Tensile deformation characteristic and SASH modeling of Superni 625 alloy: Synergistic effects of grain size, ageing time, and deformation micromechanisms

The present study aims to understand effects of large grain size (av. size >1 mm), secondary phase transformation during ageing (at 700 ^oC up to 1500 h) and deformation micromechanisms of homogenized cast alloy Superni 625 samples subjected to tensile loading. The precipitation followed by growth of γʺ (Ni₃Nb) enhanced its yield strength by more than two times (from 290 to 610 MPa) after 650 h of ageing accompanied by a drastic reduction in ductility (from 82% to 20%). Further ageing deteriorated strengthening contribution of γʺ phase due to a non-conventional slip-to-twin transition in shearing of γʺ particles. Simultaneously, development of continuous secondary-carbide network along grain boundaries provided an easy crack propagation path that resulted in lowering of ductility and premature failure of the aged samples. In contrast to the usual strain-hardening trend of most polycrystalline materials, the homogenized alloy Superni 625 exhibited a regain in strain hardening rate (SHR). This is accomplished by sluggish activation of single→double→multiple slip during plastic deformation caused by its low stacking fault energy (SFE) in conjunction with its large-grained structure (>1 mm). Furthermore, the aged samples also demonstrated a similar SHR behavior as coherent/semi-coherent ordered intermetallic phase (γʺ) in the matrix further promoted the planar dislocation movement through shearing of γʺ precipitates, which resulted in consequent localization of plastic deformation. A physics-based constitutive phenomenological model, known as ‘slip activity-based strain hardening’ (SASH) model found to track flow stress curves effectively and regain in the SHR of the large-grained homogenized and aged samples. Hence, it can be concluded that the SASH model could convincingly predict the shift from single to multiple slips during plastic deformation, which is accomplished through analysis of deformation micromechanisms.