Acta Metallurgica et Materialia最新文献

The effect of heat treatment variables such as initial microstructure, isothermal reaction time and cooling rate on the phase transformations occurring at 1420°C in Ti-48A1 and Ti-48Al-2Mn-2Nb alloys was studied. The main effect of the initial microstructure, which comprised either lamellae of α₂and γ or equiaxed γ grains, was to alter the kinetics, through a change in the chemical driving force of the phase transformations. Therefore, the equiaxed γ grains transformed to α much faster than the lamellar structure and in the initially lamellar structure, growth of α resulted in the delineation of the initial dendritic structure formed during solidification. The effect of the rate of cooling from the heat treatment temperature on the final morphology of these alloys was drastic and resulted in a change in morphology from lamellar grains obtained on furnace cooling to a feathery and mottled morphology obtained on water quenching. TEM analysis of water quenched Ti-48Al-2Mn-2Nb revealed complex morphologies including a structure which consisted of equiaxed γ grains and residual α₂and abutting colonies of γ and α₂. Based on the TEM results, the early stages of formation of γ from α were studied and mechanisms of nucleation and growth discussed. The relative importance and the coexistence of massive and martensitic transformation products is also discussed.

The presence of hydrides in the microstructure can substantially reduce the tensile ductility of Zr and Ti alloys. For treating hydride-induced embrittlement in these alloys, a fracture model has been developed by considering the hydrides to crack readily under tensile loading so that an array of microcracks form in the microstructure. Interaction of the plastic fields of the microcracks leads to fracture of the matrix ligaments, and a loss in the tensile ductility. Application of the proposed model to Zircaloys reveals that hydride-induced embrittlement depends on the hydride size, morphology, and distribution, as well as the continuity of the hydride network, in accordance with experimental observations.

The phenomenology of Lüders bands formation in a rapidly solidified Ni-20Al-12Cr-1.8Mo intermetallic alloy ribbon in the temperature range of 300-770 K is discussed. It was observed that strength and Lüders bands aspect on the specimen were irrespective of temperature. The flow characteristics in the Lüders region of the load-elongation curve were, however, very temperature sensitive. At low temperatures (<470 K), a flat plastic region with few instabilities was seen; but at higher temperatures (>470 K), a clear serrated behavior was manifested and the amplitude of serration increased with temperature. It is suggested that yielding occurs by dislocation generation at grain boundaries and that the stress required for dislocation generation (σ_eff) is athermal. A temperature dependent stress originated by the dynamic pile-up of dislocations at grain boundaries (dynamic stress) is, however, introduced as rate controlling for Lüders front motion and responsible for serration appearance.

The elastic strain energy of perfectly coherent ellipsoid of revolution, which has the cube-cube orientation relationship with the matrix, has been calculated as a function of the orientation of the axis of revolution and of shape factor in anisotropic cubic crystalline materials. The minimum strain energy condition occurs at four different shapes and orientations, i.e. sphere, rod along 〈001〉 axis, disc on 001 plane and disc on 111 plane, depending on the two shear moduli of precipitate, i.e. μ*₁((C*₁₁—C*₁₂)/2) and μ* (C*₄₄). This is true regardless of the elastic property of the matrix phase when its anisotropy factor is larger than 1. The conditions of the occurrence of each shape and orientation are greatly affected by the difference in the misfit accommodation behavior depending on the shape of precipitate. A review of the experimental observations indicates the presence of all four different shapes and orientations in the case of GP zones in Al alloys. The conditions of their appearance are in good agreement with the prediction of the present calculation.

The mechanical properties of a 6061-T6 aluminum alloy reinforced with a 20 vol.% fraction of alumina particles and of an unreinforced 6061-T6 alloy are studied over a range of strain rates (10^-4to 6 x 10⁵s^-1) using quasistatic compression, compression and torsion Kolsky Bars, and high strain rate pressure-shear plate impact. At a given strain rate the composite displays increased strength but essentially the same strain hardening as the matrix. However, the composite displays a stronger rate-sensitivity than does the unreinforced alloy at high rates of deformation (>10³s^-1). The rate-sensitivity of the unreinforced alloy is shown to be largely the result of the imposed strain rate rather than of the rate history. For quasistatic deformations, a model proposed by Bao et al. (1991) describes the behavior of the composite fairly accurately given the behavior of the unreinforced alloy. This paper presents an extension of the model that is able to predict the dynamic behavior of the composite given the dynamic response of the monolithic alloy.

We investigate the sintering of two touching circular particles by surface and grain boundary diffusion. Typical examples for the evolution of the shape of the particles, their surface curvatures, and their surface fluxes are given. The sintering kinetics are evaluated as a function of the dihedral angle at the grain boundary-surface junctions and the grain boundary to surface diffusivity ratio. In particular, the growth rates of the neck between the two particles, the growth rate exponents, and the changes in the lengths of the particle pairs are monitored. The times needed to reach certain fractions of the final equilibrium neck sizes are tabulated for typical experimental dihedral angles and diffusivity ratios. Our simulation is based on a rigorous mathematical system modeling the sintering of the two particles, and a rigorous numerical method for solving this system is adopted.

Strengthening of aluminium-rich aluminium-lithium single crystals by spherical, coherent particles of Ll₂-long-range ordered δ'-phase has been investigated experimentally and theoretically. The total critical resolved shear stressτ₁ has been measured as a function of the radiusr and the volume fraction f of the δ'-particles. r and f covered the ranges 0.0–17.6 and 0.0–0.11, respectively. The δ'-particles' contribution τ_p to τ_t is analysed with reference to a model, which had originally been developed for the description of the strengthening effect of L1₂-long-range ordered γ'-particles in nickel-base superalloys. The experimental data τ_p(r,f) of the present aluminium-lithium single crystals are well represented by this model. This specific antiphase boundary energy of the δ'-particles has been found to be 0.070 ± 0.020J/m². This value refers to {111}-planes.

Crack growth in ceramic matrix composites with creeping fibers has been investigated using a time dependent bridging law to describe the effect of fibers bridging a matrix crack. The fibers were assumed to creep linearly and the matrix was assumed to be elastic. Time dependent crack growth was predicted assuming that matrix crack growth occurs when the stress intensity factor at the matrix crack tip reaches a constant critical value. Crack growth rates are presented as a function of crack length and time. Domains of stable and unstable crack growth are outlined. The solutions illustrate that stable crack growth consists of a relatively brief period of decerelation followed by acceleration to large crack lengths, with crack velocity approaching constancy only at loads very near the matrix cracking stress and for very long cracks. Finally, the time needed to grow a long matrix crack is compared with a rough estimate for the time needed to rupture fibers. A transition is expected from life dominated by matrix crack growth at low stress to life dominated by fiber creep rupture after crack growth at higher stresses.

The formation and growth of Ni₅Al₃ precipitates found inside an annealed B2 matrix in Ni_62.5Al_37.5 samples is described on the basis of conventional and high resolution electron microscopy images and electron diffraction patterns. Short anneals introduce three-pointed star shaped precipitates consisting of twin related segments with different variants of the Ni₅Al₃ structure. Longer anneals result in separate plates growing from these wings and developing microtwinning in order to accommodate stress built-up at the interfaces with the surrounding matrix. Two different growth mechanisms were observed and are discussed. The lattice distortions are found to be much higher in the small precipitates than in the large microtwinned plates.

Cast Ni-30Cr and Ni-30Cr-0.5Y alloys were oxidized at 1000°C in pure O₂ for various times, then were either furnace cooled to room temperature, or thermally cycled between 1000°C and different lower temperatures. The isothermal oxidation rate of the Ni-30Cr alloy was reduced by about a factor of 3.6 by the addition of 0.5% Y. Acoustic emission signals, which are generated by scale fracture events, were collected during isothermal oxidation, during continuous furnace cooling and during thermal cycling. These data showed, as others have shown, that the scale formed on Ni-30Cr-0.5Y was significantly more resistant to fracture than that on Ni-30Cr. This advantage of the Y-containing alloy was evident for comparisons based on equal oxidation times, and more importantly, at equal scale thicknesses. SEM and EDAX analyses show that continuous Cr₂O₃ scales were formed on both Y-bearing and Y-free alloys after a short time of oxidation (2 h), but after a longer period of oxidation and thermal cycling, a NiO or NiCr₂O₄ outer layer was found. This outer scale created a new interface with the Cr₂O₃ scale where thermal stresses will be generated during cooling due to the thermal expansion difference between Cr₂O₃ and NiO or NiCr₂O₄. Spallation at the inner scale/outer scale interface, as well as at the metal/scale interface, was observed. X-ray measurements of scale strains at equal scale thicknesses showed that the growth strains (at the end of the isothermal oxidation period) were larger on the Y-containing alloy, and that this alloy also sustained larger residual strains upon cooling to room temperature. Using a model based on elastic strain energy, estimates of the surface energy for scale fracture (a measure of scale adhesion) were significantly higher for the Y-containing alloy at equal scale thicknesses. Both the AE and the strain measurements are consistent with the proposal that Y improves the inherent strength of the metal/scale interface. The smaller rate of scale cracking for Y-containing alloys, combined with their slower scale growth rate, offers the further benefit of delaying the onset of NiO or NiCr₂O₄ overgrowth layers, which themselves may degrade the integrity of the scale.