δ phase driven flow stress size effect of Inconel 625 ultra-thin plates: An experimental and modeling study

Exploring the impact of the precipitate phase on the size effect is of crucial importance for the production of high-performance ultra-thin sheets. In this paper, the effects of characteristic size (the ratio of sample thickness to grain size, t/d) and the precipitation of δ phase on the mechanical properties of Inconel 625 alloy ultra-thin plate were studied by tensile test at room temperature. The results indicate that at room temperature, for the specimens without δ phase, the flow stress decreases with the reduction of the t/d, and a “smaller is weaker” (SW) size effect occurs when t/d < 5.01. For the specimens containing δ phase, the flow stress also decreases with decreasing t/d. Although a SW size effect still occurs when t/d < 5.01, the strengthening effect of the δ phase at the mesoscale leads to a “smaller is stronger” (SS) size effect when t/d < 3.59. The analysis of GND density shows that the precipitation of δ phase hinders the slip of dislocation from the surface layer grains, weakens the surface weakening effect, and then increases the flow stress. Considering grain boundary strengthening, solid solution strengthening and precipitation strengthening, the flow stress size effect model is established by introducing J-C model, the average absolute relative error is only 2.15 %, which indicates that the calculated values are in good agreement with the experimental values. This study identifies the influence and mechanism of the δ phase on the size effect in Inconel 625 ultra-thin plates, establishes the corresponding constitutive model, provides new insights into controlling size effects in metallic ultra-thin plates, and lays the foundation for the performance optimization and stress prediction of ultra-thin plates.