Superplastic Behavior of Hot Extruded Gamma TiAl (Mo, Si) Alloys

Superplastic behavior of hot extruded intermetallic Ti-46Al-1.7(Mo,Si) (at%) alloys was studied by stress change tests in compression and tensile tests at temperatures ranging from 700 to 1050degreesC. The material produced by are melting exhibited a structure of coarse lamellar grains in the as-cast condition that transforms to an equiaxic near gamma microstructure after processing by hot extrusion at 1250degreesC. This microstructure consists of zones of gamma grains finer than 1 mum and band like regions with coarser grains, ranging from 5 to 20 gm. In addition to gamma grains, a volume fraction of more than 20 vol% of alpha(2)-Ti3Al particles finely dispersed are also present in the fine-grained zones. Compression tests of the extruded material at stresses ranging from 4 to 825 MPa showed values of the strain-rate-sensitivity exponent near 0.5 at low stresses and/or high temperatures. The microstructure in the fine-grained areas remains essentially constant during deformation. TEM analysis of deformed samples in this regime leads to relate grain boundary sliding as the mechanism controlling the deformation process. High elongation to failure, characteristic of superplasticity, was achieved at 975 and 1050degreesC at an initial strain rate of 4.6 x 10(-4) and 4.6 x 10(-3) s(-1).