In situ observation of competitive growth of α grains during β → α transformation in laser beam manufactured TiAl alloys

Low ductility has long been the bottleneck for high temperature application of TiAl alloys. It is reported that grain refinement through boride could improve its mechanical property. However, this refinement is suppressed at high cooling rate. This article delineates for the first time an in situ observation by synchrotron X-ray diffraction. It illustrates the mechanism of competitive nucleation and grain growth of Burgers and non-Burgers alpha grains during beta -> alpha transformation in a Ti-45Al-5Nb-0.2C-0.2B alloy (TNB-V5). Comparing with those in base material, the volume fraction and size of borides are significantly reduced in the welding zone. The non-Burgers alpha grains nucleate earlier than Burgers alpha. However, Burgers alpha grains grow much faster than non-Burgers alpha during beta -> alpha transformation in the welding zone, due to a high thermodynamic driving force of Burgers alpha grains. No orientation relationships between alpha and borides or between beta and borides are observed in the fast cooling.

Automated summary

For the first time, this study conducted in situ observation of the grain refinement mechanism in TiAl alloys under high temperature conditions using synchrotron X-ray diffraction. The results showed a significant reduction in volume fraction and size of borides in the welding zone, with non-Burgers alpha grains nucleating earlier and Burgers alpha grains growing faster during the beta -> alpha transformation process.