Martensite Variants Research Articles

Thermal stability of Ag-doped Ni-Mn-Ga high-temperature shape memory alloy

In the present study, the microstructure features and martensitic transformation behaviour of a Ni-Mn-Ga alloy were tailored by adjusting the Ag content. In addition, the effect of Ag content on the thermal stability was also explored. The results revealed that the non-modulated martensite phase and Ag particles coexisted in the Ni-Mn-Ga-Ag alloys. Different martensite variants were in the {112} type I twinning relationship. With increasing Ag content, not only all transformation temperatures but also the thermal hysteresis decreased continuously in a quasilinear manner. This was attributed to the introduction of small and soft Ag particles, which were applied to release the elastic energy during the martensitic transformation. The decreasing trend of elastic energy was highly nonlinear. The alloy with 4 at% Ag addition was the percolation threshold. In addition, the relationship of energy dissipation against Ag is not monotonic. While the energy dissipation of Ni48Mn25Ga19Ag8 is almost zero, the others were much higher. Furthermore, unlike others, Ni48Mn25Ga19Ag8 featured good martensitic transformation stability upon thermal cycling.

Elucidating the role of stress-induced martensite on the tensile behaviour of metastable β Ti-10V-2Fe-3Al alloy

Stress-induced martensitic transformation (SIMT) during loading in metastable alloy systems result in strength-ductility enhancement via transformation-induced plasticity (TRIP). The present study aims to investigate the influence of stress-induced martensites (SIM) on the tensile behaviour of metastable β Ti-10 V-2Fe-3Al (Ti-1023) alloy. The solutionized, and thermo-mechanically processed (i.e., cold rolling and annealing at 820 °C for 5 to 30 min) condition of Ti-1023, with single β phase microstructure, has been considered. The β grain size of solutionized Ti-1023 was ~625 μm, while the thermo-mechanically processed Ti-1023 was in the range of ~48 μm to ~106 μm as function of annealing time. The room temperature tensile behaviour of considered samples show SIMT, which results in strength enhancement (~7.5%), along with an increase in the trigger stress (~30%) as a function of β grain size (i.e., ~48 μm to ~625 μm). However, the % strain to failure show a significant decrease. A detailed microstructural characterization indicates the relation of SIM formation capability with β grain size. For finer β grain size, SIM displays a pronounced embrittling effect on the Ti-1023 tensile behaviour. Fracture surfaces show distinct regions with lath morphologies, which are further confirmed by surface as well as cross-sectional transmission electron microscopy (TEM) of the tensile fractured surfaces, illustrating the embrittling effects induced by SIM. In comparison, the dislocation-induced plasticity regions in the surrounding remnant β matrix result in dimple morphology. Thus, the coupling of enhanced martensite transformability, with reduced grain size, and the eventual decrease in martensitic laths spacing (i.e., from ~48 μm to ~7 μm), increase the density of martensite/matrix interfaces and the embrittling action. This results in pronounced dislocation pileups and formation of multiple martensitic variants facilitating premature failure by micro-void nucleation (at the martensite/matrix interface), growth and coalescence mechanism. The observations suggest that grain boundary engineering be optimized in order to fully exploit SIM's potential in enhancing the strength-ductility relation in Ti-1023 alloy.

From mechanical behavior and elastocaloric effect to microscopic mechanisms of gradient-structured NiTi alloy: A phase-field study

The introduction of gradient structure has been recently reported as a practical way to tailor the mechanical behavior of NiTi alloy while improving its elastocaloric effect simultaneously. In this research, a thermo-mechanically weakly coupled phase-field model considering crystal plasticity is employed to investigate the microscopic mechanisms of the elastocaloric effect related to the gradient structure, especially from the point of view of the two-way interaction between stress and martensite variants. A set of polycrystalline systems are established to accomplish the gradual transition from a fine-grained specimen to a coarse-grained one by the grain-size gradient. The gradually changed mechanical response and phase transformation characteristics during the superelastic simulations embody the effectiveness of the grain structure as a regulator of the material properties. The Brayton cycle simulations conducted next revolve around further validation of the proposed model and variant-related interpretation of the effects of the grain structure. Predictions relevant to histories of stress and temperature agree well with the experimental results, and more remarkably, the linear stress-temperature relationship observed during the stress drop of the first hold in the Brayton cycle conforms perfectly to the classical Clausius–Clapeyron relation. Together with martensite reorientation, the propagating martensite phases, where the increasing non-major variants are necessary complements to the major one preferentially aligned with the external stress, are largely responsible for this stress drop. Regarding the elastocaloric effect, the gradient structures achieve very similar cooling capacity but higher cooling efficiency compared to the coarse-grain one, taking advantage of more martensite, particularly the major variant, produced if provided with the same transformable area (excluding the grain boundaries). Besides, a steeper gradient in structures is prone to alter the composition of martensite variants and more non-major variants are demanded to alleviate the developing deformation mismatch among layers with different grain sizes.

Water quenching cracking mechanism and prevention of steels

ABSTRACTQuenching is often an essential step in heat treatment of steels for enhancing strength. Water quenching which is low cost and free of pollution risk instead of oil quenching or polymer aqueous solution quenching which is high cost and has high pollution risk will be widely used in heat treatment enterprises. However, the problem of easy cracking of low alloy steels (especially medium carbon low alloy steels) quenched in water has long been a concern for many researchers. The finite element simulation (FES) is used to predict transient stress at cracking position during quenching of an AISI 4140 steel cylinder sample and found that the transient tangential tensile stress at cracking position was only 350 MPa which is smaller than half of the yield strength (775 MPa) of AISI 4140 steel. Such a low tensile stress causing quenching cracking is surprising. To ascertain the underline reasons, a phase-field finite element (PFFE) model is established to investigate the microscopic stress distribution produced by martensitic transformation. The results indicate that the microscopic stress at the junction of the lath martensitic variants in AISI 4140 steel exceeded 800 MPa, and thus such a high microscopic stress is considered as the origin of quenching cracking. By using two examples, our work further clarifies the basic principle of the water–air alternative timed quenching (ATQ) process invented by us as a process suitable for engineering applications with the ability to avoid water quenching cracking and ensure mechanical properties.

Coexistence of five domains at single propagating interface in single-crystal Ni[sbnd]Mn[sbnd]Ga shape memory alloy

Coexistence of both austenite and martensite during phase transformation is a common feature of all Shape Memory Alloys (SMAs). The martensite has different variants featuring characteristic deformations rotationally linked to each other due to the symmetries of the austenite parent phase, and the martensite variants can form twins with different mean characteristic deformations. Multiple-domain microstructures (consisting of austenite, martensite twins and individual martensite variants) evolve collectively within an SMA sample during the phase transformation, contributing thus to the material's macroscopic response (e.g., its global deformation). Particularly, the multiple domains can exist at the diffuse austenite-martensite interface nucleating and propagating in a single crystal in certain conditions. This implies an energy barrier for this interfacial structure, influencing the interface kinetics and the driving force (energy dissipation) of the phase transformation. In this paper, we report an experimentally observed interface consisting of five domains (austenite, one martensite variant and three twins) in a Ni–Mn–Ga single-crystal initially consisting of one martensite variant and subjected to a uniaxial thermal gradient. The compatibility analysis (performed from the characteristic strains of the three martensite variants having approximately a tetragonal symmetry) reveals that the five-domain interface is not a perfectly compatible pattern like the basic habit plane (consisting of only one twin compatible with austenite). However, its level of non-compatibility is similar to that of the quite common X-interface (four-domain coexistence) which is observed in many SMAs. Further, the significant effects of the thermal loading path and the material initial state (the initial martensite variant) on the domain pattern formation are demonstrated and analyzed. The experimental observation and the theoretical analysis of the domain patterns can provide hints to better understand diffuse interface kinetics and phase transformation hysteresis.

Microstructure evolution and mechanical properties of friction stir welded Fe50Mn30Co10Cr10 high-entropy alloy

Metastable Fe50Mn30Co10Cr10 HEA has a significant strength-ductility synergistic effect and a wide application prospect in nuclear power. The present work reports the microstructure evolution and tensile behavior of friction stir-welded joints for Fe50Mn30Co10Cr10 high entropy alloy (HEA) at various rotational speeds. Defect-free welds of Fe50Mn30Co10Cr10 alloy were obtained by a W-Re alloy welding tool. Significant grain refinement occurs in the nugget zone, where the most of HCP phases in the base metal transformed into FCC phase. Additionally, strain-induced Σ3 mechanical twins are generated in the FCC austenite grains. The microstructure of thermo-mechanically affected zone exhibited multiple deformation mechanisms, including stacking faults, martensite lath, martensite variants, and reverse transformation. The microhardness of the joint primarily depends on the grain size of the FCC phase and the fraction of twins and HCP phase. In the nugget zone of the joint welded at 300 rpm, a high microhardness zone ranging from 266.7 to 310 HV was detected. While the reverse transformation and fresh twinning in the joint at 600 rpm contribute to the enhancement of tensile properties, the increase in grain size and ε martensitic fraction have resulted in a noteworthy reduction in ultimate tensile strength (UTS) and elongation (EL). Conversely, the joint of 300 rpm exhibited optimum UTS and EL owing to the grain refining after FSW and the transformation induced plasticity (TRIP) effect. These insights confirm that FSW is an ideal approach for achieving superior joint performance in dual-phase HEAs.

Molecular dynamics study on martensitic transformation behavior of SLM-NiTi alloy induced by temperature and stress

Selective laser melting (SLM) technology is currently one of the most promising additive manufacturing technologies for complex metal components. NiTi alloy has been highly regarded in advanced applications due to its excellent shape memory and good biocompatibility. However, as a new material, SLM-NiTi alloy is far from being applied in actual advanced fields. In the actual processing, such as grinding, turning, polishing, electrical discharge machining, all involve changes in temperature and stress, Therefore, it is very important to study the martensitic phase transition caused by temperature and stress changes in the precision machining process of SLM-NiTi alloy. However, it is difficult to observe the martensitic phase transition changes directly in the actual processing, so the method of molecular dynamics is adopted in this paper. Moreover, in the process of preparing NiTi alloy by selective laser melting, the ratio of Ni to Ti is very important, which determines the final forming quality. Therefore, this paper studied the martensitic transformation behavior induced by temperature and stress under different nickel proportions, different initial temperatures and different model sizes, and expounded the variation laws of stress–strain, potential energy, volume and dislocation. The microstructure and shear strain were demonstrated on the atomic scale. The results show that temperature plays an important role in the martensite transformation of SLM-NiTi alloy, low temperature will largely inhibit martensite transformation, and high temperature will promote martensite transformation. The stress induced martensite reorientation in SLM-NiTi alloy is accomplished by the migration of the interface between different martensite variants. When the nickel content is 52% and 55%, there is no inflection point between volume and potential energy with the change of temperature, when the nickel content is 50.8%, there is an obvious jump between volume and potential energy. The research in this paper is helpful to guide the processing technology of SLM-NiTi alloy, and also broadens the application of additive manufacturing materials.

Martensite Variant Identification Method for shape memory alloys by using graph neural network

Detailed microstructure evolution in shape memory alloys (SMAs) can be studied by molecular dynamics (MD) simulations. However, the conventional post-processing methods for atomistic calculations, such as Common Neighbor Analysis (CNA), fail to identify distinct crystal variants and to reveal twin alignments in SMAs. In the current work, a powerful and efficient post-processing tool based on GraphSAGE neural network is developed, which can identify multiple phases in martensitic transformation, including the orthorhombic, monoclinic and R phases. The model was trained by the results of sets of temperature- and stress-induced martensitic transformation MD calculations. The accuracy and generality were also verified by the application to the cases which did not appear in the training dataset, such as the unrecoverable nanoindentation process. The proposed method is rapid, accurate, and ready to be integrated with any visualization tool for MD simulations. The outcome of the current work is expected to accelerate the pace of atomistic studies on SMAs and related materials.

Investigations of Shape Deformation Behaviors of the Ferromagnetic Ni-Mn-Ga Alloy/Porous Silicone Rubber Composite towards Actuator Applications.

Ferromagnetic shape memory alloys (FSMAs), which are potential candidates for future technologies (i.e., actuators in robots), have been paid much attention for their high work per volume and rapid response as external stimulation, such as a magnetic field, is imposed. Among all the FSMAs, the Ni-Mn-Ga-based alloys were considered promising materials due to their appropriate phase transformation temperatures and ferromagnetism. Nevertheless, their intrinsic embrittlement issue and sluggish twin motion due to the inhibition of grain boundaries restrict their practicability. This study took advantage of the single-crystal Ni-Mn-Ga cube/silicone rubber composite materials to solve the two aforementioned difficulties. The single-crystal Ni-Mn-Ga cube was prepared by using a high-temperature alloying procedure and a floating-zone (FZ) method, and the cubes were verified to be the near-{100}p Ni-Mn-Ga alloy. Various room temperature (RT) curing silicone rubbers were utilized as matrix materials. Furthermore, polystyrene foam particles (PFP) were used to provide pores, allowing a porous silicone rubber matrix. It was found that the elastic modulus of the silicone rubber was successfully reduced by introducing the PFP. Additionally, the magnetic field-induced martensite variant reorientation (MVR) was greatly enhanced by introducing a porous structure into the silicone rubber. The single-crystal Ni-Mn-Ga cube/porous silicone rubber composite materials are considered to be promising materials for applications in actuators.

Modulated martensite in NiTi shape memory alloy exposed to high stress at high temperatures

NiTi shape memory wire was deformed in martensite state at low temperature −125 °C up to 750 MPa stress and subsequently heated far above the Af temperature while the stress 750 MPa was held constant. Under such conditions, the heated B19’ martensite in the wire was driven towards its thermomechanical stability limits and eventually transformed into plastically deformed austenite. In order to evaluate the mechanism by which such highly constrained reverse martensitic transformation proceeds, martensite variant microstructures and lattice defects evolving in the heated wire were analyzed by TEM and HRTEM.It is found that, the oriented B19’ monoclinic martensite deformed plastically via kwinking deformation involving coordinated dislocation slip and twinning and simultaneously adopted a long period modulated crystal structure, before it transformed to austenite upon heating under applied stress. It is proposed that the modulated martensite structure forms from the B19’ structure via coordinated slip of partial dislocations with Burgers vector b = a/2 [100] on (001) crystal planes. Coordinated slip was enforced by the kwinking deformation and partial dislocations started to glide because the martensite was exposed to elevated temperatures. Plastic deformation of oriented B19’ martensite enabling the reverse martensitic transformation upon the constrained heating is discussed in view of its impact on NiTi functionality in engineering applications.

Plastic deformation of B19’ monoclinic martensite in NiTi shape memory alloys: HRTEM analysis of interfaces in martensite variant microstructures

Martensite variant microstructures in grains of nanocrystalline NiTi shape memory wire deformed in tensile test at room temperature up to 15% strain were analyzed in transmission electron microscope (TEM) with the aim to reveal the mechanism of plastic deformation of B19’ monoclinic martensite which created these microstructures. The analysed microstructures consisted of parallel and/or wedge arranged deformation bands within the martensite matrix oriented in a common <010> low index zone. High resolution TEM (HRTEM) analysis of 35 interfaces within a single grain was performed. The observed interfaces were reasonably planar on mesoscale of individual grains but highly irregular on atomic scale. Majority of analysed interfaces displayed interface planes and lattice misorientations similar to (100), (20-1) and (10-1) martensite twins, but not exactly, many interfaces deviated from the exact twin planes and misorientations. Some interfaces displayed crystal lattices only rotated within the <010> low index zone, i.e. lacking the mirror symmetry of adjoining lattices characteristic for twins. On the other hand, traces of (001) crystal planes were mirror symmetric across all observed interfaces. These experimental observations were rationalized by assuming that the analysed martensite variant microstructures were created by the plastic deformation of B19’ martensite by kwinking deformation proceeding via coordinated [100](001) dislocation slip based kinking combined with (100) twinning.

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.

Effects of additive manufacturing process parameters and heat treatment on texture evolution and variant selection during austenite-martensite transformation in 18%Ni-M350 maraging steel

Variant selection and texture evolution of 18%Ni M350 maraging steel produced by laser-based directed energy deposition (DED) additive manufacturing (AM) technology are investigated in this study. The effects of heat treatment and AM processing parameters, including energy area density (EAD), powder feed rate, and laser power, on variant selection and texture evolution are discussed. Theoretically, 24 martensite variants tend to be formed during a cubic-to-cubic transformation based on the Kurdjumov–Sachs orientation relationship. Identifying the preference of variants in a thermo-mechanical process can pave the way for tailoring the desired product texture. Grain graph and local neighbor voting algorithms using measured electron backscatter diffraction (EBSD) were employed to reconstruct the parent austenite grain structure in this study. Even though no evidence of global variant selection is found, the variant selection tends to take place locally in small parent grains of heat-treated samples. Strong rotated Copper 112110γ and Z 111110γ components are observed in the parent austenite phase of the low EAD sample after heat treatment due to a high degree of recrystallization and second-order twins. A Goss texture 110001α in child martensite transitioned from these strong components is also observed to be strongly derived from packet 3 of this sample. This finding indicates that some low-misorientation variants of packet 3 are more energetically favorable to develop in a heat-treated M350 alloy produced by a low EAD.

Study of ageing and size effects in Nickel–Titanium shape memory alloy using molecular dynamics simulations

ABSTRACT The effect of ageing process at 1073.15 K on the athermal phase transformation in different nanograins (4.5, 6 and 9 nm) of Nickel–Titanium shape memory alloy is studied using molecular dynamics simulations. The martensitic transformation start temperature and transformed martensite volume decrease with decreasing grain size. The two-stage phase transformation with intermediate phase formation is observed only in the smallest grain (4.5 nm) where a single martensite variant is formed. On the contrary, larger grains show a self-accommodated twinned martensite variants with one-stage transformation from austenite to martensite. The ageing process hardly influences the morphology of the martensite phase, however, it rises the martensite start temperature and reduces the austenite start temperature. The nano grain size models demonstrate the constrained transformation due to the transformation barrier and the accumulation of internal stress, while the ageing process shows the tendency for stress relaxation.

Controllable selection of martensitic variant enables concurrent enhancement of strength and ductility in a low-carbon steel

A novel strategy through controlling the selection of martensitic variant in metallic materials is proposed to break through the trade-off between strength and ductility. Here we prepared a high-strength low-carbon steel by applying the two-stage mechanical processing combined with high-temperature large-reduction compression and the subsequent compression above the low critical point of austenitization. The steel has the lowest degree of static recrystallization and dynamic recrystallization after the two-stage mechanical processing, leading to the smallest prior austenite grains with an average diameter of 16.4 μm and the highest dislocation density of 10.21 × 1014 m−2. With the refinement of the prior austenite grains caused by the adjustment of the two-stage deformation process, the driving force required for martensitic transformation increases and variants selectivity becomes stronger, leading to a large number of fine martensitic laths and nanoscale twins after martensitic transformation. Consequently, the best combination of strength and ductility is obtained, with the yield strength of 871 MPa, tensile strength of 1054 MPa and the total elongation of 25%. The use of variant selection exploits the strengthening of both grain boundaries and dislocations, resulting in simultaneous enhancement of strength and ductility. These findings demonstrate how multiple deformation mechanisms can be deliberately activated via the controllable selection of martensitic variants.

Kwinking as the plastic forming mechanism of B19[formula omitted] NiTi martensite

Irreversible plastic forming of B19′ martensite of the NiTi shape memory alloy is discussed within the framework of continuum mechanics. It is suggested that the main mechanism arises from coupling between martensite reorientation and coordinated [100](001)M dislocation slip. A heuristic model is proposed, showing that the (201̄)M deformation-twin bands, commonly observed in experiments, can be interpreted as a combination of dislocation-mediated kink bands, appearing due to strong plastic anisotropy, and reversible twinning of martensite. We introduce a term ’kwinking’ for this combination of reversible twinning and irreversible plastic kinking The model is subsequently formulated using the tools of nonlinear elasticity theory of martensite and crystal plasticity, introducing ’kwink interfaces’ as planar, kinematically compatible interfaces between two differently plastically slipped variants of martensite. It is shown that the (201̄)M kwink bands may be understood as resulting from energy minimization, and that their nucleation and growth and their pairing with (100)M twins into specific patterns enables low-energy plastic forming of NiTi martensite. We conclude that kwinking makes plastic deformation of B19′ martensite in polycrystalline NiTi possible despite only one slip system being available.

In-situ high energy X-ray diffraction study of deformation behavior in Nb nanoparticle-martensitic NiTi composite wire

This study investigated the deformation behavior in Nb nanoparticle-martensitic NiTi composite wire during tensile loading by means of in-situ high-energy X-ray diffraction. Compared to Nb nanowires, the dispersedly distributed Nb nanoparticles have little effect on the transformation and deformation behaviors of NiTi matrix, causing the absence of R phase transformation and the occurrence of deformation twinning in NiTi matrix. Meanwhile, the occurrence of deformation twinning in NiTi matrix can reduce the shape strain to be coordinated by the unfavorable orientated martensite variants, and thus decreases the internal stress among NiTi martensite variants generated during martensite reorientation process. In addition, TEM result proved that Nb nanoparticles can achieve exceptionally large elastic strain in B19′-NiTi matrix (∼4%), just as Nb nanowires. However, this large elastic strain (stress) will be released with the occurrence of NiTi deformation twinning.

Microstructure of macrointerfaces in shape-memory alloys

Shape-memory alloys exhibit a rich microstructure, characterized by large regions containing relatively regular laminates mixing different martensitic variants. Macrointerfaces separate these regions, and are often composed of arrays of needle-shaped martensitic domains. We formulate a model for the geometry of arrays of needles and present numerical simulations, appropriate for two compound twinned variants of martensite in a plane stress setting. One important ingredient is a new class of polyconvex energy densities with cubic symmetry, that is able to reproduce the austenitic elastic constants of the relevant materials. The energy-minimizing needle geometry is determined with tools from shape optimization. Our main finding is the “transparency effect”, which characterizes the effect of needles on domain boundaries located in front of their tip. Our numerical results are in good agreement with experimental observations.

Tension induced detwinning of hierarchically twinned non-modulated martensite in Ni-Mn-Ga alloys

Detwinning of martensite accounts for the macroscopic deformation in shape memory alloys, yet the corresponding microstructural and crystallographic evolution remains less understood. Here, the detwinning behaviors of hierarchically twinned non-modulated (NM) martensite in a directionally solidified Ni54Mn24Ga22 alloy under uniaxial tension along the <001>A direction of austenite are systematically investigated. Based on the interrupted in-situ EBSD measurements, it is demonstrated that tensile loading leads to the thickening of favorable variants with the <001>NM parallel to the loading direction through intensive detwinning, which follows the route of detwinning of internal nanotwins as well as collective detwinning of internal nanotwins and inter-plate major-major variant pairs. Besides, the internal detwinning is also coupled with the rigid body rotation owing to the constraints of inter-plate boundaries, resulting in the refinements in the inter-plate correlation as the variation of thickness ratio of nanotwins. Consequently, the hierarchically twinned microstructure finally evolves into a single-variant state with the <001>NM along the loading direction. This work provides clear demonstrations on the detwinning behaviors of hierarchically twinned NM martensite induced by tensile loading, which allows us to gain deep crystallographic insights into the stress-induced rearrangement of martensite variants in association with the macroscopic deformation.

Effects of Predefined Thermomechanical Procedure on the Microstructure and Mechanical Properties of the Two-Way Shape Memory Effect in the NiTi Alloy

In the current paper, a predefined thermomechanical procedure has been applied to the two-way shape memory effect (TWSME) in a NiTi alloy to study the effect of two different applied load conditions on the induced martensitic state. The microstructure of the strips was studied using optical microscopy (OM), scanning electron microscopy (SEM) fitted with an EDS microprobe, and microhardness tests at the end of both the training and thermal cycles. Inducing internal stresses along specified directions during training cycles results in the formation of oriented martensitic variants rather than expected twinned martensitic variants upon cooling. It was found that the microstructure is made up of interlocking martensitic lathes, including the fine martensite colony next to the coarse martensite lathes. Furthermore, the results of the average hardness tests for bending at one point and three points were 241 and 247 HV0.2, respectively. It was shown that only the cubic austenitic phase (B2) and the martensitic monoclinic phase (B19′) experience transformation. The results reveal that homogeneous bending in three locations leads to achieving the best difference between high- and low-temperature curvatures after training.