STRUCTURE AND DEFORMATION-BEHAVIOR OF TI-AL ALLOYS CONTAINING NEARLY 60 AT-PERCENT AL

An alloy of composition 38at.%Ti, 57at.% Al and 5at.%Cr has been prepa red both by casting and by spray forming, and its structure and deform ation mechanisms examined. This alloy is essentially a high-Al-content gamma-TiAl intermetallic susceptible to decomposition by forming Al2T i phase. Microstructures are very different for cast and sprayed mater ial because of casting segregation leading in one case to a homogeneou s but unstable matrix with a Cr-rich interdendritic phase and in the o ther case to an intimately mixed two-phase gamma-TiAl + Al2Ti structur e. The two-phase material is very strong but both cast and sprayed sta tes are brittle. Microstructural instabilities during high-temperature testing lead to deformation instabilities seen as serrations on the s tress-strain curves. Dislocation structures at room and elevated tempe ratures are the same as those seen in lower-Al-content gamma-TiAl allo ys but show significant variations in character and extent of dissocia tion with test temperatures. Variations in fault energies are examined over a wider range of Al content (48-75at.%) and as a function of tem perature and are discussed in terms of atomic bonding and local chemic al segregation.