Deep cold rolling (DCR) and mechanical shot peening (MSP) are two surface hardening techniques of particular interest for improving the fatigue performance of aerospace engine turbine blades. These techniques both induce extensive plastic deformation in the surface region of the sample, generating a layer of cold worked material, which provides protection against fatigue crack propagation from surface flaws. Using local misorientation analysis of electron backscattered diffraction (EBSD) data we have been able to quantify the microstructural consequences of these two techniques on samples of a single crystal nickel-based superalloy used to manufacture turbine blades.

DCR produces a surface cold worked layer typically twice as deep as that produced by MSP, with significantly lower average cold work levels throughout the affected region. In DCR, cold work is seen to be concentrated in narrow near surface bands, while in MSP it is relatively evenly distributed, with much more pronounced slip bands. These observations will help to inform the analysis of future fatigue testing on samples treated by DCR and MSP to determine whether the intense shallow cold work produced by MSP is more or less beneficial to fatigue life than the deeper but more moderate cold work induced by DCR.

Figure: Local misorientation maps of samples treated by DCR (left) and MSP (right) showing the characteristic distributions of cold work.