The Effect of Withdrawal Rate on the Microstructure of Ni-based Superalloy GTD-111 through the Directional Solidification

Effects of withdrawal rate on the structure of directionally solidified GTD-111 Ni-based superalloy were investigated. To this end, DS specimens were first obtained by the Bridgman Furnace (equipped with a graphite cooling zone) at withdrawal rate of 1 and 10 mm/min. Then, structural investigation was carried out by optical microscopy and field emission scanning electron microscopy at longitudinal and transverse sections with respect to solidification direction at the bottom (location of starting solidification) and top (location of finishing solidification) zone of specimens. The results showed that the polycrystalline grain zone at bottom of the specimen enlarged with increasing the withdrawal rate. Furthermore, with increasing withdrawal rate from R=1mm/min to R=10mm/min, the primary and secondary arm spacing decreased from 445μm to 252μm and from 110μm to 60μm, respectively. The γʹ size also decreased from 468μm to 421μm at top of specimens and from 711μm to 604μm at bottom of specimens with increasing the withdrawal rate from R=1mm/min to R=10mm/min. In addition, due to the direction of heat transfer during directional solidification at a constant withdrawal rate, the bottom zone of specimen was exposed to heat output from top zone. This led to increase the γʹ size at bottom zone (711μm at R=1mm/min and 604μm at R=10mm/min) compared to the top zone (468μm at R=1mm/min and 421μm at R=10mm/min) of specimen. The best microstructure belonged to the top zone of DS specimen with withdrawal rate of 10mm/min which has a small gamma-prime average size, homogenous distribution, and regular morphology.