Preparation And Performance Of W-based W-ETM (ETM=Zr,Nb,Hf,Ta) Solid Solution Alloys And Oxide Dispersion Strengthened Materials

In the future magnetic-confinement fusion reactors,the targets of divertors and the first wall are required to withstand extremely high thermal loads and radiation of high flux ions and neutron.Tungsten(W)is currently considered the most promising facing plasma materials(PFMs).Manufacturing of pure W has been a problem due to its high ductile-to-brittle transition temperature(?100-400°C),and the low recrystallization temperature(?1150-1350°C)has limited its high temperature application as the PFMs.Solid solution alloying and dispersion strengthening of second phase particles(oxides or carbides)are two possible means to improve the performance of advanced W materials.Powder sintering is mostly employed for the synthesis of W materials,and in most cases solid solution alloying of W matrix and dispersion strengthening are not concurrently reached.Novel methods in which the two material mechanisms can be jointly realized are highly desirable for the fabrication of advanced W materials.Refractory early transition metals(ETMs)are the main alloying elements of W-based solid solution alloys.The W solid solution parts of the W-ETM binary phase diagrams are incomplete,and very limited data is available to conduct solid solution alloying experiment.In the present work,W-ETM(Early Transition Metals,ETM=Zr,Nb,Hf,Ta)alloys were prepared by means of non-consumable vacuum arc melting,and the non-equilibrium microstructures of the ingot alloys are comparatively examined.The solid solution limits thus determined in these systems were taken into account in the design of new powder metallurgy W materials.Spark plasma sintering(SPS)has been carried out by using the Zr-Fe amorphous alloys as intermediate alloys,together with W powder as the raw materials.The microstructures and component phases of the arc-melted and SPS samples are investigated by optical metallography(OM),X-ray diffraction(XRD),Scanning Electron Microcopy(SEM)and Transmission Electron Microscopy(TEM).The room-temperature mechanical properties of the W materials are studied by microhardness and quasi-static compression tests.The main experimental findings of this work are as follows:(1)A chain of well designed experimental procedures have been developed for preparation of small-size W button alloys using non-consumable vacuum arc-melting,and a series of W-ETM alloys have been made,namely,W_100-xTa_x(x=0,1,3,5,6,7,10;at.%),W_100-xNb_x(x=1,3,5,10;at.%),W_100-xZr_x(x=0.25,0.4,0.5,0.75,1,2,3;at%)and W_100-xHf_x(x=0.4,0.6,1,2,3,6,9;at%).The total mass loss of the alloys after arc-melting is controlled to be less than 0.5 wt.%.The upper part of the as-cast W-Ta ingots are composed of millimeter-scaled grains,and the grain size showed subtle changes with the increase of Ta content;all the samples are single-phase BCC alloys,and the lattice constant(y)exhibits a linear relationship with Ta content(x)):y=0.001x+3.1657,following the Vegard's law;Micrometer scaled grains are formed in the upper part of the W-Nb ingots,and the grain size is refined with the increase of Nb content.When the Nb content x?3 at.%,secondary phases precipitated in the interior of the grains,which indicates that the solid solubility of Nb was less than 3 at.%in the non-equilibrium solidified W alloys.Micrometer sized grains are formed in the upper part of W-Zr ingots,and the grain size is refined by increasing Zr contents.The grain size in the center region of the ingots is of millimeter size;OM observation find that round particles of 1-2?m in size precipitated in the 2 at.%Zr alloy.The number density of particles increases with the increase of Zr content.The secondary phase is identified to be the W₂Zr phase by XRD.The OM and XRD evidence indicates that the solid solubility of Zr in W was less than 0.5 at.%;the upper part grains of W-Hf ingots are of micrometer size,and are refined with the increase of Hf content.OM observation shows the presence of a large number of spherical or needle-like particles in the interior and at the grain boundaries of the 2 at.%Hf sample,the grain sizes being 1-2?m and 5-10?m,respectively.As the Hf content increases,the precipitated phase changes into flake shape with a size of 20-300?m.W₂Hf phase is found to precipitate when the Hf content exceeds 1 at.%.Microhardness analysis shows that the microhardness of W_100-xTa_x,W_100-xNb_x,W_100-xZr_xand W_100-xHf_x alloys increase with the increase of alloying content,while Zr and Hf deliver greater strengthening effect than Ta and Nb do.(2)Using an Zr-Fe amorphous intermediate alloy,W materials with nominal compositions of W-0.4 at.%Zr and W-1 at.%Zr are designed,and high-density dispersion-strengthened W materials are prepared by spark plasma sintering technology.The results show that the relative densities were 93.8%and 96.6%,the grain sizes of 3.4?m and1.3?m,and the microhardness of 378 H_V and 457 H_V,respectively,for the two W materials.Tetragonal Zr O₂ as a second phase particle with a size of 20-300 nm is formed in the interior of grains and at grain boundaries in both materials,and no specific coherent crystallographic relationship is found between Tetragonal Zr O₂ and the matrix W.Under room temperature compression,the materials exhibit yield strengths of 1210 MPa and 1520 MPa,respectively,along with a large plastic strain of 45%.In a nutshell,tetragonal Zr O₂ dispersion-strengthened W materials with high density and high room temperature strength and large compressive plasticity are fabricated with a Zr-Fe amorphous intermediate alloy.In the future study,high performance W materials are to be made by tailoring the alloying addition of the intermediate alloy,and by optimizing the ball milling parameters and the sintering process as well.