Coarsening of γ (Ni-Al Solid Solution) Precipitates in a γ' (Ni_3Al) Matrix: Preliminary Results

摘要

We present the results of initial experiments on the coarsening of coherent precipitates of the γ phase (Ni-Al solid solution) in a matrix of off-stoichiometric γ' (Ni_3Al). The compositions of the γ' phase were 22.25, 22.33 and 22.6 at. % Al. The alloys were aged at 650℃. The volume fractions of the γ precipitates, f_γ, were calculated to be 0.053, 0.046 and 0.023, respectively. The longest aging times exceeded 750 h. Precipitates of the γ phase are spherical when very small, but become cuboidal at small sizes (< 7 nm), with interfaces parallel to {100}. They also coalesce readily because there are no antiphase relationships among nearest neighbors, unlike the situation for γ' precipitates in a γ matrix. The γ precipitates are thus highly non-equiaxed at relatively small sizes compared to γ' particles. At short aging times the data obey the classical rate law for diffusion-controlled coarsening, ~3 ≈ kt, where (r) is the average radius at time t and k is the rate constant. For these data k increases with increasing f_γ. However, if the data on all aging times are included in the analysis the rate law is not nearly so well obeyed, but the values of k are nearly independent of f_γ. The correct dependence of k on f_γ requires additional experiments for resolution.