Strong interactions between hydrogen in solid solution and stress-induced martensite transformation of Ni–Ti superelastic alloy

The role of the dynamic interactions between hydrogen in a solid solution and the stress-induced martensite transformation in hydrogen embrittlement has been investigated using trained Ni–Ti superelastic alloy. In a cyclic tensile test in the stress plateau region caused by stress-induced martensite and reverse transformations after hydrogen charging, a further decrease in the critical stress for the martensite transformation is observed. In addition, the number of cycles to fracture for a trained specimen is significantly larger than that for a non-trained specimen. Since most of the charged hydrogen is preferentially trapped in defects induced by training, the hydrogen embrittlement is considerably suppressed as a result of decreasing interactions between the hydrogen and the transformation. The present results indicate that hydrogen in a solid solution more strongly interacts with the stress-induced martensite transformation than hydrogen trapped in defects, thereby further enhancing the hydrogen embrittlement related to phase transformations.     

After-effects induced by interactions between hydrogen and the martensite transformation in Ni–Ti superelastic alloy

The effects of dynamic interactions between hydrogen and a stress-induced martensite transformation on the recovery of deteriorated tensile properties by ageing in air at room temperature have been investigated for a Ni–Ti superelastic alloy. A specimen is subjected to single stress-induced martensite and reverse transformations immediately after hydrogen charging. Upon tensile testing, brittle fracture occurs in the latter half of the elastic deformation region of the martensite phase after the stress-induced martensite transformation. Upon ageing before the tensile test, fracture occurs during the stress-induced martensite transformation. In addition, the nano- and micro-morphologies of the brittle outer part of the fracture surface of the specimen are changed by ageing. Thus, the tensile properties markedly deteriorate, rather than recover, by ageing. The present results clearly indicate that dynamic interactions between hydrogen and the stress-induced martensite transformation have serious after-effects on hydrogen embrittlement of Ni–Ti superelastic alloy.     

Hydrogen-assisted damage in austenite/martensite dual-phase steel

For understanding the underlying hydrogen embrittlement mechanism in transformation-induced plasticity steels, the process of damage evolution in a model austenite/martensite dual-phase microstructure following hydrogenation was investigated through multi-scale electron channelling contrast imaging and in situ optical microscopy. Localized diffusible hydrogen in martensite causes cracking through two mechanisms: (1) interaction between {1?1?0}_M localized slip and {1?1?2}_M twin and (2) cracking of martensite–martensite grain interfaces. The former resulted in nanovoids along the {1?1?2}_M twin. The coalescence of the nanovoids generated plate-like microvoids. The latter caused shear localization on the specific plane where the crack along the martensite/martensite boundary exists, which led to additional martensite/martensite boundary cracking.     

Strain rate response of a Ni–Ti shape memory alloy after hydrogen charging

In this work, we investigate the susceptibility of Ni–Ti superelastic wires to the strain rates during tensile testing after hydrogen charging. Cathodic hydrogen charging is performed at a current density of 10?A/m² during 2–12?h in 0.9% NaCl solution and aged for 24?h at room temperature. Specimens underwent one cycle of loading-unloading reaching a stress value of 700 MPa. During loading, strain rates from 10^?6 to 5?×?10^?2?^?s^?1 have been achieved. After 8?h of hydrogen charging, an embrittlement has been detected in the tensile strain rate range of 10^?6 to 10^?4?s^?1. In contrast, no embrittlement has been detected for strain rates of 10^?3?s^?1 and higher. However, after 12?h of hydrogen charging and 24?h of annealing at room temperature, the embrittlement occurs in the beginning of the austenite-martensite transformation for all the studied strain rate values. These results show that for a range of critical amounts of diffused hydrogen, the embrittlement of the Ni–Ti superelastic alloy strongly depends on the strain rate during the tensile test. Moreover, it has been shown that this embrittlement occurs for low values of strain rates rather than the higher ones. This behaviour is attributed to the interaction between the diffused hydrogen and growth of the martensitic domain.     

First interactions between hydrogen and stress-induced reverse transformation of Ni–Ti superelastic alloy

The first dynamic interactions between hydrogen and the stress-induced reverse transformation have been investigated by performing an unloading test on a Ni–Ti superelastic alloy subjected to hydrogen charging under a constant applied strain in the elastic deformation region of the martensite phase. Upon unloading the specimen, charged with a small amount of hydrogen, no change in the behaviour of the stress-induced reverse transformation is observed in the stress-strain curve, although the behaviour of the stress-induced martensite transformation changes. With increasing amount of hydrogen charging, the critical stress for the reverse transformation markedly decreases. Eventually, for a larger amount of hydrogen charging, the reverse transformation does not occur, i.e. there is no recovery of the superelastic strain. The residual martensite phase on the side surface of the unloaded specimen is confirmed by X-ray diffraction. Upon training before the unloading test, the properties of the reverse transformation slightly recover after ageing in air at room temperature. The present study indicates that to change the behaviour of the reverse transformation a larger amount of hydrogen than that for the martensite transformation is necessary. In addition, it is likely that a substantial amount of hydrogen in solid solution more strongly suppresses the reverse transformation than hydrogen trapped at defects, thereby stabilising the martensite phase.     

Cooperative strain accommodation over grains in martensitic transformation from Fe-Ni nanocrystalline austenite

This paper reports an investigation of martensitic transformation behaviour from austenite with various grain sizes ranging from 290 nm to 34 μm in an Fe–Ni alloy fabricated by electrodeposition and subsequent heat treatment. We confirmed that martensite morphology changed from lath to thin plate with decreasing the austenite grain size. Crystallographic orientation analysis revealed that the variants of thin plate martensite formed in the austenite with relatively coarse grains achieved self-accommodation of the transformation strain inside one austenite grain. In contrast, the transformation strain accompanying martensitic transformation from the ultrafine-grained or nanocrystalline austenite was not accommodated by the martensite variants formed in one austenite grain but accommodated cooperatively by those formed in the several adjacent austenite grains.     

Microstructure and martensitic transformation behaviors of a Ti–Ni–Hf–Cu high-temperature shape memory alloy ribbon

The Ti₃₆Ni₄₁Hf₁₅Cu₈ melt-spun ribbon undergoes a B2 ? B19′ transformation upon cooling and heating. When the Ti₃₆Ni₄₁Hf₁₅Cu₈ melt-spun ribbon is annealed at 873 K for 1 h, the spherical (Ti, Hf)₂Ni particles with a diameter of 20–40 nm precipitate in the grain interior. The fine (Ti, Hf)₂Ni precipitates improve the stability of phase transformation temperatures and cause martensite domains, with (001) compound twins in three orientations dominant instead of (011) type I twins. {111}-, {113}- and (001)//{111}-type boundaries are observed among these martensite domains. When the (Ti,Hf)₂Ni precipitates coarsen, (011) type I twins become main martensite structures in the ribbon annealed at 973 K for 1 h.     

Influence of interface migration during annealing of martensite/austenite mixtures

Tempering of martensite in the absence of carbide precipitation leads to carbon partitioning into retained austenite. If the martensite/austenite interface is assumed to remain stationary during this process, the phase compositions reach a condition that has recently been called constrained carbon equilibrium. If iron atoms are sufficiently mobile at the interface, longer partitioning times may lead to migration of the ferrite/austenite interface. The interface may be expected to move in either direction, depending on the specific details of the phase fractions and compositions controlling the chemical potential of iron at the interface. If interface migration occurs during carbon partitioning, the situation is more complicated and conditions could exist where the interface moves first in one direction and then the other.     

A novel lattice correspondence of B19 → B19′ transformation in Ti–Ni–Cu shape memory alloy thin film

In this study, we found a novel lattice correspondence of the B19–B19′ transformation in a Ti–Ni–Cu thin film: (1?1?1)_B19′//(0?0?1)_B19, [0, 1, 1]_B19′//[1?0?0]_B19. Near the coarse precipitate and the grain boundaries, the B19′ martensite forms with the novel lattice correspondence to product the (1?1?1) type I twinning instead of the usual (0?0?1) compound twinning. Crystallographic analyses show that the novel lattice correspondence results from the local stress concentration.     

High-strain induced reverse martensitic transformation in an ultrafine-grained Ti-Nb-Ta-Zr alloy

We report on a novel phenomenon, that is a high-strain-induced reverse martensitic transformation in an ultrafine-grained Ti–36Nb–2Ta–3Zr (wt.%) alloy processed by equal channel angular pressing (ECAP) at room temperature. Our results show that a martensitic transformation from body-centred cubic β matrix to orthorhombic α″ martensite occurs under low-strain ECAP conditions and that a large portion (~34%) of martensite transforms into a matrix phase (i.e. reverse martensitic transformation) with increasing ECAP strain to a high value of 4 (i.e. 6 passes) with a corresponding reduction in the α″-lath thickness and a refinement of grain size in the matrix phase.     

Strain localization in nickel single crystals cyclically deformed at 77 K

Nickel monocrystals oriented for single slip have been cyclically deformed at 77 K at plastic strain amplitude between 5 x 10^-4 and 1 x 10^-2 up to saturation of the stress amplitude. After unloading from maximum compression, the slip markings on the surface of the specimens were removed and the deformation continued for one half cycle in tension. As previously observed for room-temperature deformation, the plastic strain was found to be localized in narrow slip bands (SBs). Using atomic force microscopy at a given imposed strain amplitude, a wide spectrum of local plastic strains was found. The averaged resolved shear strain of the SBs was found to be independent of the imposed plastic strain amplitude and turned out to be a factor of three larger than the upper plateau strain limit of the cyclic stress-strain curve.     

Relaxation of the elastic strain energy of misfitting inclusions due to diffusion of vacancies

Vacancy diffusion is investigated as a mechanism for relaxation of the elastic strain energy caused by a misfitting inclusion. The kinetics of reduction of the total eigenstrain by the deposition or removal of an atom layer along the interface is derived. The time evolution, as well as an estimate for the characteristic time of the relaxation process, is presented. The relaxation times are compared with recent in situ measurements of stress relaxation times in aluminum with small lead-alloy inclusions after their solidification. Experimentally observed relaxation times and those theoretically predicted agree very well.     

Stress relaxation in cobalt between 15 and 300 K

Abstract

The slope of the logarithmic stress–relaxation curve for a well-annealed cobalt polycrystal of 99·999% purity has been measured as a function of the initial stress level from which relaxation at constant strain was allowed to start at a given temperature between 15 and 300 K. A pronounced undulation was observed in the plot of the relation between the inverse of the stress sensitivity of the relaxation rate and temperature, with a maximum and a minimum at about 75 and 50 K, respectively. The ‘classically unexpected’ behaviour below about 80 K seems to arise from the progressive inhibition of dynamic recovery process as T→0 K, which necessitates the use of stresses higher than that applied in the basic equations describing the mode of deformation.     

Compositions of different phases appearing during devitrification of Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk metallic glass

The devitrification process of Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 metallic glass during annealing in the supercooled liquid region has been studied by conventional, high-resolution and analytical transmission electron microscopy (TEM). Two kinds of particle appear during devitrification: quasicrystals and crystalline precipitates. Nanoanalysis with energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy reveal that quasicrystals are Be free, while the crystallites have the ZrBe₂ hexagonal structure. These experiments confirm the key role of Be during devitrification. They also indicate that quasicrystals can form in the system Zr–Ti–Cu–Ni (without Be), as further evidenced by direct synthesis and an in situ neutron diffraction investigation, according to the chemical composition deduced from the TEM analysis.     

Critical factors governing the thermal stability of austenite in an ultra-fined grained Medium-Mn steel

The time-evolution of retained austenite at 650 °C is shown to be a fair indicator of the factors governing the austenite stability. The grain size contribution to austenite stability is evidenced and a martensite start temperature (M_s) law applicable to medium-Mn steels and including both the chemical and size effects is determined.     

Texture development during cross rolling of a dual-phase Fe–Cr–Ni alloy: experiments and simulations

Texture development during multi-step cross rolling of a dual-phase Fe–Cr–Ni alloy has been investigated. X-ray diffraction was used to investigate changes in crystallographic texture of both the constituent phases (austenite and ferrite) through changes in orientation distribution function. After deformation, rotated brass (rotated along φ₁, i.e. the sample normal direction ND), along with a weak cube texture was observed in austenite, while a strong rotated cube texture was obtained in ferrite. Texture was also simulated for various strains using a co-deformation model by self-consistent visco-plastic (VPSC) formulation. Simulations showed strong rotated brass texture in austenite and a strongly rotated cube, α-fibre (sample rolling direction RD //<1 1 0>) and γ-fibre (ND //<1 1 1>) in ferrite after highest strain (ε_t = 1.6). VPSC models could not effectively capture the change in crystallographic texture during cross rolling. In ferrite, simulations showed an overestimation of γ-fibre component and an underestimation of rotated cube component. Simulated texture of austenite, on the other hand, showed an overestimation of rotated brass with an absence of cube component. The results are rationalised based on the possible role of shear banding and activation of non-octahedral slip system during cross rolling, both of which are not incorporated in conventional VPSC models.     

NiTi superelasticity via atomistic simulations

The NiTi shape memory alloys (SMAs) are promising candidates for the next-generation multifunctional materials. These materials are superelastic i.e. they can fully recover their original shape even after fairly large inelastic deformations once the mechanical forces are removed. The superelasticity reportedly stems from atomic scale crystal transformations. However, very few computer simulations have emerged, elucidating the transformation mechanisms at the discrete lattice level, which underlie the extraordinary strain recoverability. Here, we conduct breakthrough molecular dynamics modelling on the superelastic behaviour of the NiTi single crystals, and unravel the atomistic genesis thereof. The deformation recovery is clearly traced to the reversible transformation between austenite and martensite crystals through simulations. We examine the mechanistic origin of the tension–compression asymmetries and the effects of pressure/temperature/strain rate variation isolatedly. Hence, this work essentially brings a new dimension to probing the NiTi performance based on the mesoscale physics under more complicated thermo-mechanical loading scenarios.     

Influence of Guinier-Preston zones on deformation in Ti-rich Ti-Ni thin films

The deformed microstructure of a Ti-48.9at.%Ni thin film has been investigated by transmission electron microscopy. It was found that Guinier-Preston (GP) zones exist in the thin film and the martensite has (001) compound twins as substructure. The microstructure of the martensite shows that the GP zones do not stop both the shear deformation of martensitic transformation and the twinning shear of (001) deformation twin in the martensite phase. These results give a microstructural explanation for the previous result that Ti-rich Ti-Ni thin films with GP zones show a large transformation strain despite the presence of the GP zones.     

TEM in situ straining experiments in Fe at low temperature

In situ transmission electron microscopy straining experiments were carried out in pure Fe to investigate the origin of the discontinuity observed at 250 K in the temperature variation of the deformation activation parameters. The results show that the motion of screw dislocations is steady at 300 K, in agreement with a kink-pair mechanism, but jerky at 110 K. This change has been attributed to a transition from a kink-pair mechanism to a locking–unlocking mechanism, similar to that observed previously in Ti.     

Creep strengthening by lath boundaries in 9Cr ferritic heat-resistant steel

The interactions between dislocations and lath boundaries in Grade 91 steel were observed by an in situ transmission electron microscopy tensile test at 973 K. Dislocations glided slowly and bowed out in a martensite lath interior. The ends of the dislocation were connected to the lath boundaries. In a tempered specimen, the pinning stress caused by the lath boundary was estimated to be >70 MPa with a lath width of 0.4 μm. In crept specimens, lath coarsening reduced the pinning effect.