The effect of chromium on the microstructure and micromechanical properties of TiAl-base alloys

The microstructure of the TiAl-base alloys containing up to 10 at.% Cr and the micromechanical properties of gamma grains and the lamellar microconstituent (alpha(2)-Ti3Al/gamma-TiAl) after solidification as well as after additional annealing for 6 and 48 h at 1200 degrees C, were investigated. The apparent interlamellar gamma/gamma spacing decreases with increasing Cr content. No clear effect of Al on the interlamellar spacing is observed. The Cr content in the gamma-TiAl and alpha(2)-Ti3Al lamellae increases with decreasing apparent interlamellar spacing. The Cr content in the gamma lamellae of alloys with 3, 5 and 10 at.% Cr slightly decreases with increasing annealing time from 6 to 48 h at 1200 degrees C. The Cr content in the alpha(2) lamellae of alloy with 10 at.% Cr increases with increasing annealing time at 1200 degrees C. Vickers microhardness (VHN) (at 1000 g) of the lamellar constituent vs. the apparent interlamellar spacing (IS) data were fitted by the linear regression method to the Hall-Fetch relationship in the form: VHN = H + Y-H (IS)(-n) for three n values: 1, 1/2 and 1/3. The best fit is obtained for n = 1/2 irrespective of the annealing treatment of alloys. However, the other two exponents also exhibit a good fit to the experimental data. Microhardness of gamma grains in a duplex microstructure increases almost linearly with increasing Cr content in grains (solid solution strengthening effects).