Microstructure, Microtexture and Grain Boundary Evolution in Heat-Treated Cast Haynes 25 Alloy

Heat treatment procedures such as solution annealing at 1200 °C for 3 h, single-step age-hardening at 870 °C for 500 h and double-step age-hardening at 870 °C for 500 h followed by 760 °C for 500 h are investigated in this study to determine how the microstructure and mechanical properties of as-cast Haynes 25 superalloy change using a different heat treatment process. This work describes the mechanisms of precipitation strengthening that affect the behaviour of the alloy using a thorough approach that includes optical microscopy, field emission scanning electron microscopy-energy dispersive X-ray spectrometer (FESEM-EDS), electron back scatter diffraction (EBSD) analysis and mechanical testing. Age-hardened samples show the presence of Co₃W intermetallic phases, which are confirmed by X-ray diffraction. It is observed that there are notable variations in the shape, size and distribution of Laves phase precipitates (Co₃W), especially in samples that have been age-hardened in one or two steps. Aged samples show signs of microtexture degradation, such as {111} <110> fiber evolution, which suggests that annealing twins predominate over precipitates as a result of prolonged heat exposure and dislocation dipole destruction during recovery. The double-step age-hardened sample (760 °C, held for 500 h) exhibits a 30% increase in ultimate tensile strength (UTS) to 1012 MPa, 20% increase in yield strength (YS) to 586 MPa and a 16% increase in microhardness to about 392 HV_0.1 compared to the cast sample. The double-step age-hardening process enhances the strength of the Haynes 25 alloy through grain refining and effective dislocation motion impediment via interaction with Co₃W precipitates. In both the cast and solution-annealed samples, the tensile fractographic study shows transgranular fracture, whereas in the age-hardened samples, it reveals intergranular fracture.