Acta Metallurgica et Materialia最新文献

This work is devoted to the prediction of the constitutive steady-state creep behavior of matrix inclusion composites. Both phases are characterized by power-law constitutive equations. The three phase model is extended to viscoplastic equations. If both phases have the same strain rate sensitivity, the effective behavior of the composite is characterized by an effective prefactor. If not, an effective strain rate sensitivity is defined, which is a function of the applied strain rate and of the volume fraction of the phases. All the results are compared with the classical self-consistent ones. A limit case which may be related to the grain boundary sliding accommodated by intragranular power-law creep is also studied.

The cyclic stress-strain curve of a 2618 Al alloy reinforced with 15 vol.% SiC particulates was measured through the incremental step method in the naturally aged and peak-aged conditions. The mechanical response was also simulated by means of the finite element analysis of a unit cell to determine the average stresses acting on the matrix and on the reinforcements during cyclic deformation. The stresses on the reinforcements were used to calculate the fraction of broken reinforcements by assuming that the reinforcement strength follows the Weibull statistics. Then the cyclic stress-strain curve, including the influence of reinforcement fracture, was obtained by neglecting the load carried by broken reinforcements. Finally, the predictions of the model (the cyclic stress-strain curve and the fraction of broken reinforcements) were compared with the experimental results.

The static grain growth has been investigated in ultrafine grained copper with an initial grain size of 160 nm. It has been revealed that its kinetics follows to normal grain growth behaviour, but a grain growth starts at a relatively low temperature (0.32 T_m). Good fits with experimental data for several ultrafine grained metals have been obtained if the activation energy for grain boundary diffusion is assumed to be lower than for coarse grained materials, but increases during grain growth. It is suggested that this unusual behaviour of the activation energy is caused by the presence of non-equilibrium grain boundaries in ultrafine grained materials and their recovery during heating.

The calculation results of a cluster variation model and two different Monte Carlo models that have been used to deduce the atomic arrangement in a concentrated solid solution of nitrogen in an austenitic alloy (Fe-39.9Ni-14.96Cr-0.25N wt%) are compared. Quantitatively, the predictions of all three models are in good agreement. In the absence of nitrogen, the arrangement of the metal atoms at 1273 K was found to be close to random. Addition of nitrogen markedly increases the number of octahedral lattice clusters containing four or more chromium atoms, with the result that a large fraction of the nitrogen atoms occupy interstitial sites in these high-chromium clusters. The association of nitrogen and chromium atoms predicted at 298 K was found to be even more pronounced, the nitrogen being exclusively in clusters with six chromium atoms. The validity of the assumptions made in each of the models and the significance of the results are discussed.

A new blister test is proposed to measure the specific work of adhesion W between a thin flexible film on a rigid substrate. In contrast to the conventional blister loaded by constant fluid pressure which leads to catastrophic crack propagation, the new test is driven by an internal expansion of a fixed mass of working gas which leads tostable crack growth. The new technique is demonstrated by measuring W of an interface with a commercial sticky tape serving as the thin film and aluminium as the rigid substrate.

Average growth rates and misorientations between recrystallization nuclei (or grains) and neighbouring deformed matrix material have been studied for partially recrystallized samples by the electron back scattering pattern (EBSP) technique in heavily cold rolled aluminium and copper. It was studied how the annealing time and the crystallographic orientation of nuclei/grains affects the growth rates and distribution of misorientations. The two materials, aluminium and copper, develop a weak and a strong recrystallization cube texture respectively. Information about effects of cube texture strength was therefore also obtained. It was found that grains of cube orientation grow faster than grains of other orientations. A wide distribution of misorientation relationships was observed to exist between the growing grains and the neighbouring deformed matrix, and this distribution was not significantly affected by the annealing time. The faster growth of the cube oriented grains may be ascribed to a larger misorientation between cube grains and deformed matrix than that between other grains and the matrix.

A stochastic model is formulated to analyse crack tip shielding from the applied load, as a function of microstructural parameters and loading conditions, in nontransforming polycrystalline ceramics. The model recognizes the random nature of the microstructural elements, such as grains, inclusions or fibers, which are traversed by the propagating crack. The role of distribution of grain size, and strength of grains and interfaces in the development of the crack interface bridging is emphasized, and numerically evaluated. The standard model parameters are chosen to represent aluminium oxide, as an extensive experimental data base is available for this material. Quantitative predictions of toughening and closure stresses within the bridging process zone are in agreement with experimental data quoted in the literature. It is found that a typical coarse-grained alumina with geometric average grain size of 10 μm and geometric standard deviation of 1.3 exhibits a 5 mm long bridging zone, with the maximum closure stress of 86 MPa, and the maximum toughening due to crack bridging of 90 J/m². The R-curve has been confirmed to depend both on the average grain size and on the grain size distribution, as well as on the level of residual stresses, single grain strength, interfacial roughness and the grain boundary strength. The validity of the relatively simple Monte Carlo model proposed in this work opens up a possibility for optimization of microstructures of monolithic and composite ceramics for maximum resistance to fracture.

Different twinning arrangements in Ni₅Al₃ plates grown inside the B2 phase in Ni_62.5Al_37.5 samples are descrubed on the basis of conventional and high resolution electron microscopy images and electron diffraction. The plate morphology is comparable with that of pure 2M martensite plates. The choice of ordered variants within a plate is dictated by the ordering coherency of the twin plane. No dislocations were observed at the primary plate-matrix interfaces. An explanation using the streaking due to aperiodic twinning is given for unexpected reflections.

High temperature crack growth in ceramics often occurs by the nucleation, growth and coalescence of cavities in a region ahead of the crack tip known as the damage zone. Models describing this type of behaviour generally assume that the presence of cavities does not affect the stress distribution ahead of the crack tip. In this study, a crack growth simulation has been developed which incorporates the effects of cavity nucleation, growth and coalescence on the stress field ahead of the crack. Cavity growth dominated both by grain boundary diffusion and by surface diffusion has been modelled. The models follow both the transient effects which occur following initial loading and the development of a steady-state regime under conditions of constant applied stress intensity factor K_I. In general, the wedging action due to cavity growth reduces the stress field near the crack tip. However, this is largely compensated for by an increase in the damage zone size, as a result of load transfer from the crack tip to the end of the damage zone. We have therefore demonstrated that the established analytical models which do not account for stress redistribution give a much better description of steady-state crack growth behavior than one would expect.

A synergistic effect of hydrogen and stress on a corrosion rate was analyzed with thermodynamics. The results showed that an interaction of stress and hydrogen could increase the corrosion rate remarkably. Stress corrosion cracking (SCC) of austenitic stainless steel (ASS) was investigated in boiling chloride solution to confirm the analysis. Hydrogen could be introduced into the specimen and concentrated at the crack tip during SCC in boiling LiCl solution (143°C). The concentrating factor is about 3 which is consistent with calculated results according to stress induced diffusion.