Two types of dendrite tip splitting including dendrite orientation transition and twinned-like dendrites in Fe-C alloys were investigated by phase-field method.In equiaxed growth,the possible dendrite growth directions and the effect of supersaturation on tip splitting were discussed;the dendrite orientation transition was observed,and it was found that the orientation regions of anisotropy parameters were reduced from three to two with increasing the supersaturation,which was due to the effect of interfacial anisotropy controlled by the solute in front of S/L interface changing with the increase of supersaturation.In directional solidification,it was found that the twinned-like dendrites were formed with the fixed anisotropy couples and no seaweed dendrites were observed;these were concluded from the results of competition between process anisotropy and inherent anisotropy.The formation process of twinned-like dendrite was investigated by tip splitting phenomenon,which was related to the changes of dendrite tips growth velocity.Then,the critical speed of tips splitting and solute concentration of twinned-like dendrites were investigated,and a new type of microsegregation in Fe-C alloys was proposed to supplement the dendrite growth theories.
First-principles calculations have been carried out to investigate the structural stabilities, electronic structures and elastic properties of Mg17Al12, Al2 Ca and Al4 Sr phases. The optimized structural parameters are in good agreement with the experimental and other theoretical values. The calculated formation enthalpies and cohesive energies show that Al2 Ca has the strongest alloying ability, and Al4 Sr has the highest structural stability. The densities of states(DOS), Mulliken electronic populations, and electronic charge density difference are obtained to reveal the underlying mechanism of structural stability. The bulk modulus, shear modulus, Young's modulus, and Poisson's ratio are estimated from the calculated elastic constants. The mechanical properties of these phases are further analyzed and discussed. The Gibbs free energy and Debye temperature are also calculated and discussed.
