The role of primary Laves phase on the crack initiation and propagation in Thermo-Span alloy

Abstract

Thermo-Span alloy is an oxidation resistant low thermal expansion superalloy and shows good mechanical properties, which has been used to fabricate components in gas turbine engine to control the clearance between static and rotating part, and thus improve the efficiency. Owing to the insufficient Cr, it was not applicable for this alloy to employ long homogenisation treatment at high temperature, and consequently, the primary Laves phase formed during the solidification process might be remained in this alloy. However, few studies were carried out on studying the role of primary Laves phase in this alloy. This paper studied the effects of primary Laves phase particles on the crack initiation and propagation during tensile test at room temperature, 650 and 750°C of the rolled bar made from casting Thermo-Span alloy. It was found that at room temperature, cracks were mainly related with the Laves phase larger than 5 μm and the Laves clusters. At 650°C, cracks were formed preferentially at large Laves particles, but they would propagate faster at grain boundaries. At 750°C, as the hardening effect of γ’ phase was not effective, and the stress concentrated at large Laves phase would be released by grain deformation, so that large cracks were no longer widely spread and more intergranular cracks especially at surface of the specimen were found.