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Spontaneous growth of metal whiskers has been studied for around 70 years, but still resists interpretations. Herein, the extrusion of whiskers from MAX phases, a group of nanolaminate ceramics, is surveyed using a combination of experiment and simulation methods. It was once believed that the excess A in MAX substrate provided element source for whiskers, while in this work, however, direct evidence from isotope experiment of Ti₂SnC/¹²⁰Sn demonstrates that the extruded Sn whiskers are composed of the atoms from both the excess Sn and the lattice Sn. Moreover, the Ti2AC/A' (A, A' = Sn, Ga) crossover experiment indicates that the extrusion of A whiskers is always companied with the substitution process of excess A (or A') for lattice A atoms, with the driving force of which concluded to be the chemical potential gradient (or reduced formation enthalpy). Accordingly, a lattice diffusion based atomic motion mechanism is proposed in this work, to explain the whiskering behaviors in MAX phase.

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The generation of vacancies at a crystal-liquid interface propagating under conditions of undercooling below the equilibrium melting temperature is investigated in molecular dynamics simulations performed under well-controlled temperature and pressure conditions for two representative metals, bcc Cr and fcc Ni. The results of the simulations reveal that, for both metals, vacancy concentrations produced in the course of the steady-state propagation of crystallization fronts can exceed the equilibrium values at the corresponding temperatures by orders of magnitude. Different trends in the temperature dependences of vacancy concentration observed for the two metals, namely, the continuous increase with increasing undercooling in Ni and a nonmonotonous temperature dependence in Cr, are related to the qualitatively different temperature dependences of the crystallization front velocity predicted for the two metals. The general character of the computational predictions is confirmed in simulations performed for Ni with four different interatomic potentials and for both metals with (001), (011), and (111) interface orientations. A detailed analysis of atomic rearrangements at the crystallization front suggests that the level of vacancy supersaturation is largely defined by the ability of atoms to migrate within the interfacial region and to fill the numerous vacant sites produced through the simultaneous construction of several atomic crystal planes within the interfacial region. While the majority of the transient vacancies generated during the construction of the crystal planes are annihilated by the atomic flux coming from the liquid side of the interface, a small fraction of the vacancies are trapped behind the crystallization front. Under conditions of strong undercooling, the fast movement of the interface and decreased vacancy mobility prevent the equilibration of vacancy concentration in the newly built crystalline region, thus creating a strong vacancy supersaturation. Analysis of the temperature dependence of the atomic rearrangements occurring at the crystallization front suggests incomplete relaxation of the disordered phase at and in front of the crystal-liquid interface rapidly advancing under conditions of strong undercooling.

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The aluminum-zirconium (Al-Zr) alloys are of interest because of their superior thermal stability and good electrical conuctivity, but their main drawback is their rather low hardness (<75 Hv) even after severe plastic deformation (SPD). These alloys, which are actually composites of Al and Al₃Zr intermetallics (D0₂₃ tetragonal structure), do not show age-hardening behavior due to the immiscibility of Zr in Al. In this study, ultra-SPD with shear strains up to 40,000, realized by high-pressure torsion (HPT), is employed to generate supersaturated solid solution in an Al-Zr alloy. Subsequent aging leads to unusual precipitation of AlZr precipitates at grain boundaries (Bf orthorhombic structure) and of coherent metastable Al₃Zr precipitates within grain interiors (L1₂ cubic structure). It is found that the supersaturation of Zr in Al by ultra-SPD is controlled by an ultrafast dynamic diffusion which is comparable to surface diffusion, while the formation of precipitates by static aging is controlled by lattice and pipe diffusions. These microstructural features provide exceptional increase in hardness up to 148 Hv, great thermal stability up to 523 K and reasonable electrical conductivity up to 35 %IACS. Detailed analysis shows that ~30% of this hardening is caused by precipitation strengthening, while extra hardening is achieved due to nanograin formation, grain-boundary segregation and dislocation accumulation. This study introduces a rational approach to produce novel age-hardenable Al alloys with high thermal stability and reasonable electrical conductivity by application of ultra-SPD.

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向上滑动阅览英文摘要

A single Cr-rich σ-phase alloy with a composition of Co17Cr46Fe16.3Mn15.2Ni5.5 (at.%) and a tetragonal lattice structure was produced. The tracer diffusion coefficients of Ni and Fe were measured by secondary ion mass spectroscopy using the highly enriched ⁶⁴Ni and ⁵⁸Fe natural isotopes. On the homologous temperature scale, Ni and Fe diffuse in the σ phase faster as compared to the corresponding diffusion rates in the equiatomic and face-centered cubic CoCrFeMnNi alloy. In contrast, on the absolute temperature scale, these elements diffuse roughly at the same rates in both materials. Factors influencing element diffusion and phase stability of the σ phase compared to the equiatomic alloy are discussed.

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向上滑动阅览英文摘要

This paper presents a multi-level crystal plasticity-based simulation framework for modeling mechanical response and microstructure evolution of Ti-6Al-4V with α-lath/lamellar microstructures. The model is a correlated structure visco-plastic self-consistent (CS-VPSC) formulation linking three scales: a single crystals micro-scale, a lath/lamellar colony meso-scale, and a lath/lamellar aggregate macro-scale. A selected hardening law for the evolution of critical resolved shear stress per slip system used in CS-VPSC is phenomenological. However, it adjusts the resistances of basal and prismatic slip systems based on the geometry of slip transfer between adjacent lamellae. Consistent with experimental evidences, the resolved shear stress on the pyramidal slip planes is dependent not only on the stress in the direction of slip but also on the two orthogonal shear stress components and the three normal stress components (non-Schmid effects). Electron backscatter diffraction (EBSD) data in conjunction with a procedure relying on α→β phase transformation is used to construct paired variants of α-laths/lamellae satisfying their local crystallographic correlations. The procedure fits volume fractions of individual laths/lamellae with the experimental EBSD data and selects a distribution of habit planes between adjacent variants with respect to the loading direction. The simulation framework is applied to interpret the deformation behavior in tension and compression along two sample directions of Ti-6Al-4V fabricated via laser powder bed fusion. Moreover, the model is used to simulate texture evolution during rolling of the material to large strains. It is demonstrated that the model is capable of predicting plastic anisotropy/asymmetry and the concomitant texture evolution. While the model reveals a significant effect of habit plane inclination with respect to the loading direction on yield stress, the comparison of the data and model predictions shows that a random distribution of habit planes fits the flow response. It is further inferred that the tension-compression asymmetry arises from the non-Schmid effects.