Grand boundary structure as a function of aluminium level in Ni3Al

Understanding the boron effect in Ni3Al currently centers on determining the structure of the region near grain boundaries, especially the presence of a disordered γphase. In this paper, a series of alloys are examined by transmission electron microscopy (TEM) as a function of aluminum level, boron level, and cooling rate for the presence of any grain boundary phases. The base alloy series all contained 0.5 at.% Hf. At 21.5 at.% Al, γ formed in the matrix as expected. However, in slowly cooled specimens with 22 and 22.5% Al, a second phase with thicknesses from 25 to 50 nm formed on some of the grain boundaries. At high aluminum levels no evidence of any second phase was observed. The minimum width of grain boundary images formed by normal diffraction contrast imaging was generally 0.5 to 1 nm for grain boundaries that presumably had no second phase and were parallel to the electron beam. Therefore, a second phase with a thickness of 1 nm or less would not be discernable by TEM