Abstract
The nature and variety of the dislocations passing through the two-phase γγ′ microstructure of Ni-based superalloys is key to the properties of these materials. The chemistry, size and arrangement of the precipitates greatly affects the nature of these dislocations. We present High Angle Annular Dark Field (HAADF) TEM observations of the structure of dislocations entering, passing through the γ′ precipitates in the single-crystal superalloy CMSX-4®. The creep deformation of the sample was interrupted after 8 hours at 750 °C and 750 MPa, a critical stage just as secondary creep was being established, and shows a range of defects in both phases, not always those predicted by the Schmid factor for the deformation geometry. We show that dislocations lodged in the γγ′ interfaces have a significant effect on the structure of the interface and that they combine to produce stacking faults which cut through the γ′. The implications of these observations for secondary creep deformation are discussed.
Highlights
High-resolution techniques are enabling us to look in unprecedented detail at dislocation structures and interactions
In this paper we examine the microstructure of the alloy CMSX-4 R crept to 8% strain just at the onset of secondary creep
In a previous paper we established the structure of the superlattice partial dislocations, a/3 112 and the way in which the lattice dislocations are embedded in the γ γ interface [7]
Summary
High-resolution techniques are enabling us to look in unprecedented detail at dislocation structures and interactions. Single crystal superalloys are ideal subjects for this as the geometry of deformation can be closely controlled. Their creep strength derives from the regular arrays of ordered L12 precipitates that resist the ingress of regular FCC lattice dislocations [1, 2]. At high temperatures the role of the precipitates is largely to exclude dislocations, forcing them to move in the remaining 30% of γ phase by a combination of glide and climb [3]. A large part of the strain derives from the cutting of the precipitates by suitable partial dislocations [4, 5]. In this paper we look at how dislocations combine and contribute to the creep and the implications this has for creep under these conditions
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