| Three solid phases are involved in the phase equilibria of the important intermetallic compound Ni₃Al near its melting point, β, γ` (Ni₃Al) and γ. The generally-accepted phase diagram involves a eutectic reaction between γ` and γ, but some recent studies agree with an older diagram due to Schramm, which has a eutectic reaction between the β and γ` phases. This study uses diffusion couple experiments to evaluate the equilibria at temperatures just below the liquidus. The Ni/NiAl diffusion couples were prepared at 1249, 1340, 1355, 1365℃. where the γ` phase formed between β and γ after diffusion heat treatment. The formation of the γ` layer was found to be controlled by volume diffusion. The composition range of the γ` phase with increasing temperatures were increased to the higher Ni composition, which agrees with the Schramm diagram. The interdiffusion coefficient in the γ and β phases has been also determined from the composition profiles of the diffusion couples. |
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