Acta Metallurgica最新文献

Microscopic and microstructural analyses of fatigue damage evolution in copper single crystals vs temperature down to 4.2 K and environment revealed that the spacing between slip bands becomes smaller at lower temperatures and in inert environments whereas the thickness of a slip band decreases with the temperature but is independent of environment. The thickness of a slip band was found to correspond to the ladder rung spacing. Under all conditions, just initiated fatigue cracks are very short and small in depth.

The influence of thermal activation on the nature of the interactions between an array of particles and a migrating grain boundary is considered, adapting concepts from the theory of thermally activated deformation. Thermally activated unpinning of boundaries is found to become significant at small particle sizes and high temperatures. For a given particle array, this leads to a time dependent average grain size which is coarser than the predictions of the traditional “Zener-Gladman” static equilibrium model. For grain sizes and temperatures of interest in ferrous alloys, substantial reductions in particle size and volume fraction of grain refining dispersions can be achieved before thermal activation interferes with the effectiveness of boundary pinning.

Clustering and ordering processes often occur concomitantly or synergistically during the decomposition of supersaturated solid solutions. However, the occurrence of spinodal decomposition in conjunction with an order → disorder transformation thermodynamically of first order under equilibrium conditions is generally not well understood. In this paper a simple graphical approach using free energy-composition diagrams and loci of thermodynamic instability delineating regions of continuous transformation is used to develop a framework for understanding the interplay between clustering and ordering occurring in metallic solid solutions where the ordered precipitate stems from a phase diagram configuration in which the ordered and disordered states are connected by a first-order phase transition. The approach is applied to the NiAl, NiTi and AlLi binary alloy systems in which the interaction of clustering and ordering appears to play an important role.

A discrete lattice plane, regular solution, broken bond model, previously used to calculate the interfacial energy of f.c.c.:f.c.c. and b.c.c.:b.c.c. interphase boundaries, is extended to the situation in which there is a change in crystal structure across the interphase boundary, specifically to an interface between an f.c.c. and h.c.p. crystal. The particular interface studied, {0001}_h.c.p. • {111}_f.c.c., 〈11$\text{2}$0〉_h.c.p.• 〈1$\text{1}$0〉_f.c.c. was made fully coherent by choosing the appropriate lattice parameter ratio. Both f.c.c. and h.c.p. phases are taken to be regular solutions. Minimization of the grand potential yields a set of non-linear equations in the equilibrium composition of the planes in the interface region whose solute concentration differs from that of the bulk composition in either phase. These values of the compositions are then used to compute the interfacial free energy of the boundary. The variations in the concentration profile and interfacial free energy with temperature and regular solution constants were investigated. The concentration profile was found to be diffuse and also asymmetric. The diffuseness does not vary in any simple manner with temperature. The interfacial energy decreases with increasing temperature in the various situations considered.

The internal friction peak appeared around 310°C (l Hz) in single crystals and bamboo crystals of high-purity aluminium introduced by twisting deformation as previously reported has been further studied especially concerning its relaxation properties. This peak is thermally-activated having a very high relaxation strength up to 0.3 and a very high activation energy of about 2 eV. The fact that the peak disappears after rapid cooling of the specimen leads to the proposition that dislocation climb does not play a dominating role to the non-elastic motion of the dislocations giving rise to this peak. TEM observations show that the stable dislocation configuration in the twisted specimen after the 600°C anneal always consists of uniformly distributed dislocation networks. Subsequent tensile deformation destructs such dislocation networks and converts it into cell structures so that the twisting peak is suppressed. It is suggested that the dislocation slip enhanced by the non-conservative shift of the node of the dislocation network contributes dominately to the high relaxation strength of the twisting peak and the dislocation climb controls the relaxation time of the relaxation process.

The deformation characteristics of ceramic whisker- and particulate-reinforced metal-matrix composites were studied experimentally and numerically with the objective of investigating the dependence of tensile properties on the matrix microstructure and on the size, shape, and distribution of the reinforcement phase. The model systems chosen for comparison with the numerical simulations included SiC whisker-reinforced 2124 aluminum alloys with well-characterized microstructures and 1100-o aluminum reinforced with different amounts of SiC particulates. The overall constitutive response of the composite and the evolution of stress and strain field quantities in the matrix of the composite were computed using finite element models within the context of axisymmetric and plane strain unit cell formulations. The results indicated that the development of significant triaxial stresses within the composite matrix, due to the constraint imposed by the reinforcements, provides an important contribution to strengthening. Systematic calculations of the alterations in matrix field quantities in response to controlled changes in reinforcement distribution give valuable insights into the effects of particle clustering on the tensile properties. The numerical results also deliver a mechanistic rationale for experimentally observed trends on: (i) the effects of reinforcement morphology and volume fraction on yield and strain hardening behavior of the composite, (ii) the pronounced influence of reinforcement clustering on the overall constitutive response, (iii) ductile failure by void growth within the composite matrix, (iv) the insensitivity of the yield strength of the composite to changes in matrix microstructure, and (v) the dependence of ductility on the microstructure of the matrix and on the morphology and distribution of the reinforcement. The predictions of the present analyses are compared and contrasted with current theories of elastic and plastic response in multi-phase materials in an attempt to develop an overall perspective on the mechanisms of composite strengthening and of matrix and interfacial failure.

Mössbauer spectrometry was used to measure short range correlations between ⁵⁷Fe and Mo atoms, and between ⁵⁷Fe and Co atoms in crystalline Fe-49 at.%Co-2 at.%Mo alloys. After a rapid quench from the melt, both types of correlations were characteristic of those of a nearly disordered alloy. After annealing, we found FeCo correlations characteristic of B2 order, and a Co-rich environment around Mo atoms. We found that the Warren-Cowley SRO parameters, α_Mo|Fe and α_Co|Fe, do not change at the same relative rates when rapidly quenched specimens are annealed at different low temperatures. At lower temperatures we observe a smaller ratio, R(T), where $\text{R(T)≡}\text{d}\text{d}\text{t}\text{α}{}_{\mathrm{<mtext>Mo|Fe</mtext>}}\text{d}\text{d}\text{t}\text{α}{}_{\mathrm{<mtext>Co|</mtext>}}$ Because the critical temperature for B2 ordering in FeCoMo is only weakly dependent on Mo concentration, we attribute the temperature-dependence of R(T) to a high activation barrier for the diffusive jumps of Mo atoms.

The dislocation structures of directionally solidified (DS) René 80 superalloy in γ′ phase during tensile deformation at 760°C and 850°C, and during creep deformation at 760°C under a stress of 618MPa have been studied by transmission electron microscopy. It has been shown by an extensive analysis of the dislocations bounding a superlattice stacking fault that γ′ shearing occurs by viscous slip of pairs of SISF separated $\text{1}\text{3}\text{〈112〉}$ superpartials, but the $\text{1}\text{2}\text{〈110〉}$ screw components of them have moved away from the $\text{1}\text{6}\text{〈112〉}$ edge components on [111] plane, leaving a stress stabilized APB in its wake in tensile deformation. In contrast to the tensile glide mechanism, in creep deformation γ′ shearing occurs by viscous slip of pairs of SISF separated $\text{1}\text{3}\text{〈112〉}$ superpartials with net slip vector 〈110〉 while the cores of the $\text{1}\text{3}\text{〈112〉}$ superpartials do not separate observably.

It is supposed that the correspondence relationship between atoms of the new and parent phases as shown in Bain mechanism exists in the martensitic transformation of CuZn alloys. Applying the known values of thermodynamic properties and treating the ordering problem of CuZn alloys by means of the BWG model, a general formula for the driving force ΔG^{β′→α′} of the phase transformation is derived. This formula is used to calculate the equilibrium temperature T₀ and the starting temperature M_s of the martensitic transformation in CuZn alloys. The calculated M_s values are in very good agreement with the experimental ones. Calculations also indicate that the degree of ordering of the parent phase has a tremendous influence upon the values of M_s in CuZn alloys.

Eshelby's equivalent inclusion approach is used to provide a rigorous theoretical basis for the prediction of the mechanical properties of short fibre composites. The equivalent inclusion construction which is central to this method is described in detail. The elastic, thermoelastic and plastic behaviour of short fibre metal matrix composites is predicted, and, taking the Al/SiC system as an example, compared with experiment. Finally, it is shown that relaxation phenomena play an important role in the development of internal stresses, and that the energetics and the resultant stress redistribution between the two phases can be understood within the framework of the Eshelby model.