NiAl/Ni3Al 2상 합금의 상변태 및 미세조직에 미치는 제 3원소 첨가효과

Abstract

The microstructural evolution of b-NiAl/γ` -Ni3Al two-phase (Ni_(66)Al_(34))_(100-x)X_(X)(X=Ti, Si, Nb) alloys during various heat treatment (quenching and aging) were systematically studied. It was found that the microstructural features of the two-phase NiAl/Ni₃Al alloys could be divided into three types : lamellar, mesh and Widmansttten microstructures. The reason why those three types of microstructure were formed could be explained by the change of austenite start temperature(As temperature) depending on the ternary elements. In case of As>250℃, the lamellar microstructure was formed by following phase transformation: Martensite→Ni5Al3→β+γ` . In this case, the NiAl martensite was quickly transformed into the Ni5Al3 phase at 250℃ by a re-ordering reaction. In case of 20℃ <A_(s)<250℃, two types of mesh microstructure were formed depending on the ternary element. When Ti and Nb were added as a ternary element, the β→Ni_(5)Al3 transformation occurred very quickly, but not in the case of Si addition. Hence, the final mesh microstructures showed somewhat different microstructural features. On the other hand, in case of A_(s)<20℃, the direct β→β+γ` transformation occurred because the martensitic transformation could not occur due to its very low M_(s) temperature, resulting in the Widmanstatten type microstructure.