Impurity-induced embrittlement of heat-affected zone in welded Mo-based alloys

Characteristics of strength and plasticity, fracture mode and grain boundary segregation for two Mo-based alloys with different bulk compositions, recrystallized by either furnace annealing or rapid heating followed by quenching, are studied as a function of heating temperature by mechanical test, scanning electron microscopy, Auger electron spectroscopy and computer simulation. There exists an essential difference in both segregation behaviour and mechanical properties between as-annealed and as-quenched structural states. The rapid quenching causes strong oversaturation of the grain boundaries. In this case, intergranular enrichment is approximately twice as high as that in as-annealed alloys, and spontaneous nucleation of brittle microcracks is observed at certain embrittled boundaries. The proposed high-speed heat treatments are considered as a promising method for modelling of the structural states of the heat-affected zone of weldments. The results obtained are discussed from the viewpoint of possible reasons of impurity-induced embrittlement of Mo-based alloys.     

Limiting state of two-phase composites under uniaxial tension

We propose a model based on the statistical analysis of fracture processes and a concept of effective medium when a system of structural elements of different kinds is regarded as a quasihomogeneous medium with certain effective properties. This approach enables one to compute parameters of the stable stage of fracture of composite materials. The suggested model is used for the prediction of critical values of the parameters corresponding to the transition to unstable fracture under uniaxial tension. We also compare two-phase composites with brittle and plastic phases with strong and weak interfaces between the structural elements, respectively.Frantsevich Institute for Problems in Materials Science, Ukrainian Academy of Sciences, Kiev. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 6, pp. 51–58, November–December, 1995.     

Principles of the design of highly porous layered composites working in the bending mode

A comparative analysis is made of the spesific mechanical characteristics of a sandwich-type laminated porous composite under bending. The external layers of the composite are compact while the internal layer consists of a highly porous material made by using a pore-forming agent. The specific stiffness, strength, and yield load as a function of the volume fraction of pores Θ and the porous/compact-layer thickness ratio λ are considered. The stiffness, strength, and yield load are shown to be affected by variations of Θ and λ when the weight of the composite is constant. Translated from Poroshkovaya Metallurgiya, Nos. 3/4(412), pp. 70–78, March–April, 2000.     

Influence of random pore-type mesodefects on the strength of brittle materials

The effect of pore-type mesodefects on the mean bending strength of brittle materials is calculated by a statistical method. The combined effect of defect amount in a specimen is analyzed and so is the effect of their mean size on the mathematical expectation of the breaking stress. The breaking point is found as a function of the pore size at constant porosity and various degrees of material homogeneity. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(406), pp. 102–106, March–April, 1999.     

Effect of the structural state on the cold brittleness and fracture toughness of low-alloyed molybdenum

We investigate the effects of the chemical and phase compositions of grain boundaries and the state of an excess phase on the fracture toughness, cold brittleness, and strength of low-alloyed molybdenum by the methods of scanning electron microscopy, transmission electron microscopy, and Auger electron spectroscopy. The structural state of the material was changed without changing grain sizes by annealing in various modes at different temperatures. It was shown that the level of segregation of interstitials on grain boundaries is determined by both the annealing temperature and distinctive features of the evolution of excess phases. Embrittlement of grain boundaries results in a decrease in the strength and fracture toughness of the material and in an increase in its temperature of cold brittleness. In the case of transcrystalline fracture, the material exhibits better mechanical properties correlated with distinctive features of the evolution of excess phases inside the grains.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 5, pp. 34–44, September–October, 1994.     

Bifurcation of Cracks in Laminated Ceramic Composites with Rigid Interlaminar Bonds

Bifurcation of cracks in symmetrical laminated two-component composites was analyzed. Geometric factors limiting this effect were established. The bifurcation area is shown to be determined by the following parameters: number of layers, cooling temperature range, elastic constants, and thermal expansion coefficients of the layers. Bifurcation conditions in the composite specimen with a given number of layers are compared with corresponding conditions for the specimen of the fixed total thickness.