Acta Metallurgica最新文献

Superplastic alloys possess either a quasi-single phase or a microduplex microstructure: in quasi-single phase alloys, cavities are observed to nucleate predominantly at coarse grain boundary particles whereas in microduplex alloys, cavities tend to form at interphase boundaries and at triple point junctions. A general analysis is presented for cavity nucleation, in both microstructures, under the stress concentrations caused by bursts of grain boundary sliding during superplastic deformation. In quasi-single phase alloys, calculations indicate the cavities nucleate at coarse particles located at grain boundaries because local interphase diffusion creep cannot accommodate the stress concentrations sufficiently rapidly. The analysis demonstrates that it is possible for cavities to nucleate at grain boundary ledges under some limited experimental conditions. It is demonstrated also that the present analysis is in agreement with the available experimental data on a quasi-single phase Cu-based superplastic alloy and a microduplex superplastic Zn-22% Al eutectoid alloy. Calculations show that small pre-existing cavities, if present, are likely to be sintered rapidly prior to superplastic déformation at elevated temperatures.

The temperature dependence of the initiation and growth of small fatigue cracks has been studied by microscopic observations in polycrystalline copper during the early stages of fatigue. The primary procedure was to examine the same site of the specimen by SEM and TEM using surface replicas taken periodically during the fatigue tests. A periodical array of pit-shaped fatigue cracks as small as 0.1 ,um in depth forms along slip bands at 298, 77 and 4.2 K, even in an inert environment. A dislocation gliding mechanism of dislocation dipoles in the slip bands has been suggested as a possible process for the formation of these cracks. It was found that the configuration of small fatigue cracks is related to the periodically spaced dislocation regions in fatigued Cu. The growth rate of small fatigue cracks during the early stages of fatigue was measured and found to be approximately 0.1 nm/cycle at 298 and 77 K. A quantitative analysis of the net effects of temperature and environment on the fatigue crack initiation was performed.

The binding energy between zirconium atoms and vacancies in aluminium has been measured to be 0.24 ± 0.02 eV. Although the equilibrium solid solubility of zirconium in aluminium is very low the high binding energy results in substantial vacancy trapping in solution treated and quenched alloys. It is demonstrated that this results in modification of precipitation of S′-phase in dilute AlCuMg alloys of solute concentrations similar to that employed in commercial alloy 8090 when lithium is also present.

This article summarizes the current understanding of relationships between microstructure and mechanical properties in ceramics reinforced with aligned fibers. Emphasis is placed on definition of the micromechanical properties of the interface that govern the composite toughness. Issues such as the debond and sliding resistance of the interface are discussed based on micromechanics calculations and experiments conducted on both model composites and actual composites.

Grain boundary segregations of impurities like P, S, Sb, Sn and others are the origin of temper embrittlement in low alloy steels. Till now it has been assumed that segregation kinetics is determined by bulk diffusion. But some recent experimental results and a more sophisticated account for diffusion behaviour of substitutional solutes in ferromagnetic iron show such an interpretation to be invalid at least for the embrittlement by phosphorus segregations. In contrast one has to start from the formation of a miscibility gap below 600°C within the grain boundary due to weak repulsive interaction between phosphorus and carbon. The growth of the P-rich phase by grain boundary diffusion should be rate determining to develop embrittlement. For energetical reasons the P-rich phase is favourably formed at prior austenite boundaries and may reach their dimensions. The growth kinetics of this phase is described by one-dimensional diffusion at grain boundaries. This model is successfully applied to experimental results of Möller et al. [Archs Eisenhütt. 55, 543 (1984)]. It allows an unique interpretation of many observed details connected with segregation induced embrittlement.

The development of rolling texture and structure of an alloy Al 1.8wt% Cu was analyzed as function of the precipitation state, which was extensively varied by systematically changing the annealing treatment. While the type of the texture (Copper-type texture) was independent of the precipitation state, by means of the quantitative analysis of the orientation distribution function (ODF) significant differences in the texture development could be revealed. From these conclusions about the differences of the deformation modes for different precipitation states could be drawn.

The structure of the ∑ = 13 (θ = 22.6°) [001] twist boundary in gold is computed using the Embedded Atom Method (EAM). The atomic positions near the boundary are computed and found to be in good agreement with the recent X-ray diffraction results of Fitzsimmons and Sass. The amplitude of the thermal displacements is also computed from the EAM using Monte Carlo simulations and compared with the results for the average Debye-Waller factor of the boundary obtained by Fitzsimmons. Burkel and Sass. The vibrational amplitudes of the atoms adjacent to the boundary are found to be larger than in the bulk and the enhancement is greater for vibrations in the plane of the boundary than for vibrations normal to the boundary. These results obtained using the EAM are also compared to results obtained using two different pair interactions. The main difference between the results using the many-body EAM interactions and the pair interactions is that the overall expansion of the boundary is found to be smaller using the EAM.

Analyses are developed for three processes observed in the preceding paper. These are: (i) the formation of thickening ledges at the intersection of plate-like precipitates, (ii) the thickening kinetics of plate-like precipitates when much of the solute supersaturation has been released by the formation of metastable GP zones (iii) the kinetics of the interfacial energy driven shape change of plate-like precipitates relaxing back to towards their equilibrium shape. Conventional models for the first process are shown to be incapable of accounting for the observed effects. In the second process the analysis developed here was found to be appropriate. In the third process a numerical model developed was developed and this demonstrated that the observed shape change was occuring at rates that were much slower than expected for volume diffusion control.

The grain-size distribution is investigated in the nucleation and growth process. The results are obtained by a computer simulation taking into account different nucleation kinetics. The grain-size distributions and other characteristic quantities strongly depend on the kinetics of nucleation.