Creep behaviour of Ni-base single-crystal superalloys with various γ′ volume fraction

Creep tests at 900 degreesC, 392 MPa and 1100 degreesC, 137 MPa were performed on a third generation Ni-base single-crystal superalloy TMS-75 and its gamma/gamma' line alloys. These tie line alloys were designed to contain various volume fractions of gamma', while the compositions of two individual phases were kept the same. The longest rupture life was attained by an alloy with gamma content in vicinity of 65% in both creep condition, which is similar trend has been observed in polycrystalline Inconel 713C through an earlier study by one of the authors. However, in comparison with measured amount of gamma', the creep rupture lives were the longest at about 70% and 55% measured gamma' volume fraction at 900 and 1100 degreesC, respectively. In addition, the dependence of the creep rupture life oil the amount of gamma' was more evident in single crystal than in polycrystal. Rafting was observed in the ruptured 80% gamma' alloy in the direction parallel to the stress axis, unlike the rafting commonly observed in the 40-60% gamma' alloys, perpendicular to the loading axis. This parallel raft structure can be explained by the interchanges in the relative roles of the matrix and the dispersed phase. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.