Structure and formation mechanism of α/α interface in laser melting deposited α + β titanium alloy

The α/α interfaces caused by the nucleation and growth of secondary α (αs) phase in laser melting deposited (LMD) TC21 titanium alloy were investigated by SEM, TEM and EBSD. There were two kinds of αs phases in conventionally heat treated LMD TC21 alloy, regular αs phase and irregular αs phase. The regular αs nucleated through surface instability and the protuberance of αp phase and spread into β matrix with a plate like shape while the irregular αs phase nucleated at the defects of β matrix. A gradient orientation region between αs phase and αp phase was made after nucleation and growth of regular αs phase. When different αs phases met each other, small-angle αs/αs interfaces were made, as a result of misorientation. These interfaces were made up by a series of dislocations. Large-angle αs/αs interfaces which were caused by twinning, although not observed, were confirmed in theory. After aging treatment, the αs/αs interfaces disappeared as a result of diffusional reaction.