Zhang, Qinqin (2011)
Ph.D. thesis, University of Birmingham.
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The response of powder of the Nickel-based superalloy CM247LC to a range of Hot Isostatic Pressing (HIPping) conditions and post-HIP heat treatments has been investigated as a preliminary step in the assessment of net shape HIPping as a possible process-route for component production. A HIPping pressure of 150MPa was used at temperatures of 1100°C, 1200°C, 1260°C and 1320°C. 1260°C as-HIPped specimens had the best tensile and HCF fatigue properties at both room temperature and 750 °C; the influence of post-HIP solution treatment conditions and of subsequent ageing on the properties of samples which had been HIPped at 1260 °C was thus investigated. A solution treatment at 1100 °C for 1h followed by forced air cooling and ageing at 870 °C for 16h resulted in the optimum tensile, high cycle fatigue at both room temperature and 750 °C. The optimized microstructure consists of a ‘necklace’ structure of coarse γ′ particles along the grain boundaries; cuboidal γ′ distributed inside grains with hyper-fine γ′ precipitated in γ channels; and fine carbides homogeneously disperse in the matrix. The properties of samples with this microstructure were comparable with those of cast, directionally solidified CM247LC. Solution treatment at 1260 °C gives better creep resistance at 760 °C/ 350MPa. It was also concluded that with gas-atomised powder, solution treatment must be carried out below the HIP temperature to minimise porosity. Analytical scanning electron microscopy showed that Hf-rich inclusions, some of which had alumina cores, initiated failure in almost all tensile samples and in all fatigue samples. It was shown that this type of inclusion can be avoided by removal of Hf from the alloy; but the removal of Hf – which is added to the alloy to improve castings – degrade the creep properties over the temperature range of 700 °C to 950 °C and the stress range of 150 MPa to 550 MPa. Further work is required to assess the influence on the tensile and fatigue properties.
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