Re-precipitation of coherent γ-Fe particles following annealing of equal-channel angular pressed Cu–Fe alloy

The transformation of second-phase particles in a Cu–Fe alloy following equal-channel angular extrusion and annealing has been determined by transmission electron microscopy. Equal-channel angular pressing of the Cu–Fe alloy transformed coherent γ-Fe particles in the Cu matrix into incoherent α-Fe. Upon annealing, numerous coherent γ-Fe particles were observed. A dislocation–particle interaction mechanism is suggested to explain the re-precipitation of coherent γ-Fe particles following annealing.     

Formation of ω -Al7Cu2Fe phase during laser processing of quasicrystal-forming Al–Cu–Fe alloy

The formation of an ω-Al₇Cu₂Fe phase during laser cladding of quasicrystal-forming Al₆₅Cu_23.3Fe_11.7 alloy on a pure aluminium substrate is reported. This phase is found to nucleate at the periphery of primary icosahedral-phase particles. A large number of ω-phase particles form an envelope around the icosahedral phase. On the outer side, they form an interface with an α-Al solid solution. Detailed transmission electron microscopic observations show that the ω phase exhibits an orientation relationship with the icosahedral phase. Analysis of experimental results suggests that the ω phase forms by precipitation on an icosahedral phase by heterogeneous nucleation and grows into the aluminium-rich melt until supersaturation is exhausted. The microstructural observations are explained in terms of available models of phase transformations.     

Formation of Cu precipitates and vacancy clusters in neutron-irradiated Fe–Cu alloys

Two Fe–Cu binary model alloys, Fe–0.3Cu and Fe–0.6Cu, were irradiated with fission neutrons at doses ranging from 4?×?10^?6 to 0.16 dpa (displacements per atom) at ～573 K to investigate the formation of Cu precipitates and microstructural evolution. The Cu content only affected the formation of Cu precipitates and microvoids at low doses. In Fe–0.3Cu, the formation of microvoids and Cu precipitates initiated at doses of 1.2?×?10^?4 and 4?×?10^?5 dpa, respectively. On the other hand, the formation of microvoids started at a dose of 4?×?10^?4 dpa in Fe–0.6Cu, and Cu precipitates were formed even after irradiation to 4?×?10^?6 dpa. On further irradiation, the difference in the formation of Cu precipitates and microvoids was small. Microvoids grew with increasing irradiation dose up to 3?×?10^?3 dpa in both alloys. Prominent aggregation of Cu atoms occurred upon irradiation from 3?×?10^?3 to 1.6?×?10^?2 dpa and the microvoids shrank. The Cu precipitates no longer grew, and microvoids nucleated and grew in the matrix above a dose of 1.6?×?10^?2 dpa in both alloys. The present studies clearly reveal the relationships between the formation and growth of Cu precipitates and microvoids with irradiation dose.     

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.     

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.     

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.     

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.     

Precipitation in a Cu–4Ti–2Be alloy

It is known that the binary Cu–2?wt.% Be alloy can be strengthened by ageing its supersaturated solution at 523?K. The increase in the strength arising from ageing is because of the formation of GP zones. The same level of strength is achieved by ageing the supersaturated solid solution of a dilute binary Cu–4?wt.% Ti alloy at 723?K. In contrast to Cu–2?wt.% Be alloy, the increase in strength in this alloy occurs on account of spinodal decomposition of the as-quenched microstructure through the mechanism of clustering and ordering. In the present study, decomposition of a supersaturated solid solution of the ternary Cu–4?wt.% Ti–2?wt.% Be alloy was studied. Whereas ageing of this alloy at 523?K resulted in the formation of GP zones like in a Cu–2?wt.% Be alloy, ageing at 723?K resulted in a spinodally decomposed microstructure as in a Cu–4?wt.% Ti alloy. The decomposition of the ternary alloy at the two temperatures by two different mechanisms indicates that Be and Ti in solution in Cu act, more or less, independently of each other during ageing.     

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.     

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.     

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.     

Shape of prismatic dislocation loops in anisotropic α-Fe

Prismatic dislocation loops are the primary manifestation of radiation damage in crystals, and contribute to the phenomenon of radiation embrittlement. This undesirable effect, most serious for materials used in high-dose environments such as next-generation fission and future fusion reactors, results from the strong interaction between gliding dislocations, the carriers of plasticity, with the population of radiation-induced prismatic loops. Ferritic–martensitic steels, the most promising candidate materials for future high-dose applications, are based on iron and are known to become highly elastically-anisotropic at the high temperatures (>500°C) at which they must operate. In this article, we develop a novel modelling approach based on anisotropic elasticity theory to predict the shapes of prismatic loops in anisotropic crystals, paying particular attention to the technologically important case of α-iron. The results are compared with transmission electron microscope observations of the damage structure sustained by ultra-high-purity iron irradiated to a dose of approximately two displacements per atom.     

Rapid dendritic growth within an undercooled Ni–Cu–Fe–Sn–Ge quinary alloy

A liquid quinary alloy with composition Ni–5%Cu–5%Fe–5%Sn–5%Ge has been prepared from a containerless state by undercooling. Dendritic growth of α-Ni phase took place with a velocity of 28 m s^?1 at the maximum degree of undercooling, which was as high as 405 K (0.24T _L). All of the four solute elements Cu, Fe, Sn and Ge exhibited a significant solute trapping effect during the rapid dendrite growth. Segregation-less solidification is consequently realized when the degree of undercooling is sufficiently large. The lattice constant of α-Ni solid solution phase is found to increase with the amount of multicomponent solute trapping.     

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.     

High-resolution characterization of the precipitation behavior of an Al–Zn–Mg–Cu alloy

The metastable particles in an Al–Zn–Mg–Cu alloy have been examined at atomic-resolution using high-angle annular dark field (HAADF) imaging. In under-aged conditions, thin η′ plates were formed with a thickness of seven atomic planes parallel to the {111}_Al planes. The five inner planes of the η′ phase appear to be alternatively enriched in Mg and Zn, with two outer planes forming distinct Zn-rich interfacial planes. Similar Zn-rich interfacial enrichment has also been identified for the η phase, which is a minimum 11-plane thick structure. In rare instances, particles less than seven planes were found indicating a very early preference for seven-layer particle formation. Throughout the aging, the plate thickness appears constant, while the plate radius increases and no particles between 7 and 11 planes were observed. Based on the HAADF contrast, our observations do not support the η′ models previously set forth by other authors. Clear structural similarities between η′ and η were observed, suggesting that drawing distinctions between η′ and η phases may not be necessary or useful.     

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.