High strain rate tensile testing of a nickel superalloy

Abstract

In this thesis a novel nickel-based superalloy was studied to determine whether it is suitable to be used as a turbine blade material of a jet engine based on its tensile properties both at room temperatures and at elevated temperatures. Turbine blades encounter temperatures as high as 700 °C. The material is tested at high strain rates to examine the dynamic behaviour of the studied nickel-based superalloy. The theory part includes Chapters 2-4. In Chapter 2, the effect of strain rate and temperature on material properties is discussed, followed by the basic working principles of a turbine engine and the materials used in turbine engines in Chapter 3. Chapter 4 discusses different dynamic testing methods, focusing on Split Hopkinson bar, since Split Hopkinson tensile bar was used in the experiments of the thesis. Chapter 5 presents the experimental setup and the used material. The experimental setup was a Split Hopkinson tensile bar and the tensile pulse was created with a striker bar at a strain rate range of 320-450 1/s. At the elevated temperatures, a furnace was used to heat the sample and a water-cooling system was placed on both sides of the furnace to keep the temperature of the bars at room temperature. The testing temperatures were room temperature (20 °C), 650 °C and 760 °C. The samples were 20 mm long and had a diameter of 4 mm. Altogether 18 samples were tested, 6 at each temperature. The samples were mounted to place with a screw at both ends of the samples. The chemical composition nor the heat treatment history of the material is not known, since it is a proprietary industrial material. It is known, however, that the material is a nickel-based superalloy intendent to be used in turbine blades. The data of the experiments was processed with Excel and the steps are also explained in Chapter 5. The results were presented and discussed in Chapter 6. The ultimate tensile strengths were 1460 ± 11 MPa at room temperature, 1200 ± 10 MPa at 650 °C, and 1180 ± 12 MPa at 760 °C. The ultimate tensile strengths were determined from the stress strain curves. The yield strengths were impossible to determine from the stress strain curves since the screw mounting of the sam-ples caused oscillations in the beginning of the stress strain curves. For this reason, a Johnson-Cook model fitting was carried out with Matlab to estimate the yield strengths. Also, Johnson-Cook model parameters make it easier to compare the material to other nickel-based superalloys used in turbine engines. The yield strengths were 900 ± 1 MPa at room temperature, 830 ± 1 MPa at 650 °C and 780 ± 2 MPa at 760 °C. The yield strength and especially the ultimate tensile strength of the studied nickel-based superalloy were high at room temperature when compared to other nickel-based superalloys. However, both strengths decreased with increasing tempera-ture which is not typical for nickel-based superalloys at a temperature range of 20-800 °C. The Johnson-Cook model fitting was representative of the experimental results. The parame-ters were A = 900 MPa, B = 3377 MPa, n = 0.75, C = 0.01, and m = 3.5. The strain hardening related parameters B and n were high, when compared to parameters of other nickel-based sup-eralloys found in literature. The strain rate related parameter C was in the same range as the other nickel-based superalloys. Temperature softening related parameter m was also high when compared to other nickel superalloys.