Multi-phase field simulation of multi-grain peritectic transition in multiple phase transformation

Taking Fe-C binary alloy as an example,based on the multi-phase field model,the nucleation and growth ofδphase,peritectic reaction,peritectic transformation,and the growth of subsequent austenite are simulated.Effects of the nucleation site of austenite on the peritectic reaction rate and the starting time of the peritectic transformation were studied.The simulation results show that theγphase,as a shell,surrounds theδphase and grows rapidly when the peritectic reaction occurs between the dendriticδgrains,and a layer ofγphase shell is formed aroundδphase after the peritectic reaction.After theδphase is surrounded byγphase completely,the membrane shell separates the L phase from theδphase,so that the phase transfers from peritectic reaction to peritectic transformation.During the peritectic transformation,since the solute diffusion coefficient of the liquid phase is much greater than that of the solid phase,the average growth rate of austenite in the liquid phase is visibly higher than that of theδphase.The peritectic reaction rate is related to the curvature of the nucleation site of theγphase on theδphase grains.The peritectic reaction rate at the large curvatures is faster than that at small curvatures.

Microstructure control and mechanical properties of directionally solidified TiAl-Nb alloys

The double directional solidification（DS） technique was developed to control the lamellar microstructures in primary β TiAl-Nb alloys.Polysynthetically twinned（PST） crystals with lamellar boundaries parallel to or inclined 45o to the growth direction were achieved due to the complete peritectic transformation during directional solidification of the alloys with the dendritic solid/liquid interface.The PST crystals with aligned lamellar boundaries only parallel to the growth direction were produced when lamellar grains with lamellar boundaries in the same orientation were seeded by themselves under appropriate growth conditions.Low boron addition is harmful to align the lamellar orientation because of the growth of non-peritectic α phase.Due to the larger yttria particles and boride ribbons in the directionally solidified TiAl-Nb alloys,the tensile plastic elongations of the alloys are only close to 2%.

Numerical Simulation on Irregular Shrinkage of Initial Shell in Continuous Casting of Steel

Based on the microstructures of steel, a thermo-elasto-plastic stress model of continuously casting billets is established to study hailal solidifying process in molds. Results show that peritectic phase transformation contributes greatly to the irregular shrinkage ofinitial shell, which makes the billets vulnerable to surface defects.