Solute stabilization of nanocrystalline tungsten against abnormal grain growth

• Published in: Journal of materials research Vol. 33; no. 1; pp. 68 - 80
• Main Authors: Donaldson, Olivia K., Hattar, Khalid, Kaub, Tyler, Thompson, Gregory B., Trelewicz, Jason R.
• Format: Journal Article
• Language: English
• Published: New York, USA Cambridge University Press 15.01.2018; Springer International Publishing; Springer Nature B.V
• Subjects: Allotropic transformation; Alloys; Annealing; Applied and Technical Physics; Biomaterials; Chromium; Coarsening; Early Career Scholars in Materials Science 2018: Invited Feature Paper; Energy; Grain boundaries; Grain Boundary Segregation; Grain growth; Grain size; High temperature; Inorganic Chemistry; Laboratories; Materials Engineering; Materials research; Materials Science; Microstructure; Nanocrystals; Nanotechnology; Phase transitions; Signaling; Thermal stability; Thin films; Transmission electron microscopy; Tungsten base alloys; United States > US; grain growth; nanocrystalline metals; tungsten; grain boundary segregation
• ISSN: 0884-2914; 2044-5326
• DOI: 10.1557/jmr.2017.296

Microstructure and phase evolution in magnetron sputtered nanocrystalline tungsten and tungsten alloy thin films are explored through in situ TEM annealing experiments at temperatures up to 1000 °C. Grain growth in unalloyed nanocrystalline tungsten transpires through a discontinuous process at temperatures up to 550 °C, which is coupled to an allotropic phase transformation of metastable β-tungsten with the A-15 cubic structure to stable body centered cubic (BCC) α-tungsten. Complete transformation to the BCC α-phase is accompanied by the convergence to a unimodal nanocrystalline structure at 650 °C, signaling a transition to continuous grain growth. Alloy films synthesized with compositions of W–20 at.% Ti and W–15 at.% Cr exhibit only the BCC α-phase in the as-deposited state, which indicate the addition of solute stabilizes the films against the formation of metastable β-tungsten. Thermal stability of the alloy films is significantly improved over their unalloyed counterpart up to 1000 °C, and grain coarsening occurs solely through a continuous growth process. The contrasting thermal stability between W–Ti and W–Cr is attributed to different grain boundary segregation states, thus demonstrating the critical role of grain boundary chemistry in the design of solute-stabilized nanocrystalline alloys.