Effect of Al content on the microstructure and stress rupture properties of DS Ni₃Al based alloy

The DS Ni₃Al based alloy was prepared by using directional solidification process and the effect of the Al content on the microstructure of as-cast and heat treatment specimens was investigated. Employing SEM, EDS, and XRD, the stress rupture of the alloy at high temperature was measured and the microstructure of stress rupture specimens was observed. The results show that the as cast microstructure of alloy is composed of γ', γ and MC carbide as the Al content is in the range of 6.5-9.5 mass percent. A large amount of primary prime y and a small amount of β-NiAl phases were found in the alloy of 9.5 mass percent Al content. The solid solution heat treatment of 4 hrs at 1300°C can make secondary prime y dissolve completely, but eutectic prime y only dissolve a little. Solid solution of primary prime γ and β-NiAl were not still occur when the alloy started initial melting, this would greatly decrease the high temperature strength of alloy. As-raft structure can appear when the stress ruptured time is over 10 hrs under the condition of 100°C/70 MPa. The cracks mainly initiated and extended along cross segment or interdendritic. The proper range of aluminum content of DS Ni₃Al based alloy after adding W, Mo, Ti, Ta etc elements is 7.5 to 8.5 mass percent.