Columnar grains formation and mechanical properties improvement of Ti44Al alloy by cyclic directional heat treatment

Abstract

Ti44Al alloys were conducted cyclic directional heat treatment (DHT) for 1, 4 and 6 cycles at 1750 K to obtain directional microstructure and research their microstructure evolution. Microstructure was observed and mechanical properties was tested. Results show that grains of all treated Ti44Al alloys grow up obviously, the width of grains increases and the number of grains decreases with increasing of cycles. More columnar grains are obtained and spacing of lamellar γ and α2 phases decreases to 0.60 μm approximately after cyclic DHT for 4 cycles. Refinement of lamellar phases is caused by increasing of nucleation sites for γ phases. And it is noted that the spacing of lamellar phases is homogenized after DHT, compared with that of as-cast Ti44Al alloy. In addition, there are about 80% columnar grains with small angles (less 60°) between their growth directions and orientation of lamellar phases after DHT. Micro-hardness of Ti44Al alloys increases firstly and then decreases with increasing of cycles. Micro-hardness value is the highest about 312.9 HV for Ti44Al alloy after 4 cycles of DHT, which increases by 28% compared with that of as-cast alloy (244.8 HV), due to thin and uniform lamellar phases. Compression strength of Ti44Al alloy also increases from 900 MPa to 1200 MPa after DHT, due to directional microstructure and jagged grain boundary.

Keywords

• TiAl alloy
• Columnar grains
• Heat treatment
• Grains growth
• Microstructure
• Mechanical properties