The mechanical behaviour of an ultrafine grained Ti–47Al–2Cr (at%) alloy in tension and compression and at different temperatures

Abstract

A bulk ultrafine grained (UFG) Ti–47Al–2Cr (at%) alloy has been produced using a powder metallurgy process that combines high energy mechanical milling (HEMM) of a mixture of Ti, Al and Cr powders to produce a Ti/Al/Cr composite powder and hot isostatic pressing (HIP) of the composite powder compact. The purpose of the present study is to determine the mechanical behaviour of the alloy in tension and compression at room temperature (RT) and elevated temperatures, and also to compare the compression behaviour of the material with its tensile behaviour. It has been found that due to the residual pores, lack of full level interparticle bonding and high oxygen content (0.87 wt%) in the consolidated samples, the UFG TiAl based alloy has a very low room temperature tensile fracture strength of ∼100 MPa and shows no tensile ductility. However these microstructural defects and high oxygen content have much less significant effect on the room temperature compressive mechanical properties, and the alloy shows a high compressive yield strength of ∼1410 MPa, and some ductility (plastic strain to fracture ∼4%). At elevated temperatures of 800 °C and above, the alloy shows high tensile and compressive ductility as demonstrated by 75% tensile elongation to fracture and no cracking in upset forging with a height reduction of 50% at 900 °C. The yield strength of the alloy at 900 °C is 55 MPa in tension and ∼33 MPa in compression, both of which are lower than those of coarse grained TiAl based alloys with similar compositions at 900 °C. This is due to a higher creep rate of the UFG alloy caused by the small grains. The good formability of the UFG TiAl based alloy as reflected by the lower critical temperature above which the alloy becomes highly formable indicates that the material can be used as a suitable precursor for secondary thermomechanical processing and super-plastic forming.