As-cast titanium aluminides microstructure modification

Two cast <FONT FACE="Symbol">g</font> titanium aluminides, Ti47Al and Ti50Al (at. %), were obtained by arc melting under an argon atmosphere. The as-cast microstructure modification by annealing under protective argon atmosphere and hot isostatic pressing (HIP) was analysed by optical and scanning electron microscopy with microanalysis facilities. As-cast structures presented strong microsegregations, being composed of primary <FONT FACE="Symbol">a</font> dendrites (with low Al content), which transformed into lamellar <FONT FACE="Symbol">a</font>2 + <FONT FACE="Symbol">g</font> during solid state cooling, and interdendritic <FONT FACE="Symbol">g</font> phase (with high Al content). Annealing in the <FONT FACE="Symbol">a</font> + <FONT FACE="Symbol">g</font> field was responsible for a partial reduction of microsegregation, a decreasing of the volume fraction of <FONT FACE="Symbol">a</font>2 + <FONT FACE="Symbol">g</font> lamellar dendrites, an increasing of <FONT FACE="Symbol">g</font> volume fraction and the occurrence of a small fraction of <FONT FACE="Symbol">a</font>2 particles and plates in some of the <FONT FACE="Symbol">g</font> grains. Annealing in the <FONT FACE="Symbol">a</font> field at 1400 °C for one hour was insufficient to eliminate the as-cast microstructure in the Ti50Al alloy and produced large equiaxed lamellar colonies of <FONT FACE="Symbol">a</font>2+<FONT FACE="Symbol">g</font> in the Ti47Al alloy. HIP in the <FONT FACE="Symbol">a</font> + <FONT FACE="Symbol">g</font> field (to avoid excessive grain growth) was responsible for microstructure modifications similar to the ones obtained by heat treating under similar soaking conditions