I. P. Mishin

On the micromechanical modelling of the effective diffusion coefficient of a polycrystalline material

This study focuses on calculation of the effective diffusion coefficient of a polycrystalline material accounting for the grain size and shapes. Polycrystal is modelled as a composite consisting of a matrix with high diffusivity (grain boundaries and triple junctions) and inhomogeneities with low diffusivity (bulk grains including crystal defects like dislocations). The segregation at the grain boundaries is accounted for. Typical micromechanical models are re-written for diffusivity assuming that the grains have the shape of ellipsoids of revolution (spheroids). The results are compared with (1) numerical results for hydrogen diffusion in an imaginary polycrystalline material and (2) experimental results for diffusion of hydrogen in nickel polycrystal available in the literature. The approach can be used for extraction of information on diffusivity along the grain boundaries.

Structure, Thermal Stability and Properties of Grain Boundaries of Submicrocrystalline Mo Obtained by Severe Plastic Deformation

The structure of submicrocrystalline Mo, obtained by high pressure torsion, its thermal stability and the state of grain boundaries have been studied by transmission electron microscopy and emission Mössbauer spectroscopy.

Effect of Hydrogen on the Development of Superplastic Deformation in the Submicrocrystalline Ti–6Al–4V Alloy

Hydrogenation effect on the development of superplastic deformation in the submicrocrystalline Ti–6Al–4V alloy at temperatures (0.4–0.5)Тmelt is investigated. Hydrogenation of the submicrocrystalline Ti–6Al–4V alloy to 0.26 mass% during superplastic deformation is found to result in solid solution strengthening, plastic deformation localization, and as a consequence, decrease of the deformation to failure. Possible reasons for the decrease of the flow stress and increase of the deformation to failure in the submicrocrystalline Ti–6Al–4V–0.26H alloy during deformation under conditions of superplasticity and simultaneous hydrogen degassing from the alloy are discussed.

Evolution of misorientation spectrum of grain boundaries of submicrocrystalline molybdenum upon deformation under conditions of grain-boundary diffusion of nickel

Comparative investigations of the concurrent evolution of the structure and the misorientation spectrum of grain boundaries of submicrocrystalline molybdenum have been performed upon free annealing and under the creep conditions at a temperature of 1023 K. It has been established that changes in the misorientation spectrum of grain boundaries of submicrocrystalline molybdenum upon annealing and creep are observed concurrently with boundary migration. The grain-boundary diffusion of nickel upon annealing leads to an increase in the fraction of grain boundaries of the special type with Σ17a in the grain-boundary ensemble of submicrocrystalline molybdenum. Under the creep conditions grain-boundary diffusion fluxes of nickel atoms favor transformation of low-angle boundaries into high-angle boundaries and growth in the angle of misorientation of the high-angle boundaries of submicrocrystalline molybdenum to 45°–60°.

Effect of deformation and heat treatment on the structure, the mechanical properties, and the fracture characteristics of an ultrafine-grained Zr–1Nb alloy

The effect of cold working and annealing for various times on the structure, the mechanical properties, and the fracture characteristics of an ultrafine-grained Zr–1 wt % Nb alloy after three-dimensional pressing is studied. Possible causes for increasing the thermal stability of the strength properties of the ultrafine-grained Zr–1 wt % Nb alloy during prerecrystallization annealing after additional cold working by rolling are discussed.

Effect of Grain Boundary State on Diffusion and Diffusion-Controlled Processes in Ultrafine-Grained Materials Processed by Severe Plastic Deformation

Experimental studies on the grain boundary diffusion and processes controlled by it in the ultrafine-grained metallic materials produced by various methods of severe plastic deformation are reviewed. Correlation between the increased diffusion permeability of grain boundaries and features of recrystallization and deformation development in these materials possessing the non-equilibrium state of grain boundaries formed during severe plastic deformation in the temperature range of T < 0.35Tm is demonstrated and analyzed.

Deformation behavior and fracture of the ultrafine-grained titanium alloy of Ti-Al-V-Mo system

Effect of prerecrystallization annealing on the evolution of structural and phase state, deformation and failure behavior of the ultrafine-grained structure of VT16 grade titanium alloy is studied. In the ultrafine-grained VT16 alloy during prerecrystallization annealing such processes as the β→α phase transformation and redistribution of alloying elements is found to take place along with defect structure relieving, leading to strength properties decrease. These processes promote retention of high level of alloy strength properties. Failure behavior of the alloy qualitatively is in agreement with the wave theory of fracture.