Phase Transformation and Microstructural Features of NiAl/Ni_3Al Two-Phase Alloys Containing Ternary Elements

摘要

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 (Ni_(66)Al_(34_(100-x)X_x(X=Ti, Si, Nb) depended on the A_s (austenite start) temperature of beta'-martensite with alloying elements. For A_s>250 deg C, a lamellar microstructure was found to form by the following phase transformation: Martensite- > Ni_5Al_3- > beta + gamma'/. For A_s<250 deg C, 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- > Ni_5Al_3 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/Ni_3Al two-phase alloys are determined by not only the A_s temperature but also by the beta- > Ni_5Al_3 transformation.