Determination of the DBTT of nanoscale ZrC doped W alloys

Among the refractory metals, tungsten (W) is preferred for high-temperature applications due to its distinct properties such as thermal conductivity, low spattering yield, and high neutron load capacity. However, the high temperature required for the ductile-to-brittle transition of tungsten, limits the applications of tungsten materials in various fields. Instead, it confines its use in regions with temperatures higher than the ductile-to-brittle transition temperature (DBTT) due to the failure of the material caused by brittle fracture at temperatures lower than DBTT.

Generally, the low-temperature and mechanical properties of tungsten materials depend on the processing, purity, and microstructure of the materials. They too influence the DBTT of tungsten materials. Consequently, internal friction caused by grain boundaries, crack initiation, and dislocations are very sensitive for investigating the mechanical properties and material defects states. To continue exploring the broad applications of tungsten materials especially the nuclear applications, there is a need for proper investigation of the mechanical properties of tungsten aimed at reducing the DBTT temperature.

Researchers at the Chinese Academy of Sciences, Institute of Solids State Physics led by Professor Tao Zhang and Professor Q.F. Fang developed a new technology for investigating the mechanical properties and in particular to determining the DBTT of W-0.5wt%ZrC alloy. The techniques were based on amplitude-dependent internal friction (ADIF). The 1 mm thick W-0.5wt%ZrC alloy plates used in the experiment was prepared by mechanical milling, hot pressing sintering and rolling processes. The use of ADIF enabled investigation of the yield strength, physical mechanisms as well as the determination of the critical amplitude corresponding to the plastic behavior. The obtained yield strength and DBTT through ADIF were compared to the tensile results. The research work is published in Materials Science and Engineering A.

The newly developed amplitude-dependent internal friction technique was observed to be a reliable and efficient method for determination of DBTT of metals and alloys materials. For instance, the DBTT obtained for the W-0.5wt%ZrC alloy was in the range of 500C -800C which was confirmed by the conducted tensile tests. Consequently, the critical strain amplitude in ADIF and the yield strength in tension were observed to decrease with an increase in testing temperature simultaneously.

From the microstructure analysis results, Professor Tao Zhang and colleagues concluded that the high strength and relatively low DBTT of the W-0.5wt%ZrC alloy plates were as a result of the nanoscale particle pinning (on the dislocations and grain boundaries) effects brought about by multistep hot and cold rolling during preparation. Additionally, the impact of the structural parameters such as the grain sizes, shapes, boundaries, and dislocations were also observed. For instance, the obtained particle distribution from the 500 counted particles showed that most particles had an average size value of 51nm and in the range of 10-160 nm. The ZrC particles dispersed in tungsten inhibited grain growth during hot pressing sintering and rolling processes.

The authors are optimistic that the study would help in the determination of low DBTT temperature for tungsten and its alloys hence promote its applications in various fields especially in nuclear fusion reactors which were initially difficult due to the high DBTT temperatures.