Key Considerations of Electro-thermal Mechanical Testing in Ni-Base Superalloy Phase Transformations

Abstract

Electro-thermal mechanical testing (ETMT) with direct current and Joule heating was used to study the dissolution and formation of γ′$$\gamma '$$ precipitates in miniaturised samples of the single crystal nickel-base superalloys, CMSX4 and CMSX10N. Alloys were subjected to heating and cooling cycles with resistivity simultaneously measured to infer γ↔γ′$$\gamma \leftrightarrow \gamma '$$ phase transformations. Temperature-resolved resistivity measurements exhibit notable variations between samples; however, when normalized, the resistivity changes become systematic, with trends suitable to infer phase transformation behaviour. Specifically, dissolution and precipitation behaviour of the γ′$$\gamma '$$ phase with respect to heating or cooling rates can be determined. The ETMT measured γ′$$\gamma '$$ solvus is lower compared with the calorimetric/thermodynamic value and approaches the latter, when the resistivity values exceed a threshold value, which is temperature dependent. The differences in the resistivity curves cannot be explained by the range in γ′$$\gamma '$$ size distribution within the specimens; rather, the difference becomes prominent above a threshold current density, which occurs above a given temperature. Like in the case of dissolution, the nucleation and precipitation of γ′$$\gamma '$$ is dependent on the normalized resistivity, but independent of stress, if cooling occurs under restraints in case of the latter. A greater γ′$$\gamma '$$ solvus corresponds to an increased nucleation temperature, implying a varying undercooling for nucleation when calculated with respect to the thermodynamic solvus. When stress develops during cooling, it increases rapidly above a critical γ′$$\gamma '$$ mole-fraction when precipitation hardening becomes prominent. Given these features are endemic to ETMT, some guidelines are offered for use of ETMT tests in specific applications.