Stress-induced Martensite Formation Research Articles

Synchrotron analysis of toughness anomalies in nanostructured bainite

High-resolution synchrotron X-ray diffraction has been used to characterise the notch root regions of Charpy impact test specimens of a superbainitic steel, both before and after loading. The changes in the volume fraction of austenite induced by the application of a three-point-bending load were quantified. Analysis of diffraction peak shifts revealed the extent of residual tensile and compressive strains present due to both machining and an applied load. The results lend support to the hypothesis that the comparatively low energies absorbed during Charpy impact testing of superbainitic steels, <7 J, are due to the formation of stress-induced martensite at the notch root, prior to crack initiation.

Superelasticity in Polycrystalline Ni-Mn-Ga-Fe Microwires Fabricated by Melt-extraction

Ni48Mn26.4Ga19.7Fe5.9 microwires with grain size of 1-3 micron were successfully fabricated by melt-extraction. The superelastic effects in the microwires under various temperatures and loads were systematically demonstrated. The as-extracted microwires displayed partial superelasticity when attended at relatively high temperature. The critical stress for stress-induced martensite formation increases linearly with temperature and follows the Clausius-Clapeyron relationship. The temperature dependence of the as-extracted polycrystalline Ni48Mn26.4Ga19.7Fe5.9 microwires is 16.4 MPa/K, which is higher compared with Ni-Mn-Ga single crystals. In addition, the as-extracted microwires display excellent shape memory behavior with the recovery strain and recovery ratio of 1.26% and 86%, respectively, when the total strain reaches 1.47% at 310 K.

Deformation-induced martensitic transformation in a new metastable β titanium alloy

A new metastable β titanium alloy, Ti-4Al-7Mo-3V-3Cr (wt.%), was designed using d-electron method, aiming to tailor the deformation mechanism. Microstructural and X-ray diffraction analysis of deformed specimens conforming with the theoretical prediction of d-electron method, confirmed the formation of stress-induced martensite (SIM) and mechanical twinning as the deformation mechanisms. The effect of initial grain size, strain level and strain rate on the formation of SIM were investigated. The results showed that in a given grain size, the volume fraction of SIM initially increases intensively and then follows with lower rate reaching a saturation at 35% reduction. It was observed that after a decrease in the martensite laths interspace down to less than 2 μm as a result of increasing the strain, some secondary martensite laths forms within the primary ones. It was found that with an increase in grain size from 150 μm to 250 μm, the volume fraction of SIM increases while a further increase in grain size up to 500 μm leads to a decrease in SIM volume fraction. The compression test results at different strain rates ranging from 0.7 × 10−4 to 0.7 × 10−1s−1 showed SIM transformation occurs at all strain rates and although the triggering stress for SIM transformation increases continuously with an increase in strain rate, the volume fraction of SIM is independent of the strain rate.

Effect of chemical ordering annealing on martensitic transformation and superelasticity in polycrystalline Ni–Mn–Ga microwires

Polycrystalline Ni–Mn–Ga microwires of diameter 30–80μm were prepared by melt-extraction technique on a large scale. The rapidly solidified microwires exhibit a fairly high ductility and excellent shape memory property. Here, with the aim to reduce the defect density, internal stress and compositional inhomogeneity in the as-extracted microwires, a stepwise chemical ordering annealing heat treatment was carried out and the effect of annealing on martensitic transformation, magnetic properties and superelastic behavior were investigated. The results indicate that annealing increase the transformation temperature and decrease the transformation hysteresis. These are related to composition homogenization, increase of atomic ordering and decrease in internal stress and defects. During mechanical tests, the stress-induced martensite (SIM) formation took place at a much lower stress after annealing treatment. The annealed microwires also demonstrate a lower superelastic hysteresis and a higher recovery rate compared to the as-extracted microwires. The temperature dependence of SIM stress is weaker after annealing, which is related to the enthalpy change (ΔH) and phase transformation temperature change according to the Clausius–Clapeyron relation.

Sudden stress-induced transformation events during nanoindentation of NiTi shape memory alloys

This study investigates the stress-induced formation of martensite during nanoindentation of an austenitic NiTi shape memory alloy, where stress-induced martensite is stable at room temperature. An individual grain with a [111] surface normal was selected for spherical ex situ and in situ nanoindentation in a scanning electron microscope. The in situ load–displacement curves show several pop-ins which occur concomitantly with the formation of traces around the contact zone between the indenter tip and the sample. These traces exhibit a threefold symmetry around the remnant indent. A detailed study of the indentation-induced surface relief by atomic force microscopy before and after shape recovery allows to identify the formation of six twinned martensite plates. Post-mortem microstructural characterization shows that these twinned martensite plates are growing as the applied load is increasing. The activation of the experimentally observed twinned martensite plates is rationalized by analytical calculations of resolved shear stress and mechanical interaction energy density. Finally, the in situ nanoindentation results in combination with the post-mortem microstructural characterization show that the most likely deformation mechanism responsible for pop-in events corresponds to sudden increases of the thicknesses of twinned martensite plates.

Orientation dependence of stress-induced martensite formation during nanoindentation in NiTi shape memory alloys

In the present work we used nanoindentation with a spherical indenter tip to study the formation of stress-induced martensite in NiTi shape memory alloys. Prior to nanoindentation, orientation imaging was performed to select austenite grains with specific crystallographic orientations, including the principal crystallographic directions [001], [101] and [111]. We studied a material where stress-induced martensite is stable at room temperature and found surface patterns with four-, two- and threefold symmetries for the [001], [101] and [111] crystallographic indentation directions, respectively. Atomic force microscopy investigations of the topography showed that the surface patterns were associated with sink-ins. The crystallographic sink-in patterns disappeared during heating, which proved their martensitic origin. Our results provide clear experimental evidence which shows that the crystallographic anisotropy of nanoindentation is governed by the crystallographic anisotropy of the stress-induced formation of martensite.

A Comparative Study on the Mechanical Behavior of Porous Titanium and NiTi Produced by a Space Holder Technique

NiTi and Ti porous specimens with appropriate pore characteristics for biomedical applications are produced by space holder method. Porosities of the specimens linearly increase from 14 to 65 and 42 to 70% for the Ti and NiTi specimens, respectively, with the urea space holder. Mechanical properties such as stiffness, fracture strain, and strength of the porous NiTi and Ti are adjustable with pore characteristics. The apparent elasticity modulus of NiTi specimens decrease from 3.5 to 0.73 GPa as porosity increases. Since the initial linear part of the stress-strain curve consists of elastic behavior, formation of stress-induced martensite, deformation and/or detwinning of martensite variants, and plastic deformation, the unloading slope of stress-strain curves is a better approximation for the elasticity modulus of the NiTi porous specimens as it is proved by an isotropic cubic cell model. The unloading slope of the NiTi specimen with 61% porosity is 3.1 GPa, while the apparent elasticity or loading slope is 0.85 GPa. In comparison to Ti, the high, recoverable strain of NiTi improves capability of it as a good candidate for bone replacement. Moreover, in contrast to Ti specimens, hysteresis loops are clearly observed in the stress-strain curves of NiTi specimens.

Modelling and design of stress-induced martensite formation in metastable [formula omitted] Ti alloys

The temperature dependence of the stress-induced martensite (SIM) formation in a Ti–10V–2Fe–3Al (Ti-1023) alloy under compressive loading has been studied. At low temperatures, the stress level at which martensite starts to form increases linearly with the deformation temperature, while the stress at which the deformation switches to regular plastic deformation is roughly temperature independent. A thermostatistical model for dislocation evolution is employed to describe deformation twinning in martensite. Combined effects of twinning induced plasticity and solid solution strengthening are considered in terms of temperature variations. The SIM effect disappears on deformation at temperatures beyond ~233°C, which is close to the predicted Ms temperature of 240°C. The thermostatistical model predicts a transition from twinned martensite to pure slip at 250°C. By providing a model to predict the martensite formation, and by describing deformation twinning, the present work provides a number of tools that may be employed to conceive new titanium alloys combining improved strength and ductility.

The effect of texture on stress-induced martensite formation in nickel–titanium

An experimental study was performed to investigate the effect of texture on stress-induced martensitic phase transformation in the shape memory alloy (SMA) nickel–titanium (Nitinol). Thin sheet specimens of Nitinol were examined under uniaxial tensile loading using three-dimensional digital image correlation in order to spatially and temporally track strain localization indicative of martensitic transformation. Tensile specimens were fabricated along directions oriented 0° (RD), 45°, and 90° (TD) to the rolling direction of the sheet and subjected to 50 cycles at prescribed strain rates of , and 10−2 s−1. It was found that upon loading, specimens unfavorably oriented for transformation (TD specimens) nucleated a greater number of deformation bands due to a smaller difference between nucleation and propagation stresses, and also accommodated less axial strain inside the band and more axial strain outside the band. The unfavorable (TD) specimens also exhibited a stronger cycle-to-cycle similarity in the strain accommodated inside the deformation band, which has important implications for the design of SMA structures for fatigue applications. Finally, the (primarily martensite) region of the deformation band(s) consistently showed significantly stronger cycle-to-cycle similarity than the (primarily austenite) region outside the band(s), regardless of specimen texture.

Internal friction measurements of phase transformations during the process of deep cryogenic treatment of a tool steel

This paper reports internal friction measurements in die steel with the aim of clarifying phase transformation mechanisms during deep cryogenic treatment (DCT). Internal friction peaks appeared in the quenched sample and the DCT treated material during the process of cooling down to −160°C from room temperature (20°C) and then heating up to room temperature for multiple cycles in high vacuum. However, internal friction peaks do not appear in annealed material. It is concluded that the internal friction peaks are attributed to stress-induced martensite formation based on thermal hysteresis phenomena. That is, retained austenite is mostly transformed into martensite during the DCT.

On the crystallographic anisotropy of nanoindentation in pseudoelastic NiTi

We use a nanoindenter with a Berkovich tip to study local mechanical properties of two polycrystalline intermetallics with a B2 crystal structure, NiAl and NiTi. We use orientation imaging scanning electron microscopy to select a relevant number of grains with appropriate sizes and surface normals parallel to 〈001〉, 〈101〉 and 〈111〉. As a striking new result, we find a strong crystallographic orientation dependence for NiTi. This anisotropy is less pronounced in the case of NiAl. For NiTi, the indentation force required to impose a specific indentation depth is highest for indentation experiments performed in the 〈001〉 direction and lowest along the 〈111〉 direction. We consider transmission electron microscopy results from cross-sections below the indents and use molecular dynamics simulations and resolved shear stress calculations to discuss how this difference can be accounted for in terms of elementary deformation and transformation processes, related to dislocation plasticity (NiAl and NiTi), and in terms of the stress-induced formation and growth of martensite (NiTi). Our results show that the crystallographic anisotropy during nanoindentation of NiTi is governed by the orientation dependence of the martensitic transformation; dislocation plasticity appears to be less important.

Effect of thermo-mechanical treatment on superelastic behavior of Ti–19Nb–14Zr (at.%) shape memory alloy

The effect of thermo-mechanical treatment on shape memory effect and superelastic behavior of Ti–19Nb–14Zr (at.%) shape memory alloy is investigated. The annealed specimens are aged at 300 °C for different times from 10 min to 20 h followed by ice water quenching. Both reverse and forward martensitic transformation peaks are identified in the annealed and aged specimens. Fine ω particles with a dimension of ∼20 nm in β grains are clearly observed in the TEM micrograph. The strains of the annealed specimen are completely recovered by heating, starting from the 2nd cycle to the 5th cycle due to the reverse martensitic transformation. Non-martensitic superelasticity strain of about 6% is obtained in the aged-10 min specimen. It is noted that the ω phase hinders the formation of stress-induced martensite from the β phase, suggesting that the non-martensitic superelasticity should be attributed to the presence of ω + β phases. Also, the non-martensitic superelasticity can be improved through increasing the critical stress for dislocation slip deformation by controlling the size and amount of ω phase and/or introducing plastic strain. It is found that this non-martensitic superelasticity can be obtained from the present alloy with combined ω + β phases by introducing additional precipitation hardening and/or work hardening. The elastic modulus of the aged-10 min specimen after aging and cyclic training is approximately of 14 GPa, which is close to that of cortical bone of human beings (7–35 GPa).

High damping capacity in porous NiTi alloy with bimodal pore architecture

Porous NiTi exhibits pseudoelasticity, shape memory and low modulus, and thus shows application potential as high damping and bio-materials. Here Ni50.5Ti49.5 foam with bimodal pore structure was prepared by elemental powder sintering and high damping capacity demonstrated. NaCl particles of 500–600μm and 75–90μm diameters were adopted as space holder to create large and small pores with porosity of 40%. Incomplete densification of NiTi alloy created even smaller pores of 10–50μm and contributed another 21.47% porosity. This NiTi foam containing bimodal pores has a compressive modulus of 1.83GPa, smaller than that of foam with single pores or estimation by Gibson–Ashby model. This low modulus is attributed to the stress-induced martensite transformation under compressive load. The foam exhibits high damping capacities during phase transformation and at austenite region because of deformation, dislocation motion in thin nodes/walls and stress-induced martensite formation.

Influence of α morphology and volume fraction on the stress-induced martensitic transformation in Ti–10V–2Fe–3Al

This paper reports an investigation into the influence of α morphology and volume fraction on the critical stress levels for the stress-induced martensitic transformation in Ti–10V–2Fe–3Al. Control of the stress-induced martensite transformation offers a new route to the optimization of the load bearing capabilities of titanium alloys. Various heat treatments in α + β and β + (α + β) conditions were imposed to study their effect on both microstructure and mechanical properties. The results show that in samples with globular α, the β matrix stability is the main factor controlling the stress-induced martensitic transformation. For samples with acicular α, the “grain size” (i.e. the size of the retained β domain), which governs the mean length and the width of martensitic plates, also plays an important role in such transformations. The linear combination of both effects provides a good prediction of the stress-induced martensite formation stress as a function of the volume fraction and morphology of the α phase.

In situ study of structural integrity of low transformation temperature (LTT)-welds

We discuss the stability of weld residual strain under static and quasi cyclic transverse tensile loading in the elastic and elastic–plastic region. The test welds were joined with low transformation temperature weld filler materials with 10 wt% Cr and varying Ni-content from 8 to 12 wt%. Using neutron diffraction the residual lattice strain in the martensitic α′- and austenitic γ-phase in the fusion zone as well as the ferritic α-phase in the heat affected zone and base metal as (1) induced by welding, (2) superimposed by stepwise tensile loading and (3) after unloading was measured. The amount of retained austenite in the fusion zone increases with increasing Ni-content, but it decreases with increasing load level due to stress induced martensite formation. In the as-welded condition the transverse macroscopic residual lattice strain was found to be in low compression in the fusion zone in each weld, while the heat affected zone was in tension. Local plastic deformation of the γ-phase as a result of yielding during tensile loading in combination with the change in phase fraction resulted in increased macroscopic compression in the fusion zone. The reduced yield strength in the heat affected zone resulted in plastic deformation and a considerable shift into compression. Comparison with the cross weld distribution of the hardness and FWHM of the neutron diffraction interference lines supported the assumption of plastic deformation of the γ- and α-phase in the fusion and heat affected zone, respectively, while the α′-phase in the fusion zone was stressed within the elastic regime only. Microstructural observations as well as measurement of the local γ-phase fraction by means of laboratory X-ray diffraction in the fusion zone strengthen these observations.

Plasticity induced transformation in a metastable β Ti-1023 alloy by controlled heat treatments

Investigations into the possibility of improving the strength–ductility relation in a metastable β-titanium alloy (Ti–10V–2Fe–3Al) through plasticity induced transformation (PiTTi) have been carried out. Various heat treatments in the β and/or α+β condition were performed to study their influence on both the microstructure and solute partitioning, which eventually control the PiTTi effect. Stress induced martensite formation promoting such effect has been observed upon compression testing for β and β+(α+β) microstructures. The stress–strain curves exhibiting stress induced martensite show a ∼20% increase in strength, while still retaining a reasonable ductility level. Microstructural parameters such as grain size and solute concentration (especially V) in β have been related to the alloy's ability to exhibit PiTTi.

Influence of Zr content on phase transformation, microstructure and mechanical properties of Ti 75− xNb 25Zr x ( x = 0–6) alloys

Ternary Ti–Nb–Zr alloys consisting of biocompatible alloying elements have been prepared to investigate the effect of Zr content on β → α phase transformation, microstructure and mechanical properties. The study showed that element Zr stablized the β phase and that its effect on β phase transus exhibited a “margin effect”. With increasing Zr content, the microstructure of the Ti–Nb–Zr alloy was refined and the strength of ternary Ti–Nb–Zr alloys was also improved while the plasity significantly decreased. The Ti 72Nb 25Zr 3 alloy exhibited the maximum ultimate tensile strengh of 775 MPa and Ti 73Nb 25Zr 2 exhibited the lowest elastic modulus of 62 GPa. The low elastic modulus of Ti 73Nb 25Zr 2 was attributed to the formation of stress-induced martensite (α″ phase).

Notched tensile tests of cold-rolled 304L stainless steel in 40 wt.% 80 °C MgCl 2 solution

The effects of cold-rolling (20% thickness reduction) and sensitization treatment (600 °C/10 h) on the microstructure, tensile properties and susceptibility to stress corrosion cracking of 304 stainless steel in 80 °C MgCl 2 (40 wt.%) solution were investigated. The increase in hydrogen traps, which retarded hydrogen diffusion to the strained region, accounted for the low loss in notched tensile strength (NTS) of such a cold-rolled specimen, as compared to the solution-treated specimen in the corrosive environment. By contrast, the high NTS loss of sensitized specimens in MgCl 2 solution was attributed mainly to the formation of stress-induced martensite near grain boundary regions.

A transmission electron microscopy procedure for in-situ straining of miniature pseudoelastic NiTi specimens

Abstract The in-situ transmission electron microscopy technique allows direct observations of formation and growth of stress-induced martensite in pseudoelastic NiTi shape memory alloys. The present paper reports on the development of a miniature test procedure for in-situ straining experiments with specimens taken from small components. The deformation of an ultra-fine grained NiTi specimen is characterised by transmission electron microscopy (at early loading stages) and by optical microscopy (at larger strains). A complementary finite element analysis of the complex strain state in the specimen rationalises why the stress-induced martensitic transformation first occurs in the thin foil region of the specimen before spreading towards the outer rim of the specimen.

Mechanisms of stress generation and relaxation during pulsed laser deposition of epitaxial Fe–Pd magnetic shape memory alloy films on MgO

Mechanical stress generation during epitaxial growth of Fe–Pd thin films on MgO from pulsed laser deposition is a key parameter for the suitability in shape memory applications. By employing in situ substrate curvature measurements, we determine the stress states as a function of film thickness and composition. Depending on composition, different stress states are observed during initial film growth, which can be attributed to different misfits. Compressive stress generation by atomic peening is observed in the later stages of growth. Comparison with ex situ x-ray based strain measurements allows integral and local stress to be distinguished and yields heterogeneities of the stress state between coherent and incoherent regions. In combination with cross-sectional TEM measurements the relevant stress relaxation mechanism is identified to be stress-induced martensite formation with (111) twinning.