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 revealed by the application of selected metallographic preparation techniques and characterized using light microscopy. Examination of the as-polished specimen surface under polarized light was highly effective in revealing the equiaxed gamma grain structure and twins within the gamma grains, but it did not delineate alpha-two phase present at gamma grain 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 equiaxed gamma grain structure and likewise did not reveal the alpha-two phase. Furthermore, the application of Kroll's reagent resulted in localized dissolution in the form of fine grooves or microcracks oriented in preferred directions within the equiaxed gamma grains. Under light microscopy, gamma grains that experienced this attack resembled the lamellar gamma/alpha-two constituent. The alpha-two phase was most clearly revealed using an etching solution comprised of 30mL lactic acid + 30mL HNO3 + 3mL HF, while the gamma grain and twin boundaries were most effectively 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 revealing both the gamma and alpha-two phase morphologies.