Publications

Solute stabilization of nanocrystalline tungsten against abnormal grain growth

Donaldson, Olivia K.; Hattar, Khalid M.; Kaub, Tyler; Thompson, Gregory B.; Trelewicz, Jason R.

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.