METALLOGRAPHY OF GAMMA-TITANIUM ALUMINIDES

The microstructures of forged and heat treated Ti-48A1-2Nb-2Mn (at. %) and Ti-48A1-2Nb-2Cr (at.%) gamma titanium aluminides have been reveal ed by the application of selected metallographic preparation technique s and characterized using light microscopy. Examination of the as-poli shed specimen surface under polarized light was highly effective in re vealing the equiaxed gamma grain structure and twins within the gamma grains, but it did not delineate alpha-two phase present at gamma grai n boundaries or within a lamellar gamma/alpha-two constituent. Bright- field and differential-interference contrast light microscopy analyses of specimens chemically etched with Kroll's reagent (100mL H2O + 4mL HNO3 + 2mL HF) were marginally effective in characterizing the equiaxe d gamma grain structure and likewise did not reveal the alpha-two phas e. Furthermore, the application of Kroll's reagent resulted in localiz ed dissolution in the form of fine grooves or microcracks oriented in preferred directions within the equiaxed gamma grains. Under light mic roscopy, gamma grains that experienced this attack resembled the lamel lar gamma/alpha-two constituent. The alpha-two phase was most clearly revealed using an etching solution comprised of 30mL lactic acid + 30m L HNO3 + 3mL HF, while the gamma grain and twin boundaries were most e ffectively revealed using an etching solution comprised of 30mL HCL 10mL HNO3 + 5mL H2O2 + 3mL HF. An etching solution of 25mL H2O + 50mL glycerol + 25mL HNO3 + 2mL HF was very effective in simultaneously rev ealing both the gamma and alpha-two phase morphologies.