Size effects on the plastic behavior of polycrystalline materials: Grain size, precipitation state and free-surface effects

Surface effects were investigated using ultrathin specimens with thicknesses in the order of the grain size of the material. The candidate material was a polycrystalline Ni-based superalloy (Alloy 718) purposely heat treated to document both the effects of the grain size and the metallurgical state, $i.e.$, solid solution and precipitation hardened state, on the polycrystalline-to-multicrystalline behavior. Ultrathin tensile specimens were prepared with a dedicated technique to obtain specimens with thicknesses ranging between 20 and 550 $\mu \text{m}$, then tensile tested at room temperature. The polycrystalline-to-multicrystalline transition (PMT) was found to depend on the material grain size relative to the specimen thickness and to impair severely the tensile strength of the material. The yield strength, ultimate tensile strength (maximal stress on the stress–strain curve) and strain-to-failure severely dropped for specimens thinner than approximately two times the grain size of the material regardless of the metallurgical state. Such a decrease in tensile properties is mainly attributed to free-surface effects acting as an escape sink of dislocations, thus leading to a significant decrease of the primary dislocations density within the surface grains in comparison with the core grains. Interestingly, difference in work-hardening behavior with size reduction was found between both precipitation states, the solid solution state being more sensitive with the size reduction. The decrease in tensile properties was not found as expected from the commonly reported “thickness/grain size ($t/D$)” ratio. Therefore, a numerical approach using a modified Berveiller–Zaoui self-consistent model based on a continuum crystal plasticity approach was conducted in the present paper to distinguish microstructural features acting as strengthening (dislocation accumulation) and softening (dislocation escape at the free-surface) features. 3D numerical materials were produced using Voronoi tessellation methods to represent the fraction of “core grains” versus “surface grains”. These fractions were then used as microstructural parameters for the identification of a crystal plasticity model using mean-field homogenization with different populations of grains, $i.e.$, core versus surface features. The present work aimed at distinguishing the mechanical behavior of surface grains from core grains in Alloy 718 Ni-based superalloys using various thicknesses of specimens and different microstructure and metallurgical state variants.