Preparation And Properties Of Dispersion-strengthened Cu-Zr And Cu-Cr-Zr Alloys

High performance Cu alloys with high thermal conductivity,high strength and high resistance to ion and neutron irradiation are highly demanding by the fabrication of divertors of future fusion reactors.The presently known precipitation-hardening and oxide-dispersion-strengthening Cu alloys do not fulfill the requirements of the heat-sink materials of the divertor.In the present work,we propose a new method of making dispersion-strengthened Cu alloys,i.e.,preparation of Cu-Zr and Cu-Cr-Zr alloys by using oxygen-containing Cu-Zr amorphous intermediate alloys in vacuum arc melting,followed by cold rolling and solid solution and ageing（SSA）.The microstructures,compositions,constituent phases,and the thermal stability of alloys have been studied by using optical metallography,scanning electron microscopy,electron probe analysis,X-ray diffraction,transmission electron microscopy,and thermal analysis.The mechanical and electrical conductivity properties of the alloys are examined by means of microhardness indentation,room-temperature tensile test and electrical conductivity tests.The main results achieved in this study are presented in the following:（1）Cu₅₀Zr₅₀ amorphous ribbons were prepared by means of melt-spinning,and by oxidizing the amorphous alloys at a specific temperature for a favorable duration of time,intermediate alloys containing a certain amount of oxygen were reached,the forms of presence of oxygen in the sample being the respective constituent element of the amorphous structure and Zr（Cu）oxides.（2）Using pure Cu,Cr and the oxygen-bearing Cu₅₀Zr₅₀ alloys,Cu alloys with nominal compositions of DS Cu-[0.1,0.2,0.3,0.4,0.5]Zr are prepared by arc melting.In the as-cast alloys,a dispersive distribution of FCC（a=0.37 nm）nanoparticles of 5 nm size are formed,and are pinned by dislocations.This nano-phase shows no coherent orientation relationship with the Cu matrix structure.Micrometer scaled precipitates are formed in the interior of grains and at grain boundaries.The precipitates are of spherical and long-strip like forms,which are enriched in Zr.Phase identification reveals that the precipitates are a mixture of the nano FCC phase and an unknown body-centered tetragonal（BCT）phase（a=b=0.50nm,c=0.71 nm）of coarsened grain size.After SSA by a chain of procedures of 980?C-1 h+water quenching+475?C-2 h+in-furnace cooling,the long-strip like precipitates disappeared,the number density and shape of the spherical precipitates of 1-3μm in size remained almost unchanged,in the interior of grains,however,needle-like precipitates were formed,and triangle grain boundary appeared.To examine the thermal stability of the precipitates,as-cast DS Cu-Zr alloy was subjected to 80%cold rolling and then annealed at 400?C,500?C,600?C and 700?C for 1 h.A slight increase of¹0 HV in microhardness was observed for the 400?C annealed sample,and the other samples exhibited decreasing microhardness with the rise of annealing temperature till the value（60 HV）comparable to that of the as-cast alloy is reached at 700?C.The room temperature electrical conductivity of cold rolled samples is improved compared with the as-cast state,and similarly,the electrical conductivity showed a decreasing tendency with increasing Zr content.Among the rolled samples,the DS Cu-0.5Zr alloy exhibited a room temperature tensile strength of 416 MPa,a small plastic strain of 3.0%.A number of dimples were observed on the fracture surfaces of the rolled samples,in which the precipitated previously found in the interior of grains were seen to exist.（3）By taking a certain amount of Cr as the third alloying element,and using the same preparation method,Cu alloys with nominal compositions of DS Cu-0.75Cr-[0.1,0.2,0.3]Zr were made.The as-cast microstructures are largely similar with Cr-free as-cast DS Cu-Zr alloys.The difference lies in that Cr-rich particles of several hundreds to several micrometers size were distributed in the Cu matrix and in the vicinity of the Zr-rich precipitates in the Cr-bearing alloys.After SSA,some Cr-rich particles survived,but the Zr-rich precipitates disappeared in the DS Cu-0.75Cr-[0.2,0.3]alloys.TEM observation showed that fine nano-particles with a high number density are distributed in the Cu matrix,and are pinned by dislocations.Two kinds of precipitates were found:a thin strip-like phase of² nm thick with nearly the same orientation and parallel arrangement were homogeneously distributed in the Cu matrix;the nano FCC phase（a=0.37 nm）previously found in the Cu-Zr alloys was enriched at the thin and straight grain boundaries.It is noted that the compositions and structure of both the strip-like and nano FCC phases are not clarified in this study.While it can be expected that the cooperated hardening of the two kinds of nano precipitates would give rise to the greatly increased alloy hardness and strength.The microhardness of the DS Cu-0.75Cr-0.2Zr SAA alloy delivers a room temperature hardness as high as 149 HV and an electrical conductivity of84.8%IACS.The strength property of DS Cu-Cr-Zr alloys could be improved by cold rolling,for instance,DS Cu-0.75Cr-0.1Zr exhibiting a tensile strength of 412 MPa,a plastic strain of9%,and an electrical conductivity of 31.8%IACS.After SSA,the material properties of Cu-Cr-Zr alloys have been significantly improved,and especially for the DS Cu-0.75Cr-0.2Zr alloy,a tensile strength of 382 MPa,a large plastic strain of 32%,and an electrical conductivity of 83.8%IACS have been obtained.The strength and electrical conductivity of this alloy are comparable to the ITER grade Cu-Cr-Zr,but the plasticity is greatly improved.Dispersion-strengthened Cu-Zr and Cu-Cr-Zr alloys have been fabricated in the present study by using a new processing means.The Cu alloys exhibit good mechanical and electrical conductivity properties.The strengthening mechanism in the new alloys needs to be clarified.It is expected that the properties of the alloys can be further improved by adjusting the composition of the intermediate alloy and the optimization of processing procedures.The new alloys are promising candidates for the high temperature creep,and ion and neutron irradiation studies for developing suitable heat-sink materials used in the future fusion reactors.