Effect of wall thickness on the microstructure and properties of a fourth-generation single crystal superalloy

Abstract

This study investigated the effect of various thicknesses (0.3–1.5 mm) on the as-cast and heat-treated microstructures of the fourth-generation single crystal superalloy. The microstructures and fracture characteristics of samples with various thicknesses were analyzed after high-temperature stress rupture tests at 1100 ℃/137 MPa. The results showed that as the alloy thickness increased from 0.3 mm to 1.5 mm, the degree of segregation between dendritic core and inter-dendritic significantly increased. The increased segregation was attributed to the slower solidification rate of the 1.5 mm-thick sample compared to the 0.3 mm-thick sample. Lower solidification rates and higher degrees of dendritic segregation enhanced the area fraction of eutectics, the primary dendrite arm spacing (PDAS), the and area fraction of pores. After stress rupture tests, the rupture life of the alloy increased from 81 h to 333 h. It was observed that in the 0.3 mm-thick sample, microcracks initiated at the surface and extended into the matrix. On the contrary, as for the 1.5 mm-thick sample, microcracks initiated at the matrix and propagated towards the surface. To evaluate the influence of oxidation, interrupted stress rupture tests were conducted in air atmosphere to investigate the thickness debit effect. Surface oxidation led to the initiation of microcracks on the surface and change the morphology and area fraction of the internal γ′ phase, as well as the effective load-bearing area of the samples. TEM and EBSD were employed to characterize the deformation characteristics of samples with various thicknesses, and the influence of thickness on deformation mechanisms was discussed as well.