Nanocrystalline NiTi Research Articles

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

Temperature-dependent mechanical properties and elastocaloric effects of multiphase nanocrystalline NiTi alloys

We investigate the temperature-dependent mechanical properties and elastocaloric effects of different multiphase nanocrystalline NiTi alloys with grain sizes of 10 nm, 15 nm, 20 nm, 27 nm, 34 nm and 55 nm. These multiphase nanocrystalline NiTi alloys mainly consist of B2, B19′ and R phases and can undergo the thermally induced phase transition over a wide temperature range from 208 K to 353 K. Their superelastic deformation (loading to a strain of 4%) is contributed by combinations of the elastic deformation of B2 phase, the R → B2 phase transition, the reversible martensite reorientation and the B2 → R and B2/R → B19' phase transitions. Such complex deformation mechanism together with the microstructure evolution with temperature brings strong temperature-dependences of the mechanical properties and elastocaloric effects. As results, at the grain sizes of 15 nm ∼ 20 nm, the stress under a constant superelastic strain can show a significant V-shaped variation with temperature. At the grain size of 55 nm, the adiabatic temperature drop can even change from −8.5 K to −29 K ∼ −35.8 K as the ambient temperature rises from 293 K to 333 K. By comparison of the mechanical properties and elastocaloric effects of the different multiphase nanocrystalline NiTi alloys, it is found that the one with the grain size of 20 nm exhibits the best comprehensive cooling performances, which has a soft near-linear superelasticity (a moderate driving force), a very wide refrigeration temperature window (> 245 K) with considerable adiabatic temperature drops (∆Tmax= −17.1 K), and a good functional stability. This work provides an experimental basis for using the multiphase nanocrystalline NiTi alloys as solid-state refrigerants.

Thickness Effects on the Martensite Transformations and Mechanical Properties of Nanocrystalline NiTi Wires.

This paper studied the features of the martensitic transformations and mechanical properties of 40, 60, and 90 µm thick NiTi wires with nanocrystalline B2 structures. It was established that the wires were composites and consisted of a TiNi matrix and a TiO2 + TiNi3 surface layer. Structural methods showed that the wire matrix was formed by grains of up to 20 nm in size. The method of measuring the electrical resistivity during cooling and heating revealed a two-stage nature of the martensitic transformation. Cyclic loading-unloading demonstrated that all the samples exhibited superelasticity effects and completely restored their shape when unloaded from a 4-8% relative strain at room temperature. An increase in mechanical characteristics with respect to the wire thickness was experimentally established. This was due to the change in the composition of the TiNi matrix during drawing.

Grain boundary and dislocation strengthening of nanocrystalline NiTi for stable elastocaloric cooling

We fabricate a nanocrystalline NiTi with an average grain size of 31 nm and high-density (4.7 × 1015 m−2) dislocations via cold rolling (38% thickness reduction) and annealing (310 ℃, 2 min) for elastocaloric cooling. It is found that the high-density-dislocation nanocrystalline NiTi shows a high yield strength of 2.09 GPa, a large and stable average temperature drop (ΔT) of 17.3 ℃ over 106 phase-transition cycles under the stress of 1.4 GPa at room temperature. The high cyclic stability originates from grain boundary and dislocation strengthening mechanisms, where the grain boundaries and the high-density dislocations suppress the nucleation of transformation-induced dislocations. The simple and effective processing route via cold rolling and low-temperature annealing is applicable to mass production of bulk nanostructured NiTi with stable elastocaloric cooling performances.

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.

Grain size effect on the temperature-dependence of elastic modulus of nanocrystalline NiTi

The effect of grain size on temperature-dependent elastic modulus (TDEM) of polycrystalline NiTi over a temperature span of 200 °C was investigated by dynamic thermomechanical analysis (DMA). It is found that the TDEM rapidly reduced with grain size down to nanoscale. In-situ neutron diffraction and molecular dynamics (MD) simulation reveal that the volume fraction and thermal stability of austenite phase at low temperature gradually increased with the reduction of grain size due to the mechanical constraint of grain boundary, which results in the two-phase coexistence during cooling. The compensation of the intrinsic opposite TDEM of austenite and martensite leads to the overall reduced TDEM in nanocrystalline NiTi. This study strongly implies that the TDEM of shape memory alloy can be tailored by grain size.

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.

Shear-induced amorphization in nanocrystalline NiTi micropillars under large plastic deformation

Shear-induced amorphization is a phenomenon observed in polycrystalline NiTi shape memory alloys under severe plastic deformation, but the underlying mechanism of such a microstructural change remains elusive. To study the isothermal large-strain plastic deformation behavior and the associated mechanisms, uniaxial compression is performed on nanocrystalline NiTi cuboidal micropillars with initial grain sizes of 35 and 110 nm. It is found that the micropillars demonstrate high deformability with plastic strains up to 110% via shear-induced amorphization. It is shown that plastic strain localization in shear bands of the 35 nm-grain-size sample prompts notable amorphization up to an area fraction of 90% and crystal refinement down to 5 nm. High-resolution transmission electron microscope and molecular dynamics simulations reveal that the shear-induced amorphization starts from the local martensite phase near grain boundaries through accumulation of crystalline defects. The amorphization can lead to a reduction in the crystal size and an increase in the plastic flow stress. The large plastic deformation of the nanocrystalline NiTi micropillars via shear-induced amorphization is enabled by the suppression of crack nucleation through dynamic recovery of nanovoids. Our work provides new perspectives for producing crystalline-amorphous nanostructures in shape memory alloys at small scale.

In-situ synchrotron x-ray diffraction texture analysis of tensile deformation of nanocrystalline superelastic NiTi wire at various temperatures

According to the state-of-the-art view, superelastic deformation of NiTi wires at room temperature proceeds via stress induced martensitic transformation from B2 cubic austenite to B19′monoclinic martensite. With increasing test temperature, the stress induced martensitic transformation is substituted by plastic deformation of austenite at martensite desist temperature MD. However, there are many unsolved problems with this widely accepted view. What are the texture and martensite variant microstructure in stress induced martensite and do they depend on test temperature? Does the austenite transform to martensite completely within the transformation plateau range? How the superelasticity changes into plastic deformation of austenite with increasing temperature - is it stepwise or gradual change? How the wire deforms plastically at various temperatures? Does plastic deformation occur in austenite or in martensite, via dislocation slip or deformation twinning? Are the deformation/transformation processes in nanocrystalline NiTi wires same as in large grain polycrystals? We have addressed these long standing but unsolved questions by performing series of in-situ synchrotron x-ray diffraction experiments on superelastic nanocrystalline NiTi wire subjected to tensile tests at 20, 90 and 150 °C until fracture supplemented by post mortem TEM analysis of lattice defects created by the tensile deformation.It was found that, in case of conventional superelasticity at 20 °C, austenite transformed almost completely to stress induced martensite within the transformation plateau range. The stress induced martensite displayed a sharp two fibre texture reflecting its (001) compound twinned microstructure. Stress induced martensite transformed back to the parent austenite without leaving any significant unrecovered strains and lattice defects in the austenitic microstructure. When loaded further into the plastic deformation range, this martensite deformed via combination of (20-1) and (100) deformation twinning and kinking assisted by [100](001) dislocation slip in martensite. Recoverability of tensile strains on unloading and heating remained surprisingly large (∼10%) up to wire fracture at 62% strain.The superelasticity at elevated temperatures 90 °C (150 °C) was found to be very different. The austenite transformed into a mixture of phases containing 40% (10%) volume fraction of stress induced martensite within the transformation plateau range. The stress induced martensite displayed four fibre texture, which further evolved with increasing strain. The original <111> fibre texture of austenite evolved with increasing strain towards random orientation distribution. The recoverability of tensile strains on unloading and heating sharply decreased with increasing temperature. Two alternative deformation mechanisms are proposed to explain these changes. The first mechanism assumes that martensite stress induced at elevated temperatures appears in a form of thin internally twinned CVP martensite plates surrounded by austenite deforming via dislocation slip. Requirement for strain compatibility at habit plane interfaces affects selection of martensite variants under stress and texture. The second mechanism is based on the idea that martensite stress induced at elevated temperatures immediately deforms plastically and undergoes reverse martensitic transformation to austenite leaving behind unrecovered plastic strain, slip dislocations and {114} austenite twins in the austenitic microstructure of the wire. Since the second mechanism explains the experimental observations better, it is considered to be more realistic.

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.

Micro-damage evolution under intensive dynamic loading and its influence on constitutive and state equations for nanocrystalline NiTi alloy through molecular dynamics

In this study, to comprehensively reveal the damage mechanisms of NiTi alloys, molecular dynamics (MD) simulations were applied to examine the void evolution process under uniaxial and triaxial intensive dynamic loading. A single-crystal model was first used in the MD simulations. The calculation results revealed that the single-crystal NiTi model exhibited a similar damage response to brittle fracture. The corresponding damage mechanism was the rapid growth and coalescence of voids inside the material. Meanwhile, the defect influence was also examined for the single-crystal model, and the reduction effect of the ultimate stress value due to the stress concentration was analyzed quantitatively by the MD simulations. In addition, a polycrystalline model of NiTi was used in the MD simulations. Compared with the single-crystal model, the polycrystalline model showed an evident plastic stage under uniaxial loading due to dislocation slip. The MD simulation proved that the dislocations accumulated on the grain boundaries, which led to a stress concentration effect on the grain boundaries and sequentially resulted in void generation. However, the propagation and coalescence of voids were hindered by the grain interactions, which resulted in a ductile damage behavior inside the material. Based on this mechanism, the grain size influence was also studied in the MD simulations. It was discovered that the grain size effect in the damage stage resulted in a damage ductility enhancement with the decrease in the average grain size value. Finally, based on the relationships between the stress-strain curve, void fraction, and damage behavior, novel constitutive and state equations were proposed with damage terms to consider the void evolution process during the damage stage. The prediction results showed good agreement with the MD simulation data.

Measurement of two-dimensional residual stress in nanocrystalline superelastic NiTi fabricated with pre-strain laser shock peening

The measurement of residual stress is a challenging issue in industrial fields. In this work, focused ion beam (FIB) and digital image correlation (DIC) are combined together to measure the two-dimensional residual stress in nanocrystalline NiTi plates processed with pre-strain laser shock peening (LSP). A four-point bending experiment verifies the accuracy of this measurement method. The pre-strain LSP-treated surfaces are found to have significant compressive residual stress along the pre-strain direction and tensile residual stress along the vertical to pre-strain direction, which verifies pre-strain LSP as an efficient approach to create gradient residual stress layers in nanocrystalline NiTi plates. The FIB-DIC method has also proven to be an attractive tool to measure the two-dimensional residual stress in phase transition nanocrystalline materials.

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.

Orientation dependence of mechanical behavior and phase transformation of NiTi shape memory alloy with multilayer structures by molecular dynamics simulation

Molecular dynamics simulation is used to investigate orientation dependence of mechanical behavior and phase transformation of NiTi shape memory alloy with multilayer structures, including AMA (amorphous-monocrystal-amorphous) model, AMN (amorphous-monocrystal-nanocrystalline) model, and NMN (nanocrystalline-monocrystal-nanocrystalline) model. The middle monocrystal layer has two different orientations. The [100]-oriented monocrystal is in non-preferential orientation, which does not contribute to the stress-induced martensitic transformation. The [11¯0]-oriented monocrystal is in preferential orientation and thus it can undergo martensitic transformation under a relatively low tensile stress. Furthermore, it is beneficial to inducing the formation of multiple martensite variants inside the adjacent grains. For the [100]-oriented AMN model, a small amount of residual martensite appears in the grains with non-preferential orientation or strong mechanical constraints, whereas there is almost no residual martensite in the other five models. A great number of shear transformation regions are observed in the amorphous phase, which causes the degradation of superelasticity in AMA and AMN models. A small number of dislocations are observed at the grain boundaries in nanocrystalline NiTi structure, but their contribution to plastic deformation is very small. Surface nanocrystallization can effectively improve the superelasticity of NiTi samples. For NMN structure without any amorphous phase, the irrecoverable strain can be reduced to about 0.3%.

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.

Ultrahigh cycle fatigue of nanocrystalline NiTi tubes for elastocaloric cooling

Elastocaloric cooling using superelastic NiTi shape memory alloy with reversible phase transformation is a promising environmentally friendly technology, but the limited fatigue life of conventional coarse-grained NiTi remains a crucial bottleneck. Here, we report the ultrahigh fatigue life of nanocrystalline NiTi tubes, exceeding 120 million cycles under a compressive stress of 800 MPa. The NiTi tubes demonstrate stable cyclic stress-strain responses and a stable adiabatic temperature drop of 6.6 K in the lifespan. The material coefficient of performance increases from the initial 8.8 to 11.6 of the 108th compressive cycle. The high resistance to nucleation and growth of compression-parallel cracks results in the ultrahigh fatigue life of the tubes. Our research shows the great potentials of nanocrystalline NiTi tubes with both stable thermomechanical properties and long compressive fatigue lives for reliable elastocaloric cooling.

In-situ synchrotron X-ray diffraction texture analysis of tensile deformation of nanocrystalline NiTi wire in martensite state

Tensile deformation of nanocrystalline superelastic NiTi wire in the martensite state at -90 °C until fracture was investigated by in-situ synchrotron X-ray diffraction texture analysis. The results were interpreted in terms of the activity of various transformation and deformation twinning modes. The strain components into the load axis as well as discontinuous rotations of the martensite crystal lattice due to activation of various twinning modes were calculated using an abstraction of “ideal <hkl> fibre textured polycrystal” and used to estimate the evolution of the martensite texture during the tensile test.It is found that the four fibre texture of selfaccommodated martensite transforms during the martensite reorientation stage in the tensile test into the sharp two fibre texture of the reoriented martensite reflecting its (001) compound twinned microstructure. Plastic deformation of martensite is claimed to proceed via peculiar deformation mechanism involving deformation twinning and kinking assisted by [100]/(011) dislocation slip. This deformation mechanism enables the nanocrystalline NiTi wire to deform at ∼1 GPa engineering stress up to very large plastic strains ∼50% with small strain hardening and brings about refinement of the austenitic microstructure of the NiTi wire down to nanoscale.

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.