Determination of the Volume Fraction of Microporosity in Single Crystals of Nickel-Based Superalloys

Abstract

Microporosity is a dangerous defect observed in single-crystal gas turbine blades cast from nickel-based superalloys (NBSs). The volume fraction of porosity in single-crystal alloys does not exceed several tenths of a percent; however, it can result in shortening of the lifetime of the material of gas turbine blades under fatigue loading by many times. This work presents the results of determination of the volume fraction of porosity in single-crystal NBSs. Single crystals of a CMSX-4 NBS obtained according to the industrial technology of manufacturing of single-crystal blades are used as a test object. It is found that the methods applied, except for optical microscopy, have accuracy sufficient for measuring the volume fraction of microporosity of about 0.2 vol %. The highest accuracy with a statistical error of ±0.01 vol % is demonstrated by the Archimedes method with the use of distilled water as a liquid. The method makes it possible to measure small (up to several hundredths of a percent by volume) increases in porosity in the process of high-temperature creep. The results obtained can be used for precise determination of porosity in single-crystal NBSs before and after operation. Moreover, the process of high-temperature creep can be modeled using a correlation relationship between the increase in the porosity of a single-crystal material and the accumulated creep strain.