High strain rate deformation of Alloy 718, Influence of Microstructure on Mechanical Properties

Doctoral thesis, 2016

Alloy 718 is a high temperature alloy, used where high temperature strength or oxidation resistance is required. As the alloy is precipitate heat treatable, it is possible to have a large variety of deformation and mechanical properties depending on microstructure. The influences the microstructures have on material properties affect the production processes and usage, and are therefore important to characterise. Mechanical properties of Alloy 718 with different microstructures have been studied at high strain rates ranging from 0.001 to 3000 s-1 and higher for shear localised samples, and at room temperature up to 800 °C. To describe the deformation occurring, the Johnson-Cook plasticity material model has been adopted to the test results. The different microstructures used in this study are relevant for industry and used at various stages in production and service. To study shear localisation that can occur at high strain rate, top-hat samples have been dynamically tested in compression. The formed shear localised microstructure is compared to shear localisation occurring in metal cutting chips, where it can be seen that the material adjacent to the shear localised band shows a simple shear localised deformation in both cases. The structures within the shear band are also similar in both cases and show recrystallised grains of down to 20 nm in the centre of the band and highly deformed subgrains at the edge of the shear band. The deformed microstructures are studied and the deformation patterns of the different microstructures show some differences; for small grain size, the deformation appears to be on primary and secondary glide systems and changes direction at grain boundaries, and for large grain size, the deformation seems to occur primarily on primary glide systems and the glide systems do not seem to change direction so much at grain boundaries. The adapted Johnson-Cook material model shows a good agreement with the experimental data. For aged condition, the hardening precipitates seem to give most of the strength, as the aged condition shows superior strength up to 600 °C, and at 800 °C, the different microstructures show similar strength and the hardening effects of the precipitates have vanished. Numerical simulation of shear localising samples show a fairly good agreement with observed deformation, indicating a good accuracy of the material model.