Pressureless two-step sintering of ultrafine-grained tungsten

The challenge of producing dense ultrafine-grained (UFG) tungsten is hereby addressed by a simple pressureless two-step sintering method. It provides a uniform microstructure with similar to 99% theoretical density and similar to 700 nm grain size, which is among the best sintering practice of pure tungsten reported in the literature. Benefitting from the finer and more uniform microstructures, two-step sintered samples show better mechanical properties in bending strength and hardness. While parabolic grain growth kinetics is verified, a transition in the nominal grain boundary mobility was observed at 1400 degrees C, above which the effective activation enthalpy is similar to 6.1 eV, below which grain boundary motion is rapidly frozen with unusually large activation enthalpy of similar to 12.9 eV. Such highly nonlinear behavior in activation parameters with respect to temperature suggests that activation entropy and maybe collective behavior play a role in grain growth. We believe the as-reported two-step sintering method should also be applicable to other refractory metals and alloys, and may be generalized to multi-step or continuous-cooling sintering design using machine learning. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.