Microstructural and dimensional stabilities of a potential γ/γ′-α(Mo) directionally solidified eutectic superalloy under cyclic thermal exposure to 1000° C

Abstract

A potential heat-resistant ductile eutectic composite,γ/γ′-α, in the Ni-Al-Mo ternary system has been thermally cycled in the temperature range 200 to 1000° C for up to 1000 cycles in an attempt to examine dimensional as well as microstructural stability of the composite under thermal fatigue conditions. The composite examined has two types of initial microstructure; in one, blocky γ′-Ni₃Al encircles individualα-Mo fibres (as-grown condition) whereas in the other,γ′ is in the form of fine cuboidal particles uniformly distributed in an Ni-rich fccγ matrix (heat-treated condition). Dilatometric measurements upon temperature cycling show that the composite is stable against “thermal ratchetting” irrespective of initial microstructural conditions. However, the cycling induces microstructural change, which is characterized by segmentation ofα-Mo fibres or formation of a detrimental brittle phase identified as an intermetallicδ-NiMo that consumesα-Mo fibres whether the fibres are encircled byγ′ or not. Post-cycling tensile tests at room temperature show that the fibre damage in the former has no fatal effect on tensile strength and ductility. A beneficial effect of theα-encirclingγ′ configuration is discussed on the basis of the recognition of a peritecto-eutectoid reaction:α+γ→δ+γ′ that has been disregarded.