Acta Metallurgica et Materialia最新文献

Biaxial compression experiments were carried out to investigate the mechanisms of crack nucleation in columnar freshwater ice. For strain rates higher than 10⁻³ s⁻¹ at — 10°Cm, it is observed that crack nucleation is preceded by grain boundary sliding which results in local decohesion pockets spread intermittently along the boundary facets. The experimental results are shown to support the crack nucleation model presented in the companion paper.

The dependence upon temperature of the monovacancy concentration in metals has been calculated using models for the perturbation of the atomic frequencies accompanying vacancy formation. The effect of the lattice thermal expansion has also been considered. The calculations, based on the quasi-harmonic model for solids, indicate that Arrhenius plots of the vacancy concentration against reciprocal temperature would exhibit a measurable curvature for Al. Much smaller degrees of curvature would be observable in such plots for Cu and Au over the temperature span of experimental vacancy concentration determinations.

Hydrogen induced void nucleation of 310 stainless steel was investigated. Experimental results indicated that hydrogen promoted void nucleation. A new model of hydrogen induced void nucleation was proposed. The basic idea of this model is that hydrogen induces void nucleation not only by promoting microcrack nucleation but also by promoting the transition of microcrack to microvoid; hydrogen also increases the stability of a microvoid by forming hydrogen pressure in the microvoid and by decreasing the void surface energy.

The stability of an infinitely long screw 〈111〉 superdislocation dissociated into APB-coupled superpartials on {110}, {112}, or a combination of both, has been analysed under the approximation of linear elasticity theory of dislocations. In the absence of an applied load, the variation of the configurational energy of the dislocation pair in equilibrium has been examined analytically as a function of two basic material parameters, i.e. the elastic anisotropic parameter along 〈111〉M and the ratio of {112} vs {110} APB energies λ = γ₁₁₂/γ₁₁₀. It is found that although there is no tor1ue force on the screw partials on both {110} and {112}, the torque force is present at intermediate angular positions and acts in the direction of stabilizing {112} dissociation and destabilizing {110} dissociation. In general dissociation on {112} is favoured energetically if λ > M^1/3. Since M > 1 in anisotropic elastic media, ranging from M = 1.09 for FeAl to M = 1.64 for AuCd, favourable {112} dissociation does not require γ₁₁₂ < γ₁₁₀. For compounds with $\text{M > 1.22(√}\text{3}\text{2}\text{)}$, the screw superdislocation may dissociate into stable non-planar configurations with the APB lying partly on {110} and partly on {112}, forming a locked structure.

A fracture model for spheroidized hypereutectoid steels is developed based on the concept that the stress in the ferrite matrix is the driving force for crack initiation at grain boundaries within the coarse carbides. The ferrite matrix fracture stress, δ_f,ferr is calculated by averaging the ferrite stress using upper and lower bound concepts, and by utilizing the fracture strength of the carbide. The analyses and results indicate that the fracture behavior follows a classical fracture mechanics relation in that the fracture strength is a unique function of the reciprocal of the square root of the carbide particle size with δ_f,ferr equal to zero at infinite carbide (crack) size. It is concluded that the fracture strength of the iron-iron carbide composite is enhanced by: (i) increasing the strength of grain boundaries within carbides; (ii) decreasing the average carbide size, and (iii) increasing the carbide volume fraction.

A directional dependence of the corrosion fatigue behavior of Fe-2.7wt%Si alloy Σ5[100]/(021) bicrystals was observed in a film forming solution. To understand this directional dependence, dislocation nucleation analyses at the opposite crack tips based on the Rice-Thomson concept and continuum elastic-plastic analyses, within single crystal theory, of the near tip fields by finite element method were performed. Explanations of brittle/ductile fatigue crack growth under corrosive conditions and the local crack tip rotation within the interface are offered based on these analyses.