THE RECRYSTALLIZATION OF Ni BASE DIRECTIONALLY SOLIDIFIED SUPERALLOY DZ417G

DZ417G is a directionally solidified (DS) superalloy developed for low-pressure blade applications in gas turbine engines The crystallization microstructures of DZ417G samples caused by sand-blasting and machining were investigated. The tensile properties of the alloy after crystallization were tested at room temperature and 900 V and its stress-rupture performance was examined under conditions of 980 degrees C/216 MPa and 760 degrees C/725 MPa The results show that after solution treatment equiaxed recrystallization grains form on the surface of specimens machined from directionally solidified alloy bars After aging treatment, cellular recrystallization takes place on the surface of specimens pretreated by sand blasting Both the yield strength and tensile strength at room temperature decrease after recrystallization, while those at 900 V are slightly affected by recrystallization. The recrystallization depth increases after stress-rupture tests, which may be attributed to migration of recrystallization boundaries driven by high temperature and stress. For samples without recrystallization microstructure, the fracture mode is transgranular, which is controlled by the propagation rate of the cracks initiated both on sum face and at inner microstructure discontinuities While for samples with recrystallization microstructure: the cracks prefer to be initiated on transverse recrystallization grain boundaries and propagate along the recrystallization boundaries into the matrix, which may accelerate the propagation rate TRF (transverse recrystallization area fraction) is a factor to evaluate the effect of recrystallization on the stress-ruptured performance The stress-ruptured performance is decreased with the increase of TRF between 0 and 0.5. For samples with a TRF of 0 5, second cellular recrystallization forms in the first equiaxed recrystallization grain The cracks mitated at interfaces of cellular microstructure have a high density, which impair the stress-ruptured performance of DZ417G alloy.