Effects of solidification rate and heat treatment on microstructure characterization and hot tensile strength of single crystal CMSX-4 superalloy

Abstract

Single crystal CMSX-4 Ni-based superalloy, consisting of rhenium and hafnium elements, were directionally solidified at three different withdrawal rates of 2, 3, and 4 mm/min through a Bridgman furnace. After that a full solution heat treatment was followed by two step aging at 1140 °C/6 h and 870 °C/20 h respectively. Microstructural characteristics, e.g. the primary and secondary dendrite arm spacing, the size and volume fraction of γ ́ phase, and percentage of porosity were analyzed using the optical microscopy (OM) and scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS) detector. It was found that an increase in the withdrawal rate of the as-cast specimens led to a reduction in the values of primary and secondary dendrite arm spacing, the percentage of porosity and also size of γ ́. In addition, at higher withdrawal rates, the γ/γ ́ eutectic phase is distributed from a continuous and blocky shaped to fine scattered islands. Nevertheless, the minimum volume fraction of eutectic was obtained at the withdrawal rate of 3 mm/min. Heat-treatment of the specimens associated with formation of a noticeable cuboidal γ’-precipitates, which was about 70% at withdrawal rate of 3 mm/min. Furthermore, the aged specimens indicated the lower γ/γ ́ eutectic phase compared to as-cast specimens, while the percentage of porosity increased for aged specimens. Eventually, the highest ultimate tensile strength of the single crystal CMSX-4 at 870 °C was measured to be 1170 MPa which was obtained at a withdrawal rate of 3 mm/min, due to the high volume fraction of the γ ́ phase and the lower values of γ/γ ́ eutectic phase.