Abstract
Recrystallisation in single crystal Ni-based superalloys during solution heat treatment results in a significant cost to the investment casting industry. In this paper two sources of surface nucleation have been identified in the alloy CMSX-4®. Firstly, Electron Backscattered Diffraction (EBSD) has revealed micro-grains of γ′, between 2 and 30 μm diameter in the layer of surface eutectic found in the upper part of the casting. These have high angle boundaries with respect to the bulk single crystal and a fraction coarsen during solution heat treatment. Secondly, in the lower regions where surface eutectic does not form, locally deformed regions, 5–20 μm deep, form where the metal adheres to the mould. The local strain causes misorientations up to ≈20° with respect the bulk single crystal, and after heat treatment these regions develop into small grains of similar low-angle misorientations. However, they also form twins to produce further grains which have mobile high-angle boundaries with respect to the bulk single crystal. Experiments have shown that micro-grains at the surface grow to cause full recrystallisation where there is sufficient strain in the bulk material, and by removing these surface defects, recrystallisation can be completely mitigated. Etching of the cast surface is demonstrated to be an effective method of achieving this.
Highlights
The occurrence of recrystallisation in single crystal superalloy components during the solutioning heat treatment has long posed a significant mystery: how do you nucleate mobile high angle boundaries in a single-crystal material undergoing very moderate strains? In the past investigations have focussed on the strain necessary to drive the migration of boundaries [1e3], but avoid this central issue
Work by Hill [26] has shown that the susceptibility to recrystallisation depends on the alloy composition; the critical strain for recrystallisation was nearly halved for some recrystallisation-prone alloys, which become uneconomic despite other good properties
Transmission Electron Microscopy on the sample prepared by focussed-ion beam (FIB) was performed on a JEOL 200CX microscope operating at 200 kV
Summary
The occurrence of recrystallisation in single crystal superalloy components during the solutioning heat treatment has long posed a significant mystery: how do you nucleate mobile high angle boundaries in a single-crystal material undergoing very moderate strains? In the past investigations have focussed on the strain necessary to drive the migration of boundaries [1e3], but avoid this central issue. The occurrence of recrystallisation in single crystal superalloy components during the solutioning heat treatment has long posed a significant mystery: how do you nucleate mobile high angle boundaries in a single-crystal material undergoing very moderate strains? Recrystallisation has been observed during thermo-mechanical fatigue, nucleating at the intersection of deformation twin bands [15,16]. None of these possibilities arises in cast single crystal superalloys where potential strains are low and carbides are absent by design. This work, for the first time, provides evidence of viable nuclei in the surface layers of single crystal castings and demonstrates that under suitable conditions of very moderate strain these can develop into sizable grains equipped with mobile high angle boundaries
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