Study on mechanical alloying behavior and thermal stability of HfMoNbTaTi refractory high-entropy alloy

The present study prepared HfMoNbTaTi refractory high-entropy alloy powders with uniform distribution of components by mechanical alloying (MA). Six scenarios were designed to investigate the effects of rotational speed, milling duration, ball-to-powder weight ratio (BPR), and process control agent (PCA) on the alloy powder to achieve complete alloying, controlled morphology, and refined grain size. Results showed that the alloying efficiency and lattice strain increased significantly while the grain size decreased with increased rotational speed and BPR. In addition, the content of impurity elements (e.g., C, O, and Fe) also increased. Stearic acid added as PCA effectively prevented cold-welding and refined the particle/grain size but led to the appearance of unknown carbides. Excellent thermal stability was provided by the mechanically alloyed powders when annealed below 1000 °C. In contrast, the single body centered cubic (BCC) solid solution structure decomposed at 1200 °C annealing, and the decomposition temperature range was 942.5 °C–1041.2 °C based on the DSC curve. Finally, the dissolution pattern of components, the size mismatches effect, impact energy, and the optimal range of mechanical alloying process parameters were discussed in detail.