A collaborative research team from the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences and Xi'an Jiaotong University has recently developed a high-performance bulk pure tungsten (W) material with high strength and excellent ductility at low temperature. Results were published in Acta Materialia.  

W and its alloys have attracted much attention because of their excellent properties. But pure W has inherent brittleness and is prone to operational embrittlement due to grain coarsening and neutron irradiation.  

In this study, by sintering activated W powder through a low-temperature sintering process, the researchers obtained a high-density bulk W slab with a grain size of 8.9 μm. Subsequent warm working of high-energy-rate forging further refined the grains, creating alamellar matrix grain with a large number of fine sub-grains, and a high density of mobile edge and mixed dislocations.  

The shielding and blunting effects from low-angle grain boundaries and highly mobile dislocations, and the lamellar structure reaped delamination toughening effect are the main mechanisms for the improved ductility and strength at low temperature.

The performance of this fine-grained bulk W was outstanding, according to the team. At room temperature, its yield strength is 1,302 MPa, exceeding other reported bulk tungsten materials. Obvious tensile ductility was detected. This is the first time that bulk tungsten products have reached tensile strain at room temperature.

They also tested its tensile properties at high temperatures. At 600 ℃, the tensile strength and total elongation of the alloy remain at a high level of 843 MPa and 10.2%, respectively.  At test temperatures, the strength was much higher than previously reported bulk tungsten materials.  

"The grain was refined to about 1.3 μm," said WU Xuebang, a member of the team, "which contributes to the brilliant low-temperature properties of this material."

He further explained the structure of the bulk tungsten. The synergistic effect of elongated grain structure, high proportion oflow-angle grain boundary and high mobile dislocation ensure the improvement of low-temperature ductility and strength.  

"The prepared high-performance bulk pure tungsten has good application prospects in fusion reactors and high-temperature industries," said WU, "and we believe that this strategy is a feasible and low-cost way to design high-performance refractory metals and alloys."  

This work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Anhui Provincial Natural Science Foundation and a HIRFL (Heavy Ion Research Facility in Lanzhou) User Project.

Fig. 1. Schematic diagram of multi-scale microstructure modulation and sample pictures. (Image by XIE Xuefeng) 

Fig. 2. Engineering stress-strain curves of sintered pure W and high-energy-rate forged pure W at different temperatures, and comparison of tensile properties of various W-based materials. (Image by XIE Xuefeng)