Acta Metallurgica最新文献

We have investigated the different behavior of a carbon atom in b.c.c. and f.c.c. metal lattices (α and γ iron lattices) by means of Molecular Dynamics computer simulations. We have evaluated the structural response or the host to the presence of the intrusion in terms of lattice distortion and the relevant thermodynamic properties related to the solute-solvent interaction (site energy, migration energy, diffusion paths, interaction with the host point defects).

Strain localization at near-lattice-mismatched (100) semiconductor heterojunctions results in the formation of dislocations due to plastic deformation of the epilayer at low temperatures. A geometric model for this deformation has been presented which predicts the main features of these dislocation arrays at the heterojunction. In a full analysis of these dislocations, both the dissociated nature of the dislocations and their $\text{α}\text{β}$character must be taken into account. The model has been tested by analyzing the arrangement and character of dislocations present in (Ga,In)As/GaAs and (Ga, In)P/GaAs heterostructures. It is shown that the dislocations geometries differ significantly depending on whether the epilayer was in tension or compression during growth.

The shape of ZnO grains in Bi₂O₃ liquid solution is investigated as a function of temperature. The grains of a characteristic angular shape at low temperature become spherical with the increase of temperature. The grain shape dispersed in liquid is discussed in terms of interfacial energy minimization and growth (dissolution) rate during Ostwald ripening.

The effect of grain boundaries on the cyclic deformation and fatigue crack growth in aluminum bicrystals has been studied. The incompatible primary slip by the two sides of a grain boundary creates heterogeneous slips in a narrow area beside the boundary and cracks along the boundary. The area, termed as Grain Boundary Affecting Zone (GBAZ). is characteristic of heterogeneous deformation as well as internal stresses. As an extended stage I crack grows into the GBAZ under constant cyclic stress, the crack branches and splits deviating from the favorable crack path along a primary persistent slip band (PSB). at the same time, it decelerates or even stops to grow. The microstructure-sensitive growth behavior of extended stage I cracks across grain boundaries in aluminum bicrystals accompanying the crack tip sheilding in GBAZs is refered to the compatibility requirement at the boundaries.

The experiments on the deformation of crystals of MnZn ferrite by indentation with a loaded sphere are extended to (111) planes. As shown before on (001) and (110) planes the deformation consists of coupled slip lines. The slip patterns are discussed in terms of the possible slip planes for a Burgers vector along 〈110〉. A complete description is given on the assumption of cross slip between the slip systems, {110}, {111} and {100}. The analysis is supported by calculations of the resolved shear stress (RSS) on the basis of the elastic equations for a halfspace loaded by a sphere. The source of dislocations is located where the RSS is largest. On the assumption that propagation of slip is easier than nucleation, several cases of coupled slip (“pencil glide”) are considered. A check on the model is made by calculating the length of the slip lines on the surface between the start and the cross-over. The experimental data show a wide spread, but on the average they agree with the model. {100} and {111} appear as primary slip systems when the RSS is sufficient. {100} is excited only in cross-slip, at about the same RSS values as for {100} and {111}. The RSS where propagation stops, show a wide spread too, 0.6–1.8 GPa, but the lowest minimum is found for {100}.

A novel approach for the estimation of the free energies of metastable liquid and allotropie phases has been presented. The method is based on the Taylor series expansion of the free energy différence between stable and metastable phases. The procedure has been applied to arrive at allotropie transformation temperatures in iron and also the isentropic temperature for any two coexisting phases. Results obtained strongly validate the expressions derived which involve only the use of equilibrium heat capacity and entropy data.

The formation of dislocations at lattice-mismatched (100) semiconductor heterojunctions can be understood as a relaxation process involving the low-temperature plastic deformation of the epilayer. A simple model for this deformation shows that the main features of these dislocation arrays at the heterojunction can be predicted. It is shown that the dissociated nature of the dislocations and their $\text{α}\text{β}$ character must both be taken into account. The type of dislocations involved in this relaxation process depends on whether the epilayer is initially in compression or tension.

The zero-temperature energies and equilibrium volume expansions of point-defect free grain boundaries (GBs) on the third and fourth densest planes of f.c.c. Cu have been determined using an Embedded-Atom-Method (EAM) and a Lennard-Jones (LJ) potential. It is found that the energies and volume expansions of the (110) and (113) boundaries are typically about 50% larger than for the (100) boundaries and about three to four times those of the (111) boundaries investigated earlier. This corelation between planar density, i.e. interplanar spacing, and GB energy as well as volume expansion is shown to be closely related to the structural disorder in the interface region. A practically linear relationship between GB energy and volume expansion is observed. Based on the special geometry and properties of the symmetrical tilt configuration on a given lattice plane (which is obtained for a twist angle of 180°), it is proposed to consider these planar defects as generalized (i.e. inverted) stacking faults rather than high-angle grain boundaries.

To study the plateaus and peaks in the temperature dependence of the flow stress commonly observed in solid solution alloys, the flow stress of pure aluminum and three binary Al-Mg alloys has been measured at temperatures between 77 and 750 K and nonelastic strains between 0.02 and 20%. Both athermal and thermally activated processes were found to contribute to the observed behavior. Plateaus in the stress vs temperature response are largely due to the effects of solutes on the evolution of back stresses, an athermal process. Peaks in the stress-temperature response of the substitutional Al-Mg alloys are caused by the interaction of mobile solutes with dislocations, a thermally activated process which increases the effectiveness of the substructure as a barrier to dislocation motion. Based on these results. a physically plausible and simple method of incorporating solute strengthening within unified physical-phenomenological constitutive equations is outlined.