Casting Fabrication And Properties Of Dispersion-strengthened Heat-Sink Cu Alloys

The divertor as one of the key components in the future nuclear fusion reactors,is composed of plasma-facing materials and heat-sink materials.During operation the divertor component is required to withstand a thermal load as high as several tens of MW/m2 and irradiation of high fluxes of ions and neutrons.High thermal conductivity,high mechanical stability at elevated temperatures and high tolerance to ion and neutron irradiation are the properties desired by the heat-sink materials playing a crucial role of heat removal.These property requirements are hard to be fulfilled by the present industrial Cu alloys of both the precipitation-hardening and dispersion-strengthening types.Microstructures design along with novel preparation techniques are being intensively studied for the development of advanced heat-sink Cu alloys as divertor materials.In the present work,a novel method for syntheses of dispersion-strengthening Cu alloys is proposed,and has been realized by melting of pure Cu with oxygen containing amorphous intermediate Alloys.Two series of amorphous alloys,namely,Cu-Zr-O and Cu-Zr-Ti-O,were first prepared by means of vacuum melting and melt-spinning,and were used with pure Cu to make Cu alloys by melting,followed by cold rolling and heat treatment for microstructure modification.X-ray diffraction(XRD),differential scanning calorimeter(DSC)and transmission electron microscopy(TEM)were employed to examine the structure,thermal stability and microstructure of the amorphous intermediate alloys and the Cu alloys as well.The metallographic morphology,chemical composition,room temperature and high temperature tensile properties and electrical conductivity of the Cu alloy were studied by using optical microscopy(OM),electron probe micro-analysis(EPMA),microhardness indentation,tensile test and electrical conductivity test.The experimental results of alloy preparation,microstructure observation and property evaluation are presented in the following:(1)Alloys with the nominal compositions of(Cu64Zr36)100-xOx(x=0,1,2,3,4)and(Cu60Zr20Ti20)100-xOx(x=0,1,3,5,7)were made in ribbon form by vacuum arc-melting and melt quenching.XRD and TEM results showed that in the as-quenched(Cu64Zr36)100-xOx(x=0,1,2,3,4)alloys,the Cu64Zr36 alloy is fully amorphous,and a derisive distribution of ZrO2 particles of 10 nm in size in the amorphous sustrate is obtained in all the oxygen-containing alloys(x=1,2,3).The DSC curves of the ribbon alloys show that the onset temperature of crystallization(T?)increases with the increase of nominal oxygen content,indicating that oxygen atoms have been partially alloyed into the amorphous structure,and a Cu-Zr-O ternary amorphous substrate is formed.The as-quenched(Cu60Zr20Ti20)100-xOx(x=0,1,3,5,7)alloys exhibited similar microstructural and crystallization features.The as-quenched(Cu64Zr36)98O2 and(Cu60Zr20Ti20)97O3 alloys were determined as the intermediate alloys for the synthesis of oxide dispersion strengthening Cu alloys by melt casting.(2)Cast Cu alloys with the Zr contents of 0.15 wt.%,0.2 wt.%,0.3 wt.%and 0.15 wt.%,0.3 wt.%,0.6 wt.%were made with(Cu64Zr36)98O2 and(Cu60Zr20Ti20)97O3 intermediate alloys,respectively.The thermostatically treatment procedures are the following:homogenization at 980? and water quenching,cold rolling to 60%and isothermal annealing at 475-950?.The grain size finally reached in the Cu alloys is about several hundred of micrometers.Two kinds of nano dispersion phases were formed in the Cu-0.3 wt.%Zr alloys made from(Cu64Zr36)98O2 and subjected to annealing at 475? for 1 h or at 550? for 2 h,one of the anopheles being 4-10 nm in size and evenly distributed in the Cu substrate,and the other the particles of tetragonal ZrO2 with a size of several hundred nm to several ?m located in the interior of grains and grain boundaries.Aggregation of anopheles particles occurred in the Cu alloys made with(Cu60Zr20Ti20)97O3 and subjected to annealing at 475? for 1 h.(3)The electrical conductivity of the Cu alloys made from(Cu64Zr36)98O2 was measure to be 87-95%IACS.The Cu-0.3 wt.%Zr alloy subjected to annealing at 550? for 2 h exhibited an IACS of 90%,a microhardness of 110 Hv,a room temperature tensile strength of ?b=350 MPa,a total elongation of ?=23.2%,a tensile strength ?b=220 MPa an elongation of ?=18%at 450?,and a tough fracture mode;The Cu-0.3 wt.%Zr alloys made from(Cu60Zr20Ti20)97O3 and subjected to annealing at 475? for 2 h exhibited an IACS of 20-49%,?b=415 MPa and ?=11.5%at room temperature,and ?b=300 MPa and ?=10.2%at 450?,along with a river-like fracture surface pattern.In summary,oxygen containing amorphous intermediate alloys were designed for the fabrication of dispersion strengthened Cu alloys by casting.Among the Cu alloys synthesized in this way,the Cu-0.3 wt.%Zr alloy made from a(Cu64Zr36)98O2 amorphous intermediate alloy exhibited a combination of high electrical conductivity with high strength and ductility at room temperature and at 450?.The coarse-grain problem in the Cu alloys remain unsolved.In the future study,cast Cu alloys of kilogram scale are to be made,followed by suitable thermomehcnical heat treatment for grain size refinement and property regulation.The novel dispersion-strengthened Cu alloys are promising candidate heat-sink materials of the divertors of future fusion reactors.