Brittle-to-ductile transition temperature and its controlling mechanism in Ti-47Al-2Mn-2Nb alloy

Brittle-to-ductile transition (BDT) temperature (T-BD) has been evaluated according to temperature dependence of tensile properties under different strain rates from 10(-5) to 10(-1) s(-1) in a two-phase Ti-47Al-2Mn-2Nb alloy with near lamellar microstructure. Tensile fractography was observed using a scanning electron microscope while deformation substructures were investigated using a transmission electron microscope. It was found that T-BD, when defined as the temperature corresponding to 7.5% elongation, increases from 1023K to more than 1373K, the strain rate increases from 10(-5) to 10(-1) s(-1). Based on the strain rate dependence of T-BD (and using the Zener-Hollomon factor) an apparent activation energy of 324kJ/mol was obtained, which is approximate to the self- and inter-diffusion activation energies in the gamma-TiAl phase. Transgranular fracture and dimple fracture were found to dominate in fracture surfaces below and above T-BD, respectively. Furthermore, the most popular 1/2<110] ordinary dislocations were found to begin to climb around T-BD. All this evidence, as well as a theoretical calculation using the Nabarro Model, add up to a conclusion that the BDT of the alloy is controlled by dislocation climbing.