Nanocrystalline NiTi Shape Memory Alloys Research Articles

Physical mechanisms for dependence of temperature-induced phase transition and shape memory effect on grain size in nanocrystalline NiTi shape memory alloys

Physical mechanisms for dependence of temperature-induced phase transition and shape memory effect (SME) on grain size (GS) of nanocrystalline NiTi shape memory alloys (SMAs) were revealed by combining experiment and molecular dynamics simulation. Influences of GS on phase transition temperature and stress level during tension loading below martensite transition finish temperature Mf are interpreted by core-shell theory that energy of shell (grain boundary (GB)) is higher than that of core (grain interior), so shell becomes an obstacle to deformation of the core and the obstacle increases with reducing GS because the fraction of GB increases with decreasing GS. Due to the constraint of shell, martensite transition during cooling is so incomplete that stress-induced martensite occurs in nanocrystalline NiTi SMAs deformed below Mf. After unloading, some stress-induced martensite phases are retained due to the lack of driving force for reverse martensite transition at low temperature. This is why SME occurs in the alloy with GS below 10 nm when no temperature-induced phase transition happens. Under the same tension strain, inelastic strain after unloading increases with increasing GS. The increase of GS causes the increase of SME strain, and the residual strain after heating the deformed nanocrystalline NiTi SMAs stems from the plastic deformation of martensite and austenite in grain interior during tension.

Effects of porosity and cyclic deformation on phase transformation of porous nanocrystalline NiTi shape memory alloy: An atomistic simulation

Utilizing molecular dynamics simulation, this study aims to explore the phase transformation behavior of porous nanocrystalline (NC) NiTi shape memory alloys (SMAs) when subjected to cyclic deformation. The influences of porosity and cyclic deformation on the phase transformation of NC NiTi SMAs are examined and discussed. The simulation results show that the increase in the porosity and number of cycles leads to a decrease in both the critical phase transformation stress and peak stress whereas an increase in the residual martensite, phase boundary, and interstitial atoms; the related results can be supported by previous experiments. After cyclic deformation, the reduction in the potential energy for the entire system during the tensile phase occurs at an earlier stage, indicating that the martensitic transformation occurs earlier as the number of cycles increases. Notably, the dissipated energy demonstrates a decrease with an increasing number of cycles, and the potential energy during the austenite elastic unloading stage undergoes a transition from a decreasing to an increasing trend due to the presence of residual martensite increasing with the number of cycles.

Constitutive modeling of porosity and grain size effects on superelasticity of porous nanocrystalline NiTi shape memory alloy

Constitutive modeling of porosity and grain size effects on superelasticity of porous nanocrystalline NiTi shape memory alloy

Effects of Thermal Cycling and Porosity on Phase Transformation of Porous Nanocrystalline NiTi Shape Memory Alloy: An Atomistic Simulation

Herein, the effects of thermal cycling and porosity on the martensitic phase transformation behavior of porous nanocrystalline (NC) NiTi shape memory alloys (SMAs) at the atomic scale are investigated by molecular dynamics simulation. The simulation results show that thermal cycling causes the NC NiTi SMAs to repeatedly undergo martensitic phase transformation and inverse phase transformation processes. The martensitic phase transformation capability is suppressed with increasing porosity and number of cycles, but the suppression effect of thermal cycling is weakened with increasing porosity. Moreover, the start temperature of martensitic transformation, residual martensitic phase, and the fraction of interstitial atoms increase with increasing porosity. Thermal cycling causes the disordered structures and shear plastic deformation to accumulate as cycling increases. Recoverable shear deformation is generated within the grain, but shear plastic deformation is mainly concentrated at both grain boundaries and pore surfaces. Residual martensitic phase increases after thermal cycling. These results provide important explanations and references for a deeper understanding of phase transformation behavior and pore effects caused by thermal cycling.

Enhanced two way shape memory effect in nanocrystalline NiTi shape memory alloy wires

Two-way shape memory effect (TWSME) has wide application prospect in the field of lightweight actuators. In the present study, an excellent TWSME with large magnitude and small cyclic degradation of NiTi wires is achieved by combining martensite deformation and nanocrystallization. The wires attain a maximum of 7.37% TWSME strain after pre-deformed to 11% and maintain a stable strain of 6.88% after 20 cycles. Microstructural characterization reveals that the excellent TWSME originates from both the dislocation structures induced by martensite deformation and nanocrystalline strengthened matrix. On one hand, the former provides adequate oriented stress field. On the other hand, the latter guarantees the availability of high-volume fraction of preferentially oriented martensite variations in subsequent thermal cycles through suppressing the formation of deformation twins, which are frequently discussed in coarse-grain NiTi.

Effect of aspect ratio on the elastocaloric effect and its cyclic stability of nanocrystalline NiTi shape memory alloy

NiTi shape memory alloy (SMA) has been suggested as an environmentally friendly solid-state refrigerant that is expected to replace the traditional vapor compression technology. However, its various refrigeration indicators still need to be further explored and enhanced. Nanocrystalline NiTi SMA has significant advantages in terms of elastocaloric effect (eCE) and cyclic stability compared with traditional coarse-grained NiTi SMA. In this work, the influence of aspect ratio (L/W, where L and W are the gauge length and width of the samples, respectively) on the eCE and its cyclic stability of nanocrystalline NiTi SMA was investigated experimentally. The in-situ cyclic tensile tests were performed to observe the axial strain field and temperature field on the surface of the specimens with different aspect ratios. Some microscopic observations were also carried out to clarify the internal mechanism. The experimental results show that as the aspect ratio decreases, the adiabatic temperature change ΔTad (elasocaloric cooling capacity) of the nanocrystalline NiTi SMA gradually decreases, while the coefficient of the performance of material (COPmat) increases, and the cyclic stability of eCE was enhanced. However, the dependence of eCE and its cyclic stability on the aspect ratio is gradually weakened when L/W > 4. The eCE and its cyclic stability of nanocrystalline NiTi SMA are not related to the dislocation slipping but rather attributed to the cyclically accumulated residual martensite. This work can provide valuable reference for designing efficient and reliable nanocrystalline NiTi SMA based solid-state refrigeration devices.

A two-scale thermo-mechanically coupled constitutive model for grain size- and rate-dependent deformation of nano-crystalline NiTi shape memory alloy

In this paper, a two-scale thermo-mechanically coupled constitutive model is established to predict the grain size (GS)- and rate-dependent deformation of nano-crystalline super-elastic NiTi shape memory alloy (SMA). At the mesoscopic scale, the polycrystalline representative volume element (RVE) is considered as an aggregation of individual grains. Owing to the relatively large volume fraction (VF) of grain boundary in nano-crystalline NiTi SMA, an individual grain is further considered as a composite containing the grain interior (GI) and grain boundary (GB) phases. For the GI phase, a crystal plasticity-based model accounting for the internal heat generation is developed through the fundamental laws of irreversible thermodynamics. The constraint of GB phase on the martensite transformation in the GI phase is considered by introducing a scaling law between the transformation hardening modulus (THM) and GS. Owing to its un-transformable nature and relatively high yielding stress, the GB phase is assumed to behavior constitutively linear thermo-elasticity. To evaluate the thermo-mechanical interactions among the grains, GI phase and GB phase, and obtain the overall response of the polycrystalline RVE, a double inclusion self-consistent homogenization scheme involving the material nonlinearity and thermo-mechanical coupling effect is developed in an incremental form. At the macroscopic scale, the overall thermo-mechanical responses for the specimen of nano-crystalline NiTi SMA are obtained by solving the equations of force balance, deformation compatibility, and thermodynamic equilibrium through an approximation method. Finally, considering the specific texture measured in experiments, the proposed model is validated by predicting the GS- and rate-dependent deformation of nano-crystalline NiTi SMA sheet. Furthermore, the anisotropic thermo-mechanical responses of the sheet caused by the texture are predicted and discussed.

Tensile deformation of NiTi shape memory alloy thermally loaded under applied stress

Constitutive behavior of engineering materials is typically characterized by stress–strain curves from isothermal tensile and/or compression tests until fracture. Strain reversible behavior of martensitically transforming shape memory alloys (SMA) is additionally characterized by cyclic stress–strain and strain-temperature curves limited to temperatures and stresses, at which the recorded strain responses are reversible in closed loop cycles. In this work focussing coupled martensitic transformation and plastic deformation of NiTi, we deformed nanocrystalline NiTi SMA wire in isothermal and isostress tensile tests beyond the temperature and stress limits stemming from the requirement on the strain reversibility in closed loop cyclic tests. Stress–strain-temperature responses of NiTi wire in such tests were recorded and analysed. To detect and characterize deformation mechanisms activated in performed thermomechanical loads, electric resistance and dynamic elastic modulus of the wire were evaluated in-situ during tensile tests. Martensite variant microstructures and lattice defects in austenite evolving upon heating deformed NiTi wire under 750 MPa stress were analyzed by post mortem transmission electron microscopy. Stress-temperature diagram showing critical stress–temperature conditions for activation of 5 different deformation/transformation processes in thermomechanically loaded NiTi was constructed from the results of isothermal and isostress tests and the recorded stress–strain-temperature responses were discussed based on this diagram

Combined effects of grain size and training on fatigue resistance of nanocrystalline NiTi shape memory alloy wires

The structural fatigue life of nanocrystalline NiTi wires with grain size ranging from 53 nm to 125 nm was improved by stress-controlled mechanical training and the percentage of the improvement increased with grain size. The structural fatigue life decreased with the increased grain size for the untrained samples. After training, the structural fatigue life increased with grain size. The generation and movement of dislocations during training contributed to lattice rotation, improving orientation intensity of 〈111〉 parallel to drawn direction texture. The reduction of fatigue striation spacing revealed training-induced plastic deformation and enhanced 〈111〉 texture contributed to hindering crack propagation.

Constitutive model for nanocrystalline NiTi shape memory alloys considering grain size effects and tensile–compressive asymmetry

Mechanical properties of nanocrystalline NiTi shape memory alloys (SMAs) change drastically with grain size. The present contribution develops a constitutive model to reproduce the grain size dependent superelastic behavior and tensile–compressive asymmetry observed in the experiments of nanocrystalline NiTi SMAs. Effects of grain size are incorporated in the developed model by introducing the intrinsic length scale accounting for the transformation hardening as well as the grain-core and grain-boundary phase. In this work, nanocrystalline NiTi SMA is regarded as a two-phase composite material made of inclusions of the grain-core phase dispersed in the grain-boundary phase acting as a matrix. A transformation function allowing for the description of fine-grain strengthening mechanism and tensile–compressive asymmetry is proposed. In the grain-core phase, the evolution law for transformation strain during the forward and reverse transformation is determined. Besides, the constitutive relation of the grain-boundary phase is assumed to be linearly elastic. Based on the equivalent secant bulk and shear modulus of the grain-core and grain-boundary phase, the stress–strain relationship of nanocrystalline NiTi SMAs is derived by using the extended Mori–Tanaka method. Comparisons between experimental and predicted results demonstrate that the proposed model has the ability to reproduce the grain size dependent deformation and asymmetric stress–strain behavior under tension and compression of nanocrystalline NiTi SMAs. In detail, it is found that critical transformation stresses for forward and reverse transformations, dissipation energy density, transformation strain hardening, and maximum transformation strain are sensitive to the grain size and stress states.

Molecular dynamics simulation of grain size effect on mechanism of twin martensite transformation of nanocrystalline NiTi shape memory alloys

In this work, a constitutive model considering grain size and model size is established to describe the stress-strain response of nanocrystalline NiTi Shape Memory Alloys (SMAs) with various grain sizes, and the stress-strain curves of nanocrystalline NiTi SMAs with various grain sizes are obtained by Molecular Dynamics (MD) simulation. The correctness of the simulation results is verified by the comparison with the constitutive model. Then, the dependence of the deformation mechanism of the twin martensite transformation on the grain size is studied by MD simulation. The results show that neither temperature nor stress induces twin martensite transformation for grain size less than 20 nm. While twin martensite occurs in the models with grain sizes of 20 and 30 nm during phase transformation. Only one twin plane occurs for the grain size of 20 nm, but multiple twin planes occur for the grain size of 30 nm. During twin martensitic phase transformation, the proportion of grain boundary remains basically unchanged, while the proportion of phase boundary increases. With the grain size increasing, the twin plane and the proportion of phase boundary increase, and the appearance of multiple twin planes is closely related to the increase of phase boundary.

Transformation yield surface of nanocrystalline NiTi shape memory alloy

Molecular dynamics (MD) simulation is used for investigating mechanical behavior and phase transformation in nanocrystalline NiTi shape memory alloy (SMA) under complex stress states, including uniaxial, biaxial and triaxial stress states. The simulation results indicate that tension-compression asymmetry occurs in terms of transformation strain, critical transformation stress, dissipation energy density, martensite variant and martensite domain. This asymmetry is mainly aroused by the fact that transformation strains under the stress states with tensile loading in the major deformation direction are larger than the counterparts under the stress states with compressive loading in the major deformation direction for most crystallographic orientations. The phase transformation stresses in complex stress states strictly comply with neither the Von Mises criterion without considering tension-compression asymmetry nor the Bouvet and Patoor criteria considering tension-compression asymmetry. Consequently, a new transformation yield criterion on the basis of Bouvet and Patoor criteria is proposed in the present work to precisely describe the transformation stress anisotropy of nanocrystalline NiTi SMA under complex stress states.

Molecular dynamics investigation on mechanical behaviour and phase transition of nanocrystalline NiTi shape memory alloy containing amorphous surface

It is well known that surface severe plastic deformation will cause the surface amorphization of nickel-titanium shape memory alloy (NiTi SMA), so it is of significance to investigate the influence of amorphous phase on mechanical behaviour and phase transition of NiTi SMA. Nanocrystalline polycrystal (PC) models with the average grain size of 9, 12 and 15 nm, respectively, are established by molecular dynamics (MD) software. On the upper and lower surfaces of the aforementioned three PC models, the corresponding amorphous layers are formed to obtain the amorphous-nanocrystalline-amorphous (ANA) counterparts. MD simulation is used to investigate mechanical behaviour and phase transition of nanocrystalline NiTi SMA with amorphous surface. The phase transition stress, the maximum tensile stress and the irrecoverable strain increase with decreasing grain size. Furthermore, grain orientation has a great impact on phase transition of NiTi SMA. The existence of amorphous phase leads to superelastic degradation due to its plastic deformation, but it can greatly improve the tensile stress of NiTi SMA. The irrecoverable strain is reduced from 1.3% to 0.9% when the thickness of amorphous layer is decreased from 10 to 5 nm. The existence of amorphous phase results in reduction of martensite variants in the selected grains.

Molecular dynamics simulation of mechanical behavior and phase transformation of nanocrystalline NiTi shape memory alloy with gradient structure

Severe plastic deformation can lead to surface amorphization of nickel-titanium shape memory alloys (NiTi SMAs), and amorphous layer can be transformed into nanocrystalline layer by means of appropriate heat treatment. Consequently, molecular dynamics simulation is used to investigate mechanical behavior and phase transformation of nanocrystalline NiTi shape memory alloy with gradient structure, where surface layers possess the average grain sizes of 6, 9, 12 and 15 nm, respectively, and the middle layer possesses the average grain sizes of 20 nm. The maximum tensile stress and the irrecoverable strain decrease with the increase of the average grain size in the surface layers. As for the maximum tensile stress, the maximum value is 2.46 GPa and the minimum value is 1.92 GPa. As for the irrecoverable strain, the maximum value is 0.5%, and the minimum value is 0.25%. Amorphous phase and grain boundaries (GBs) have an important influence on the superelasticity of NiTi SMA. The degradation of superelasticity is mainly attributed to plastic deformation of the amorphous phase and the GBs.

Molecular dynamics simulations on one-way shape memory effect of nanocrystalline NiTi shape memory alloy and its cyclic degeneration

The one-way shape memory effect (OWSME) of nanocrystalline NiTi shape memory alloy (SMA) and its cyclic degeneration under thermo-mechanically coupled cyclic loading conditions were investigated by molecular dynamics (MD) simulations. Firstly, the critical temperatures of martensite transformation were obtained by carrying out the MD simulations of temperature-induced martensite transformation; then, the microscopic mechanisms of martensite reorientation-induced plasticity, the deformation during the subsequent heating and cooling, and the cyclic degeneration of OWSME were investigated based on the correspondent MD simulations. The results show that the martensite reorientation of nanocrystalline NiTi SMA occurs via the migration of the interfaces between martensite variants, the growth of martensite variants with favorable orientations and their swallowing to the variants with unfavorable orientations under specific mechanical loading conditions; the cyclic degeneration of OWSME in nanocrystalline NiTi SMA is caused by the interaction among the martensite reorientation, martensite transformation, martensite reorientation-induced plasticity and martensite transformation-induced plasticity occurred in the martensite region and at the interfaces between martensite variants. The martensite reorientation in each OWSME cycle is reflected by the migration of martensite variant interfaces and the growth and swallowing of martensite variants; the martensite transformation in each OWSME cycle is the nucleation and growth of martensite phase; and the accumulation of plastic deformation is caused by the migration of martensite variant interfaces and the dislocation slipping at the martensite variant interfaces which leads to the cyclic degeneration of OWSME. The MD simulations are verified further by comparing them with the available experimental results of NiTi SMA.

Comparison for Grain Growth Dynamics of Severely Deformed Austenite and Martensite NiTi Shape Memory Alloys after Complete Crystallization of Amorphous Phase

Severe plastic deformation (SPD) based on local canning compression is imposed on Ni-rich Ni50.7Ti49.3 (at.%) shape memory alloy (SMA) with B2 austenite phase and equiatomic Ni50Ti50 (at.%) SMA with B19′ martensite phase in order to induce the amorphization of the two alloys. SPD-processed Ni50.7Ti49.3 and Ni50Ti50 samples are annealed at 450°C for various times in order to comparatively investigate grain growth dynamics of severely deformed austenite and martensite NiTi SMAs after complete crystallization of amorphous phase. The grain growth exponent of B2 austenite Ni50.7Ti49.3 SMA is 2.78, whereas the grain growth exponent of B19′ martensite Ni50Ti50 SMA is 3.18. The results show that grain growth dynamics of amorphous NiTi samples is sensitive to the chemical composition. The grain growth of equiatomic B19′ martensite Ni50Ti50 sample is inhibited to a greater extent than that of Ni-rich B2 austenite Ni50.7Ti49.3 one. The mechanisms for grain growth of NiTi SMAs deal with formation of nano-clusters due to crystallization of amorphous phase, recovery of retained nanocrystalline grains containing dislocations under thermal driving force, growth of retained nanocrystalline grains as well as new nanocrystalline grains and incorporation of nano-clusters or smaller nanocrystalline grains into larger nanocrystalline grains. The grains of Ni50.7Ti49.3 sample absorb nano-clusters more readily than that Ni50Ti50 one during grain growth. Investigation on grain growth dynamics of severely deformed NiTi SMAs lays the foundation for fabricating nanocrystalline NiTi SMAs with tailored grain size.

PHASE FIELD SIMULATION ON THE SUPER-ELASTICITY AND SHAPE MEMORY EFFECT OF GRADIENT NANOCRYSTALLINE NiTi SHAPE MEMORY ALLOY1)

A two-dimensional phase field model was established to simulate and predict the super-elasticity, one-way and stress-assisted two-way shape memory effects of gradient nanocrystalline NiTi shape memory alloy system. The simulated results show that in the super-elastic process of gradient nanocrystalline NiTi alloy, the characteristics of martensite transformation and its reverse in the traditional coarse-grained NiTi alloy, i.e., nucleation-expansion and reduction-disappearance of local martensite band, are retained in the relatively coarse-grained region, but with the decrease of grain size, a homogeneous transformation mode, i.e., without the formation of local martensite band, is observed in the fine-grained region; moreover, in the super-elastic and shape memory processes, both the martensite transformation and reorientation originate from the relatively coarse-grained region and then propagate progressively to the fine-grained one, while the reverse transformation is opposite. The gradual propagations of martensite transformation and reorientation make the stress-strain and strain-temperature curves of gradient nanocrystalline NiTi alloy show a remarkable “hardening”, which can be attributed to the grain size-dependence of martensite transformation and re-orientation in nanocrystalline NiTi alloy, i.e., with decreasing the grain size, the transformation or re-orientation barrier increases gradually, and the nucleation and expansion of martensite transformation or re-orientation becomes more and more difficult. It is concluded that the gradient nanocrystalline structure has wider transformation stress, reorientation stress and transformation temperature windows than the traditional uniform-grained NiTi alloys, which means that the controllability of the inelastic deformation of such alloy is significantly improved.

Phase field simulation on the grain size dependent super-elasticity and shape memory effect of nanocrystalline NiTi shape memory alloys

Based on the Ginzburg-Landau's theory, the free energy function of polycrystalline system was modified by introducing an extra grain boundary energy, and a new two-dimensional phase field model considering the continuous variation of temperature was proposed to investigate the grain size dependent super-elasticity (SE) and shape memory effect (SME) of nanocrystalline NiTi shape memory alloys (SMAs) and to reveal the microscopic mechanism of such a grain size dependence. The simulated results show that: in the SE process, the nucleation-expansion and reduction-disappearance mode of local martensite band observed in the nanocrystalline NiTi SMAs with relatively large grain size can be gradually converted to a uniform martensite transformation and its reverse in the ones with smaller grain size; in the process reflecting one-way shape memory effect (OWSME), with the reduction of grain size, the content of remained austenite phase in the martensitic polycrystalline system obtained by quenching an original austenite one increases, and the involved inelastic deformation mechanism during tension-unloading progressively changes from martensite reorientation to reversible martensite transformation; for the stress-assisted temperature-induced martensite transformation (SATIMT), the content of austenite phase in the polycrystalline system at the lowest applied temperature increases with decreasing the grain size, and the martensite transformation cannot occur when the grain size is below a certain critical one. Further analysis indicates that the dependence of the SE and SME of nanocrystalline NiTi alloys on the grain size can be attributed to the increased proportion of un-transformable grain boundary with decreasing the grain size, whose inhibition to the martensite transformation within grains becomes stronger and stronger, i.e., the energy barrier increases progressively.

Tension-Compression asymmetry of single-crystalline and nanocrystalline NiTi shape memory alloy: An atomic scale study

In this study, the tension-compression asymmetry of single-crystalline and nanocrystalline NiTi shape memory alloys (SMAs) was investigated by molecular dynamics (MD) simulations. Compound twinning martensite variants were simulated via thermally-induced martensitic transformation. The characteristics of the forward and reverse martensitic transformations were derived using atomic structural evolution. The tension-compression asymmetry of single-crystalline NiTi was attributed to different stress-induced martensitic variants and different deformation modes, which led to stress asymmetry and strain asymmetry, respectively. However, the phenomenological tension-compression asymmetry in nanocrystalline NiTi was mainly attributed to stress-induced martensitic variants, and no clear martensitic reorientation occurred under tension and compression loads. The corresponding atomic-scale structural evolution also explained the shorter tensile stress plateau in the nanocrystalline NiTi when compared to that in the single crystal samples. Moreover, the nanocrystalline tension-compression asymmetric behaviour was simulated under a broad temperature range to obtain the critical stress-temperature relation. Finally, the tension-compression asymmetry mechanisms of single-crystalline and nanocrystalline NiTi SMAs were numerically derived at the atomic scale.

Grain size-dependent energy partition in phase transition of NiTi shape memory alloys studied by molecular dynamics simulation

Nanocrystalline NiTi shape memory alloys show fundamental changes in phase transition behavior when the grain size is reduced down to nano-scale (grain size <30 nm). However, due to the extreme difficulties of in-situ experimental observation, universally acknowledged physical mechanisms and detailed microstructures at atomic level are still not clear. In this paper, detailed microstructure images and quantified energy partition at atomic level during stress-induced phase transition are obtained by molecular dynamics simulation. Phase transition is gradually suppressed and incomplete phase transition occurs as grain size decreases. The potential energy landscape of the nanocrystalline system changes significantly from nonconvex to convex, which leads to corresponding changes in stress–strain response and microstructural evolution. With decreasing grain size, the interface (grain boundary and phase boundary) energy makes much more contributions in variation of the system potential energy than the crystallite (austenite and martensite) energy. The mechanism of energy dissipation changes from atomic interface friction in phase transition to plastic deformation caused by grain boundary sliding. It is the gradual dominance of interfacial energy terms in total potential energy that leads to the observed fundamental changes of phase transition behavior in nanocrystalline NiTi.