Phase transformations and resulting microstructures in Ti - 47 Al -2 Cr alloy

During the last three decades, intermetallic alloys have focused attention because of their high strength to weight ratio and good creep resistance. Titanium aluminide alloys based on γ-Ti Al are potential candidates to replace Ni-based super alloys currently used in jet engine components at high temperatures because of their low density, high melting temperature, good elevated-temperature strength and modulus retention, high resistance to oxidation and hydrogen absorption, and excellent creep properties. One of the major concerns in these alloys is their poor ductility at room and intermediate temperatures which has been improved slightly by microstructure modifications through heat treatment. Thus, modification of microstructure during cooling and CCT diagram in these alloys is of vital importance. In this study, Ti - 47 Al - 2 Cr intermetallic alloy has been prepared by remelting 4 times with a vacuum arc remelting furnace. Homogenizing treatment was done at 1125^deg^C for 72 h in a sealed vacuum quartz tube. All heat treatments on the samples were carried out in a vacuum heat treatment furnace under a pressure of 10^-1 bar. The atmosphere inside the furnace was changed to that of high purity argon for each heat treatment as an added precaution against oxidation. In this paper, phase transformations in a γ-Ti Al based intermetallic alloy containing chromium were investigated. Heat treatments on samples of this alloy at temperatures above Tα and subsequent cooling with various cooling rates resulted in variety of microstructures. The schematic CCT diagram for this alloy was drawn from microstructural studies using microscopy routs and X-ray diffraction. Then, cyclic heat treatment with grain refining purpose was conducted on a sample of this alloy having massive gamma microstructure. During cyclic heat treatment, gradual dissociation of the gamma phase resulted in the formation of a Widmanstaetten type structure. Trend of microstructure evolution and formation of Widmanstaetten structure has been analyzed considering the crystallographic characteristics of the parent phase. The results show that homogenizing treatment at 1100 ^deg^C may bring about a microstructure containing γ, α² single phase grains and initial and secondary lamellar grains. Also the various cooling rate from phase region may result in formation variety microstructure, as at low cooling rate may cause formation lamellar coarse grains microstructure and by increasing the cooling rate will result in feathery, massive and fully α2 microstructures