Ternary eutectic growth during directional solidification of Ag–Cu–Sb alloy

The directional solidification process of ternary Ag_42.4Cu_21.6Sb₃₆ eutectic alloy within a wide growth rate range from 2 to 60 μm/s was accomplished at a constant temperature gradient of 50 K/cm. As growth rate increases, the ternary (θ(Cu₂Sb) + ε(Ag₃Sb) + Sb) eutectic morphology evolves from “lamellar (θ + ε) plus fibrous (Sb)” structure into “(θ + Sb) fibres in continuous ε matrix” structure. The θ and (Sb) phase spacings decrease with the increase of growth rate according to power functions with exponent values of 0.55 and 0.56, respectively. It is also found that the microhardness of directionally solidified Ag_42.4Cu_21.6Sb₃₆ alloy samples is enhanced with the increase of growth rate, and the decrease of θ and (Sb) phase spacings.     

Deformation of nanograined Ni–60Co alloy with low stacking fault energy

The evolution of deformation texture in a Ni–60Co alloy with low stacking fault energy and a grain size in the nanometre range has been investigated. The analyses of texture and microstructure suggest different mechanisms of deformation in nanocrystalline as compared to microcrystalline Ni–60Co alloy. In nanocrystalline material, the mechanism responsible for texture formation has been identified as partial slip, whereas in microcrystalline material, a characteristic texture forms due to twinning and shear banding.     

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.     

Segregation of copper in an Fe–Cu alloy under pulsed electric current

Effect of electric current on the segregation of copper precipitates in the Fe–13.6Cu alloy is evaluated. Results of this approach present two stages of segregation, namely, grain-boundary segregation during the solidification and interphase-boundary segregation during the decomposition of a solid solution. The segregation becomes apparent not only because the thermodynamic barrier for segregation is decreased, but also because the diffusion is greatly enhanced. Based on the thermodynamic and kinetic aspects, the segregation process under electric current would be of great interest and of physical importance because this kind of electric current-induced segregation was much stronger than the thermal diffusion segregation.     

Faceting of a rough solid–liquid interface of a metal induced by forced convection

The solid–liquid interface of metallic systems of small entropy of fusion is characterized by a rough interface and dendritic morphology. In contrast, systems of high entropy of fusion like semimetals and semiconductors show smooth interfaces and facetted interfaces. The present work demonstrates that, in an undercooled melt of a metal–metalloid alloy Ni₂B of intermediate entropy of fusion, a transition from a rough to a smooth interface is induced by forced convection of the melt. Electrostatic levitation is used to container-less undercool droplets in a quiescent state with no convection while electromagnetic levitation (EML) is used to undercool droplets with forced convection. The growth velocity of the solid phase is monitored as a function of undercooling by a high-speed video camera. The data are analysed within dendrite growth theory. In the case of EML, a transition from a rough to a smooth interface is indicated during dendrite growth in the undercooled melt. This is confirmed by facetted microstructures of samples solidified upon undercooling by EML. Hopper-like crystals are formed like in non-metals as bismuth, halite and ice.     

The displacement field of a triangular dislocation loop–a correction with commentary

A correction is made to a previous coordinate-free formulation of the displacement field of a triangular dislocation loop by Barnett [Phil. Mag. A 51 383 (1985)]. Then, taking account of the correction, it is shown that the formulation reduces to that given by Hirth and Lothe [Theory of Dislocations, 2nd edn (Wiley, New York, 1982) p. 146.], which is restricted to a specially chosen coordinate system.     

Production of recrystallized nano-grains in a fine-grained Cu–Zn alloy

A fine-grained Cu–30%Zn alloy sheet was rolled at 77 K to induce ultrafine mechanical twins. Subsequent annealing of the rolled alloy at temperatures up to 543 K led to the uniform appearance of recrystallized ultrafine grains (UFGs), which contained numerous annealing twins. Average grain sizes of 150 ～ 300 nm were produced in this way. The formation of such UFGs during annealing is attributed to the high nucleus density associated with the fine initial grain size as well as to the high densities of mechanical twins and dislocations produced by cryorolling. The high driving force for recrystallization enabled the use of relatively low annealing temperatures, which limited the subsequent grain growth.     

High magnetic field induction of the formation of twinned dendrites during directional solidification of Al–4.5wt%Cu alloy

It is reported that the application of a high magnetic field is capable of inducing the formation of twinned dendrites during directional solidification of Al–4.5wt%Cu alloy. Numerical results reveal that a unidirectional thermoelectric magnetic force acts on tilted dendrites during directional solidification under a magnetic field. This force should be responsible for the formation of twinned dendrites. The work may initiate a new method for inducing twinned dendrites in Al-based alloys via an applied high magnetic field during directional solidification.     

Reverse evolution in nanoscale Cu-rich precipitates of an aged Fe–Cu alloy under electropulsing

Nanoscale Cu-rich precipitates (CRPs) are one of the most important microstructural nano-features responsible for embrittlement and hardening of reactor pressure vessels (RPVs), which threaten the safe operation of nuclear power plants (NPPs) and hinder the lifetime extension of nuclear reactors. A thermally aged Fe-1.1 wt.%Cu alloy, which is used to simulate embrittlement of the irradiated RPV steels, was treated by electropulsing with various parameters. The effect of electropulsing on nanoscale CRPs was investigated by using transmission electron microscopy (TEM). Compared to the traditional heat treatment, the electropulsing treatment (EPT) can accelerate the dissolution of CRPs in an aged Fe-Cu alloy on account of the higher atomic drift flux and the additional Gibbs free energy induced by electropulsing. More importantly, EPT is likely to be a new way of eliminating irradiation-induced Cu-rich precipitates.     

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.     

Formation and analysis of amorphous and nanocrystalline phases in Al–Cu–Mg alloy under friction stir processing

Homogeneous amorphous and nanocrystalline phases formed in the nugget zone of a friction stir-processed Al–Cu–Mg alloy have been studied. X-ray diffraction analysis indicated a diffuse scattering peak with characteristics of an amorphous phase existed in the range 15°–29°. Further, TEM analysis proved the existence of an amorphous structure. Friction stir processing provides special physical conditions, such as high temperature, high hydrostatic pressure and large shear stress, which could induce the amorphization of the alloy.     

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.     

Microstructural development during transient directional solidification of a hypomonotectic Al–In alloy

Upward directional non-steady-state solidification experiments have been performed on a hypomonotectic Al–5.5?wt%In alloy. The alloy developed cellular as-solidified microstructure for tip growth rates, V _L, higher than 0.95?mm/s. The casting regions associated with V _L?<?0.95?mm/s were shown to be characterized by a microstructure formed by In droplets disseminated in the Al matrix. Tip growth rate and microstructural features, such as cell spacing and interphase spacing, have been experimentally determined. The experimental cell spacings have been compared with theoretical predictions furnished by the Hunt–Lu model. It was found that the experimental scatter lies below the minimum range of values theoretically predicted. Moreover, the experimental cell spacing evolution with V _L is characterized by a ?1.1 power law. The droplets’ interphase spacing, λ, is related to the growth rate by the Jackson–Hunt relationship (λ ² V _L?=?constant).     

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.     

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.     

Grain orientation effects on delamination during fatigue of a sensitized Al–Mg alloy

Fatigue crack growth experiments were conducted in humid air (RH~45%) at 25 °C on 29-mm-thick plate samples of an aluminium–magnesium (Al–Mg) 5083-H131 alloy in the long transverse (LT) direction. Samples were tested in both the as-received condition and after sensitization at 175 °C for 100 h. Delamination along some grain boundaries was observed in the short transverse plane after fatigue testing of the sensitized material, depending upon the level of ΔK and K_max. Orientation microscopy using electron backscattering diffraction and chemical analyses using transmission electron microscopy and energy dispersive spectroscopy of grain boundaries revealed that Mg segregation and the orientation of grains had key roles in the observed grain boundary delamination of the sensitized material.     

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.     

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.     

Polytypes of long-period stacking structures synchronized with chemical order in a dilute Mg–Zn–Y alloy

A series of structural polytypes formed in an Mg–1 at.%Zn–2 at.%Y alloy has been identified, which are reasonably viewed as long-period stacking derivatives of the hexagonal-close-packed Mg structure with alternate AB stacking of the close-packed atomic layers. Atomic-resolution Z-contrast imaging clearly revealed that the structures are long-period chemical-ordered as well as stacking-ordered; unique chemical order along the stacking direction occurs as being synchronized with a local faulted stacking of AB′C′A, where B′ and C′ layers are commonly enriched by Zn/Y atoms.