Phase composition and structure of Ni-Al-Co-(Ti, Zr, Hf, V, Nb, Ta, Cr, Mo) alloys

The effects of the IV-VI-group transition metals (Ti. Zr, Hf, V. Nb. Ta. Cr, and Mo) in contents ranging from 0.27 to 23 at. % on structure and phase composition of the (beta + gamma) Ni-Al-Co alloys were studied. The (beta + gamma) alloys may be taken as the basis for high-temperature nickel-based alloys with density of at most 7.3 g/cm(3). These alloys have microstructure consisting of the beta-phase grains surrounded by continuous grain-boundary interlayers of gamma phase (degenerate eutectic). The contents of the beta and gamma phases are greater than or equal to 70 and greater than or equal to 30 vol %, respectively. The beta-phase grains may content secondary precipitates of gamma and gamma' phases. In the Ni-(20 - 25 at. %) Al-(32 - 34 at. %) Co alloys. the content of allaying elements such as Mo, Tn. Nb. and V should not exceed 1 - 1.8 at. %, and the contents of Ti and Cr may reach 3 and 12%, respectively. This chemical composition provides: (1) an increase in the contents of Co (the main strengthening element) in the NiAl-based beta phase; (2) substantial strengthening of beta and gamma phases by alloying elements such as Cr, Ti, and, to a smaller extent, by Nb, Mo, and Ta; and (3) strengthening of the gamma phase by substantial amounts of Cr and Ti or by small amounts of Mo, Nb. or Ta. The solubilities of ail elements in the gamma phase are higher by a factor of two (Ti. Ta. Nh. Mo) or three (Cr) than these are in the beta phase. The solubilities in gamma and beta phases are maximum for Cr and Ti. and the solubilities of Cr and Ti both in the gamma and beta phases are higher than those of Mo, Nb, Ti. and Hf.