An insight into size effect on fracture behavior of Inconel 718 cross-scaled foils

Abstract

Product miniaturisation has increased the demand for cross-scaled components with various sized features covering the macroscale (mm-scale) and mesoscale (μm-scale). During the sheet metal forming process, the feature size of cross-scaled components, such as foil thickness, changes from macroscale to mesoscale. The fracture behavior of the cross-scaled components varies from that of the macroscale components due to the size effect, leading to a high scrap rate and unacceptable cost in manufacturing. To reveal size effect on fracture and predict the fracture occurring of the cross-scaled components is still a challenge urgently to be solved. Thus, taking Inconel 718 superalloy cross-scaled foils as the case material, firstly, the cross-scaled foils were prepared with the thicknesses from 0.05 to 1.5 mm, the ratio of thickness to grain size (t/d) from 1.07 to 24.26. Uniaxial tensile tests and in-situ tensile tests were performed to reveal the fracture mechanism of the superalloy cross-scaled foils. Then, several uncoupled fracture criteria were implemented into the crystal plasticity finite element modeling (CPFEM) with the cross-scaled geometric modeling and random texture to predict the fracture behavior of the cross-scaled components. The uncoupled fracture criteria include Freudenthal criterion, Cockcroft & Latham(C&L) criterion, Normalized C&L, Brozzo criterion, Ayada criterion, Rice & Tracy criterion, Tresca criterion, equivalent stress criterion, equivalent strain criterion and maximum principal strain criterion. The phenomenological crystal plasticity model and dislocation density-based crystal plasticity model were compared in terms of calculation accuracy and efficiency. Thirdly, the size effect related fracture mechanisms of the cross-scaled foils were revealed, and the relationship between the fracture strain, fracture stress and t/d were obtained. Generally, the fracture strain and stress drop with a decrease in t/d. Consequently, an abnormal phenomenon occurs where the plasticity and strength increase with the decrease of t/d in some areas. The random textures lead to fluctuations in the strength and plasticity of the cross-scaled foils. Finally, the equivalent stress fracture criterion (ESFC) was determined where the fracture threshold is less affected by random texture. The relationship between the fracture threshold of ESFC and t/d was found, and the CPFEM-ESFC model was established considering the random textures induced scattering of the fracture threshold due to random textures. The scattering distribution function of cross-scaled component fracture behavior under different t/d was obtained through CPFEM-ESFC model and hundreds of random textures designs. The study provides a statistical method to avoid the forming scattering and a theoretical basis for the process design of cross-scaled components.