Recrystallization mechanism for two nickel based superalloys

The demand for enhanced structural performances materials is growing every year and technological advancements in the sector of aerospace or nuclear are in constant need for materials with good mechanical properties. The alloys commonly used for these features are Nickel-based superalloys as they exhibit high strength and good resistance to corrosion and oxidation. To improve their mechanical behavior, recent studies have focused on grain refinement methods. Among these methods to obtain the finest grain size distribution, one is particularly advantageous for it low cost and feasibility: severe plastic deformation. In this study, the deformation mechanism of two low stacking fault energy nickel-based alloys are investigated. The first alloy, Monel400, it is a single FCC phase material. The second one is Inconel 625 which has a two-phase microstructure. During hot deformation, the gamma' precipitate may be present in the gamma phase and above a certain solvus temperature, the gamma phase exists in the material. The restoration mechanism for FCC crystals is well known, and particular attention was given to the recrystallization response and flow behavior of Inconel 625 for sub-solvus temperatures. In the introduction a brief review of the current state of literature on the deformation response of Nickel-based superalloys is provided. Samples were compressed under various temperatures and strain rate conditions using a Gleeble simulator and flow stress curves were extracted. To characterize both qualitatively and quantitatively the deformation, samples were then analyzed using standard microscopy, scanning electron microscopy and electron backscatter diffraction analysis. The resulting images and maps combined with flow stress curves have lead to the formulation of constitutive models of the recrystallization process using three parameters, the stress, grain size and recrystallized volume fraction. The data shows that deformation is first accommodated through dynamic recovery with the formation of sub-grains structures. Then, after the experimental strain reaches a critical value, recrystallized grains form within the microstructure. EBSD analysis show a trend for new recrystallized grain to grow under certain conditions. Results show a trend of increasing the grain size with increasing the strain and decreasing the Zener-Hollomon parameter and an increasing recrystallized volume fraction with increasing the strain and Zener-Hollomon parameter.