Controlling the final grain size in a uniform and controlled manner in powder metallurgy nickel-based superalloys is important since many mechanical properties are closely related to it. However, it has been widely documented that powder metallurgy superalloys are prone to suffer from growth of abnormally large grains (ALGs) during supersolvus heat treatment, which is harmful to in-service mechanical performance. The underlying mechanisms behind the formation of ALGs are not yet fully understood. In this research, ALGs were intentionally created using spherical indentation applied to a polycrystalline nickel-based superalloy at room temperature, establishing a deformation gradient underneath the indentation impression, which was quantitatively determined using finite element modelling, electron backscatter diffraction (EBSD) and synchrotron diffraction. Subsequent supersolvus heat treatment leads to the formation of ALGs in a narrow strain range, which also coincides with the contour of residual plastic strain in a range of about 2% to 10%. The formation mechanisms can be attributed to: (1) nucleation sites available for recrystallization are limited, (2) gradient distribution of stored energy across grain boundary. The proposed mechanisms were validated by the phase-field simulation. This research provides a deeper insight in understanding the formation of ALGs in polycrystalline nickel-based superalloy components during heat treatment, when subsurface plastic deformation caused by (mis)handling before super-solvus heat treatment occurs. The practical relevance of looking at small strains at room temperature this research is to understand what happens when turbine disks undergo small dents and scratches during (mis) handling before heat treatment.
Near-alpha titanium alloys are used for moderate-temperature applications in the early stages of the compressor in gas turbine engines. The quasi-static and fatigue properties of these alloys depend heavily on microstructure due to the absence of hard second phases and inclusions which can nucleate voids or cracks. Moreover, these alloys are known to exhibit a significant reduction in fatigue life when subjected to high mean stress or upon the application of dwell-fatigue cycles. Previous analysis has elucidated the microstructural features that drive these properties; the most important features are the volume fraction, size, and shape of clusters of similarly oriented alpha particles or microtextured regions (MTRs). To date, there have been few efforts to elucidate in a quantitative fashion the evolution of MTRs during thermomechanical processing (TMP). To meet this need, we have performed hot-compression tests on Ti-6Al-2Sn-4Zr-2Mo-0.1Si billet material with high-aspect-ratio MTRs at 0°, 45°, and 90°to the direction of primary metal flow during manufacture (i.e., the billet axis), thoroughly characterized the initial and final microstructures, and quantified field variables via finite-element method (FEM) process simulations for each experiment. These data can be used for a variety of purposes including the development, verification, and validation of models for microstructure/texture/microtexture evolution and defect formation.
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