Intermediate Temperature Creep Deformation in Cmsx-3 Single Crystals

Creep deformation in < 001 > oriented single crystals of CMSX-3 has been studied at temperatures around 850°C at a stress level of 552 MPa. Detailed observations of the evolution of the dislocation structure during creep have been made. TEM stereo pairs show creep deformation in the matrix phase, leaving the 7’ precipitates undeformed. In previously undeformed crystals there is a brief incubation period during which dislocations spread from widely spaced sources, bowing out between the precipitates in the narrow matrix gaps. This Orowan bowing process is the major source of creep resistance. In the early stages of primary creep, over large regions dislocations all have the same Burgers vector in spite of the fact that there are eight equivalent slip systems. Later in the primary transient dislocations with other Burgers vectors interpenetrate from neighboring regions, gradually building up a three dimensional nodal network, which is characteristic of steady state creep. At 850°C static recovery processes are sluggish, indicating that dynamic recovery may be important at this temperature and at lower temperatures. Finite element analysis shows that the constraint of the non-deforming precipitates leads to the buildup of a negative pressure in the matrix channels normal to the applied stress. The negative pressure provides load support and reduces the effective stresses driving the steady state creep process in the matrix. The 7’ precipitates are increasingly stressed as the creep process progresses, and under the conditons studied here, they are occasionally sheared in the later stages of steady state creep. Superalloys 1988 Edited by S. Reichman, D.N. Duhl, G. Maurer, S. Antolovich and C. Lund The Metallurgical Society, 1988