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.

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.

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).

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.

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.

Microstructural and textural evolution during rolling were investigated in (112)[11$\text{1}$] single crystals of Al, Cu and homogenous supersaturated All.9wt%Cu. After a rolling degre of 30% the initial C-orientation (112)[11$\text{1}$] of all three materials has rotated towards the so called D-orientation (4411)[1111$\text{8}$]. While in the non-shear banding Al the D-orientation remains stable up to high rolling degrees, in the shear banding materials Cu and AlCu it rotates back to the initial C-orientation simultaneously with the formation of shear bands. This orientation change is explained by a rigid body rotation due to the special geometry of a deformation with unidirectional shear bands. With the onset of shear band formation also strong orientation scatterings about tthe transverse direction appear in the pole figures. These scatterings are located inside the shear bands as well as in their vicinity. They are due to the strong shear deformation and the resulting reaction stresses occurring in the shear bands and in their vicinity, respectively.

An aluminum single crystal with the axial direction of [211&#x0304;0 was fatigued in push-pull at the constant resolved shear stress amplitude 4 MPa, frequency 20 Hz room temperature. Microcracks, microvoids, macrobands, extrusions and intrusions were observed on the side-surface containing the Burgers vector b. Most microcracks were opened, and were within, but approximately perpendicular to, PSBs. Slip steps were found in the extrusions and intrusions. There was net irreversible slip in one direction in most PSBs. Some short cracks along the PSBs on the side-surface were also observed at 5 × 10⁶ cycles. These observations indicate that, without the aid of the surface roughness of PSBs, cracks can still be nucleated, and that, apart from the notch effect of a PSB, there are other factors controlling crack initiation in single crystal aluminium. There may be an internal tensile stress existing in a PSB in the direction of b, and a shear stress applied by the specimen grips in the specimen due to the irreversible slip in one direction in PSBs. These stresses and the applied stress are responsible for the formation of microcracks, microvoids, extrusions, intrusions and macrobands on the side-surface.

The recently developed SEM electron back scattering technique was employed to examine the domain structure in the monoclinic phase of a rare-earth orthoniobate LaNbO₄. The monoclinic phase was identified by X-ray diffraction. The existence of the domain structure was revealed by the electron back scattering pattern, optical microscope and TEM. The orientation relationship between domains was determined by the electron back scattering technique as a rotation of 94° about the [010] axis. This result was confirmed by TEM diffraction and mathematical relations between domain orientations were established.