TiAl alloys are introduced in jet engines, turbocharger etc. due to their superior high specific strength in high temperature. However manufacturing processes of TiAl parts are still more difficult than conventional Ni based alloys because of their high reactivity of liquid and/or low toughness. As for forging process of TiAl, Takeyama advocated forging process use of beta Ti phase which has high deformability. In Ti-Al binary phase, beta Ti phase exists only high temperature and low Al content region. By addition of beta stabilizer such as Nb, beta phase can exist at relatively low temperature and alpha + beta + gamma or beta + gamma phase regions appear. Therefore forging process window becomes wider and various microstructures can be obtained by heat-treatment. In this study, the effects of microstructure on mechanical properties were investigated using forged TiAl which has different microstructure. Chemistry of experimental alloy is Ti - 43Al - 5V - 4Nb (at%). Cylindrical specimens were machined from as-cast ingot manufactured by VAR. Then specimens were forged to pancake at 1200C without canning. Four heat-treatment conditions were selected in order to obtain various microstructures. Tensile specimens and CT specimens were machined from heat-treated pancake. Tensile tests were conducted at RT and 760 degree C, and crack propagation tests were also conducted at RT. Microstructure of as-forged, heat treated and fractured specimens ware observed using SEM, TEM and EBSD. Microstructure of ingot consists of elongated alpha/gamma lamella colonies and small amount of beta and gamma grains. After pancake forging, lamella colonies were deeply distorted and ultrafine recrystallized alpha2 and gamma grains were observed along colony boundaries. After 1st step heat treatment at 1200 - 1300 degree C and air cooling, microstructure was changed to equiaxed alpha2 grains and small amount of beta phase or near lamellar structure. And gamma/gamma lamella +beta, duplex and equiaxed alpha + beta + gamma structure were also obtained by various heat treatments. Equiaxed alpha 2 ( or alpha2 lamellar structure with small amounts gamma ) + beta structure has superior 0.2% yield strength than the other microstructures at both RT and 760 degree C. As for elongation, equiaxed alpha 2 + beta + gamma structure shows larger value at both temperatures. Further analysis of the microstructure and fracture surface observations will be reported.
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