The effect of a small addition of nickel on the sintering, sintered microstructure, and mechanical properties of Ti-45Al-5Nb-0.2C-0.2B alloy

Titanium aluminide based pre-alloyed powder is difficult to consolidate by pressureless sintering. The effect of a small addition of nickel (Ni) on the sintering, sintered microstructure, and mechanical properties of a Ti-45Al-5Nb-0.2C-0.2B alloy, compacted from pre-alloyed powder, has been investigated. The relative density of the alloy, sintered at 1375 degrees C for 60 min, increased progressively from 74% to >99% with increasing Ni content from 0% to 1.25 at.%. The addition of Ni leads to the formation of an intermediate hexagonal-structured Al3NiTi2 phase (known as the tau(3) phase) during heating prior to reaching 1150 degrees C. The presence of the tau(3) phase enables the formation of a Ni-containing sintering liquid through three reactions between 1250 degrees C and 1350 degrees C: tau(3) + gamma -> Liquid, tau(3) + gamma -> Liquid + alpha and tau(3) + alpha -> Liquid. The sintering shrinkage of the powder blend Ti-45Al-5Nb-0.2C-0.2B-1.25Ni occurs mostly before reaching the isothermal sintering temperature (1375 degrees C). Consequently, the sintered density increases only marginally during the subsequent isothermal sintering at 1375 degrees C from 15 min to 60 min and remains unchanged afterwards. However, achieving a near full lamellar structure requires a longer isothermal hold (120 min) at the single alpha phase region. The as-sintered Ti-45Al-5Nb-0.2C-0.2B-1.25Ni alloy shows compression strength of 2200 +/- 50 MPa, yield strength of 750 +/- 20 MPa, and strain to fracture of 28 +/- 0.5%.