I. Iturriza

Self-passivating bulk tungsten-based alloys manufactured by powder metallurgy

Self-passivating tungsten-based alloys are expected to provide a major safety advantage compared to pure tungsten, which is at present the main candidate material for the first wall armour of future fusion reactors. WC10Si10 alloys were manufactured by mechanical alloying (MA) in a Planetary mill and subsequent hot isostatic pressing (HIP), achieving densities above 95%. Different MA conditions were studied. After MA under optimized conditions, a core with heterogeneous microstructure was found in larger powder particles, resulting in the presence of some large W grains after HIP. Nevertheless, the obtained microstructure is significantly refined compared to previous work. First MA trials were also performed on the Si-free system WCr12Ti2.5. In this case a very homogeneous structure inside the powder particles was obtained, and a majority ternary metastable bcc phase was found, indicating that almost complete alloying occurred. Therefore, a very fine and homogeneous microstructure can be expected after HIP in future work.

Manufacturing of self-passivating tungsten based alloys by different powder metallurgical routes

Self-passivating tungsten based alloys will provide a major safety advantage compared to pure tungsten when used as first wall armor of future fusion reactors, due to the formation of a protective oxide layer which prevents the formation of volatile and radioactive WO3 in case of a loss of coolant accident with simultaneous air ingress. Bulk WCr10Ti2 alloys were manufactured by two different powder metallurgical routes: (1) mechanical alloying (MA) followed by hot isostatic pressing (HIP) of metallic capsules, and (2) MA, compaction, pressureless sintering in H2 and subsequent HIPing without encapsulation. Both routes resulted in fully dense materials with homogeneous microstructure and grain sizes of 300 nm and 1 μm, respectively. The content of impurities remained unchanged after HIP, but it increased after sintering due to binder residue. It was not possible to produce large samples by route (2) due to difficulties in the uniaxial compaction stage. Flexural strength and fracture toughness measured on samples produced by route (1) revealed a ductile-to-brittle-transition temperature (DBTT) of about 950 °C. The strength increased from room temperature to 800 °C, decreasing significantly in the plastic region. An increase of fracture toughness is observed around the DBTT.

ODS ferritic steels produced by an alternative route (STARS): microstructural characterisation after atomisation, HIPping and heat treatments

The conventional PM ODS Ferritic Steel (FS) processing route includes gas atomisation of steel powder and its mechanical alloying (MA) with Y2O3 powder particles to dissolve yttrium and form, during consolidation, a dispersion of oxide nanoparticles (Y–Ti–O) in a nanostructured matrix. This work presents an alternative route to produce ODS steels avoiding MA: STARS (Surface Treatment of gas Atomized powder followed by Reactive Synthesis). STARS FS powders with composition Fe–14Cr–2W–0.3Ti–0.23Y, already containing the nanoparticles precursors, were gas-atomized. Oxygen, Y and Ti contents were tailored to the required values to form Y–Ti–O nanoparticles during processing. Powders were HIPped at 900, 1220 and 1300°C. Specimens HIPped at 900 and 1220°C were heat treated (HT) at temperatures ranging from 1200 to 1320°C. The microstructural evolution with HIP and HT temperatures, including characterisation of nanoparticles and feasibility of achieving complete dissolution of prior particle boundaries (PPBs) were assessed.