Hafnium carbide growth behavior and its relationship to the dispersion hardening in tungsten at high temperatures

The growth behavior of hafnium dispersed carbide particles and its relationship the strength of a tungsten-rhenium alloy at temperatures above 2200 K have been examined with transmission electron microscopy. From 2200 to 2600 K, HfC particles grow homogeneously with a relatively slow growth rate. In this temperature region, the activation energy for HfC particle growth is determined to be 50.2 ± 1.0 kcal mol⁻¹. The coarsening of HfC particles is a bulk diffusion-controlled process and hafnium is the less stable element in the HfC decomposition and growth process. Rapid particle growth occurs at temperatures above 2600 K. A bi-modal particle size distribution is observed at 2800 and 3000 K and selective particle growth takes place along dislocation lines and at grain boundaries, indicating that dislocation and boundary enhanced diffusion mechanisms are involved in the HfC particle coarsening process above 2600 K. Based upon the studies on the growth behavior of HfC particles, An Ashby-Orowan dispersion strengthening model is applied to the HfC hardened tungsten. Good agreement is obtained between the calculated resolved shear stress and experimental results.