Finite element modeling of temperature distribution in field activated sintered(FAS) MoSi_2-SiC composite was made on the basis of modeling for FAS process.The results show that FAS temperature field is determined by a combined effect of Joule heat of electric field,heat released by chemical reaction,and transferring heat in die-specimen system.Due to overlap of heats produced by Joule effect and chemical reactions,the center temperature is the highest in the sample,a radius and axial temperature gradients are formed,which significantly affect the uniformity of mierostructure in the sizes and densifications of grains.The simulation results provide a theoretical support on temperature gradient controlling,which help to prepare dense and fine grained bulk materials.
Homogenisation has been used to eliminate solute segregation of CrMnFeCoNi high-entropy alloys (HEAs) before thermomechanical processing. To overcome the deficiencies caused by high-temperature homogenisation, we cancelled the homogenisation and studied the solute segregation feature and its effect on the grain growth and the mechanical property. The HEA forms equiaxed grains with the solute segregation in the range of tens of micrometres after cold rolling and recrystallisation. The grain growth in recrystallisation still abides by the classical grain growth kinetics, but with a higher power index of 3.33 and activation energy of 392.4 kJ mol −1 than those of the homogenised HEA, indicating a solute-drag effect. The relationship between the yield stress and grain size follows the Hall–Petch dependence with a higher intrinsic strength.
Solidification structures are determined by the interaction between the interfacial processes and transport processes of heat and solute. In this paper, we investigate planar instability in directional solidification. Firstly, the interfacial evolution at the initial growth stage is simulated, indicating the planar instability is represented by the transition from the planar to the cellular. Secondly, to represent the history-dependence of solidification, constant thermal gradient G and varying pulling speed VP are used in the simulations. The results indicate the cooling rate R ( = G*VP) dominates the overall propagation speed of the interface, to maintain the local thermodynamic equilibrium. The solute segregation determines the stability of the interface, by changing the excess free energy at the interface and corresponding interface energy. Finally, the simulations of the grains with different preferred crystallographic orientations are performed, indicating the surface energy and its anisotropy do not affect the solute diffusion and planar growth. The results also verify the conclusion that solute segregation influences the interface energy and results in interface instability. On the other hand, for the planar-cellular transition, the minimum surface stiffness rule is more suitable than the maximum surface energy rule. The influence of the solute concentration on the excess free energy and interface energy can be applied to other solidification patterns induced by the interface instability, which will be studied in the future.
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