Abstract
The effect of size and volume fraction of cuboidal γ' (Ni₃(Al, Ti)) precipitates on Vickers hardness and tensile properties at 950 °C of a single crystal nickel-based superalloy CMSX-4 was studied. Samples with a mean size and volume fraction of the cuboidal γ' precipitates ranging from 147 to 487 nm and from 62 to 70 vol.%, respectively, were prepared by heat treatment consisting of solution annealing, quenching and ageing at a temperature of 950 °C for various time ranging from 100 to 2000 h. Both the Vickers hardness HV and 0.2% offset yield strength σ_0.2 linearly decrease with decreasing normalized volume fraction V_p and increasing normalized mean size a of the γ' precipitates according to relationships HV ∝ √V_p/a and σ_0.2 ∝ √V_p/a, respectively. A linear relationship between the σ_0.2and HV is determined which allows high-temperature yield strength to be predicted from simple Vickers hardness measurements at room temperature. Critical resolved shear stress (CRSS) τ_CRS is calculated to increase from 116 to 234 MPa by decreasing a mean width of the γ (Ni-based solid solution) channels from 71 to 32 nm. The decrease of 0.2% yield strength with decreasing normalized volume fraction and increasing normalized size of the γ' precipitates is controlled by a decrease of order stress resulting from shearing of the γ' precipitates at high tensile stresses. The theoretical calculations validated by experimental results are used to predict ageing time and mean size of the γ' precipitates to achieve a peak 0.2% tensile yield strength of the studied alloy at 950 °C.

Key words
nickel alloys, single crystal superalloy, ageing, microstructure, mechanical properties

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