Experimental characterization and crystal plasticity modeling of mechanical properties and microstructure evolution of additively manufactured Inconel 718 superalloy

In this thesis, the mechanical behavior of the additively manufactured (AM) IN718 nickel-based superalloy and their correlations with the evolution of microstructure are studied comprehensively. The effects of manufacturing parameters, build orientations, and post processing procedures, i.e. standard heat treatment and hot isostatic pressing (HIP), on various mechanical properties including monotonic compression and tension strength, low cyclic fatigue performance, high cyclic fatigue behaviour, and fatigue crack growth behavior are investigated. Due to the high temperature applications of the IN718 alloy, elevated temperature properties are examined as well. Electron Backscattered Diffraction (EBSD) technique is employed to measure the initial and deformed textures. In addition, an elasto-plastic self-consistent polycrystal plasticity model is developed to interpret the deformation behavior of the alloy in room temperature and high temperatures. The model incorporates the contributions of solid solution, precipitates shearing, and grain size and shape effects into the initial slip resistance. For activating the slip systems, the non-Schmid effects and backstress are implemented in the model. The crystal plasticity model is capable of simulating the monotonic and large-strain load reversal cycles of the material with pole figure difference (PFD) values no more than 0.2

Thermo-hydrogen refinement of microstructure to improve mechanical properties of Ti–6Al–4V fabricated via laser powder bed fusion

This paper describes the main results from an investigation into the consequences of thermo-hydrogen refinement of microstructure (THRM) after laser powder bed fusion (LPBF) of Ti–6Al–4V on the evolution of microstructure and mechanical properties using a set of experimental techniques. Porosity fraction, grain structure, phases, and crystallographic texture per phase are characterized using micro X-ray computed tomography, microscopy, and neutron diffraction. A hierarchical structure of acicular α-phase morphology formed inside the prior β grains by fast cooling during LPBF transforms into fine-grained globular microstructure by THRM, which facilitates homogeneous nucleation and growth of the low temperature phase with some retained β phase. Moreover, hydrogenated material during THRM has low activation energy for diffusion, which in conjunction with the surface energy of pores causes densification of the material, thereby closing porosity formed during LPBF. Such significant microstructural changes induced by the THRM treatment cause brittle material created by LPBF to become ductile. Significantly, the strength and ductility produced by THRM exceed the minimums set forth by the ASTM B348 standard for Ti-6Al-4V. Moreover, the treatment improves fatigue strength of the material. In particular, it improves the endurance limit and reduces the scatter in the measured fatigue strength data. Performance characteristics of the material can be further optimized for specific application requirements by tailoring microstructures using the LPBF and THRM processes