Even though tungsten exhibits outstanding properties at high temperatures in an oxygen-free atmosphere, its brittle behavior at room temperature impedes the use as a structural material. Severe plastic deformation of tungsten by a combination of hot and cold rolling can refine the grain size well down into the ultrafine-grained (UFG) regime and improve strength and room temperature ductility simultaneously. However, despite a few interesting findings, a systematic study of the deformation mechanisms is still lacking.
To investigate the emerging room temperature ductility with decreasing grain size, a batch of tungsten sheets with different thicknesses was produced out of a single sintered compact of commercially pure tungsten by subsequent cold rolling. This unique batch of samples allows investigating the mechanical properties without any influence of the chemical composition or fabrication differences.
In this presentation, the authors focus on the deformation mechanisms in UFG tungsten and their evolution over grain size and temperature. Through XRD, EBSD and TEM studies, the microstructure of eight different conditions was characterized from the grain size and texture development down to the density and arrangement of cold-work induced dislocations. In combination with tensile and strain rate jump experiments between 50°C and 800°C, the dependencies of flow stress, uniform elongation, hardening behavior and strain rate sensitivity (SRS) on temperature, as well as on microstructural changes were evaluated.
A grain refinement well down to the UFG regime has been verified, reaching a grain size of 240 nm perpendicular to the rolling direction. While the dislocation density was shown to depend mainly on the rolling temperature, a special dislocation arrangement was detected for grains below approximately 300 nm, which hints to an ordered glide of dislocations for the UFG tungsten foils. Tensile tests showed peculiar characteristics in the stres- strain curves for the UFG grain structure and proved the development of room temperature tensile ductility with decreasing grain size. A change of SRS revealed a transition of the deformation mechanism for UFG tungsten from being controlled by screw dislocations at low temperatures towards being controlled by the absorbing character of grain boundaries for higher temperatures.