Fatigue behavior and damage mechanism of copper-niobium multi-filament nanocomposite material under cyclic three-point bending

The internal structure, interface width, and residual internal stress distribution of composite materials affect their fatigue properties, and thus their service life. Copper-niobium multi-filaments nanocomposite wire, produced through bundled drawing, possess a high-density, low-energy copper-niobium interface and a nanostructured filament. As wires for winding magnetic coils, they are subjected to lateral cyclic loads in a bent state during service. In this study, the two types of copper-niobium nanocomposite wires produced via jelly-rolling-bundled drawing and melting-bundled drawing were taken as the research objects. Cyclic bending deformation tests with different applied bending load ratios of 30 %, 50 %, and 70 % are carried out on them to study the fatigue properties and fracture behavior of the copper-niobium nanocomposite wires. The effect law of different load ratios on the fatigue cycles were analyzed, and the effect mechanisms of the internal microstructure, interface density and width, and residual stress distribution at the interface on crack propagation and fracture behavior during cyclic deformation process were revealed. The results indicated that, in copper-niobium heterostructured composite wires, the tensile stress zone above neutral axis exhibits a mix of ductile and brittle fracture morphologies, along with fatigue striations. In contrast, the tensile stress zone of copper-niobium alloy composite wire predominantly exhibits brittle fracture, with observable brittle fatigue striations, cleavage steps, and fan-like river patterns. The jelly-rolling composite structure with alternating copper/niobium layers reduce the residual stress at the interface, while large plastic deformations induce twinning within the copper filaments. These effectively transfers gradient stresses and delays crack growth, leading to superior cyclic bending fatigue performance. At a 30 % bending stress level, the fatigue life of the copper-niobium heterogeneous composite material reaches 1.7 × 10⁶ cycles.