SHORT-FATIGUE-CRACK GROWTH IN A NICKEL-BASE SUPERALLOY AT ROOM AND ELEVATED-TEMPERATURE

An optical system, developed to monitor short-crack growth at elevated temperature, was used to study the fatigue behaviour of Waspaloy. Tests conducted at 19-degrees-C and 500-degrees-C revealed that the dominant mechanism of crack formation was slip band cracking. Crack formation was also associated with coarse carbide particles within the matrix. The dominant failure mechanism at 19-degrees-C and 500-degrees-C was one of mixed mode-I and -II fracture, with the mode-II shear displacements giving rise to non-closure induced by surface roughness. Oxide- and plasticity-induced non-closure processes made only a minor contribution to the overall growth process. Short-fatigue-crack growth measured at R = 0.1 was faster at 500-degrees-C than at 19-degrees-C at an equivalent value of DELTA-K1. This was attributed to a change in slip character from highly planar to one involving increasing amounts of cross slip at 19-degrees-C and 500-degrees-C, respectively.