Investigations on the Origin and Effect of Anomalous Rafting

Abstract

Anomalous rafting is a morphological transformation of the phases y and y’ occurring in monocrystalline nickel-base superalloy turbine blades which is observed under the surface of the turbine blades after service. The origin of anomalous rafting is discussed, based on microstructural investigations of turbine blades before and after service and of laboratory samples after shot peening and ageing. Microscopical investigations are completed with microhardness and X-ray diffraction measurements. Residual macrostresses and long-range internal microstresses are discussed as driving forces of anomalous rafting, induced by the plastic deformation of subsurface layers of the material, e.g. during peening. Finally, the effect of anomalous rafting is discussed on the basis of fatigue and creep experiments. Introduction and Obiectives Rafting of monocrystalline nickel-base superalloys has been investigated experimentally and modeled in detail in recent years [ 1,2]. The change of the originally cuboidal shape of they’ precipitates towards a y/y’ plate structure occurs at temperatures above about 1150 K during deformation or in samples predeformed at a lower temperature during subsequent (stress-free) ageing treatments [35]. In the regime of plastic deformation above plastic strains of ant = 0.001 to 0.002, the orientation of the plate structure depends on the lattice mismatch 6 (6 is the relative difference of the lattice parameters of the two phases y’ and y) and on the direction of the externally applied stress 6. Detailed experimental investigations have shown that the modification of the internal stress state during deformation is responsible for the type of rafting observed, i.e. whether the plate structure has an orientation parallel or perpendicular to the stress axis 13-61. Thus, it was shown that the type of rafting is governed strongly by the sign of the plastic deformation [3-61. In the hot regions of the airfoils of turbine blades, a y/y’ raft structure develops which has an orientation perpendicular to the stress axis [7-91, because the lattice misfit of technically applied turbine blade superalloys is negative at the high service temperatures, and the external stress axis acting on a blade during service is tensile due to the centrifugal stresses. In regions near the surface, however, a different microstructure, called anomalous raft structure, with an orientation of the plates parallel to the surface has been found repeatedly [7,8,10,11 J which was in some cases attributed to the influence of a surface coating [ IO,1 11. In the present work, we consider different types of anomalous rafting, occurring in . real turbine blades during service, . initially unused turbine blades during ageing and . shot-peened laboratory samples during ageing. The origin of anomalous rafting occurring in different cases is discussed on the basis of microhardness and microfocus X-ray diffraction investigations. Furthermore, its effect on the high-temperature strength properties of nickel-base superalloys is investigated in fatigue and creep experiments which are part of a more extended study of the influence of different y/y’ raft structures on mechanical high-temperature properties [12-l 53. SupemIloys *COO Edited byT.M. Pollock, R.D. Kissinger, R.R. Bowman, K.A. Green, M. McLean, S. Olson, and J.J. Schirra TMS (The Minerals, Metals&Materials Society), 2000