Formation of carbides and their effects on stress rupture of a Ni-base single crystal superalloy

Creep tests of a nickel-base single crystal superalloy with minor C addition and non-carbon were carried out at different temperatures and stresses. Correlations between microstructural change and testing temperature and stress were enabled through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), detailing the rafting microstucture and carbides precipitation. The results showed that minor carbon addition prolonged the second stage of creep strain curves and improved creep properties. Some carbide was precipitated during creep tests in modified alloy. M23C6 carbide precipitated at lower temperature (871-982degreesC), while (M6C)(2) carbide precipitated at higher temperature (>1000degreesC), all of which was considered to be beneficial to creep properties. A small amount of MC carbide formed during solidification and its decomposition product (M6C)(1) were detrimental to mechanical properties, which together with micropores provided the site of initiation of cracks and led to the final fracture. (C) 2003 Elsevier B.V. All rights reserved.