Microstructure evolution of Ti44Al alloy during directional induction heat treatment and its effect on mechanical properties

Abstract

For realizing equiaxed to columnar transition, Ti44Al alloy was processed by directional induction heat treatment (DHT) and mechanical properties were tested. The results show that columnar grains can be obtained and elongation is improved at room temperature. During this heat treatment, Ti44Al alloy was heated in single β phase domain. The microstructure is consisted of fully lamellar colonies after DHT. The average lamellar spacing is decreased, and the orientation of lamellar colonies with angle of less than 60° is more than 76%. Moreover, grain boundary presents zigzag. Tensile testing shows that strength and elongation of heat-treated alloy are 217 MPa and 1.78% respectively, they are 253 MPa and 1.63% for as-cast Ti44Al alloy. The elongation is increased about 9.2% although tensile strength is decreased. Cleavage fracture is main way in as-cast Ti44Al alloy but a mixed fracture consisting of some cleavage planes and many tearing edges is in Ti44Al alloys after DHT. The fracture morphology shows that deterioration of strength is mainly resulted from the micropores that reserved from as-cast alloy, it means DHT could not eliminate the micropores. The improvement of ductility is caused by decreasing of lamellar spacing and changing orientation lamellar colonies.