Effect of Cr and Nb on the phase transformation and pore formation of Ti-Al base alloys

The phase transformation and its relation to the porosity of Ti-Al base alloys are investigated in order to understand the effect of composition on the pore formation during sintering at low and high temperatures. Ti-Al base alloys with nominal compositions of Ti52Al48, Ti50Al48Cr2, Ti50Al48Nb2 and Ti48Al48Cr2Nb2 are prepared by the conventional elemental powder metallurgy route. The sintering of mixed powders was carried out at 600° C (low-temperature sintering below the Al melting point) and 1200° C (high-temperature sintering). Al-rich intermetallic compound TiAl3 was primarily formed during low-temperature sintering while Ti-rich intermetallic compound Ti3Al was formed during high-temperature sintering. The phase transformations increase the overall porosity and pore size of alloys to a similar extent regardless of the alloy composition during the low-temperature sintering. However, large differences in the pore size and pore volume distribution occur depending on the composition of Ti-Al alloys during high-temperature sintering. Chromium and niobium do not affect the formation of Kirkendall voids and thus show a similar void formation behavior at 600° C, but affect the void agglomeration behavior to a different extent due to the different diffusion activities at 1200° C.