for martensite transformation. The alloy plastically deforms by a complex reorientation of αʺ martensite variants via the movement of the inter-variant boundaries of <211>α′′ type Ⅱ twin, and deformation twinning of {111}α′′ type I internal twin and of newly generated {011}αʺ compound twin in primary twinned martensites. {130}α′′<3¯ 10>α′′ and {110}α′′<1 1‾ 0>α′′ plastic twins were not observed in the alloy. Such a kind of deformation induced martensite reorientation results in a good combination of strength and plasticity with yielding strength of 480 MPa, tensile strength of 1030 MPa and elongation of 23% in the alloy. The calculated results further show that the shears of {111}α′′ type I, <211>α′′ type II and {011}α′′ compound twins and the shuffle of {111}α′′ type I twin increase, while the shears of {130}α′′<3‾ 10>α′′ and {110}α′′<1 1‾ 0>α′′ twins and the shuffle of {130}α′′<3‾ 10>α′′ twin decrease with an increase of the lattice parameter ratio bα′′/aα′′. The shears of these activated twinning systems are larger than those of {130}α′′<3‾ 10>α′′ and {110}α′′<1 1‾ 0>α′′ twins, and the shuffle of {111}α′′ type I twin is also bigger than that of {130}α′′<3‾ 10>α′′ twin in the present alloy, which implies that the operative twining modes cannot be derived simply in terms of the shear and shuffle magnitude in full αʺ martensite Ti based alloys.

Peculiar martensitic microstructure and dislocation accumulation behavior in Ti-Ni-Cu shape memory alloys almost satisfying the triplet condition

Dislocation accumulation behavior at the lattice correspondence variant (CV) triplets of the Ti-Ni-Cu alloy close to the triplet condition (TC) was investigated by using in situ heating transmission electron microscopy observation. The martensitic microstructure of this alloy was peculiar because of no {011}o compound twin laminate as predicted by the phenomenological theory of martensite crystallography. The peculiar martensitic microstructure was composed of a combination of six different CVs connected by twin-relationship, including triplet and quadruple. Other types of triplets with higher incompatibility were not observed. It was revealed that few transformation-induced dislocations associated with thermal cycling were formed at the compatible martensitic microstructure satisfying the TC in the present alloy. The fact that few transformation-induced dislocations are formed in the TC microstructure in the Ti-30Ni-20Cu alloy during thermal cycling explains the good thermal cycling properties of this alloy.

Enhancing the thermal stability and recoverability of ZrCu-based shape memory alloys via interstitial doping

ZrCu-based shape memory alloys (SMAs) are regarded as emerging smart materials with high phase transformation temperatures. However, unfavorable thermal stability and poor strain recovery rate hinder commercial applications. Herein, these issues were addressed by doping interstitial B atoms, and the impacts of B content on microstructure, martensitic transformation behavior and properties of ZrCu-based SMAs were systematically investigated. It was shown that (Zr50Cu25Ni7.5Co17.5)99.98B0.02 SMAs possessed excellent thermal stability and shape memory effect. The change of martensitic transformation temperature for this alloy was 15.7 °C after several cycles. The shape recovery rate reached 92.5 % at 8 % compressive pre-strain, which was attributed to the de-twinning of (0 0 1) compound twins and the appearance of (1 1 1) type I twins. Furthermore, it was elucidated for the first time by first-principles calculations that B atoms were covalently bonded with Zr and Cu atoms occupying the octahedral interstitial positions in the B19′ and Cm phases. This study offers an available pathway to exploit efficient and advanced products.

Microstructure evolution and shape memory behaviors of Ni47Ti44Nb9 alloy subjected to multistep thermomechanical loading with different prestrain levels

Ni47Ti44Nb9 shape memory alloy (SMA) is a promising material in the aerospace field due to its wide transformation hysteresis. The application of shape memory effect depends on multistep thermomechanical loading, viz., low-temperature deformation and subsequent heating to recovery. Low-temperature deformation prestrain plays a pivotal role in shape memory properties tailoring of SMA components. However, microstructure evolution and deformation mechanisms of Ni47Ti44Nb9 SMA subjected to various prestrain levels are still unclear. To this end, microstructure evolution and shape memory behaviors of Ni47Ti44Nb9 alloy subjected to multistep thermomechanical loading with prestrain levels of 8% - 16% at -28 °C (Ms + 30 °C) were investigated. The results demonstrate that the stress-strain curve of the specimen exhibits four distinct stages at a maximal prestrain of 16%. Whereas stage II and stage III end at prestrains of ∼8% and ∼12%, respectively. In stage II, the stress-induced martensitic transformation is accompanied by the dislocation slip of the NiTi matrix and β-Nb inclusions. In stage III, in addition to the higher density of dislocations and further growth of stress-induced martensite variants (SIMVs), (0 0 1) compound twins are introduced as a result of the (0 0 1) deformation twinning in stress-induced martensite. More {2 0 -1} martensite twins are gradually introduced in stage IV. Correspondingly, after subsequent unloading and heating, a higher density of {1 1 4} austenite twins form in the specimen with a larger prestrain of 16%. With increasing prestrain from 8% to 16%, the recoverable strain εreT upon heating increases first and then decreases. The εreT obtains a maximum of 7.03% at 10% prestrain and decreases to 6.17% at 16% prestrain. The increase of εreT can be attributed to the formation of new SIMVs, the further growth of existing SIMVs, and the recoverable (0 0 1) compound twins. While the decrease of εreT is mainly associated with the irrecoverable strain by {2 0 −1} martensite twins. The effect of β-Nb inclusions on the evolution of SIMVs is also found herein that deformed β-Nb inclusions can significantly hinder the growth and recoverability of adjacent stress-induced martensite.

The atomic structure and mechanisms of formation of some geometrically incompatible interfaces within cubic B2 austenite – monoclinic B19′ martensite shape memory alloy microstructures

Microstructures of aged nickel‑titanium‑hafnium shape memory alloys (SMAs) Ni50.3Ti41.2Hf8.5 and Ni50.3Ti43.7Hf6 (at.%) are examined using high-resolution transmission electron microscopy (HRTEM) techniques. Characterizations of the structures of austenite-martensite, martensite-martensite, and martensite-precipitate interfaces provide new insights into cubic B2 austenite–monoclinic B19′ martensite phase transformation and martensite reorientation mechanisms. Specifically, in the absence of external load, theoretically unfavorable (001) compound martensite twins nucleate at H-phase precipitate interfaces. At the interface of these twins and the austenite matrix, a zig-zag-rise structure follows an extension of superdislocation theory previously developed to explain analogous zig-zag structured interfaces in non-shape-memory martensitic alloys. Additionally, 〈011〉 type II martensite twins that are theoretically favorable to form were also observed within the austenite matrices. However, instead of forming defect-free habit planes accommodated by elastic strains as predicted by theory, misfit dislocations worked in cooperation with elastic strain fields to accommodate atomic misfits between the austenite and martensite phases, and the elastic strain fields were smaller at the interfaces. Finally, the atomic scale thermoelastic mechanism of <011> type II twins reorienting to form (001) compound twins within the martensite is proposed in a manner consistent with direct observations of such interfaces. In total, connections with previous work on other binary and ternary NiTi base alloys show that these new mechanistic understandings generalize to other B2 - B19′ SMAs that should not perform well according to the elasticity-based Phenomenological Theory of Martensite Crystallography, yet still exhibit robust cyclic thermomechanical performances due to transformation-plasticity interactions.

Transitions among martensitic phases during continuous deformation in metastable β Ti-10V-2Fe-3Al alloy

Martensitic transitions as a function of cold rolling strain levels, in the range of 5 – 43 %, in the solution treated condition of metastable β Ti-10V-2Fe-3Al (Ti-1023) alloy has been systematically investigated. The formation of ω and α′′, α′ martensitic phases from β phase, for the different conditions of Ti-1023 samples, has been confirmed through X-ray diffraction (XRD), transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) techniques. The lattice parameters obtained from XRD confirmed that (c/a)α" and [b/(a√3)]α" ratios approached the value of ∼ 1.587 and unity, respectively, along with consistent variations in principal lattice strains and relative volume changes indicating β→α′′→α′ phase transformations, with increase in cold rolling strain levels. The presence of ω-phase for the 5 % cold rolling sample is confirmed via slow scan XRD and TEM. It is further inferred that with the cold rolling strains the ω-phase act as preferential nucleating sites promoting α′ martensite formation by suppressing the stress-induced β→α′′ martensitic transformation. Moreover, to accommodate strains, additional deformation processes such as martensite compound twinning formation and strain-induced α′′→α′ martensitic transformation are confirmed, through TEM and EBSD, respectively. With further increase in cold rolling strains to 43 %, the transformation of β→α′ and strain-induced α′′→α′ martensites are pronounced, which is consistent with the formation of equilibrium hexagonal close packed structure in titanium alloys. Finally, the present investigation suggests that orthorhombic-α′′ martensites and strain-induced α′′→α′ martensitic transformations give Ti-1023 alloy high cold deformability up to ∼ 43 %.

Classical and “non-classical” twins in Ni2MnGa: Phenomenological versus mechanistic modelling

The characteristic crystallography of “classical” twins was set out over 60 years ago by Bilby and Crocker: a twin boundary is an invariant plane, and the adjacent crystals are interrelated by a homogeneous simple shear. In subsequent work, Crocker and Bevis introduced the notion of “non-classical” twinning, where, as before, the adjacent crystals are interrelated phenomenologically by a homogeneous simple shear, but where the misfit-free interface plane is not invariant. Later experimental studies revealed that “classical” twins actually grow by glissile motion of interfacial line-defects with both dislocation and step character, now known as disconnections. This inhomogeneous shear mechanism has been expounded in the topological model of interfaces developed by Pond and Hirth. In the present paper, experimental observations of “non-classical” twins in Ni2MnGa by Seiner et al. are analysed using the phenomenological and topological approaches. It is concluded that the descriptor “non-classical” is not appropriate because physically feasible glissile disconnections cannot arise in these boundaries. Instead, the boundaries are immobile, misfit-free grain boundaries formed at the intersection of “classical” compound twins.

In situ synchrotron X-ray diffraction analysis of two-way shape memory effect in Nitinol

Despite the fact that the Two-Way Shape Memory Effect (TWSME) has been demonstrated in most Shape Memory Alloys, the effective application of this unique functional behaviour is hindered by the lack of a proper training methodology and understanding of its mechanisms. In this study, a novel training routine has been established together with a home-designed device, enabling TWSME of customised spline curvature to be produced. An in situ high energy synchrotron X-ray diffraction experiment has been performed on Nitinol, followed by comprehensive analysis to reveal the micromechanics of TWSME. Multiple mainstream hypotheses have been examined. The important findings are: (1) The training process has negligible influence on the texture of parent phase; (2) The preferred variant of the B19’ phase exhibits tension/compression asymmetry in TWSME; (3) (100) compound twin is the preferred deformation mode for compression TWSME; (4) The mesoscale residual strain field is the dominant factor that induces TWSME; (5) Lattice defects (dislocations) are spatially rearranged after training; (6) Compression TWSME training retards the B2 to B19’ transformation, whilst tension has the opposite effect. The implications of these findings are further discussed.

Effect of post-heat treatments on the microstructure, martensitic transformation and functional performance of EBF3-fabricated NiTi shape memory alloy

In the present work, a single solution treatment as well as solution/aging treatments were employed to regulate the precipitation behavior, martensitic transformation, mechanical and functional properties of EBF3-fabricated NiTi shape memory alloys. Under the optimized post-process heat treatment conditions, i.e., solution treatment at 1000 oC for 10 h followed by subsequent aging treatment at 500 oC for 4 h, the previously existing elongated Ti4Ni2Ox oxides were partially dissolved and interrupted into granular form and relatively uniformly distributed in the NiTi matrix. Meanwhile, the preexisting nanoscale Ni4Ti3 precipitates gradually transformed from clustered or segregated granular to a lenticular shape with the increase of the aging time. The martensite transformation routes also underwent an accompanying alteration, which changed from a one-step (B2-A ↔ B19’-M) to two-step (B2-A → R → B19’-M) transformation. Moreover, Ni4Ti3 precipitated with a coherent relationship of {123} <111>B2-A //{11–20} <0001>Ni4Ti3 which increase the material strength by hindering dislocation movement and improve plasticity by providing a transition domain for accommodating deformation at the interface. The superelastic stability and narrow response hysteresis promoted by the presence of the R-phase are affected by residual (100) compound twins martensite in the NiTi matrix and the permanent plastic deformation activated in (011) [001] slip system of the B2-A phase. This deteriorates the superelastic recovery ratio from 83.6 to 57.4%, as observed after 10 loading-unloading cycles. The post-heat treatment regime presented in the current investigation can be used to modulate the material precipitation, phase transformation, as well as the mechanical and functional properties of EBF3-fabricated NiTi alloys. These results allow to extrapolate the potential for carefully selected post-heat treatment scheduled for additively manufacturing NiTi shape memory alloys.

Thermally induced reorientation and plastic deformation of B19’ monoclinic martensite in nanocrystalline NiTi wires

Functional behavior of nanocrystalline NiTi shape memory wires having various austenitic microstructures was investigated by thermomechanical testing and TEM analysis of martensite microstructures in deformed wires. Three phenomena are reported and discussed in this work. First, it is observed that martensitic NiTi wire heated under low applied stresses elongates several percent before it shortens due to reverse martensitic transformation. This phenomenon is explained as being due to thermally induced martensite reorientation proceeding via motion of interfaces between (001) compound twin domains existing in the microstructure of selfaccommodated B19’ martensite. Second, it is shown that martensite stabilization by deformation (upward shift of the As temperature after deformation in martensite) is not caused by plastic deformation and lattice defects introduced by deformation, as frequently argued in the literature, but that it is due to change of martensite variant microstructures in polycrystal grains during the reorientation process. Finally, it is observed that generation of unrecovered plastic strains and dislocation defects accompanies all martensitic transformation/reorientation/detwinning/ processes in NiTi, though in different extents. It is found that: i) no plastic strains and lattice defects are generated by martensitic transformation proceeding in the absence of external stress, ii) plastic strains, which are generated by martensitic transformations proceeding under external stress, are significantly larger than plastic strains generated during reorientation or detwinning processes in martensite, iii) plastic strains generated while the wire shortens during reverse transformation upon heating are larger than plastic strains generated while it elongates upon forward transformation on cooling. It is proposed that plastic deformation proceeds in the oriented martensite phase via [100](001) dislocation slip. The coupled martensitic transformation and [100](001) dislocation slip in martensite constitutes a TWIP/TRIP like deformation mechanism enabling functional fatigue of NiTi taking place at low stresses below the yield stress for plastic deformation of martensite.

A large compressive recoverable strain induced by heterogeneous microstructure in a Ni50.6Ti49.4 shape memory alloy via laser powder bed fusion and subsequent aging treatment

Laser induced highly metastable microstructure provides the possibility for the tunable precipitation behavior and mechanical/functional properties via direct aging treatment. In this paper, Ni-rich NiTi-based samples were fabricated by laser powder bed fusion (LPBF) and then were directly aged for different aging times that spanning three time-scales (10 min-100 h). The results showed that through the aging treatment, a heterogeneous microstructure consisting of Ti 4 Ni 2 O x nanoparticles, nanoscale Ni-rich precipitates, dislocation structures, and martensitic twin variants formed in the matrix. Due to the change in the elastic strain field, the short time aging treatment tended to induce (001) compound twins, while the long time aging assisted in the formation of< 011 > type II twins. The temperature-induced and deformation-induced phase transformation behavior were further studied. With the progress of precipitates, it was found that a rapid evolution from a single-stage phase transformation to a multi-stage one occurred. During the cyclic compression, the sample aged for 1 h showed the most excellent superelasticity with initial recoverable strain of 0.089 and steady recoverable strain of 0.087, as well as good cyclic stability. • Microstructure evolution of LPBF-ed Ni-rich NiTi SMA with the aging time was disclosed. • Effects of aged microstructure on transformation behavior and pseudoelasticity were elaborated. • A large compressive recoverable strain and its good cyclic stability were achieved via the proper aging treatment.

In-situ synchrotron X-ray diffraction investigation on deformation behavior of Nb/NiTi composite during pre-straining process

The mechanisms responsible for deformation behavior in Nb/NiTi composite during pre-straining were investigated systematically using in-situ synchrotron X-ray diffraction, transmission electron microscopy and tensile test. It is shown that upon loading, the composite experiences elastic elongation and slight plastic deformation of B19′, B2 and β-Nb phases, together with the forward stress-induced martensitic (SIM) transformation from B2 to B19′. Upon unloading, the deformation mechanisms of the composite mainly involve elastic recovery of B19′, B2 and β-Nb phases, compression deformation of β-Nb phase and incomplete B19′→B2 reverse SIM transformation. In the tensile loading−unloading procedure, besides the inherent elastic deformation and SIM transformation, the (001) compound twins in B19′ martensite can also be conducive to the elastic deformation occurring in B19′-phase of the composite. Therefore, this composite can exhibit a large recoverable strain after unloading owing to the elastic deformation, and the partially reversible and consecutive SIM transformation together with the (001) compound twins.

Structure and substructure characterization of solution-treated Ni50.3Ti29.7Hf20 high-temperature shape memory alloy

We investigate the structure and substructure of the solution-treated martensitic Ni50.3Ti29.7Hf20 high-temperature shape memory alloy (SMA) by transmission electron microscopy and precession electron diffraction (PED). Most martensite plates contain high-density internal (0 0 1) compound twins. Four martensite variants (i.e., A, B, C, and D) were observed using PED. The orientation relationships were determined to be (1 1 0.64) Type II twins between A and B, and (1 1 1) Type I twins between C and D. B and C are divided by junction planes, in which (1 1 1) in C is parallel to (1 1 0.64) in B. These orientation relationships are rarely observed in other NiTi or NiTiHf SMAs, which offers new insight to understand the phase transformation behavior in Ni50.3Ti29.7Hf20 SMAs. Several low-angle grain boundaries were observed near the variant interfaces, which may explain the difficulty of martensite reorientation and detwinning in NiTiHf SMAs.

Effect of grain size on the microstructure and mechanical anisotropy of stress-induced martensitic NiTi alloys

Stress-induced martensite always exhibits an oriented microstructure, which leads to mechanical anisotropy in NiTi alloys. In this study, the effects of grain size on the microstructure and mechanical anisotropy of stress-induced-martensitic NiTi were investigated. Nanocrystalline NiTi exhibited single-variant and twin-free nanograins; ultrafine-grained NiTi exhibited (111)m type І, (2‾01) deformation twins, (100) deformation twins and (001)m compound twins. Microstructural differences suggest that the nanocrystalline and ultrafine-grained alloys had different textures, grains were reoriented, and alloy deformed plastically upon indentation loading. Stress-induced nanocrystalline martensitic NiTi exhibited considerably stronger elastic-modulus anisotropy than ultrafine-grained NiTi when a lower indentation load was applied, whereas stress-induced ultrafine-grained martensitic NiTi exhibited stronger hardness anisotropy.

An atomistic study of self-accommodation martensite morphologies and microstructure evolution during forward and reverse martensitic transformations in single crystal and bicrystal NiTi alloys

The forward and reverse martensitic transformations in NiTi alloys with different crystallography conditions, in terms of [100]-oriented and [110]-oriented single crystal models as well as a bicrystal model possessing twist grain boundary, are studied deeply using molecular dynamics simulations. An atomic tracing method is proposed to identify the specific numbers of B19′ martensite variants, hence the self-accommodation martensite morphologies and microstructure evolution during cooling and heating are clearly demonstrated at the atomic scale. The triangular self-accommodation morphology consisted of three correspondence variants (CVs) possessing {1¯1¯1}M type I twin relationships is formed in the NiTi single crystals at the beginning of the martensitic transformation, and stays settled till the end of the transformation. Whereas three types of self-accommodation morphologies including triangular, “herring-bone” and mixed morphologies are formed in the bicrystal, due to the geometrical constraint induced by the grain boundary. The mixed self-accommodation morphology is unstable, which occurs briefly during martensitic transformation and thus evolves into “herring-bone” morphology ultimately. During reverse transformation, B2 austenite phase nucleates preferentially at the triple-junctions of the triangular self-accommodation morphology in NiTi single crystals, and propagates along the junction plane rather than the (001)M compound twin boundaries in the bicrystal alloy.

Elastic interaction energy analysis during twin plane formation in the martensitic transformation process of α"-martensite in Ti-Nb-Al

To understand the twin selection mechanism that occurs in the initial stage of the martensitic transformation of shape memory alloys, the stress state during twin plane formation was numerically analyzed by micromechanics. The self-accommodation microstructure of α''-martensite of the Ti-23Nb-3Al (at%) shape memory alloy, in which the lattice parameter was adjusted to eliminate the formation of internal twinning due to lattice-invariant deformation, was adopted for analysis. The two lattice correspondence variants (CVi and CVj) formed a twin plane. CVi was assumed to be spheroid, and the elastic strain energy during twin plane formation was evaluated by analyzing the elastic interaction energy when CVj formed around CVi. There are three types of twins ({011}α" compound twin, {111}α" type I twin, and <211>α" type II twin) in the self-accommodation microstructure of α"-martensite. Regardless of the CVi geometry (i.e., aspect ratio (1 to 0.01) and minor axis orientation), the {111}α" type I twin exhibited minimum elastic interaction energy, indicating that minimum elastic strain energy was generated during this twin plane formation. This result is consistent with the experimental results, which showed the selective formation of {111}α" type I twins in the initial stage of martensitic transformation.