Acta Metallurgica et Materialia最新文献

The modelling of self-propagating high-temperature reactions is attempted using a finite difference model. The model is described and model results are compared with experimentally observed combustion wave velocities for the reaction 3TiO₂ + 4Al + 3C → 3tiC + 2Al₂O₃ using various processing parameters (diluents, preheating etc.). The model is extended to derive a parameter which determines under what conditions a reaction will become self-propagating.

Based on the random walk of point defect in concentrated alloys, the kinetics of the annihilation of point defects in a f.c.c. alloy is studied by computer simulation. The effect of the degree of order is especially considered by using the pair model for describing the atomistic interaction. The simulations cover nearly the whole temperature range in which short range order or short range clustering exists. Due to the special treatments, the escape probability for various trap structures (0D or 1D and with different reaction radius) can be determined as a function of the annealing time. Further evaluation yields the trapping time of point defects for broad variation of relevant parameters including the composition of alloy and the mobility ratio of the components. Empirical functions are developed for expressing these dependences.

Compared to the random alloy simplification reported in the preceding paper [1], there is no change in the essential form of the trapping kinetics. The relation between the activation energy for the annihilation of point defects and that for the self-diffusion of atoms in real alloys is revealed, which yields the background for studying the diffusion properties through the investigation of point defect annihilation.

The residual stresses generated by Vickers indentation in brittle materials and their changes due to annealing and surface removal were studied in 4 mol% yttria partially stabilized zirconia (4Y-PSZ). Three experimental methods to gain information about the residual stress field were applied: (i) crack profile measurements based on serial sectioning, (ii) controlled crack propagation in post indentation bending tests and (iii) double indentation tests with smaller secondary indents loccated around a larger primary impression. Three zones of different residual stress behavior are deduced from the experiments. Beneath the impression a crack free spherical zone of high hydrostatic streses exists. This “core” zone is followed by a transition regime where indentation cracks develop but still experience hydrostatic stresses. Finally, in an outward third zone, the crack contour is entirely governed by the tensile residual stress intensity (elastically deformed region). Annealing and surface removal reduce this crack driving stress intensity. The specific changes of the residual stresses due to the post indentation treatments are described and discussed in detail for the three zones.

The phenomenon of crack nucleation from grain triple junctions is investigated in columnar freshwater ice at rates of loading where the dislocation pile-up process is inhibited. Two mechanisms are explored. First, crack nucleation due to elastic anisotropy-induced singular stress field at the triple junction is investigated. To this end, both the singularity exponent and the energy release values associated with crack nucleation are provided for random orientations of the grain boundaries and material axes of the grains. The computed energy release rate values fall much short of those required for nucleation. Next, by assuming a linear viscous response for the boundary, the stress concentrations due to grain boundary sliding are computed, and the resulting energy release rate values are shown to be sufficiently high to overcome the barrier for crack nucleation. Experimental evidence for grain boundary sliding-induced boundary decohesions at — 10°C are presented in the companion paper. At such temperatures, sliding is activated even though the strain rate of loading invokes an overall brittle response from the polycrystal.

The problem of diffusional growth of a proeutectoid constituent in a ternary steel is considered, taking into account the interfacial diffusion of a slow-diffusing substitutional solute, under conditions which do not permit its long-range redistribution between parent and daughter phases. It is assumed that the faster diffusing interstitial solute (carbon) controls the rate of the transformation. The substitutional solute profile within (across) the interface is estimated as a function of interface velocity; the interstitial chemical potential difference is allowed to vary with, and balance, the solute drag due to the substitutional component. A transition to paraequilibrium is found at high interface velocities, and a variety of behaviour is predicted for intermediate states, depending on the relative rates of diffusion of the two solutes and their energetic interactions with each other and with the interphase boundary.

An investigation into the occurrence of reinforcement cracking within a particulate ZrO₂/2618 Al alloy metal matrix composite under tensile plastic straining has been carried out, special attention being paid to the dependence of fracture on particle size and shape. The probability of particle cracking has been modelled using a Weibull approach, giving good agreement with the experimental data. Values for the Weibull modulus and the stress required to crack the particles were found to be within the range expected for the cracking of ceramic particles. Additional information regarding the fracture behaviour of the particles was provided by in situ neutron diffraction monitoring of the internal strains, measurement of the variation in the composite Young's modulus with straining and by direct observation of the cracked particles. The values of the particle stress required for the initiation of particle cracking deduced from these supplementary experiments were found to be in good agreement with each other and with the results from the Weibull analysis.

Further, it is shown that while both the current experiments, as well as the previous work of others, can be well described by the Weibull approach, the exact values of the Weibull parameters so deduced are very sensitive to the approximations and the assumptions made in constructing the model.

In non-stoichiometric oxides (NiO, CoO, Cu₂O…) two thermodynamic parameters (temperature T and partial pression of oxygen P_o2) are necessary in order to fix the population of point defects. When one of these two parameters (T or P_o2) is suddenly changed and a physical property is continuously recorded, a “transient” can be observed due to the diffusion of the point defects in order to reach the new thermodynamic equilibrium population. Many works have been devoted to the majority point defects (non-stoichiometry). In this paper we present an overview about the high temperature transitory plastic deformation which is related with the minority point defects.

During the isothermal pearlitic transformation in vanadium alloyed low and medium carbon steels the formation of proeutectoid ferrite precedes that of pearlitic formation. Within this proeutectoid ferrite interphase precipitation of vanadium carbide (VC) occurs in a form of linear uniformly sized and shaped arrays of particles. When pearlite subsequently forms, two distinctive pearlitic morphologies exist. Lamellar and non-lamellar spheroidised types of pearlite. Within the pearlitic constituent of both of these pearlitic microstructures VC interphase precipitation occurs. However, in the lamellar type of pearlite curved arrays of uniformly sized and shaped VC particles whose direction is perpendicular to the cementite long axis are observed while in the non-lamellar type of pearlite apparently random distributions of VC particles exist. It was consistently found that VC interphase precipitation always occurred on the moving austenite/ferrite interphase boundaries at all temperature of isothermal pearlitic transformation.