Fracture modes and grain growth of tungsten during extreme cyclic heating

Abstract

To investigate the fracture mechanisms resulting from the thermomechanical loading in fusion reactors, cyclic heating experiments were carried out on tungsten discs (W). These experiments involved subjecting the samples to an arc-jet plasma gun, which can deliver a heat flux exceeding 20 MW m−2. The W discs had varying thicknesses and pulse durations. To gain a better understanding of the thermal environment, finite element (FE) simulations were performed to model heat transport through exposed specimens and the cooling assembly, and these were compared with experimental measurements. The temperature at the edge of the sample was found to reach a maximum of 1500 °C, and the temperature at the center of the upper surface of the disc fluctuated between a maximum of over 2800 °C and a minimum of 500 °C, depending on the thickness of the disc and the thermal contact between the sample and the cooling structures. Cracks were found to propagate through different modal regimes, starting with ductile growth, followed by a combination of ductile/intergranular growth, intergranular growth, and finally brittle transgranular fracture (cleavage fracture). Steep temperature gradients (∼1.6 × 106 °C/m) were found to induce grain growth, with a grain boundary mobility of 7.5 eV under moderate thermal stresses. Alternatively, crack formation and growth can occur due to residual stresses. Cumulative residual stress has been shown to correlate with the duration of exposure to elevated temperatures, leading to increased hardening, although with noticeable grain growth. Consequently, the strength of the sample is primarily driven by thermal stress rather than grain growth.