Length scale effects on the macroscopic behaviour of single and polycrystalline FCC crystals

In this work, a recently proposed strain gradient and rate-dependent crystallographic formulation is used to investigate length scale effects on the macroscopic behaviour of two-phase FCC single crystal and polycrystalline alloys. The multi-scale constitutive approach incorporates strain gradient phenomena at both the microscopic (i.e. reinforcing phase) and mesoscopic (i.e. grain) scales and relies on strain gradient concepts to account for the additional strengthening mechanism caused by presence of geometrically necessary dislocations (GNDs). The total slip resistance of the homogenised single crystal at the mesoscale is assumed to be due to a mixed population of mobile and sessile forest obstacles arising from both statistically stored (SSDs) and GNDs, and their evolutionary behaviour is formulated in terms of material parameters which depend explicitly on the characteristics of the reinforcing phase at the microscale (e.g. precipitates). A parametric study is conducted on a 3D specimen of a polycrystalline superalloy to quantify the effects of precipitate and grain sizes on its deformation behaviour.