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.     

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.     

Recrystallization in a low-density low-alloy Fe–Mn–Al–C duplex-phase alloy

The recrystallization behaviour of a cold-rolled, low-density, low-alloy duplex-phase alloy (Fe–6.57Al–3.34Mn–0.18C, wt.%) has been studied. Temperature-resolved X-ray diffraction and dilatometry showed that the alloy recrystallizes at 850?°C during continuous heating. However, electron back-scattered diffraction investigations using Kernel average misorientation revealed that during annealing ferrite recrystallizes at lower temperatures while austenite remains strained up to 1200?°C. This study underlines the complexity of recrystallization of a microstructure comprising of constituents with high and low stacking fault energy.     

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.     

Prediction of termination migration of a rod-type particle in Ti–6Al–4Fe alloy

The kinetics for the termination migration of a rod-type alpha particle in a two-phase Ti alloy was predicted on the basis of the edge recession theory. The developed model quantified the effect of geometrical dimensions and diffusional factors on the spheroidization rate. Comparison with the experimental results of Ti–6Al–4Fe showed that the model can provide a reasonable prediction of the time to complete the static spheroidization of rod-type particles.     

TEM observation of oxide scale formed on a Ti–Al–Zr alloy oxidized at 360°C in alkaline steam

Ti–Al–Zr alloy has been oxidized at 360°C in alkaline steam at a pressure of 10.3?±?0.7?MPa. Cross-sectional transmission electron microscopy (TEM) observations indicated that the oxide scale of Ti–Al–Zr alloy was composed of outer and inner subscales, in which the outer layer consists of anatase-TiO₂ and the inner layer a mixture of TiO and Ti₂O. The thickness of the Ti₂O, TiO and anatase-TiO₂ were approximately 50, 100 and 400?nm, respectively. These results were confirmed by X-ray energy dispersive spectrometry (EDS) measurements. The enhanced corrosion of titanium alloys in LiOH solution is attributed to a high hexagonal Ti₂O to tetragonal TiO₂ phase transformation rate induced by the substitution of Li⁺ for Ti⁴⁺ in the oxide layer.     

Effect of Si on the formation and stability of the icosahedral quasicrystalline phase in Al–Fe–Cr–Ti alloys

The formation and stability of the quasicrystalline icosahedral (i) phase in melt-spun Al_{93– x} Fe₃Cr₂Ti₂Si _x (x?=?0–5) ribbons are reported. Samples were characterized by X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. Primitive (P-type) ordered i phase particles were found to be homogeneously distributed in an fcc α-Al matrix in the as-melt-spun ribbons. The size of the i phase particles decreased and their thermal stability increased with increasing substitution of Al by Si. The i phase had a decomposition temperature of approximately 480°C in an Al₉₃Fe₃Cr₂Ti₂ ribbon whereas that in an Al₉₂Fe₃Cr₂Ti₂Si₁ ribbon was approximately 500°C. The i phase particles are resistant to coarsening prior to decomposition into crystalline phases. The presence of a small quantity of Si (up to 1.0?at.%) is beneficial to both the thermal stability and the hardness of nanoquasicrystalline Al–Fe–Cr–Ti alloys.     

Phase transformation of Cu precipitate in Fe–Cu alloy studied using self-guided molecular dynamics

The self-guided molecular dynamics (SGMD) method, which can enhance the conformational sampling efficiency in MD simulations, was applied in investigating the phase transformation of Cu precipitate in α-iron that took place during thermal aging. It was shown that the SGMD method can accelerate calculating the bcc to 9R structure transformation of a small precipitate (even 4.0 nm in size), enabling the transformation without introducing any excess vacancies. The size dependence of the transformation also agreed with that seen in previous experimental studies.     

Microstructural features of phase transformation in a binary Pd–Si metallic glass

Glassy ribbons of Pd–Si alloys were prepared by a combination of melt spinning and flux treatment. The crystallization behaviour of a Pd₈₁Si₁₉ glassy alloy was studied through isothermal annealing at temperatures ranging lower than the glass-transition temperature T _g to around the onset of crystallization. The evolution of microstructures arising from isothermal annealing was investigated by X-ray diffraction (XRD) and (high-resolution) transmission electron microscopy ((HR)TEM). XRD spectra showed that, after the sample was annealed at a sub-T _g temperature, its first diffraction peak was split into two overlapping broad peaks. TEM analysis revealed the formation of a spherical, particle-like glassy phase embedded in the glassy matrix together with a finely connected network morphology within both. Combining these observations with compositional analysis suggested that phase separation had taken place during sub-T _g annealing. When the glassy alloy was annealed at temperatures higher than T _g, nanocrystalline structures, composed of Pd₃Si and Pd phases plus a Pd₉Si₂ phase with a lamellar structure, was formed.     

Synthesis of a single phase of high-entropy Laves intermetallics in the Ti–Zr–V–Cr–Ni equiatomic alloy

The high-entropy Ti–Zr–V–Cr–Ni (20 at% each) alloy consisting of all five hydride-forming elements was successfully synthesised by the conventional melting and casting as well as by the melt-spinning technique. The as-cast alloy consists entirely of the micron size hexagonal Laves Phase of C14 type; whereas, the melt-spun ribbon exhibits the evolution of nanocrystalline Laves phase. There was no evidence of any amorphous or any other metastable phases in the present processing condition. This is the first report of synthesising a single phase of high-entropy complex intermetallic compound in the equiatomic quinary alloy system. The detailed characterisation by X-ray diffraction, scanning and transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the existence of a single-phase multi-component hexagonal C14-type Laves phase in all the as-cast, melt-spun and annealed alloys. The lattice parameter a = 5.08 Å and c = 8.41 Å was determined from the annealed material (annealing at 1173 K). The thermodynamic calculations following the Miedema’s approach support the stability of the high-entropy multi-component Laves phase compared to that of the solid solution or glassy phases. The high hardness value (8.92 GPa at 25 g load) has been observed in nanocrystalline high-entropy alloy ribbon without any cracking. It implies that high-yield strength (~3.00 GPa) and the reasonable fracture toughness can be achieved in this high-entropy material.     

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.     

Distribution of Cr atoms in a strained and strain-relaxed Fe89.15Cr10.75 alloy: a Mössbauer effect study

An Fe_89.15Cr_10.75 alloy in a heavily strained (by cold rolling) state and in strain-relaxed states was studied by means of conversion electrons Mössbauer spectroscopy. Analysis of the spectra in terms of a two-shell model revealed significant differences between the studied samples, particularly in values of the hyperfine field and the distribution of Cr atoms within the first two neighbour shells. The latter is quantified in terms of short-range order parameters.     

HRTEM study of the {252}γ austenite–martensite interface in an Fe—8Cr–1C alloy

Abstract

High-resolution transmission electron microscopy has been used to image the atomic structure of the (2S2)_γ austenite-martensite interface. By imaging along [101]_γ ∥[111]_α, the interface was viewed edge-on and seen to consist of facets on the close-packed (111)_γ, planes. From the correspondence of atoms in the close-packed planes across the austenite-martensite interface, the magnitude of the shear can be analysed as (a/24)<112> on every close-packed plane in the plane of projection. Comparison with theory indicates that this is an (a/12)<112> Burgers vector out of the plane of projection. Hence, each atomic facet can be viewed as a structural ledge containing an (a/12)<112> transformation dislocation.     

Formation of stacking fault tetrahedra around α-Fe particles in a Cu–Fe alloy

Abstract

The formation of stacking fault tetrahedra in a Cu–Fe alloy containing α-Fe particles has been examined by in situ observation in a high-vacuum and high-voltage electron microscope. A sudden disappearance of secondary defect contrast has been noted during prolonged electron irradiation. Concurrently, a moiré pattern develops inside the α-Fe, indicating the alignment of the low-index direction along the electron beam by rotation of the particles.     

Application of the stagnant stage concept for monitoring Mn partitioning at the austenite-ferrite interface in the intercritical region for Fe–Mn–C alloys

The length of the stagnant stage during the new ferrite growth starting from a mixture of austenite and ferrite has been investigated for a Fe-0.17Mn-0.023C (wt%) alloy. It was found that the stagnant stage depends on the thermal path followed to create the mixture, and deduce that the tie-lines for austenite to ferrite transformation are quite different from those for ferrite to austenite transformation. The length of the stagnant stage is determined by the very local partitioning effect at the interface, and it can be used as a tool to monitor the Mn partitioning.     

Mechanically driven alloying forces in the fabrication of a Cr–Zr nanocomposite

A highly supersaturated nanocrystalline Cr–25?wt% Zr alloy has been prepared by mechanical alloying of elemental crystalline powders. High-purity powders of Cr and Zr were milled for up to 20?h. The development of the microstructure was investigated by X-ray diffraction (XRD) and scanning electron microscopy. XRD patterns confirmed complete alloying of the Zr and Cr. The contribution of grain boundaries, the chemical potential of a solute atom induced by dislocations, and the elastic strain energy arising from the different sizes of Cr and Zr atoms have been calculated. The alloy formation is discussed with respect to the thermodynamic conditions of the material. The role of internal strains and stored enthalpies by dislocations on solute atoms is the major mechanical driving force for alloying and this is critically assessed in this article.     

Determination of parent β-phase orientation from inherited orthorhombic phase in β → O + B2 phase transformation of Ti–22Al–25Nb alloy

We propose and describe a method for determining the orientation of parent body-centred cubic (bcc) β grains at high temperatures from the orientations of the orthorhombic variants observed at room temperature as applied to the case of high-Nb-containing Ti₃Al-based alloys. The method is based on knowledge of the orientation relationship between the parent and inherited phase. By averaging, the procedure enables determination of the most probable parent orientation using an approximate orientation relationship. The β?→?O transformation in Ti₃Al-based Ti–22Al–25Nb alloy is very suitable for checking the relevance and effectiveness of the method because, in this case, after certain processing, some β-phase is retained at room temperature.     

Kinetic study of hydrogen-induced direct phase transformation in Y2Fe17 magnetic alloy on the basis of Kolmogorov’s model

The kinetics of the hydrogen induced direct phase transformation in Y₂Fe₁₇ magnetic alloy has been analysed within the framework of Kolmogorov’s kinetic model. It is established that the transformation can be classified as a diffusion-controlled transformation, which occurs by a mechanism of nucleation and growth with a decreasing nucleation rate of new phases, namely α-Fe and YH₂. A kinetic equation has been obtained that well describes the isothermal kinetic curves of the phase transformation in Y₂Fe₁₇ as a function of the transformation temperature.     

Cognitive–emotional distinctiveness: Separating emotions from non-emotions in the representation of a stressful memory

Current theories on autobiographical memory and recent neurological evidence suggest that emotional and non-emotional features of a memory may be retrieved by separate systems. To test this notion, 207 participants who had experienced the break-up of a significant romantic relationship in the last 12 months completed a Multidimensional Scaling (MDS) procedure in relation to the previous relationship. The resulting MDS model revealed two dimensions: a valence and an emotional/non-emotional dimension. Further, participants who associated a high level of distress with their relationship break-up perceived less dissimilarity between emotional and non-emotional features than participants who associated a low level of distress with their relationship break-up. Theoretical and methodological implications for stress and memory are discussed.     

Applicability of Scheil–Gulliver solidification model in real alloy: a case study with Cu-9wt%Ni-6wt%Sn alloy

The present work explores the possibilities of the application of Scheil–Gulliver equation in modelling the solidification of a real alloy. For this study, Cu-9 wt%Ni-6 wt%Sn alloy was chosen which exhibits profuse micro-segregation during solidification, and hence easy to quantify experimentally. Also, this alloy is spinodally strengthened high strength copper alloy and has industrial importance. In this study, thermodynamic assessment using Scheil–Gulliver solidification model was carried out. Subsequently, the assessed result was compared with the experimentally obtained results from energy-dispersive X-ray spectroscopy analysis, and a good agreement was observed between these results. Therefore, it could be concluded that the solidification of this particular alloy system can be modelled using Scheil–Gulliver equation.