Powder metallurgy route to ultrafine-grained refractory metals.

Ultrafine-grained refractory metals are promising materials for applications in aerospace, microelectronics, nuclear and many others under extreme environment. Powder metallurgy (PM) allows to produce such materials with well controlled chemistry and microstructure at multiple length scales and near-net shape manufacturing. However, sintering refractory metals to full density while maintaining a fine microstructure is still challenging due to the high sintering temperature and the difficulty to separate the kinetics of densification versus grain growth. Here we seek to provide an overview of the sintering issues, microstructural design rules and PM practices towards ultrafine-grained and nanocrystalline refractory metals. We shall review the previous efforts to address the processing challenges, including the use of fine/nano powders, second-phase grain growth inhibitors, and field-assisted sintering techniques. Recently, pressureless two-step sintering has been successfully demonstrated in producing dense ultrafine-grained refractory metals down to ∼300 nm average grain size with a uniform microstructure and this technological breakthrough shall be reviewed. PM progresses in specific materials systems shall be next reviewed, including elementary metals (W and Mo), refractory alloys (W-Re), refractory high-entropy alloys and their composites. Lastly, future developments and the endeavor towards ultrafine-grained and nanocrystalline refractory metals with exceptional fine microstructure and improved properties are outlined. This article is protected by copyright. All rights reserved.