[en] The microstructure of the 2091 alloy (Al-Li-Cu-Mg-Zr) has been studied and compared to that of two simpler Al-Li and Al-Cu-Mg alloys. The SAXS, in situ SAXS, and TEM techniques have been used for heat treatments at room temperature and at 150 C. The time evolution of the size and of the volume fraction of the delta-prime precipitates and of the Cu-rich GPB zones have been determined. The delta-prime precipitates are shown to follow a classical Lifschitz-Slyosov-Wagner law. Their interfacial energy has been estimated. S and S-prime precipitates have also been characterized on grain boundaries and on helical dislocations, respectively, by TEM. 16 refs

[en] In this paper the interaction between slip dislocations and grain boundaries in-hypo-stoichiometric Ni₃Al, with and without boron, is investigated by using the in situ TEM deformation technique. In both alloys, the slip dislocations were incorporated into the grain boundaries and remained at the point of entry. The difference between the alloys was in the dominant response mode of the grain boundary to the stress concentration associated with the dislocation pileup. In the boron-free material, the stress was relieved primarily by the nucleation and propagation of a crack along the grain boundary. In contrast, in the boron-doped material, relief occurred by the emission of dislocations from the grain boundary. These results are consistent with boron increasing the cohesive energy of the grain boundary. The slip system activated at grain boundaries in the ductile ordered alloy was shown to satisfy the same slip transfer criteria that operate in f.c.c. disordered alloys

[en] A new model is developed to describe that stage of nano-indentation creep prior to 'pop-in', i.e. when the indentation load induces purely elastic stresses within the sample. Expressions for the stress fields due to a conical indenter are derived and the resulting chemical potential and diffusion fluxes established. The considerations lead to a creep rate equation which is found to fit well with existing experimental data on tungsten. The model is hence used to predict nano-indentation creep rates at various temperatures. It is found, for example, that at a temperature as low as 100 C, the indentation creep rate of tungsten can be considerable

[en] Niobium and molybdenum silicides were synthesized by the passage of high-amplitude shock waves through elemental powder mixtures. These shock waves were generated by planar parallel impact of explosively-accelerated flyer plates on momentum-trapped capsules containing the powders. Recovery of the specimens revealed unreacted, partially-reacted, and fully-reacted regions, in accord with shock energy levels experienced by the powder. Electron microscopy was employed to characterize the partially-and fully-reacted regions for the Mo-Si and Nb-Si systems, and revealed only equilibrium phases. Selected-area and convergent beam electron diffraction combined with X-ray microanalysis verified the crystal structure and compositions of the reacted products. Diffusion couples between Nb and Si were fabricated for the purpose of measuring static diffusion rates and determining the phases produced under non-shock condition. Comparison of these non-shock diffusion results with the shock synthesis results indicates that a new mechanism is responsible for the production of the NbSi₂ and MoSi₂ phases under shock compression. At the local level the reaction can be rationalized, for example in the Nb-Si system under shock compression, through the production of a liquid-phase reaction product (NbSi₂) at the Nb-particle/Si-liquid interface, the formation of spherical nodules (∼2μm diameter) of this product through interfacial tension, and their subsequent solidification

[en] Dissolved hydrogen in zirconium diffuses up the hydrostatic stress gradient and forms hydride platelets in the region of high stress. The hydride formation accompanied by an expansion on the elastoplastic stress, strain and displacement fields, is investigated in this paper by a finite-element discretization technique. It is shown that hydride formation causes an elastic unloading in the crack tip (reduction of the peak stress) and appearance of a peak stress at the front end of the hydride. Further unloading occurs along the hydride platelet, and depending on the hydride expansion, reversed plastic deformation may take place. Effects of the hydride length, its location with respect to the crack tip, and geometry of the front and extremity of the hydride, are also investigated

[en] Sintering in compacted powders of Mo, Nb, Ta, and alumina is investigated using small angle neutron scattering. The scattering data permits to follow the evolution of the total pore surface F together with the total pore volume V in the course of the entire densification process. Characteristic changes in the relation between F and V suggest that sintering proceeds in three different stages. Quantitative evidence is presented that, in particular, third stage sintering (taking place if the pores can be considered isolated from each other) is governed by a universal relation between F and V

[en] In this paper, it is shown that the equilibrium shape of a twin or martensite plate can be calculated by treating another interface as a distributed array of dislocations and by calculating their equilibrium distribution. The shape of an isolated twin is an ellipse to first approximation. Second approximation treatments show that the tip of the twin sharpens with increasing stress for the screw dislocation case while it becomes blunter with increasing stress for the edge dislocation case. A colony of twins is treated in much the same way as a stacked array of piled-up dislocations for the screw dislocation case. It is found that the twins flatten out with decreasing separation. Expressions are derived for the stress-field and for the strain energy of such stacked twins. It is shown that the twin spacing should be proportional to the square root of the twin length, from which the twin boundary energy can be calculated

[en] In this paper, toughening mechanisms in a laminate composite composed of alternating layers of brittle γ-TiAl and ductile TiNb reinforcements are studied. The TiNb phase comprising about 20% of the composite volume, contributed to toughening by both crack renucleation and bridging mechanisms, yielding a steep resistance curve and effective toughness more than ten times higher than the matrix value. In part, the extraordinary toughening is derived from large scale bridging effects, which occur when the size of the bridging zone s not small compared to the crack and specimen dimensions. Large scale bridging model predictions based on independent evaluations of the fundamental composite properties, including the reinforcement stress---displacement function, were in good agreement with the experimental observations. The authors demonstrate how the models can be used to optimize the composite properties for specific applications

[en] This paper reports that Ni-25wt%Cr Ni-25wt%Cr-1wt%Al and Ni-25wt%Cr-0.2wt%Y alloys were oxidized at 1000 degrees C in O₂ to form similar thicknesses of Cr₂O₃ scales. After cooling to ambient temperatures, the residual strains in these oxide films were measured using X-ray diffraction, and qualitative indications of the interfacial strength were obtained using a micro-indentation technique. compressive elastic strains of the order of 0.4% were measured on the Y- and the Al-doped alloys. This value agrees well with calculated thermal strains which arose form cooling. Only 0.1% compressive elastic strains were observed for the undoped Ni-25Cr alloy. Localized scale buckling and separation from the substrate is believed to have allowed relaxation of the strain in this case. The Y- and the Al-doped alloys on the other hand, have extremely strong scale/alloy interfaces, and therefore were able to retain higher strain levels

[en] The influence of initial particle size distribution on coarsening during liquid phase sintering has been experimentally investigated using W-14Ni-6Fe alloy as a model system. It was found that initially wider size distribution particles coarsened more rapidly than those of an initially narrow distribution. The well known linear relationship between the cube of the average particle radius bar r^-3, and time was observed for most of the coarsening process, although the early stage coarsening rate constant changed with time, as expected with concomitant early changes in the tungsten particle size distribution. The instantaneous transient rate constant was shown to be related to the geometric standard deviation, 1nσ, of the instantaneous size distributions, with higher rate constants corresponding to larger 1nσ values. The form of the particle size distributions changed rapidly during early coarsening and reached a quasi-stable state, different from the theoretical asymptotic distribution, after some time. A linear relationship was found between the experimentally observed instantaneous rate constant and that computed from an earlier model incorporating the effect of particle size distribution. The above results compare favorably with those from prior theoretical modeling and computer simulation studies of the effect of particle size distribution on coarsening, based on the DeHoff communicating neighbor model