Precipitation Within a Precipitate—Nickel-Rich Alloys of Nickel, Aluminum and Titanium

Abstract

Figure 1 shows how a precipitate of γ might form within a precipitate of γ' in a given Ni-Al-Ti alloy. The alloy reverts to single phase fcc γ on heating above the solvus temperature TS. Upon aging at T1 an LI2 γ' Ni3 (Al,Ti) phase of composition is formed and is in equilibrium with a γ matrix of composition. . If the alloy is subsequently aged at a lower temperature T2, not only does additional γ' form within the matrix, but γ can also precipitate within γ', provided that the γ' phase boundary is as represented.In this study a Ni-10.7Al-5.1Ti (at. %) alloy was air cooled from a solution temperature of 2250°F to form coarse γ' particles. The cooling rate was sufficiently rapid that equilibrium was not. achieved at temperatures corresponding to T2 in Figure 1. Samples were then aged for 16 hours at l400°F and their microstructure examined. As evident in Figure 2(a,b), the large γ' particles contained small plate-like precipitates after aging and these appeared to lie on {100} planes.