Effect of Load Direction on Crack Propagation in High-Temperature Fatigue of Directionally Solidified Superalloy
Directionally solidified (DS) superalloys, which have elongated large grains, are used for gas-turbine blades. Since the grain size is not small enough in comparison with the crack size observed often in a real component, the inhomogeneous microstructure due to the aligned grains may strongly affect the crack propagation property. Center-cracked-plate specimens with three different orientations, TP0 with the parallel DS axis to the load, TP90T with the perpendicular DS axis to the load and the crack propagation direction, and TP90L with the perpendicular DS axis to the load and parallel to the crack propagation direction, are subjected to high-temperature (1143K) fatigue. The specimens after the tests show the transgranular cracking perpendicular to the load axis in TP0, intergranular one parallel to the DS axis in TP90L, and intergranular / transgranular one parallel to the DS axis in TP90T. The crack propagation rate da/dN shows a good correlation with effective stress intensity factor range Keff in each cases. However, the magnitude of da/dN at a same Keff in TP90L is relatively higher than that in TP0, and that in TP90T remarkably fluctuates. A microscopic observation of TP90T reveals that the high da/dN is caused by the intergranular cracking, while the low da/dN is observed on the transgranular cracking. A procedure for the prediction of crack propagation is proposed on the basis of the two types of cracking; transgranular and intergranular cracks perpendicular to the load axis. The validity of the proposed procedure is discussed by the comparison of tested and predicted results of crack propagation in a fatigue condition.
Wei Yang, Mamtimin Geni, Tiejun Wang and Zhuo Zhuang
M. Yamamoto et al., "Effect of Load Direction on Crack Propagation in High-Temperature Fatigue of Directionally Solidified Superalloy", Advanced Materials Research, Vols. 33-37, pp. 205-216, 2008