Effect of Mg on the microstructure and properties of TiAl alloys

The effect of Mg on the microstructure and mechanical properties of TiAl-xMg (x = 0, 0.5, 1.0 at.%) alloys was studied. The microstructures were investigated by optical microscopy, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (TEM). Tensile test were conducted at room-temperature and at 900 degreesC in air and creep test were performed at 800 C under a stress level of 240 MPa in air. The results show that the addition of Mg led to a grain refinement in the Ti52Al48-0.5Mg and Ti52Al48-1.0Mg, and led to a formation of Mg-rich phase within the matrix or at the grain boundaries. This latter phase exhibited either nanometer-sized grains or was amorphous. Compared with the binary TiAl alloy, the yield and ultimate tensile strengths of the Ti52Al48-0.5Mg alloy at room- and elevated-temperature were higher, a result of the grain-refinement strengthening and solid-solution strengthening of Mg, but an addition of 1.0 at.% Mg resulted in a decrease in the tensile strengths at room- and elevated-temperature of the Ti52Al48-1.0Mg. This is attributed to the presence of the Mg-rich particles at the grain boundaries, causing intergranular failure at room- and elevated-temperature. Compared with the binary alloy, addition of Mg to TiAl at both the 0.5 and 1.0 at.% levels increased the creep-rupture-life under conditions of 800 degreesC/240 MPa. (C) 2002 Published by Elsevier Science B.V.