DETERMINATION OF THE VOLUME FRACTION OF THE MICROPOROSITY IN NICKEL-BASED SUPERALLOY SINGLE CRYSTALS

Abstract

Microporosity is a dangerous defect of single crystal gas turbine blades cast from nickel-based superalloys (NBS). The volume fraction of the porosity in single crystal alloys does not exceed a few tenths of a percent, however, it can result in manyfold shortening of the lifetime of the blade material under fatigue loading. We present the results of determining the volume fraction of the porosity in single crystal NBS. Single crystals of NBS CMSX-4 obtained according to the industrial technology of manufacturing the single crystal blades of gas turbines were used as a test object. It is found that the applied methods, with the exception of optical microscopy, have an accuracy sufficient for measuring the volume fraction of the microporosity of about 0.2 %vol. The highest accuracy with a statistical error of about ±0.01 %vol. was attained using the Archimedes method using distilled water as a liquid. The method provides the determination of small (up to a few hundredths of a percent by volume) increase of the porosity during high-temperature creep. The results obtained can be used for precise determination of the porosity in NBS single crystals before and after service. Moreover, the process of high-temperature creep can be modeled using a correlation relationship between an increase in the porosity of single crystal material and the accumulated creep strain.