Long range order in Ni 3Mo based ternary alloys—I. Isothermal aging response

Abstract

In an attempt to understand the constitution of quaternary Ni Al X superalloys, the ordering reactions in Ni 3(Mo, X) alloys have been studied. The effects of additions of Al, Ta, V or W on the ordered structures produced during isothermal aging have been determined by transmission electron microscopy. Quenching the alloys from 1300°C resulted in a f.c.c. matrix which exhibited diffuse diffraction intensity at the [1½0] locations of reciprocal space. Aging at 600, 700 and 800°C caused various long range ordered structures to evolve from this matrix including Dla, DO 22 and Pt 2Mo prototype. The phases observed were dependent upon which ternary element was substituted for Mo. The diffraction evidence was consistent with a spinodal ordering transformation mechanism for all of the alloys aged at temperatures ⩽ 700°C. At 800°C, the Al-bearing ternary formed the Pt 2Mo structure via a homogeneous nucleation mechanism. These results are interpreted in terms of the statistical thermodynamic (pairwise interaction) theory as it has been applied to ordering systems.