Interdiffusion Studies in the Co-Sb System

CoSb based compounds have gained much importance in the fields of thermoelectric devices. In this work, we have conducted the solid–state conventional bulk diffusion couple experiments. To study the phase evolutions, Co/Sb diffusion couples are annealed at 450–550 °C. The interdiffusion zone is analysed using field emission gun equipped scanning electron microscope and the composition measurements are done in electron probe micro−analyser to confirm the growth of various product phases. The marker experiment indicates that the CoSb3 phase grows mainly by diffusion of Sb in the binary Co–Sb system. Growth of the CoSb3 phase is discussed based on assessment correlating the difference in mobilities of species with the high homologous temperature, crystal structure of the phase, and the concept of sublattice diffusion mechanism in line compounds.

Microstructure evolution of the Gleeble-simulated heat-affected zone of Ni-based superalloy

The formation of liquation cracking in a simulated heat affected zone of René108 is reported. The stress controlled thermo-mechanical experiments were carried out on a Gleeble®3800 testing system. The base alloy was lost-wax cast and then solution treated and aged. Light and scanning electron microscopy of this material revealed high volume fraction of γ' precipitates in the dendrite arms and residual eutectic γ/γ' islands in the interdendritic areas. As a result of short-term exposure to high homologous temperature, the volume fraction of γ' phase was significantly decreased due to the dissolution of precipitates in the surrounding matrix. The thin non-equilibrium liquid film, formed locally along grain boundaries, was a key-factor favoring initiation of cracks and their spreading during the Gleeble testing. The liquid appeared as a result of constitutional liquation, mainly of the γ' precipitates.

On the Nuances in the Power Law Description and Interpretation of High Homologous Temperature Creep and Superplasticity Data

“Power law’’ representation is used to describe minimum creep rate and “steady state” superplastic deformation. In creep isothermal log stress – log strain rate relationship is linear for so long as the rate controlling mechanism remains unchanged. During optimal superplastic flow the slope of this curve changes even when there is no change in the rate controlling mechanism, i.e. the stress exponent, n, at a constant temperature and grain size is a function of strain rate. For a constant rate controlling mechanism, in both the phenomena, n decreases with increasing temperature. Grain size has no effect on creep, but its effect is significant in superplasticity. Therefore, analyzing creep and superplasticity data by treating n for any given mechanism as a constant independent of stress and temperature is questionable. In this analysis stress is normalized with respect to a reference stress, rather than the shear modulus. The microstructure dependence comes through the Buckingham Pi theorem. When contribution from microstructure terms to isothermal strain rate is constant, Laurent’s theorem helps generate a set of values for n. It is shown that the simplest solution, viz. n is independent of stress, but is a linear function of temperature, describes steady state creep. (The case n is independent of both stress and temperature follows as a special case.) The second simplest solution, viz. n is a linear function of both temperature and stress corresponds to steady state superplasticity. Using the equations, the values of n, activation energies for the rate controlling processes and strain rates at different temperatures and stresses could be estimated for both creep and superplasticity. The analysis is validated using experimental results concerning many systems. iiThis lecture is dedicated to the sacred memory of late Prof. Oleg D. Sherby.

Strain heterogeneities at the ductile to brittle transition; a case study on ice

This paper presents, for the first time, the evolution of local strain fields around intragranular cracking in polycrystalline ice, at the onset of tertiary creep. Owing to the high homologous temperature conditions and relatively low compressive stress applied, stress concentration at crack tips is relaxed by plastic mechanisms associated with dynamic recrystallization. Strain field evolution followed by Digital Image Correlation indirectly shows the redistribution of stresses during crack opening, but also driven by crack tip plasticity mechanisms and recrystallization. Such redistribution induces modifications in the local strain deformation bands, and crack closure during deformation. A strong interaction between cracking and dynamic recrystallization is therefore evidenced at the ductile to brittle transition in ice deformed at high homologous temperature.

On the Choice of the Power Law Flow Rule and its Consequences in Crystal Plasticity

Three types of power law flow rules are commonly used in classical crystal plasticity. These laws are purely phenomenological. The foremost point is how to define operative or effective stress and drag or slip system resistance. Specific choice of the definition leads to a unique number of implications including lattice rotation and slip activities, and we will highlight a few of them. We examined these three flow rules within finite strain framework with a single crystalline Al-rich TiAl binary alloy at very high homologous temperature with three strain rate controlled experimental data . It is revealed that two internal variables based flow rules give better results with a wide variety of applicability in plasticity and related phenomena.