O-phase and carbide precipitation in intermetallics based Ti-Al

초록

In Ti-Al based intermetallics, precipitates of several ternary phases, such as the orthorhombic O-phase, perovskite P-phase and hexagonal H-phase structures, have been experimentally identified during aging. These phases are believed to develop from the matrix of 2-phase as a result of the characteristic diffusion of the alloying elements that involves changes in chemical composition and lattice structure. In this paper, the formation mechanism of these phases was studied in the matrix of Ti-Al based intermetallic compounds. The O-phase was found to form in the primary equiaxed 2 grains in multivariant forms and as fine 2+O mixtures in Ti-24Al-14Nb-3V-0.5Mo alloy during solution treatment at 1000C/1h followed by water quenching and aging at 650C and 850C. Supersaturation of Nb, the  stabilizer, resulted in a segregation of Nb-rich region and Nb-poor region, which incurred formation of O-phase as a consequence of the 2-phase decomposition. A crystallographic orientation relationship between the 2-phase and the O-phase was observed. In Ti-46.6Al-2Mn-1.4Mo-0.3C alloy, P-phase carbides formed mainly at the 2/ interface whereas some H-phase carbides were also found within the  grain and at prior  grain boundaries. The presence of the ledges between the P-phase and 2-phase interfaces indicated that the nucleation stage of the P-phase involved an interface-controlled process with a coherency relationship at the interface.