Acta Metallurgica et Materialia最新文献

The results of measurements of the reciprocal recalescence rise time (Δt)⁻¹ of undercooled melts of an Ag-Cu alloy with 65 at.% Cu are presented. It is shown that (Δt)⁻¹ is a semiquantitative measure of the growth rate. In agreement with observations already made on alloys with other compositions in this binary system, the growth rate as a function of the undercooling rises sharply at an undercooling corresponding to the T₀ temperature. This behaviour can be explained in the framework of the theories of rapid dendritic growth, when the kinetic displacements of the solidus and liquidus are taken into account. A simple empirical expression for this kinetic effect is proposed. For the Ag-65 at.% Cu alloy the predictions of the theory are expressed as a plot of growth rate against undercooling and also in the form of a kinetic phase diagram. The predictions are in good qualitative agreement with the experimental results.

Nitrogen alloying of iron-based metals greatly improves mechanical and corrosion properties. However, nitrogen solubility is less than 0.1 weight percent (wt%) in b.c.c iron. In this study nitrogen concentrations in excess of 1 wt% were obtained by high-energy milling of pure iron powders in a nitrogen gas environment. The nitrogen concentration in the powder, the grain size, and internal strain in the mechanically processed iron powder were determined as functions of processing time in both nitrogen and argon gas environments. The contributions of plastic deformation and interstitial nitrogen to changes in lattice d-spacing, and the distribution of nitrogen between interstitial sites and defect sites were estimated. Approximately 25% of the incorporated nitrogen was contained in interstitial sites, with the remainder associated with defect structures and surfaces.

Tensile specimens of Type 316L stainless steel having grain sizes in the range 3.1–86.7 μm were deformed to 34% strain at temperatures 24, 400 and 700°C and strain rate 1 × 10⁻⁴s⁻¹ to investigate the Hall-Petch (H-P) relationship, the nature of stress-strain curves and the substructure development. Upto ∼5% strain the H-P relationship exhibits bi-linearity whereas the single Hall-Petch relation is exhibited at larger strains. The presence of bi-linearity is explained by the back stress associated with the difference in the dislocation densities in the vicinity of grain boundary and in the grain interior. The log stress (σ)-log strain (ε) plots depict three regimes and follow the relationship σ = Kεⁿ in each regime, but with varying magnitudes of the strength coefficient (K) and strain-hardening exponent (n).

Part of the dislocations which have participated in the plastic deformation of a polycrystalline metal are stored in dislocation boundaries in a two- or three-dimensional arrangement. The dislocations in such boundaries can be analysed by determining the misorientation between neighbouring crystallites and the boundary orientation. Information about the dislocations in the boundaries can also be obtained by an analysis of active slip systems based on the crystallite orientation and the imposed stress or strain state in combination with appropriate constraint conditions. In the present paper an analysis of the boundary dislocation structure and of the slip systems has been conducted for pure aluminium cold-rolled to a von Mises strain of 0.41. The results show that a substantial majority of dislocations in different types of dislocation boundaries are from the primary and conjugate slip system in the adjoining crystallites. A basis is therefore provided for integrating deformation structure observations with plastic deformation behaviour.

Misfit dislocations at the ErAs/GaAs interfaces grown by molecular-beam epitaxy have been investigated using the weak-beam technique of transmission electron microscopy (TEM). The observed dislocation configurations are significantly different from those at heterojunctions between “diamond-cubic” structured materials. Networks of nearly orthogonal dislocation, with dislocations lying approximately along [010] and [001] dislocations, and honeycomb-like dislocation networks have been observed. The dislocation density increases as the ErAs layer thickness increases. Different dislocation reactions between the a/2〈110〉 type dislocations, which result in complex dislocation configurations, are discussed. Slight misalignment of the epilayer with respect to the substrate is possible if there are uneven distributions of inclined Burgers vectors in different orientations or screw components in the dislocation network at the interface.

Yielding and plastic flow behavior of B2-type Fe-39.5 mol.% Al were investigated by deforming single crystals in the temperature range from room temperature to 1073 K. Yield stress exhibits a distinct positive temperature dependence followed by a peak for all the orientations examined. The temperatures of the anomalous peak are located between 823 and 873 K for all the orientations except the near-[111] orientation. Only for the near-[111] orientation the peak temperature is located between 773 and 823 K. The slip transition from 〈111〉 direction at intermediate temperatures to 〈100〉 at high temperatures occurs at the peak temperatures. The yield stress at 773 K exhibits a strong orientation dependence and has a good correlation with respect to non-glide stress component. Specimens having compression axes of χ ⪖ 0° exhibit serrations in stress-strain curves below the peak temperatures, whereas the serrations are not observed in those of χ < 0°. In addition, a yield drop is observed around the peak temperatures for all the orientations. Below the peak temperatures, even as low as at room temperature, the yield stress hardly depends on the applied strain rate. This indicates that the motion of 〈111〉-type superdislocations has very small strain-rate sensitivity in the temperature range. On the other hand, there is a strong strain-rate dependence at the peak temperature and above, indicating that the motion of 〈100〉-type dislocations is strongly rate sensitive. The positive temperature dependence of yield stress in B2 FeAl is discussed on the basis of the present results.

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.

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.

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.