Effects of ternary elements on phase transformation and microstructure of NiAl/Ni3Al two-phase alloys

The microstructural evolution of the beta-NiAl/gamma'-Ni3Al two-phase was investigated in (Ni66Al34)(100-chi)X-chi (X Ti, Si, Nb; X=0-4 at.%) alloys. Three different microstructures, such as lamellar, mesh and Widmanstatten, were obtained with the phase transformation procedure which depends on the A(s) (austenite start) temperatures and on the ternary elements. In the case of A(s) > 250 degreesC, lamellar microstructure was formed by the phase transformation, beta'-martensite-->Ni5Al3 --> beta + gamma', in all the alloys. In the case of 20 degreesC < A(s) < 250 degreesC, the phase transformation behavior and the microstructure changed with addition of the alloy elements. Mesh type microstructure was formed in the alloys containing Ti or Nb where the beta --> Ni5Al3 transformation occurred quickly, while Widmanstatten structure was formed in the alloys containing Si where the beta-Ni5Al3 transformation occurred very slowly. This phase transformation showed a significant phase transformation procedure in the formation of the mesh type microstructure, which was different from the morphology of the original martensite plates. In the case of A(s) < 20degreesC the beta-NiAl remained unchanged at room temperature because the martensitic temperature decreased below room temperature. Widmanstatten microstructure formed through the single phase transformation, beta --> beta + gamma'. (C) 2004 Elsevier Ltd. All rights reserved.