Development of irradiation tolerant tungsten alloys for high temperature nuclear applications.

Development of refractory metals for application as plasma-facing armour material remains among priorities of fusion research programmes in Europe, China and Japan. Improving the resistance to high temperature recrystallization, enhancing material strength to sustain thermal fatigue cracking and tolerance to neutron irradiation are the key indicators used for the down selection of materials and manufacturing processes to be applied to deliver engineering materials. In this work we investigate the effect of neutron irradiation on mechanical properties and microstructure of several tungsten grades recently developed. Neutron irradiation campaign is arranged for screening purposes and therefore is limited to the fluence relevant for the ITER plasma facing components. At the same time, the neutron exposure covers a large span of irradiation temperatures from 600 up to 1000 degrees C. Four different grades are included in the study, namely: fine-grain tungsten strengthened by W-carbide (W-4wt.% W2C), fine-grain tungsten strengthened by Zr carbides (W-0.5% ZrC), W alloyed with 10 at.% chromium and 0.5 at.% yttrium (W-10Cr-0.5Y) and technologically pure W plate manufactured according to the ITER specification by Plansee (Austria). The strengthening by W2C and ZrC particles leads to an enhanced strength, moreover, the W-0.5ZrC material exhibits reduced DBTT (compared to ITER specification grade) and is available in the form of thick plate (i.e. high up-scaling potential). The W-10Cr-0.5Y grade is included as the material offering the self-passivation protection against the high temperature oxidation

Beneficial Effects of a WC Addition in FAST-Densified Tungsten

The particle reinforcement of fusion-relevant tungsten through the incorporation of tungsten sub-carbide W2C particles at the grain boundaries is demonstrated as an effective way of eliminating the harmful W oxide, enhancing densification and stabilising the composite's microstructure and flexural strength at room and high temperatures. The W2C particles are formed in situ during the sintering by carbon diffusion from WC nanoparticles added as a precursor to the W matrix. Even in an extremely fast sintering process using Field-Assisted Sintering Technology (FAST, 1900 °C, 5 min), the added WC completely transforms to W2C, resulting in a W–W2C composite. While at least 5 vol % of WC nanoparticles are needed to eliminate the oxide, approximately 10 vol % result in a W–W2C composite with favourable characteristics: high density, high flexural strength at RT (>1200 MPa) as well as at elevated temperatures, and high thermal conductivity, which remains above 100 W/mK up to 1000 °C

Beneficial Effects of a WC Addition in FAST-Densified Tungsten

The particle reinforcement of fusion-relevant tungsten through the incorporation of tungsten sub-carbide W2C particles at the grain boundaries is demonstrated as an effective way of eliminating the harmful W oxide, enhancing densification and stabilising the composite's microstructure and flexural strength at room and high temperatures. The W2C particles are formed in situ during the sintering by carbon diffusion from WC nanoparticles added as a precursor to the W matrix. Even in an extremely fast sintering process using Field-Assisted Sintering Technology (FAST, 1900 °C, 5 min), the added WC completely transforms to W2C, resulting in a W–W2C composite. While at least 5 vol % of WC nanoparticles are needed to eliminate the oxide, approximately 10 vol % result in a W–W2C composite with favourable characteristics: high density, high flexural strength at RT (>1200 MPa) as well as at elevated temperatures, and high thermal conductivity, which remains above 100 W/mK up to 1000 °C

The role of grain boundary scattering in reducing the thermal conductivity of polycrystalline XNiSn (X=Hf, Zr, Ti) half-Heusler alloys

Thermoelectric application of half-Heusler compounds suffers from their fairly high thermal conductivities. Insight into how effective various scattering mechanisms are in reducing the thermal conductivity of fabricated XNiSn compounds (X = Hf, Zr, Ti, and mixtures thereof) is therefore crucial. Here, we show that such insight can be obtained through a concerted theory-experiment comparison of how the lattice thermal conductivity κLat(T) depends on temperature and crystallite size. Comparing theory and experiment for a range of Hf0.5Zr0.5NiSn and ZrNiSn samples reported in the literature and in the present paper revealed that grain boundary scattering plays the most important role in bringing down κLat, in particular so for unmixed compounds. Our concerted analysis approach was corroborated by a good qualitative agreement between the measured and calculated κLat of polycrystalline samples, where the experimental average crystallite size was used as an input parameter for the calculations. The calculations were based on the Boltzmann transport equation and ab initio density functional theory. Our analysis explains the significant variation of reported κLat of nominally identical XNiSn samples, and is expected to provide valuable insights into the dominant scattering mechanisms even for other materials