Martensitic Transformation In NiTi Research Articles

First-principles phonon-based model and theory of martensitic phase transformation in NiTi shape memory alloy

Remarkable shape memory property of Nitinol (NiTi) originates from a diffusion-free reversible phase transition it exhibits upon cooling from the cubic austenite to a low-symmetry martensite structure. Atomistic mechanisms of this martensitic transformation (MT) and consequent emergence of its various low-symmetry structures are not understood yet. Starting with first-principles density functional theoretical calculations, we present here a phonon-based model and its statistical mechanical analysis to obtain atomistic insights into martensitic phases and transformation in NiTi. We uncover seven order parameters that are relevant to the MT in NiTi. From Monte Carlo simulations of an effective model Hamiltonian derived to capture its low energy landscape, we determine its soft phonons and establish the cell-doubling M5′ mode as the primary order parameter. Using Landau theoretical analysis, we show that relative strengths of its third-order coupling with secondary order parameters (e.g. strain) determine the symmetry of low-T structures emerging at its MT. These couplings can be used as the descriptors of stability of martensitic phases that will guide the strategies to improve the shape memory of NiTi through substitutional alloying.

Beyond the strain recoverability of martensitic transformation in NiTi

Tensile deformation of a medical grade NiTi wire was investigated in a wide temperature range from −100 °C to 450 °C. Supplemental in-situ electrical resistance, synchrotron x-ray diffraction, digital image correlation and ex-situ TEM methods were employed to characterize deformation/transformation processes acting at high temperatures and stresses. Conventional superelastic deformation due to stress induced martensitic transformation taking place around room temperature becomes gradually accompanied by dislocation slip in the temperature range 30–80 °C. With further increasing temperature, stress induced martensitic transformation still proceeds in localized manner but the length of the forward stress plateau increases, volume fraction of the martensite phase at the end of forward stress plateau decreases, unrecovered strain increases and {114} austenite twins appeared in the microstructure of deformed wires. These observations were explained by the activity of a new deformation mechanism - stress induced B2=>B19´=>B2T martensitic transformation into twinned austenite coupled with dislocation slip. Thermodynamic and crystallographic aspects of this B2=>B19´=>B2T martensitic transformation breaking the strain recoverability limit of cubic to monoclinic martensitic transformation were outlined. To rationalize the observed thermomechanical responses of the wire at elevated temperatures, a TRIP like deformation mechanism based on this transformation was incorporated into an existing constitutive model of thermomechanical behaviors of NiTi. The model was numerically implemented into finite element code, simulations were performed and compared with the experimentally observed behaviors. It was found that the B2=>B19´=>B2T martensitic transformation destroys the shape memory functionality of NiTi but renders it excellent ductility in thermomechanical loads, introduces nanoscale heterogeneity into its austenitic microstructure and allows for its low temperature processing and shape setting.

The effect of crystallographic texture on stress-induced martensitic transformation in NiTi: A computational analysis

NiTi׳s superelasticity is exploited in a number of biomedical devices, in particular self-expanding endovascular stents. These stents are often laser-cut from textured micro-tubing; texture is the distribution of crystallographic grain orientations in a polycrystalline material which has been experimentally shown to have a marked influence on mechanical properties. This study offers a computational examination into the effect of texture on the stress-induced martensite transformation (SIMT) in a micro-dogbone NiTi specimen subject to tensile loading. Finite Element Analysis (FEA) is employed to simulate the transformational behaviour of the specimen on a micro-scale level. To represent a realistic grain structure in the FEA model, grains present in a 200µm×290µm test site located at the centre edge of the specimen were identified using Scanning Electron Microscopy (SEM). Grains are assumed to have homogenous behaviour with properties varying according to their crystallographic orientation to the loading direction. Required material properties were extracted from uniaxial stress–strain curves of single crystals for each crystallographic orientation for input into the in-built UMAT/Nitinol. The orientation of each grain in the test site was identified using Electron Back-Scatter Diffraction (EBSD) techniques. In this way, a quantitative explanation is offered to the effect of crystallographic texture on SIMT. Finally, the evolution of grains in the specimen, during the transformation process, was experimentally investigated by means of an in-situ SEM tensile test.

Elastic and Irreversible Energies of a Two-Stage Martensitic Transformation in NiTi Utilizing Calorimetric Measurements

Elastic energy and irreversible energy are quantified based on calorimetric measurements. We analyze energetics for each stage of the stress-free, thermally induced two-stage phase transformation A ⇔ R ⇔ B19′ in an aged Ni-rich NiTi shape memory alloy. Heating/cooling rates are imposed from 1 K/min up to 100 K/min. We compare energetic analysis after multiple thermal cycles to virgin (i.e., first-cycle) material. Fundamental thermodynamic formulations are applied from two perspectives: the free energy change dG, and the rate of change of free energy expressed as dG/dfm. Two measures of irreversible contributions are defined: the difference between the forward and reverse transformation heats, and the product of the entropy and the thermal hysteresis. Higher values are determined for the former. For scan rates of 10 K/min and greater, the energetic values become relatively stable. Substantial variations are evident at 1, 5, and 10 K/min. The scan rate impacts the elastic strain energy and irreversible energy of the B19′ markedly compared with the R-phase transition. The findings are rationalized considering morphologic changes at the lower scan rates and the impacts on elastic and irreversible energies.

Ab initiostudy of point defects in NiTi-based alloys

Changes in temperature or stress state may induce reversible B2$\leftrightarrow$(R)$\leftrightarrow$ B19' martensitic transformations and associated shape memory effects in close-to-stoichiometric nickel-titanium (NiTi) alloys. Recent experimental studies confirmed a considerable impact of the hydrogen-rich aging atmosphere on the subsequent B2 austenite $\leftrightarrow$ B19' martensite transformation path. In this paper, we employ Density Functional Theory to study properties of Ar, He, and H interstitials in B2 austenite and B19' martensite phases. We show that H interstitials exhibit negative formation energies, while Ar and He interstitials yield positive values. Our theoretical analysis of slightly Ni-rich Ni--Ti alloys with the austenite B2 structure shows that a slight over-stoichiometry towards Ni-rich compositions in a range 51--$52\,\text{at.%}$ is energetically favorable. The same conclusion holds for H-doped NiTi with the H content up to $\approx6\,\text{at.%}$. In agreement with experimental data we predict H atoms to have a strong impact on the martensitic phase transformation in NiTi by altering the mutual thermodynamic stability of the high-temperature cubic B2 and the low-temperature monoclinic B19' phase of NiTi. Hydrogen atoms are predicted to form stable interstitial defects. As this is not the case for He and Ar, mixtures of hydrogen and the two inert gases can be used in annealing experiments to control H partial pressure when studying the martensitic transformations in NiTi in various atmospheres.

Stress-induced martensitic transformation in nanometric NiTi shape memory alloy strips: An in situ TEM study of the thickness/size effect

Abstract Ultrathin NiTi miniature strips of 40–83 nm in thickness were fabricated by means of focused ion beam milling from a polycrystalline NiTi shape memory alloy. The NiTi strips were subjected to tensile deformation inside a transmission electron microscope using a self-designed tension apparatus for in situ examination of the effect of thickness on the stress induced martensitic transformation behavior in the strips. The study revealed that the transformation was completely suppressed in a strip of 40 nm in thickness whereas it was possible in thicker strips. In these strips, the stress induced martensitic transformation was found to commence sequentially in thicker strips first and then in thinner strips at higher strain (stress) levels, demonstrating the size effect. This size effect is attributed to the effect of damaged surfaces, including a Ga + -impregnated amorphous layer on one side of the strip caused by sample fabrication using FIB and oxidation affected layers on both sides. This means that the observed “size effect” is not an intrinsic behavior of the martensitic transformation in NiTi but of extrinsic influences.

Study of the martensitic transformation in NiTi–epoxy smart composite and its effect on the overall behavior

In this work, the effect of wire phase transformation on the overall thermo-mechanical behavior of NiTi–epoxy composites has been investigated. The shape memory wire received in as drawn condition was subjected to three heat treatments which results to different transformation characteristics. Composite specimens were manufactured by casting followed by curing and post curing process. The mechanical behavior of samples has been determined using standard tensile test. The effect of wire volume fraction and test temperature was investigated as well.It is found that the martensitic transformation occurring in the wire affects the mechanical behavior of the composite specimens. In this way, using the wire with higher transformation stress improves the composite tensile strength. This is achieved either by increasing the test temperature or by using the wires heat treated at lower temperatures. From the experimental results, the martensitic transformation can change the debonding mode. It seems that on the constraint of matrix, the transformation occurs simultaneously at several points in wires that result in regular debonded/undebonded pattern.

Isothermal and athermal martensitic transformations in the B2–R–B19′ sequence in Ni–Ti shape memory alloys

The kinetics of accumulation of martensite/austenite during the direct/reverse two-stage B2–R–B19′ martensitic transformation in NiTi has been studied by means of resistance measurements during interruptions of cooling/heating scans. Experimental results indicate that the R–B19′ transformation is isothermal, whereas both direct and reverse B2–R and R–B2 transformations are perfectly athermal. The isothermal accumulation of the R-phase is also detected during B19′–R transformation. A good correspondence exists between the rate of the isothermal transformation and the transformation rate in an uninterrupted cooling cycle.

Martensitic transformation path of NiTi

We present the structural evolution mechanism during the NiTi martensitic transformation and show the origins of this behavior in electronic and phononic anomalies. By employing highly precise all-electron density-functional theory calculations, we establish a barrierless transformation path for equiatomic NiTi consisting of a basal shear composed of bilayer $⟨100⟩{011}$ stacking faults to the B2 phase followed by another basal shear which causes a relaxation of the structure's monoclinic angle and results in the $\text{B}1{9}^{\ensuremath{'}}$ phase. This path is traced to evolving Fermi-surface nesting regions, which drive the structural transformation between the austenitic and martensitic phases.

Pseudoelastic cycling and ageing effects at ambient temperature in nanocrystalline Ni-rich NiTi wire

It is well known how the features of a stress–strain hysteresis during pseudoelastic pull–pull cycling of NiTi evolve: plateau stresses for the forward (loading) and reverse transformation (unloading) decrease, the width of the stress–strain hysteresis decreases and irreversible strain accumulates. This evolution is strong during early cycling. It is generally assumed that these effects, which saturate after about a hundred cycles, are associated with the elementary processes which govern dislocation plasticity and stress-induced martensitic transformations. Dislocations can do both, stabilize martensite (through their stress fields) and contribute to accumulation of irreversible strain. While it is accepted that dislocations may alter the cyclic stress–strain behavior during stress-induced martensitic transformations in NiTi, it is generally assumed that atomic reordering processes (which are known to significantly affect martensitic transformations in Cu-base shape memory alloys) play no role. In the present investigation we show that hold times in saturated stress–strain cycles show significant effects which are smaller than, but similar in type to those in Cu-base alloys. Further work is required to fully rationalize the microstructural origin of our findings.

Strain dependence of the Clausius–Clapeyron relation for thermoelastic martensitictransformations in NiTi

The temperature–stress dependence of thermoelastic martensitic transformations inshape memory alloys is a critical design tool for applications of these alloys insmart structures. This dependence has been most fundamentally expressed in aClausius–Clapeyron linear relationship. This paper presents a theoretical analysis of thisrelationship, with particular attention given to the effect of the strain parameter inthe equation for the case of equiatomic NiTi alloys. The theory is refined in twodifferent forms describing two distinctive conditions corresponding to completeand incomplete stress-induced martensitic transformations. The conclusions ofthe analysis are used to clarify the mechanisms of post-plateau deformation ofNiTi.

Effect of pseudoelastic cycling on the Clausius–Clapeyron relation for stress-induced martensitic transformation in NiTi

This study evaluates the strain-dependence of Clausius–Clapeyron relation of the stress-induced martensitic transformation in a Ti–50.8 at.% Ni. The transformation strain is altered by means of pseudoelastic cycling. It is found that the σ– T linearity coefficient of the relation is greater for the reverse transformation than for the forward transformation, consistent with the smaller transformation strain of the reverse transformation, and that the coefficient increases with mechanical cycling, resulting from decreases of the transformation strains caused by the mechanical cycling. The thermodynamic analysis indicates that the reduction in transformation strain with cycling is caused partially by a reduction in the transformation volume and partially by a reduction in the degree of orientation of the martensite variant.

Transformation Cyclic Damage of Near-Equiatomic Ni-Ti

Near-equiatomic Ni-Ti alloys are known to exhibit shape memory effect associated with a B2↔B19’ martensitic transformation. These alloys are often used in various cyclic modes in application, typically as actuators and sensors. The B2↔B19’ martensitic transformation in Ni-Ti is accompanied with a large lattice distortion. Cycling through this transformation, induced thermally, mechanically or by the combination of the two, is found to cause structural damage to the alloys, hence changes their functional properties. This study investigates the effect of transformation cycling and heat treatment on the property stability of near-equiatomic Ti-Ni. It was found that in the case of thermally induced transformation cycling, incomplete transformation cycles caused less structural damage to the matrix than full transformation cycles whereas in the case of mechanically induced transformation cycling via pseudoelasticity in tension, partial or full transformation cycling caused similar property changes. The indifference of the case of pseudoelastic cycling is attributed to the localisation of the deformation, commonly known as the Lüders-type deformation.

Effect of Cu and Hf additions on NiTi martensitic transformation

The effect of the combination of two different additions on NiTi shape memory alloy is not straightforward. In order to investigate the effect of Hf and Cu additions on the martensitic transformation, a set of five alloys has been prepared with the following compositions: Ni 49.8− x Cu x Ti 40.2Hf 10 with x = 0, 6, 10, 15 and 20 at.%. The transformation temperatures and the microstructures obtained have been analyzed by differential scanning calorimetry and transmission electron microscopy. While the transformation hystereses are smaller with the addition of Cu like in the ternary Ti 50Ni 50− x Cu x alloys, the martensitic transformation temperatures decrease rapidly. The structure of the B19′ martensite has also been modified with a decrease of the unit cell monoclinicity. The twinning modes are not modified, but the martensite plates are longer and contain fewer defects in comparison with the ternary NiTi(Hf) alloy. Internal friction analysis has shown that the addition of Cu to NiTiHf10 improves the damping properties.

Comparative study of deformation-induced martensite stabilisation via martensite reorientation and stress-induced martensitic transformation in NiTi

This paper presents a comparative study of the effects of deformation via stress-induced martensitic transformation and via martensite reorientation on the transformation behaviour of a polycrystalline NiTi alloy. It was found that the two deformation modes had very similar effects on the thermomechanical behaviour of the alloy after the deformation, including the recovery of deformation upon subsequent heating, the development of two-way memory effect, and the stabilisation of the martensite. It was established in this study that the transformation deformation, either stress-induced martensitic transformation or the martensite reorientation, is incomplete at the end of the stress plateau and continues in the homogeneous deformation afterwards. This study also provided a direct comparison of the effects of the two deformation modes on the transformation heat. It was found in both cases the heat effect of the reverse transformation of the oriented martensite, stress-induced or reoriented, decreased with increasing pre-deformation, in agreement with previous study on deformation via stress-induced martensitic transformation but in contradiction to a previous study on deformation via martensite reorientation.

Lüders-like deformation associated with stress-induced martensitic transformation in NiTi

Stress-induced martensitic transformation in NiTi may proceed in a Lüders-type manner. This paper presents thermal analysis and TEM evidences of the presence of austenite after deformation to beyond the end of the Lüders-type stress plateau, suggesting that the end of the stress plateau is not the end of the phase transformation.

Process during heat treatment on martensitic transformation in Ni-Ti and Ni-Ti-Cr alloys

This paper presents the effect of cooling process during heat treatment on martensitic transformation in 45% cold worked, Ti-rich, Ni-rich Ni-Ti and Ni-Ti-Cr shape memory alloys, annealed in the temperature range of 100° - 800°C. Phase transformation in water quenched and furnace cooled samples is studied in the range from +140° to -170°C, employing four-probe electrical resistivity measurement technique. The heat treatment process has a distinct effect on the martensitic transformation in these alloy systems. In the case of Ti-rich alloy two-stage martensitic transformation, A→R→M during cooling and M→A on heating, are observed in the as-received sample. For both the quenched and the furnace cooled samples this behavior persists even after annealing at 420°C. The process spreads over a temperature interval of 60°, above 0°C, whereas, after annealing above 420°C only A↔M transformation is observed. Ni-rich quenched alloy annealed below 500°C exhibits a two-stage martensitic transformation, spreading over a wide temperature interval of 150°, below +50°C. For that alloy annealed above 500°C, R-phase gradually decays narrowing the transformation region. In the furnace cooled sample, R-phase region steadily narrows, for annealing at temperatures up to 580°C, suddenly broadens for annealing above 580°C and persists prominently thereafter. In the case of Ni-Ti-Cr, though for both the processes two-stage martensitic transformation is exhibited, for quenched one, R-phase disappears for annealing temperatures above 600°C. Thus, furnace cooling is found to promote the intermediate R-phase in Ni-rich Ni-Ti and Ni-Ti-Cr alloys annealed at higher temperature also.

In situ neutron diffraction studies of martensitic transformations in NiTi

Martensitic transformations in commercial near equiatomic Ni49.5Ti (at. %) polycrystalline thick wires induced by thermal and/or mechanical loading were investigated by in-situ high-resolution neutron powder diffraction techniques with the aim to develop neutron diffraction methods of stress and phases fraction evaluation in NiTi transforming in thermomechanical loads.

The role of phonons in the thermodynamics of the martensitic transformation in NiTi

Abstract Measurements of neutron inelastic scattering, thermal expansion and low-temperature heat capacity were used to study the phonon entropy of the martensite-to-austenite transformation in NiTi, which is found to be (0.5 ± 0.05) k B atom−1. The inelastic scattering spectra were corrected for the differing scattering amplitudes of Ni and Ti through a lattice dynamics simulation. The phonon density-of-states (DOS) curves so obtained account for all the transformation entropy measured by calorimetry. From simulations and the measured DOS, the vibrational entropy of austenite was found to be larger than that of martensite because the transverse acoustic (TA2) and longitudinal acoustic modes of austenite are softer than those of martensite. Simulations suggest that this originates with a large and negative first-nearest-neighbour transverse force constant. This φT lnn may also be responsible for the soft modes involved in the mechanism of the martensitic transformation.

The role of phonons in the thermodynamics of the martensitic transformation in NiTi

The role of phonons in the thermodynamics of the martensitic transformation in NiTi