Mechanical characterization of mechanically alloyed ultrafine-grained Ti5Si3+40 vol% γ-TiAl composites

Ultrafine-grained ceramic-based composites of Ti-31.6Al-21.6Si (at%) consisting of 60 vol% of ζ-Ti5Si3 and 40 vol% of γ-TiAl were produced by high-energy ball milling followed by hot isostatic pressing (HIP). Because of the cleanliness of the powder and full densification of the HIPed product, the mechanical behavior of the composite could be unambiguously related to the microstructure and chemistry. The starting microstructure after HIPing consisted of intermixed ζ-Ti5Si3 and γ-TiAl phases of approximately equal grain size, the size ranging from about 300 nm to 1 μm depending on the HIP temperature. High-temperature mechanical testing of this ultrafine-grained composite exhibited a strain-rate sensitivity of >0.3. Further, the equiaxed microstructure was retained after mechanical testing, suggesting the possibility of achieving superplastic deformation. Consequently, tensile testing demonstrated elongations of about 150% at 950 °C and a strain rate of 4×10−5 s−1. Considering that the present alloy has the ceramic (silicide) phase as the matrix, this temperature at which superplastic deformation is observed is significantly lower than that reported for conventional coarse-grained ceramic materials.