Low-temperature hot isostatic pressing of porous tungsten: Grain growth behavior and diffusion-controlled densification under high pressure

Abstract

Porous tungsten is widely employed as a thermionic electron-emitting cathode material in electron guns, where precise control of porosity and microstructural stability is critically required. Conventional powder metallurgy based sintering of tungsten typically requires temperatures above 1800 °C, leading to high energy consumption and severe grain coarsening that degrades the stability of the porous structure. In this study, a low-temperature hot isostatic pressing (HIP) process combined with a canning technique was applied to fabricate porous tungsten compacts under an isostatic pressure of 200 MPa within a temperature range of 900–1200 °C. At 900 °C, sintering was initiated; however, densification and mechanical strength remained limited, indicating potential constraints in long-term reliability. In contrast, specimens processed at 1000–1200 °C exhibited mechanically stable porous structures while maintaining the intended porous morphology. Field-emission scanning electron microscopy and electron backscatter diffraction analyses revealed that the average grain size remained below 3.7 µm up to 1200 °C, with no evidence of abnormal grain growth or pronounced crystallographic texture. Grain growth behavior was analyzed using a kinetic model, which showed the highest linearity for a grain growth exponent of n = 4, indicating diffusion-controlled grain growth under conditions where grain boundary migration is suppressed by residual pore pinning and high isostatic pressure. Arrhenius analysis yielded an apparent activation energy for grain growth of 153.1 ± 48 kJ·mol⁻¹, significantly lower than values reported for pressureless sintering or spark plasma sintering. This suggests that atomic diffusion is effectively activated at low temperatures, while grain boundary mobility remains constrained. As a result, hardness increased monotonically with increasing HIP temperature without degradation associated with grain coarsening. These results demonstrate that low-temperature, high-pressure HIP enables diffusion-assisted densification while suppressing grain growth, offering an energy-efficient processing strategy for high-performance porous tungsten thermionic cathodes.