Study of defects in potassium-doped tungsten alloy by positron annihilation technique

Abstract

Tungsten alloy is known as a promising plasma-facing material (PFM) in IETR because of high strength, high-temperature stability, low sputtering erosion, low tritium retention, etc. However, tungsten has some disadvantages, such as high ductile-brittle transition temperature, low temperature brittleness, and radiation embrittlement. For the severe environment of PFM, various techniques have been adopted to improve W-based materials, among which the potassium doping is an effective bubble strengthening method, it can bring in nano-sized K bubbles, and enhance the toughness and strength, thermal shock performance, irradiation resistance of the materials. The K bubbles, which can pin grain boundaries (GBs) and dislocations, are the most characteristic defects in W-K alloy and have been widely reported. However, little attention is paid to other defects such as vacancies, GBs and dislocations. In fact, high-density dislocations exist in W-K alloy and vacancies play a considerable role in forming the K bubbles. Thus, positron annihilation technique (including the positron annihilation lifetime spectrum and slow positron beam Doppler broadening spectrum), which is a useful technique for detecting defects in solids, can be used to study these defects in W-K alloy samples. The positron lifetime of potassium bulk is about 376 ps and the positron lifetime of tungsten bulk is about 110 ps. But by simulating positron lifetime of defects in tungsten, it is found that potassium atoms in tungsten lattice do not exhibit the characteristic positron lifetime. Therefore, potassium is not considered in analyzing positron annihilation lifetime spectra of W-K alloy samples with different potassium content (46, 82, 122, 144 ppm). Three-state capture model is established in this paper, the dislocation density and vacancy cluster concentration of these samples are obtained. From the results, the dislocation densities in all samples are very high, but vacancy cluster concentrations are relatively low, and the vacancy cluster concentration in the sample with 82 ppm potassium content is the lowest in all samples. The behavior of potassium atoms in the sintering process is also discussed. Then the slow positron beam Doppler broadening spectra of W-K alloy samples and pure tungsten samples are measured and the obtained data are fitted by VEPFIT. It is noted that the defects in W-K alloy samples are much more than those in pure tungsten sample, and are distributed homogeneously with depth. The positron diffusion length information simultaneously obtained is compared with these values computed by dislocation density and vacancy cluster concentration, confirming the positrons trapped by potassium bubbles and grain boundaries are existent.

Vacancy‐Type Defects in H+6%He Neutral Beam Irradiated WK Alloy Probed by Slow Positron Beam

“…Previous research has determined that positron annihilation avoids K atoms when a small amount of K atoms is doped, and only represents the information contained in vacancy‐type defects. [ 44,45 ] Therefore, a rough comparison of the Doppler broadening of slow positron annihilation spectra between WK samples and pure W samples is made possible, and the effects of defects introduced by K‐doping on the behavior of H/He atoms at different surface temperatures of irradiation can be analyzed. Compared to pure W, K bubbles in WK alloys are deemed to resist the movement of dislocations and grain boundaries, especially at high temperatures.…”

Microstructure and High-Temperature Performance of High K-Doped Tungsten Fibers Used as Reinforcement of Tungsten Matrix

“…The amount of energy stored in tungsten wire by the traditional process with a larger pass deformation is more than that by the optimized process; hence, the grain boundaries overcome the K bubble pinning obstacles more easily with the traditional process and, thus, form recrystallized grains, which leads to a lower recrystallization temperature. On the other hand, tungsten wire with a high K content is easily formed into large-sized potassium bubbles at high temperatures [34] (refer to K bubble size of sample 2#-0). Large-sized K bubbles tend to merge and grow continuously and finally become a kind of fracture source, which is a potential failure for high K-doped tungsten wire.…”