Influence of Shrinkage on the Mechanical Properties of Cast M247LC

The complex geometries and the high operating temperatures of aerodynamic components on gas turbines require the use of cast Ni base superalloys. Amongst these alloys, M247LC is widely used on its equiax form in the manufacture of the first stages of low pressure turbine (LPT) nozzle guide vanes (NGVs). M247LC is a gamma prime strengthened alloy with significant amounts of interdendritic gamma - gamma prime eutectics and carbides. These microstructural inhomogeneities lead to big differences between the solidus and liquidus temperatures of this alloy, which promote the formation of shrinkage. Although the casting parameters can be adjusted to minimize the presence of shrinkage, there are many circumstances where shrinkage cannot be avoided. Then it was necessary to assess the influence of shrinkage on the mechanical properties of this alloy. This has been carried out with the manufacture and subsequent testing and post processing of test pieces with different levels of shrinkage. Tensile, load controlled LCF and strain controlled LCF showed a clear relationship between the level of shrinkage present on the test piece and the drop of properties observed. This correlation has been modeled and relationships between shrinkage and property reductions have been generated. The results have shown significant differences in the behavior of concentrated areas of shrinkage surrounded by sound metal and the behavior of shrinkage spread over the entire gauge section of the specimen. The influence of shrinkage has then been classified in two main groups depending on its distribution.