Formation of property gradient in coarse-grained niobium using a wedge tool: Experiment and analysis

We introduce a novel severe plastic deformation process for coarse-grained niobium, which employs a tool with an inclined (wedge) surface for deforming the material by a reverse shear scheme. The process increases the intensity of shear deformations and the depth of plastic deformation in the body of the workpiece when a wedge tool acts on its surface. The essence of the process is in the repeated displacement of the workpiece material in opposite directions during the asymmetrical introduction of a wedge tool until the required degree of deformation is accumulated in the tool-affected volume. This deformation scheme applies a 15° angle wedge tool to a 21-mm high workpiece. After nine cycles of plastic deformation, a gradient of the accumulated degree of deformation in the range of true strain e = 0.3–4.5 was created. At maximum deformation, the microhardness of the workpieces increased by 1.86 times and the tensile strength by 1.6 times. Fractograms show a significant influence of the accumulated degree of deformation on the nature of the fracture. The finite element method simulation of the deformation process showed that creating a uniformly strengthened layer requires at least five deforming operations. For example, the proposed reverse shear process with a wedge tool can be applied to improve the structure of the surface layers of niobium ingots for subsequent forming. Due to the creation of a significant gradient of properties, the reverse shear process can be used as an express method for determining the mechanical characteristics of different materials in a wide range of accumulated degree of deformation.