Solidification and grain refinement in Ti45Al2Mn2Nb1B subjected to fast cooling

Solidification of Ti45Al2Mn2Nb1B in the cooling rate range from 10² K s^-1 to that of gas atomisation occurred via beta phase and the peritectic reaction, L+β→α, followed when the cooling rate was at the lower end of this range. At the cooling rate of about 10² K s^-1 the solidification pathway was basically the same as that during slow solidification but the onset of boride precipitation from liquid was delayed, causing boride precipitates to segregate to the interdendritic areas. In fine powder particles (<100 mm) the peritectic reaction was not observed and boride precipitation from liquid was partially suppressed. Instead, an Al-rich alpha phase saturated with boron was formed in the interdendritic area consuming the last liquid and fine boride precipitation occurred within it upon further cooling. The boron addition was found to effect on nucleation of peritectic alpha during solidification at about 10² K s^-1 and to help breaking down the Burgers orientation relationship for the subsequent beta-to-alpha transformation during solidification at higher cooling rates. In all the cases, boron addition led to grain refinement.