Evolution of microstructure and 800 °C/294 MPa stress rupture property of cast Ni-based superalloys during long-term thermal exposure: Role and behavior of primary MC carbide

The role and behavior of primary MC carbide and their effects on the microstructure and stress rupture property of the cast Ni-based superalloys during long-term thermal exposure at elevated temperature are systemically investigated in this work. It is found that the chemical compositions of primary MC are proportional to the Nb/Ti and (Ti + Nb)/(Mo + W) ratios of alloys. During long-term thermal exposure, the thermal stability of primary MC can be quantitatively evaluated by its decomposition degree defined as Dt=(V0-Vt)/V0 × 100%, which is closely associated with the Nb/Ti and (Mo + W)/Cr ratios of alloys. The decomposition process, degree, form and mechanism of primary MC are revealed in detail. The STEM observations with the EDS analyses show that primary MC decomposition is an inter-diffusion process of elements between the primary MC carbide and γ matrix, which can increase the volume fraction of γ' phase, restrain the precipitation of topologically close-packed (TCP) phases and promote the coarsening of grain boundaries (GBs). The combination of these microstructural evolutions, together with the appropriate Al + Ti + Nb and W + Mo contents in the matrix, eventually results in a remarkable improvement in the stress rupture property of long-term thermally exposed cast Ni-based superalloys.