Microstructural degradation and its corresponding mechanical property of wrought superalloy GH4037 caused by very high temperature

Abstract

Overheating exposures of turbine blades during service increased the risk of service safety in aircraft engines. Nevertheless, limited investigations about microstructural degradation induced by the overheating temperature and the effects on mechanical property of wrought superalloys were reported. In this paper, wrought superalloy GH4037 sectioned from an un-used 1st stage turbine blade of an aircraft engine was adopted to investigate the microstructural degradation caused by short-time thermal exposure at very high temperature (higher than normal service temperature) and the microstructural evolution during the mechanical tests after the short-time thermal exposures. The effects of microstructural degradation on mechanical properties including hardness, high temperature tensile properties and creep properties were also analyzed. The results indicate that gradual dissolutions of γ′ phase and grain boundary (GB) carbides were the typical manifestation of microstructural degradation in GH4037 alloy during the short-time thermal exposures at 1000–1140 °C for 3 min and 5 min. The dissolved γ′ phase and GB carbides re-precipitated in a very short time (less than 12 min) as ultra-fine particles and continuous/cellular GB carbides during 850 °C tensile tests and creep tests at 850 °C/196 MPa. Due to the re-precipitated ultra-fine γ′ phase, the tensile strength at 850 °C after exposing to 1140 °C for 3 min was analogous to those of specimens without the thermal exposure. But the re-precipitated continuous/cellular GB carbides would cause the significant decrease of tensile ductility at 850 °C and the creep elongation at 850 °C/196 MPa. On the other hand, the dissolution of γ′ phase and GB carbides during the short-time thermal exposures reduced the creep properties at 700°C/471 MPa significantly, as it took much longer time (more than 18.1 h) for the re-precipitation of γ′ phase when the overheated alloys were exposed at 700°C/471 MPa. The work provides the guidance for overheating-inspection and diagnosis of service safety for blades made of wrought superalloys.