Acta Metallurgica et Materialia最新文献

Fatigue crack growth (FCG) tests have been conducted in air at 650°C and 850°C on 〈001〉 oriented single crystals of SRR99 having the γ′ particles in the form of: (A) 0.3 μm cuboids; (B) 0.2 μm ogdoadical cuboids; and (C) a coarse, rafted γ′ structure. In general, reducing the frequency and increasing the temperature enhances crack-tip shielding at low ΔKs due to increasing oxide induced crack closure. In material A at 650°C the crack path changes from one of γ′ precipitate cutting on {001} to propagation within the matrix as ΔK increases. Enhanced crack branching at 850°C improves the Paris regime behaviour compared with that seen at 650°C. In material B at 650°C, greater cross slip at lower frequency reduces slip reversibility, thus enhancing the fatigue crack growth rate (FCGR). At 850°C crack tip blunting and meandering, associated with γ′ cutting, improves the high ΔK FCG response and on a strength/modulus normalized basis is comparable with that seen for material A. Material C shows a similar FCG resistance to A at 650°C, but there is an acceleration in FCGR at 850°C, which can be accounted for in terms of the lower proof stress and modulus of this microstructure.

The phenomenological theory of martensite crystallography has been used to determine habit plane/shear direction combinations for stress-induced transformation of NiTi, Cu-Ni-Al and NiAl shape memory alloys (SMA) to twin-related martensite correspondence variant pairs. By considering the habit plane/shear direction combinations as unidirectional shear systems, generalized Schmid's law is then used to predict the mechanical response of unconstrained single crystals of each SMA. Model results include axial transformation strain, and plane stress transformation surfaces as a function of crystal orientation. Comparison of the predicted mechanical response results with the habit plane/shear direction combinations reveals a link between the anisotropy and asymmetry of the mechanical response of SMA single crystals, and the crystallography of the martensitic transformation.

Cracks in a chromium coating on a steel substrate which are caused by residual stresses developed during an electroplating process are examined. The chromium coating, formed as a multilayer by alternating electroplating utilizing direct current (DC) and periodic current reversal (PR), is in a state of biaxial tensile stress due to a volume contraction in the successive DC layers which occurs during deposition. A uniform biaxial misfit strain idealizes this layerwise contraction. The state of stress in the multilayer is modelled using laminate theory. Special emphasis is given to the influence of the substrate flexibility on the stress build-up. It is shown that the flexibility of the substrate produces an equ al biaxial bending moment in the coating. At a critical coating thickness, the chromium multilayer cracks and spalls off the substrate. The radius of curvature of detached coating fragments provides a measure of the size of the bending moment and, indirectly, of the misfit strain. The observed fracture mechanism is qualitatively divided into cracks channelling in the coating and debonding craks running in the interface between the coating and the substrate. Long crack asymptotic solutions for the two distinct crack types are presented. The fracture analyses of the multilayered chromium coating show the functional dependence of relative layer and substrate thicknesses and flexibility on the energy release rate for crack propagation.

A constitutive description of creep curves is derived from the time evolution of the internal stress in creep determined by means of strain transient dip test technique and from the kinetic equation between this stress and the instantaneous creep rate. Several constitutive equations have been tested on creep curves of aluminium. It is shown that fitting of the curves and its statistical judgement cannot be used as the only criterion for an investigation of the evolution and role of the internal stress in creep. Such procedure must be completed by another independent procedure. As an appropriate procedure, the measurement of the internal stress in the steady state creep has been successfully applied.

The strength of heavily wire drawn Cu-20 mass% Nbin situ composites considerably exceeds the predictions of the linear rule of mixtures (ROM). An analytical model for the calculation of the yield strength of Cu-20 mass% Nb wires is suggested. The approach is a modified linear rule of mixtures (MROM). It regards the yield strength of the composite as the sum of the volumetric weighted average of the experimentally observed yield strengths of the individual pure phases and a Hall-Petch type contribution arising from the impact of the Cu-Nb phase boundaries. The latter term is described in terms of dislocation pile-ups in the Cu matrix and dislocation movement and multiplication in the Nb filaments. The crystallographic texture and filament geometry of both phases is incorporated. The predictions of the model are in very good accordance with experimental data.

High-temperature crack growth behavior of a polycrystalline alumina was examined under Mode-I tension-tension cyclic loading. Locally at the crack tip, the fatigue crack was found to advance is shear by fractional sliding of grains on alternating sets of planes of the maximum shear. Evidence of a shear-driven crack growth was given in terms of topological and morphological analyses of the fatigue crack surface, grain sliding, frictional debris, and temperature-dependence of fatigue crack growth kinetics. Based on experimental observations, a new model of fatigue crack growth by alternating shear was proposed.

A novel criterion to distinguish between stable (homogeneous) and unstable (localized) plastic flow in tensile specimens is proposed which is based on a consideration of the structural evolution on characteristic time-dependent intrinsic or extrinsic length scales. Macroscopic localization is predicted to occur if fluctuations grow with respect to these length scales. Previous instability criteria by Hart, Estrin and Kubin, and Molinari are recovered as special cases of the present generalized criterion. Moreover, it allows one to distinguish between homogeneous and heterogeneous nucleation of slip. Two applications are discussed: (i) the peculiar necking resistance during superplastic flow is shown to depend not only on a high value of the strain-rate sensitivity but also on a large amount of deformation accommodated by grain boundary sliding; (ii) the instability criterion is applied to creep damage described in terms of a model of the Kachanov-Rabotnov type. This allows for a determination of the critical damage parameter at failure.

The deformation of Si-Al and Si-Al-SiO₂ multi-layered thin films in response to controlled sequences of constant- and variable-amplitude thermal cycling and isothermal exposures has been studied experimentally by recourse toin situ measurements of curvature changes which made use of the laser scanning technique. In an attempt to systematically isolate salient mechanistic features, a select set of companion experiments have also been conducted on the Si-SiO₂ bi-layer system. In some cases, the layered solids have been subjected to as many as 14 thermal cycles between 20 and 450°C to examine the stability of thermally induced deformation. It is found that the variation of curvature with temperature reaches saturation after the first thermal cycle for the Si-Al bi-layer system. The presence of the SiO₂ passivation layer, however, drastically alters the plastic deformation characteristics of the Al layer with the result that: (i) sharp transitions arise in the variation of curvature with temperature during constant-amplitude thermal cycling; (ii) as many as 12 thermal cycles are needed to attain saturation in the curvature-temperature hysteresis loops; (iii) the extent of stress relaxation is significantly reduced during isothermal hold periods in the heating or the cooling phase of the thermal cycle; and (iv) the effects of certain types of variable-amplitude thermal cycling on elastoplastic deformation are essentially suppressed. An elastoplastic analysis, presented by Sureshet al. (J. Mech. Phys. Solids42, 979, 1994) for multi-layer systems, has been used to interpret some of the experimental results obtained in this paper. The predictions of this analysis for curvature changes during thermal cycling (without isothermal hold periods) are found to capture many trends experimentally observed in the Si-Al and Si-Al-SiO₂ layered systems. It is seen, however, that continuum analyses based upon assumptions of steady-state, power-law creep response for thethin Al film fail to capture the measured effects of the passivation layer on creep relaxation even at saturation.

Grain boundary facets forming at the intersection between a grain boundary and the free surface in diffusion bonded Σ3〈011〉Ag bicrystals during prologed annealing have been characterized crystallographically by metallographic methods. It is shown that the observed faceting has qualitatively the same character as that in Σ3〈011〉 grain boundaries in Cu. The energy of an incoherent Σ3 grain boundary in Ag (210 mJ/m²) is determined from the dihedral angle of the thermal groove and the extrapolated literature data on the surface tension of Ag. The facet geometry is discussed with respect to computer simulation data on the inclination dependence of the energy of Σ3 grain boundaries in Cu. The geometrical stability of a grain boundary near the free surface is considered.

In this paper we demonstrate the power of artificial neural networks in predicting strengthening in the transverse direction of metal matrix composites by regularly arranged strong fibers. A neural network is trained in different ways based on a numerical study in which the fiber volume fraction and the matrix hardening ability was studied systematically for fibers in a hexagonal arrangement loaded at 0 and 30° transverse direction and for a square arrangement of fibers loaded at 0 and 45° transverse directions. Strengthening predictions are then made for hardening cases of both fiber arrangements which were not covered by the finite element calculations as well as for arbitrary loading directions not achievable by simple finite element unit cell calculations in the case of square fiber arrangements.