Fabrication And Properties Of Carbide/Oxide Dispersion Strengthened Tungsten Based Alloys

Plasma facing materials(PFMs)are exposed to high thermal loads,high thermal stress and high dose irradiation in nuclear fusion facilities.The comprehensive service performances of PFMs are related to the safe operation of fusion devices.Tungsten(W)has some excellent properties such as high melting temperature,high sputtering resistance and high thermal conductivity,and thus it has been considered as one of the most promising candidates for PFMs in fusion reactors.However,a long-standing problem for tungsten alloys is that they suffer from serious embrittlement,i.e.,low-temperature embrittlement(relatively high ductile-brittle transition temperature(DBTT)>400 ?),recrystallization embrittlement and radiation embrittlement,which limit the usability of W for structural applications.In order to solve these problems,we developed a powder metallurgical method that achieves high performance W alloys which contain simultaneous high ductility/strength/plasticity.This strategy has combined the advantages of grain-boundary(GB)purifying and strengthening,nano-sized oxide/carbide dispersion-strengthening and micro-alloying.A methodical and innovative powder metallurgical method was developed from the "down to up"preparing small size W alloy tablets to the large-scale "up to down" fabrication of engineering-applied high performance W bulk alloys.The "down to up" preparing route provides a high fabrication efficiency,like the spark plasma sintering(SPS)with fast heating rate and short sintering time,and allows the consolidation of powder materials into dense fine-grained products at lower sintering temperature,which is undoubtedly suitable for component optimization and the broad exploration of the strengthening/toughening mechanism.The SPSed W-0.2wt.%Zr-1.0wt.%Y2O3(WZY)and W-0.5wt.%ZrC(WZC)have been prepared.It is found that the addition of Y2O3 and/or ZrC nano-particles could significantly pin the GBs and hinder the grain growth of tungsten.Adding a small amount of Zr or ZrC could capture impurity oxygen in tungsten and form ZrOx or Zr-C-O particles during sintering,which would undoubtedly reduce the detrimental oxygen concentration in GBs,strengthen the GBs and thus increase the toughness/strength of W based materials.Basing on optimized compositions and the discovered strengthening/toughening mechanism,a further "up to down" architecture was developed for industrial production of engineering-applied bulk W alloys.This "up to down" architecture combined the conventional sintering and a subsequent high temperature plastic deformation,like high temperature swaging or rolling:WZY and WZC alloys billets were firstly prepared as the precursor by traditional sintering and then the hot plastic working was used to deform the billets to high dense bulk plates with refining grains due to the extensive dynamic recrystallization.A 6.5 mm thick R-WZY alloy plate with high tensile performances was achieved through hot rolling process.Its DBTT is about 150 ?.It has an ultimate tensile strength(UTS)of 911 MPa and a total elongation(TE)?3.2%at 150 ?.And its UTS are always larger than 580 MPa from room temperature(RT)to 500 ?.These high performances originate from the special microstructure design with achieving refined equiaxed sub-grains,elongated mother grains and dispersed nanoparticles.Another 8.5 mm thick R-WZC alloy plate shows a combination of high mechanical properties,high thermal shock resistances and high resistance for plasma irradiation.Its DBTT is about 100 ?.It shows a high flexural strength of 2.5 GPa at RT.The UTS and TE are about 991 MPa and 1.1%at RT,respectively.At 500 ?,the UTS is 582 MPa and the TE is up to 41%.In addition,the R-WZC alloy plate can sustain 4.4 MJ/m2(RT)or 100 shots of 1.0 MJ/m2(1 MJ/m2/shot,200 ?)transient thermal load without any cracks.It also has a higher resistance for He+(100 eV)plasma irradiation than that of other W materials.Its excellent performances come from the synergistic effect of the designed interfaces:stable small angle GBs(fine equiaxed sub-grains)resulting in fine grain strengthening,coherent phase boundaries(PBs)between the nano size ZrC and W matrix leading to a significant strengthening of PBs and GBs,ZrC capturing impurity oxygen resulting in purifying/strengthening GBs.These preparing routes and multi-scale microstructure design thoughts in this paper will provide useful guidance for developing high-performance tungsten-based PFMs.