Tensile properties and fracture behaviour of an ultrafine grained Ti-47Al-2Cr (at.%) alloy at room and elevated temperatures

An ultrafine grained (UFG) Ti-47Al-2Cr (at.%) alloy has been synthesized using a combination of high energy mechanical milling and hot isostatic pressing (HIP) of a Ti/Al/Cr composite powder compact. The material produced has been tensile tested at room temperature, 700 and 800 degrees C, respectively, and the microstructure of the as-HIPed material and the microstructure and fracture surfaces of the tensile tested specimens have been examined using X-ray diffractometry, optical microscopy, scanning electron microscopy and transmission electron microscopy. The alloy shows no ductility during tensile testing at room temperature and 700 degrees C, respectively, but very high ductility (elongation to fracture 70-100%) when tensile tested 800 degrees C, indicating that its brittle to ductile transition temperature (BDTT) falls within the temperature range of 700-800 degrees C. The retaining of ultrafine fine equiaxed grain morphology after the large amount of plastic deformation of the specimens tensile tested at 800 degrees C and the clear morphology of individual grains in the fractured surface indicate that grain boundary sliding is the predominant deformation mechanism of plastic deformation of the UFG TiAl based alloy at 800 degrees C. Cavitation occurs at locations fairly uniformly distributed throughout the gauge length sections of the specimens tensile tested at 800 degrees C, again supporting the postulation that grain boundary sliding is the dominant mechanism of the plastic deformation of the UFG TiAl alloys at temperatures above their BDTT. The high ductility of the UFG alloy at 800 degrees C and its fairly low BDTT indicates that the material a highly favourable precursor for secondary thermomechanical processing.