Phase transformation and microstructural features of NiAl/Ni3Al two-phase alloys containing ternary elements

Various ternary elements were added to observe the effects on the microstructural features of beta+gamma' two-phase alloys. The microstructural features of beta+gamma' two-phase (Ni66Al34)(100-chi)X-chi(X=Ti, Si, Nb) depended on the A(s) (austenite start) temperature of beta'-martensite with alloying elements. For A(s)>250 degreesC, a lamellar microstructure was found to form by the following phase transformation: Martensite-->Ni5Al3-->beta+gamma'. For A(s)<250degreesC, two-type microstructures, mesh and Widmanstatten, were formed depending on the ternary element. When Ti or Nb was added as a ternary element, the beta-->Ni5Al3 transformation occurred very quickly. Conversely, this transformation proceeded very slowly in the case of Si addition, and the resultant microstructure assumed somewhat different features. Consequently, it could be suggested that the microstructures, of NiAl/Ni3Al two-phase alloys are determined by not only the A(s) temperature but also by the beta-->Ni5Al3 transformation.