Irrecoverable Strain Research Articles

Stress-induced microstructural evolution of martensite in additive manufactured Cu-13Al-4Mn shape memory alloy

Cu-13Al-4Mn (wt%) shape memory alloys were fabricated by additive manufacturing and exhibits good shape memory effect. Corresponding microstructural evolution was revealed under the compressive condition. Results indicate that stress-induced twin-related transformation played an important role in deformation process. At the initial deformation stage, the reorientation of (−1–1) twin was activated. With the pre strain increased, detwinning and crossed deformation twin occurred, which accompanied by massive stacking faults, Lomer-Cottrell (L-C) locks, resulted in the formation of irrecoverable strain.

Impact of Hf alloying on the functional properties of Ni-Mn-Ga high temperature shape memory alloys

The paper reports on the functional properties and microstructure of polycrystalline Ni-Mn-Ga-Hf high temperature shape memory alloys, in which different amounts of Hf up to 4 at.% are added in substitution of Mn and Ga, while the nominal Ni content is kept constant. The increase in the amount of Hf promotes a decrease in the valence electron concentration (e/a), tetragonality (c/a) of the martensitic unit cell, and martensitic transformation temperatures, as well as a significant decrease of the transformation hysteresis. The non-modulated tetragonal martensite, typical of Ni-Mn-Ga high temperature shape memory alloys, is formed in all the alloys studied here, though the alloy with 4 at.% of Hf also contains a small fraction of modulated 14M martensite. Hf addition improves the thermomechanical response of the alloys under compressive stress up to 300 MPa and leads to nearly closed cycles with minimal irrecoverable strain and low hysteresis (8 K) for the alloys with 3 and 4 at.% of Hf. These alloys also demonstrate excellent stability under repetitive thermal cycling and little change upon aging at 870 K. A second phase rich in Hf and Ni starts to precipitate at 1 at.% of Hf addition and its volume fraction experiences an abrupt and progressive increase for higher Hf contents. The structure of the second phase looks like the usual f.c.c. γ phase reported in other Ni-Mn-Ga-based alloys, but it has a double lattice parameter. Two structural models based on the A6 face-centered tetragonal unit cell with space group I4/mmm (No. 139), equivalent to the double f.c.c. lattice, are proposed for this new phase.

Superelastic Properties of Aged FeNiCoAlTaB Cold-Rolled Shape Memory Alloys

In the present study, microstructure and cyclic tensile tests were used to measure the superelastic responses of Fe40.95Ni28Co17Al11.5Ta2.5B0.05 (at.%) shape memory alloys after 97% cold rolling. Cold-rolled samples underwent annealing heat treatment (1250 °C/1 h) followed by quenching in water or aging heat treatment (700 °C/6 h and 700 °C/12 h) followed by quenching in water. The microstructure results showed that the average grain size increased from 210 μm to 1570 μm as annealing times increased from 0.5 h to 1 h. X-ray diffraction (XRD) spectra for FeNiCoAlTaB (NCATB) showed that in cold-rolled alloys after solution, the strong peak was in the face-centered cubic (γ, FCC) <111> structure. In aged samples, a new peak (γ’, FCC) emerged, the intensity of which increased as aging times rose from 6 to 12 h. Transmission electron microscope (TEM) images showed that the average precipitate size was around 10 nm in 700 °C/6 h specimens and 18 nm in 700 °C/12 h specimens. The precipitate was enriched in Ni, Al, and Ta elements and exhibited an L12 crystal structure. Tensile samples aged at 700 °C for 6 and 12 h exhibited recoverable strains of 1% and 2.6%, respectively, at room temperature. Digital image correlation (DIC) results for the sample aged at 700 °C for 12 h showed that two martensite variants were activated during the superelastic test. Such variants can form corresponding variant pairs (CVPs), which promote tensile deformation. The tensile sample exhibited a gradual cyclic degradation, and a large irrecoverable strain was observed after the test. This irrecoverable strain was the result of residual martensite, which was pinned by dislocations.

Tailoring functional properties of a FeMnSi shape memory alloy through thermo-mechanical processing

The influence of thermo-mechanical processing on the microstructure and functional properties of a Fe–17Mn–5Si–10Cr–4Ni-1(V–C) (wt%) shape memory alloy was systematically investigated. The as-received material was subjected to 25 % cold rolling followed by a recrystallization at 925 °C and single or double aging treatments. Transmission electron microscopy revealed the formation of the ε-martensite and annealing twin boundaries and Shoji-Nishiyama orientation relationships of ε-martensite and γ-austenite in double aged specimen. Cyclic tensile testing demonstrated that the recrystallized and double aged alloy exhibited excellent pseudoelasticity. In the incremental strain test, the alloy achieved the highest peak stress and pseudoelasticity at each cycle. In the constant stresses test, the alloy accumulated a minimal residual strain of only 0.12 % over 50 cycles. This stability was attributed to a strong precipitation strengthening and the interactions between the martensite and the refined microstructural features. In addition, the recrystallized and double aged sample resulted in the greatest recovery stress of 450 MPa upon heating after pre-straining, because of its high yield strength suppressing new martensite formation during cooling process. The results of high-resolution transmission electron microscopy identified a non-Shoji-Nishiyama orientation relationship between the stress-induced ε-martensite after the stress recovery test and γ-austenite matrix, inducing additional irrecoverable strain and raising the recovery stress. Overall, the study can demonstrate that the tailored thermo-mechanical processing enables optimizing the functional performance of FeMnSi alloys.

Comparison between methods for determining the effective vertical yield stress of intermediate fine-grained soils.

Effective vertical yield stress (σ'xy) is essential in accurately describing fine-grained soils' mechanical properties and their behaviour under load over time. It helps assess settlements and stress history. In most constitutive models, this parameter indicates changes in the soil behaviour due to the development of recoverable and irrecoverable strains resulting from loading. The results of oedometric compression tests for 25 soil samples with a wide range of plasticity parameters were used for the investigation. The intermediate fine-grained soils comprised different proportions of clayey, silty and sandy fractions. An in-depth, two-staged statistical analysis was carried out to compare twelve methods of determining effective vertical yield stress, namely: Casagrande (CM), Pacheco Silva (PSM), Butterfield (BTM), Oikawa (OIM), Onitsuka (ONM), Boone (BM), Becker (WM), Morin (WPUVSM) Wang & Frost (DSEM), Tavenas (SEM), Senol (SLSEM), and Janbu (JM). It aimed to check the association of these methods and the consistency of the obtained results. Based on the difference analysis, the methods originated in the work approach (i.e. WM, WPUVSM, DSEM) and CM gave comparable σ'xy values. The methods utilised bi-logarithmic plots (i.e. BTM, OIM, ONM) received slightly greater or lesser σ'xy values than BM and JM. The remaining methods were characterised by medium to the high variability and were sensitive to even the slightest disturbances resulting from the procedure of determining σ'xy.

Reliability of Methods for Determination of Stress History Parameters in Soils

Abstract Stress history acquired by any cohesive soil influences, to a large extent, three groups of fundamental properties indispensable in geotechnical design i.e. state of soil, shear strength, and stiffness characteristics. The basic stress history parameter (from which other parameters are derived) determined directly from laboratory tests is a preconsolidation stress σ′ p. Since the first method proposed by Casagrande in 1936, value σ′ p is determined in the oedometer test as a border between overconsolidated (OC) and normally consolidated (NC) zones. Approach based on division between predominantly elastic, (recoverable) strain, and plastic (irrecoverable) strain is a main principle of several methods of σ′ p determination, which have been proposed over the past eighty-six years. Accumulated experiences have revealed that any laboratory procedure based on the oedometer test does not provide realistic value of preconsolidation stress, especially in heavy preconsolidated soils. The major reason for that results from the fact that the mechanism responsible for natural overconsolidation is more complicated than mechanical preloading. Therefore, there is a necessity to reevaluate effectiveness of standard methods and look for another solution of evaluation yield stress σ′ Y in natural soils. This article presents the comparison between σ′ Y determined for various soils with use of standard methods based on conventional oedometer test and yield stress determined on the basis of alternative procedures. The latter are represented by various approaches as e.g. based on SHANSEP procedure or initial shear modulus and others. The most promising among these alternative methods is a new concept based on dilatancy phenomenon that takes place during shearing of a dense soil. The parameter reflecting stress history is derived from pore pressure response and is based on characteristic values of Skempton's parameter A record. Consistency of data concerning stress history parameters profile obtained for deep subsoil on the basis of various methods is shown for comparison.

In-situ ternary wire + arc additive manufacturing thin-walled Ti-Ni-Cu-Al-V component: Microstructure and mechanical properties

In this study, a Ti-Ni-Cu-Al-V component was produced using wire + arc additive manufacturing (WAAM), which have involved three solid wires, i.e. Ti6Al4V, ER-Ni and ER-Cu wires. This paper presents, the microstructure and mechanical properties of the wire + arc additive manufactured Ti-Ni-Cu-Al-V component. The result shown that the deposition layer was mainly composed of Ti(Ni, Cu) and Ti2(Ni, Cu). The average hardness was 520 HV0.2. The average compressive strength was 1761 MPa, and the average strain was 7.7%. The component experienced brittle fracture. The cyclic loading–unloading compression results revealed that the irrecoverable strain and the recoverable strain were around 1.2% and 5.3%, respectively.

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 V addition on microstructure and pseudoelastic response in Fe–Mn–Al–Ni alloys

Avoiding quench cracks and controlling the formation process of NiAl precipitates are essential to regulate the microstructure and pseudoelastic properties of Fe–Mn–Al–Ni shape memory alloys (SMAs). In this study, we investigated the effect of 1.5 at.%V addition on the quench sensitivity of Fe–Mn–Al–Ni SMAs and the precipitation behavior of NiAl precipitates in air-cooled Fe–Mn–Al–Ni–V SMAs. It is found that the V addition significantly inhibits the precipitation process of non-transforming γ-phase. Polycrystalline sample of bamboo structure was prepared by abnormal grain growth and tested by incremental strain cycling tensile test at room temperature. The polycrystalline Fe–Mn–Al–Ni–V SMAs obtain fully reversible thermoelastic martensitic transformation without quenching, which also prevents the formation of quenching cracks. Single crystal samples were further prepared by directional recrystallization. High number-density and relatively large-size coherent β-NiAl precipitates were obtained to strengthen the matrix in the single crystal without aging, resulting in large stress hysteresis and irrecoverable strain. After aging Fe–Mn–Al–Ni–V single crystal for 3 h at 200 °C, both the stress hysteresis and irrecoverable strain decreased, thereby achieving good pseudoelasticity in Fe–Mn–Al–Ni–V SMAs.

Role of Pd interlayer on NiTi to Ti6Al4V laser welded joints: Microstructural evolution and strengthening mechanisms

Obtaining a reliable NiTi to Ti6Al4V dissimilar joint exhibiting NiTi's superelasticity can provide design flexibility in aerospace and biomedical fields via integrating distinct material benefits. However, this materials couple is vulnerable to severe embrittlement due to the development of excessive intermetallic compounds (IMCs), namely Ti2Ni. Pd-free and Pd-interlayered NiTi-Ti6Al4V laser joints were evaluated for their microstructure, compositional changes, thermodynamic mechanism, and mechanical properties. The presence of Pd constrained the formation of Ti2Ni IMC, and NiTi-Ti6Al4V joints with excellent mechanical properties demonstrating superelastic behavior were achieved for the first time. The tensile strength and rupture strain of the Pd-added NiTi-Ti6Al4V joint improved more than twofold, reaching 520 MPa and 5.6%, respectively. During cyclic tensile testing, the Pd-added joint demonstrated superelasticity and a comparable irrecoverable strain to NiTi (1 versus 0.65%). Multiscale characterization revealed that the fraction of Ti2Ni decreased from 83 to 10% near the NiTi boundary and 24 to 6% at the weld center compared to the Pd-free joint. The superior thermodynamic formation tendency of Ti-Pd compounds over Ti2Ni IMC favored their development, and thus Ti-Pd and NiTi compounds dominated the fusion zone (FZ) at the expense of Ti2Ni IMC, explaining the improved mechanical performance of the Pd-added joint.

A study of grain boundary effects on the stress-induced martensitic transformation and superelasticity in NiTi alloy via atomistic simulation

The stress-induced martensitic transformations and superelasticity behavior in the NiTi alloy with a single crystal model and a twist grain boundary bicrystal model at different temperatures are studied using molecular dynamics simulations. An atomic tracing method is proposed to identify specific numbers of B19′ martensite variants. Under uniaxial compressive loading, the stress-induced martensitic transformation takes place accompanied by the formation of <011>M type II twins, and the deformation process can be divided into three distinct stages based on microstructure evolution and average atomic total energy. It is found that the twist grain boundary induces an increase in the martensite start temperature, which is consistent with the experimental results. There is no residual B19′ martensite at the end of the unloading process, and the irrecoverable strain mainly results from plastic deformation at the grain boundary through the analysis of atomic local shear strains and has hardly changed with increasing deformation temperature. Remarkably, the grain boundary brings about the acceleration of martensite nucleation and an earlier occurrence of stress plateau. Further simulation results manifest that the presence of the twist grain boundary leads to weakened temperature dependence of martensitic transformation stress and a reduction in the hysteresis loop area.

A thermo-viscoelastic constitutive model addressing the cyclic shape memory effect for thermo-induced shape memory polymers

The cyclic shape memory effect of thermo-induced shape memory polymers (TSMPs) is a typical thermo-mechanical process that can be affected by thermo-mechanical loading histories. During the deformation stage of the cyclic shape memory effect, polymer chains exhibit the initial dissociation of sub-entanglements, slipping and orientation with the increase in the deformation. The strain can be recovered with the internal rotation of the dihedral angle due to the enhancing motion of polymer chains during the reheating. Based on the rheological theory, a thermo-viscoelastic model is proposed to capture the cyclic shape memory effect of TSMPs. A temperature-dependent stress threshold value is introduced to reflect the slipping of polymer chains. In order to take into account how orientation affects the cyclic shape memory effect, a relationship between the strain and orientation is constructed and introduced into the evolution equations of elastic modulus, viscosity and irrecoverable strain. By comparing the experimental and simulated results at different loading levels and numbers of cycles, the proposed model is verified. The results show that the proposed model can reasonably predict the cyclic shape memory effect of TSMPs.

Assessment of Fatigue Life for Corroded Prestressed Concrete Beams Subjected to High-Cycle Fatigue Loading

Millions of in-service concrete bridges around the world are exceeding their design life. Corrosion may lead to a sudden fracture of prestressing steels, and understanding the fatigue behavior of corroded prestressed concrete (PC) bridges is essential. Accordingly, an effective fatigue analytical model for corroded PC beams is needed. In this study, an experimental investigation of the fatigue behavior of corroded PC beams, was carried out. Thirteen PC beams had experienced acceleration corrosion before high-cycle fatigue testing. All beams failed due to fatigue fracture of the corroded tensile steel bars, and three-dimensional laser scanning results indicated that the fracture occurred at the minimum cross sections with a severe corrosion pit. A segmented linear analytical model to evaluate fatigue response and predict fatigue life was developed which considers fatigue irrecoverable plastic strain of concrete, stress concentration, and longitudinal variation in the cross-sectional areas of corroded steel bars. There is an acceptable agreement between the numerical and experimental results of corroded PC beams. The proposed model can reasonably predict the evolution of strains in concrete, corroded prestressing steel, and rebars in PC beams under high-cycle fatigue loading, thereby allowing fatigue life prediction. Further analysis studied the influence of degree of corrosion in prestressing wires/rebars, stress concentration and longitudinal variation in cross-sectional areas of corroded steel bars, load ranges, and partial prestressing ratios (PPRs). It was found that, to reasonably estimate the fatigue life of corroded PC beams, the influence of stress concentration must be considered with the proposed impact factor. A higher corrosion degree, load range, or PPR shortens the fatigue life of corroded PC beams.

Microstructure and mechanical properties of TC4/NiTi bionic gradient heterogeneous alloy prepared by multi-wire arc additive manufacturing

TC4/NiTi multi-materials structure components have been successfully fabricated via multi-wire arc additive manufacturing (MWAAM). Here we show the interface characteristics and mechanical properties of TC4/NiTi multi-materials structure component under bionic gradient interlayer build strategy. The results indicated that MWAAM TC4/NiTi gradient heterogeneous alloy with ultimate compression strength (1533.33 ± 26 MPa) was obtained. The excellent compression behaviour was mainly contributed to the excellent transition of gradient region indicated by EBSD analysis showing fine grain sizes and smaller difference Schmidt factor values. The phase composition from TC4 region to NiTi region had evolved with the increasing of NiTi content as follows: α-Ti + β-Ti → α-Ti + NiTi2 → NiTi2 → NiTi2 + NiTi → NiTi + Ni3Ti. The microhardness of the gradient heterogeneous alloy ranged from 310 ± 8 HV to 230 ± 11 HV, and the highest hardness value was 669.6 ± 12 HV in region B due to the precipitation of NiTi2 strengthening phase. The ultimate fracture stress and strain of sample were 1533.33 ± 26 MPa and 28.3 ± 6% respectively. The irrecoverable strain of MWAAM TC4/NiTi gradient heterogeneous alloy gradually approached 2.75% during 10 load/unload cycles of compression test.

Micromechanics of rock damage and its recovery in cyclic loading conditions

SUMMARY Under compressive stress, rock ‘damage’ in the form of tensile microcracks is coupled to internal slip on microscopic interfaces, such as pre-existing cracks and grain boundaries. In order to characterize the contribution of slip to the overall damage process, we conduct triaxial cyclic loading experiments on Westerly granite, and monitor volumetric strain and elastic wave velocity and anisotropy. Cyclic loading tests show large hysteresis in axial stress–strain behaviour that can be explained entirely by slip. Elastic wave velocity variations are observed only past a yield point, and show hysteresis with incomplete reversibility upon unloading. Irrecoverable volumetric strain and elastic wave velocity drop and anisotropy increase with increasing maximum stress, are amplified during hydrostatic decompression, and decrease logarithmically with time during hydrostatic hold periods after deformation cycles. The mechanical data and change in elastic properties are used to determine the proportion of mechanical work required to generate tensile cracks, which increases as the rock approaches failure but remains small, up to around 10 per cent of the net dissipated work per cycle. The pre-rupture deformation behaviour of rocks is qualitatively compatible with the mechanics of wing cracks. While tensile cracks are the source of large changes in rock physical properties, they are not systematically associated with significant energy dissipation and their aperture and growth is primarily controlled by friction, which exerts a dominant control on rock rheology in the brittle regime. Time-dependent friction along pre-existing shear interfaces explains how tensile cracks can close under static conditions and produce recovery of elastic wave velocities over time.

Cyclic compression of sands at constant stress ratio—experimental investigations and constitutive modelling

Several constitutive models for sands are based on an additive split of the effective stress rate into two terms. One term accounts for the deviatoric stress ratio varvec{r}, the other for the mean effective stress {p}. While sophisticated techniques are available to account for monotonic and cyclic variations of varvec{r}, the {p} term is usually treated by means of rather rudimentary constitutive mechanisms. In particular, no plasticity sand model seems to exist whose functions take into account cyclic changes of {p} in a realistic manner. However, cyclic changes of {p} frequently occur in geotechnical boundary value problems and can cause irrecoverable deformation in non-cohesive soils. Present sand models significantly under- or overestimate these deformations. This issue is tackled in the paper. First, results from a comprehensive series of triaxial tests on Toyoura Sand are presented. The samples were loaded with cyclic changes of {p} at constant stress ratio in order to study the effects of mean effective stress variations exclusively. The test results show that the samples accumulate significant irrecoverable strains throughout successive loading cycles. The results furthermore allow to investigate the effects of pressure level, pressure amplitude, stress ratio level and density on the strain evolution. Second, new constitutive functions for an existing Bounding Surface Plasticity model are proposed in the paper. They are intended to improve the simulation results obtained with the model for monotonic and cyclic changes of {p} at constant stress ratio. The extended model is then carefully calibrated and validated using the experimental data from the triaxial tests. The results of the validation prove that the new constitutive functions have the desired effect.

Natural Rubber Latex–Modified Concrete Pavements: Evaluation and Design Approach

Repetitive loads result in increased irrecoverable strain and, consequently, fatigue cracking failure. Fatigue resistance is the prime factor controlling the service life of the concrete pavements. This research study utilized natural rubber latex (NRL) to enhance the mechanical strength and flexural fatigue resistance of normal concrete. The effect of influence factors, such as water to cement (w/c) and dry rubber to cement (r/c) ratios, on the compressive and flexural strengths and flexural fatigue behavior was studied. Normal and NRL concretes were subjected to compressive, flexural, and flexural fatigue tests. At all w/c ratios, a higher r/c ratio resulted in a lower compressive strength but higher flexural strength. The highest flexural strength was found at the optimum r/c ratios of 0.58, 1.16, and 1.73 for w/c=0.3, 0.4, and 0.5, respectively. Concrete modified by NRL at the r/c ≤ optimum ratio had lower plastic deformation than normal concrete at the same applied flexural stress, hence, higher fatigue life (Nf). As a result, the NRL-concrete pavement at the optimum r/c ratio was found to have a thinner thickness than the normal concrete pavement for the same traffic load conditions at the same service life, which can reduce the operational cost. A cost-effective mix design method of NRL-concrete pavement with the optimized construction (operation and material) cost was also recommended. The outcomes of this research will facilitate the utilization of the NRL additive alternative to synthetic polymer for sustainable NRL-concrete pavement in Thailand and other Southeast Asian and South American countries, which are major global producers of natural rubber.

Grain size dependence of stress-assisted two-way memory effect in Ti-50.04 at.% Ni shape memory alloy

The grain size (GS) effects on stress-assisted two-way memory behavior of NiTi shape memory alloys (SMAs) were investigated by thermomechanical actuation tests over a wide GS range (from 10 nm to 230 nm). The actuation strain and transformation temperatures are observed to decrease monotonically as the GS is reduced, but the transition temperature window is enlarged. The irrecoverable strain decreases monotonically with GS reduction overall. The underlying mechanisms were investigated through microstructural and thermodynamic analysis. It is demonstrated that, as the GS is reduced, the increasing volume fraction of dislocations and grain boundaries (GB) occupying the space of austenite crystallite suppress the phase transition (PT) and lower the entropy change, leading to the decrease of transformation strain. GS reduction converts the PT from first-order type to a continuous one, and greatly expands the operating temperature range of actuation, tuning the discontinuous leap in strain changes into a continuous variation. When GS > 40 nm, transformation induced plasticity dominates the irrecoverable deformation and is monotonically affected by GS dependent hall-patch effect. Further modeling analysis reveals that, the increase in the volume of GB caused by GS reduction drastically modifies the free energy density convexity of the crystallite-GB composite system and leads to the monotonic decrease of transformation temperatures. The near-linear temperature dependency of actuation strain and wider transition temperature window of the nanostructured NiTi, compared to traditional coarse-grained NiTi, provide the opportunity of improved actuation control of SMA actuator and have the potential for some unique applications in the actuation area.

Significant improvement in the thermal cycling stability of Ni44.8Ti45.8Hf5Cu5 shape memory alloy by high pressure torsion and post-deformation annealing

A newly developed quaternary Ni44.8Cu5Ti45.2Hf5 shape memory alloy (SMA) was processed by high-pressure torsion (HPT) for five turns under a pressure of 6 GPa at room temperature and then subjected to post deformation annealing (PDA) at 450 °C for 10 min. Martensitic phase transformation behavior, thermal cycling stability of the transformation, and shape memory response of the alloy were investigated after HPT and subsequent annealing and compared with the initial as-cast material. The results showed that HPT followed by annealing decreased forward (Ms and Mf) and reverse (As and Af) transformation temperatures as compared to the as-cast state, meanwhile, significantly improved thermal cycling stability of the transformation. After 50 differential scanning calorimetry (DSC) cycles, the shift in the peak temperature (Mp) of the forward B2 (austenite) → B19′ (martensite) transformation for the as-cast alloy was measured to be 28 °C, while that for the sample processed by HPT and PDA was almost null. In addition to the substantial thermal cycling stability improvement, thermal cycling experiments under a constant stress (σ = 200 MPa) revealed that processing by HPT and subsequent annealing improved shape memory behavior of the alloy, decreased the width of thermal hysteresis from 79 °C to 53 °C, and completely removed irrecoverable strain.

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%.