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.

Characterizing the residual stress of thick nanocrystalline electrodeposits poses several unique challenges due to their fine grain structure, thickness distribution, and matte surface. We use a three-dimensional profilometry-based approach that addresses each of these complicating factors and enables quantitative analysis of residual stress with reasonable accuracy. The specific emphasis of this work is on thick (10–100 μm), nanocrystalline Ni-W electrodeposits of the finest grain sizes (4–63 nm), in which residual stresses arise during the deposition process as well as during postdeposition annealing. The present measurements offer quantitative insight into the mechanisms of stress development and evolution in these alloys, suggesting that the grain boundary structure is out of equilibrium (unrelaxed) and contains the excess free volume that controls the resulting residual stress levels in these films. There are apparently two factors contributing to this stress: the percentage of excess free volume contained in the grain boundaries, which is affected by the processing conditions, and the total volume fraction of grain boundaries, which is controlled by the grain size.

The following article is based on the Von Hippel Award presentation given by Julia Weertman of Northwestern University on December 3, 2003, at the Materials Research Society Fall Meeting in Boston.Weertman received the award for “her lifelong exceptional contributions to understanding the basic deformation processes and failure mechanisms in a wide class of materials, from nanocrystalline metals to high-temperature structural alloys, and for her inspiring role as an educator in materials science.” It has been said that “the best things come in small packages,” and that is certainly in Weertman's mind in this presentation.She has spent much of her career “in pursuit of the small.” In this article, she first looks back at her experiences studying grain-boundary cavities and life in the spaces between grains.She then fast-forwards to modern work on nanocrystalline mechanical behavior, confirming that such nanocrystalline materials are indeed strong, but also brittle.Some of her experiences in studying these phenomena are also described.