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

Brittle-to-ductile transition (BDT) temperature (T-BD) has been evaluated a ccording to temperature dependence of tensile properties under different st rain 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 invest igated using a transmission electron microscope. It was found that T-BD, wh en defined as the temperature corresponding to 7.5% elongation, increases f rom 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, w hich is approximate to the self- and inter-diffusion activation energies in the gamma-TiAl phase. Transgranular fracture and dimple fracture were foun d to dominate in fracture surfaces below and above T-BD, respectively. Furt hermore, the most popular 1/2<110] ordinary dislocations were found to begi n to climb around T-BD. All this evidence, as well as a theoretical calcula tion using the Nabarro Model, add up to a conclusion that the BDT of the al loy is controlled by dislocation climbing.