Acta Metallurgica et Materialia最新文献

Atomistic simulations of segregation to a dissociated $\text{(a}{}_0\text{/2)[1}\text{1}\text{̄}\text{0]}$ edge dislocation in the solid solution alloy Cu_0.1Ni_0.9 have been performed. Segregation to the stacking fault between the partials is minimal. Results obtained with a general embedded atom method potential and one optimized for the NiCu system differ significantly. Simulations employing the optimized potentials show significantly more Cu segregation to the dislocation cores than do simulations performed with the general potentials. When the general potentials are employed, the Cu concentration around the dislocation is well described using classical segregation isotherms based upon the stress distribution around the dislocation, except in the dislocation core region. Deviations from the theoretically predicted segregation profile around the dislocation core are largest along the slip plane. When the optimized potentials are used, the deviations from the predicted segregation behavior are significantly larger. The large deviations associated with the optimized potentials were traced to the inadequacy of describing the local heat of segregation in terms of the elastic work σ_hΔV. This can be rectified by adding a term to the heat of segregation that explicitly includes the composition dependence. The failure of the classical segregation isotherm to describe the segregation behavior around a dislocation is associated with non-ideal alloy thermodynamics and the inadequacy of linear elasticity to appropriately describe the core region of the dislocation. The failure of the classical segregation isotherm within the core appears to result from the fact that the core atoms have different atomic coordination than those in the bulk material.

Titanium matrix composites reinforced with sol-gel synthesized α-alumina fiber tows have attracted interest as a potentially low cost continuous fiber reinforced metal matrix composite system. We have conducted a detailed investigation of fiber damage during high temperature consolidation of PVD Ti6Al4V metallized sol-gel alumina fiber tows. Using both hot isostatic pressing and interrupted vacuum hot press consolidation cycles, the two principal mechanisms of fiber damage have been experimentally identified to be microbending/fracture and fiber matrix reaction. A time dependent micromechanics model incorporating the evolving geometry and mechanical properties of both the fibers and matrix has been formulated to simulate the fiber bending/failure mechanism in a representative unit cell and explore the effect of fiber strength loss due to reaction with the matrix. This model has been used to design a process cycle that minimizes damage by exploiting the enhanced superplastic deformation of the initially nanocrystalline PVD Ti6Al4V matrix.

The effects of dilute concentrations of the ternary solutes Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W on the critical temperature of the D0₃-B2 transformation in Fe₃Al were measured by differential thermal analysis (DTA). Additional studies by X-ray diffractometry showed that Mo, Ta, and W atoms occupied predominantly the 4(b) Wyckoff sites in D0₃-ordered Fe₃Al. Most solutes caused an increase in the D0₃-B2 transformation temperature, which was approximately linear in the limit of dilute solute concentrations. The efficacy of a particular ternary element in raising the D0₃-B2 transformation temperature was shown to be related to its metallic radius. We argue that the energetics of atom size mismatch are the major cause of the effects of ternary solutes on the D0₃-B2 transformation temperature of Fe₃Al.

The development of deformation microstructures is reported in reinforced powder-consolidated aluminium. This study discusses observations of the effect of additions of SiC whiskers or particles on the microstructural evolution in the strain range 0.01–0.1. For whisker-containing materials the microstructural changes introduced by thermal cycling have also been investigated, and supplemented by measurements of the dislocation density and microhardness. In plastically deformed specimens the size, shape and microstructure of the deformation zone near the reinforcement have been characterized as a function of the imposed strain. The experimental observations have been compared to finite element model predictions of the stress and strain distribution in the uniaxially loaded composite, and good qualitative agreement has been found. This comparison also points to areas for future model development.

The modelling of self-propagating high-temperature reactions is attempted using a finite difference model. The model is described and model results are compared with experimentally observed combustion wave velocities for the reaction 3TiO₂ + 4Al + 3C → 3tiC + 2Al₂O₃ using various processing parameters (diluents, preheating etc.). The model is extended to derive a parameter which determines under what conditions a reaction will become self-propagating.