References [1] S. Cruchley, et. al., Mater High Temp 2016, vol. 33, pp. 465-475. [2] S. Cruchley, et. al., Corros Sci 2015, vol. 100, pp. 242-252. [3] B. R. Barnard, et. al., Materials Science and Engineering: A 2010, vol. 527, pp. 3813-3821. [4] L. Zheng, et. al., Applied Surface Science 2010, vol. 256, pp. 7510- 7515. [5] J. H. Chen, et. al., Oxid. Met. 1997, vol. 47, pp. 381-410. [6] H. E. Evans, et. al., Oxid. Met. 1978, vol. 12, pp. 473-485. [7] S. Cruchley, et. al., Materials Science and Technology 2014, vol. 30, pp. 1884-1889. Experimental Method The alloy used in this project was RR1000 with composition (wt. %): Samples approximately 10 mm x 10 mm x 2 mm were extracted from both coarse-grained (CG) and fine-grained (FG) variants of the alloy. Oxidation exposures were performed at 650 °C for 4000 h in laboratory air on the following samples. The surfaces of samples were examined under SEM before being mounted and polished. The cross-sections of samples were then examined under SEM. Factors Affecting Oxide Formation on a Ni-Based Superalloy: The Effect of Surface Condition & Grain Size. Acknowledgements The authors thank the Centre for Electron Microscopy at the University of Birmingham for their technical expertise. This work was supported by the Rolls-Royce/EPSRC Strategic Partnership under EP/H022309/1. Their financial support, as well as the provision of materials by Roll- Royce, is most appreciated. T. D. Reynolds¹, M. P. Taylor¹, D. J. Child², P. M. Mignanelli2, M. C. Hardy2, H. E. Evans¹ ¹ School of Metallurgy and Materials, University of Birmingham, Birmingham, B15 2TT, UK. ² Rolls-Royce plc, PO Box 31, Derby, DE24 8BJ, UK. Introduction The oxidation resistance of polycrystalline nickel-based superalloys is afforded by the chromium content of the alloy resulting in the formation of an external, protective, thermally grown chromia (Cr2O3) scale. The oxidation kinetics of nickel-based chromia forming alloys have been well documented in the literature [1-7]. Differences of several orders of magnitude have been observed in oxidation rates between different chromia forming alloys. These differences have been attributed to differences in surface finish, the amount of cold work in the alloy, grain size, and oxidation environment. The coarse-grained variant of the RR1000 alloy has been extensively studied in a polished and shot-peened surface condition[1, 2, 7]. In the coarse-grained variant of the alloy shot-peening was reported to be detrimental to the oxidation properties[2]. This work extends the study of this alloy to include the fine-grained variant and at a lower temperature (650°C) where no recrystallisation was expected. Figure 2: Comparison of external oxide thickness and internal oxide penetration depth for FG samples with different surface finishes. Figure 3: Comparison of external oxide thickness and internal oxide penetration depth between FG & CG samples in the polished and shot- peened condition. Literature values are included for coarse-grained samples[2, 7]. Figure 4: SEM micrographs of CG samples. (a)(b) Polished sample. (a) planar view showing regions of delamination. (b) cross-sectional view highlighting layer of aluminium rich oxide. (c)(d) shot-peened sample. (c) planar view showing buckled regions. (d) cross-sectional view. (a) (b) Figure 1: SEM cross-sectional micrographs of FG samples. (a) polished sample. (b) shot-peened sample highlighting buckled region. (c) planar view of turned sample. (d) cross-sectional image of turned sample. Ni Co Cr Mo Ti Al Ta Hf Zr C B 52.3 18.5 15.0 5.0 3.6 3.0 2.0 0.5 0.06 0.027 0.015 Condition Fine-grained (3-5 µm) Coarse-grained (40-60 µm) Stress state Surface roughness, Ra (µm) Polished ✓ ✓ Unstressed 0.01 Shot- peened ✓ ✓ Compressive 1.42 Turned ✓ ✗ Tensile 2.33 Surface Finish Polished Shot-Peened Oxide Penetration Depth / Thickness ( m) -4 -2 0 2 4 Fine-Grained (External) Coarse-Grained (External) Fine-Grained (Intragranular) Coarse-Grained (Intragranular) Cru