Effect Of Sc And Co On The Precipitation Behaviour Of The Substable Phase Of Cu-Cr Alloys

With their high strength and high electrical conductivity,Cu-Cr series alloys play an important role in the leadframes of large-scale integrated circuits,high-speed railway contact lines,high-end electrodes and connectors for electronic components.With the continuous development of the industry,higher demands are placed on the performance of the alloys.How to further improve the comprehensive performance of Cu-Cr alloys has become the focus of research.As an age-strengthened copper alloy,the Cu-Cr system precipitates nano-sized Cr-rich phase particles during the ageing process,thereby increasing the strength of the alloy.The addition of micro-alloying elements can influence the size,morphology and crystal structure of the Cr-rich phase during its ageing process and other characteristics,thus changing the strength of the alloy.In this paper,Cu-Cr,Cu-Cr-Sc and Cu-Cr-Co alloys were studied.The evolution of Cr-rich phase structure,composition,size and volume fraction during ageing was investigated by means of TEM and APT,and the effects of the addition of Sc and Co elements on the ageing precipitation behaviour of the alloys were investigated.The effects of the deformation heat treatment process,Cr content and the addition of the third component（Sc and Co）on the structure and properties of the Cu-Cr alloy are discussed.The Cu-Cr-Sc（Co）alloy was prepared by the"casting-solid solution-aging"process and the changes in phase structure,phase size and composition during the aging process（aging temperature of 450°C）were analyzed.It was shown that when the aging time was in the range of 0 to 8 h,the precipitation aging sequence of the Cu-Cr-Sc alloy was as follows:supersaturated solid solution-with a cube-on-cube FCC-rich Cr phase-with a matrix orientation of（2?20）_Cu//（2?20）_Cr,[112]_Cu//[110]_Cr for the FCC Cr-rich phase.As the aging time increases,the Cr atoms are enriched to form a Cr-rich phase;the Sc atoms are polarized at the precipitated phase/matrix interface,and the polarization of the Sc elements hinders the diffusion of the Cr atoms and inhibits the FCC to BCC structural transformation of the precipitated phase.In addition,the addition of Sc reduces the rate of desolvation of solute atoms in the Cu-based solid solution and promotes the nucleation of the Cr-rich phase at the early stage of ageing,resulting in a smaller particle size of the precipitated phase in the Cu-Cr-Sc alloy than in the Cu-Cr alloy at the same ageing time,resulting in a high ageing strengthening effect of the Sc-containing alloy.In the ageing Cu-Cr-Co alloy,the Cr-rich phase of the FCC structure shows two orientations in relation to the Cu matrix.With increasing aging time,the orientation changes from（200）_Cu//（200）_Cr,[011]_Cu//[011]_Cr（cube-on-cube）to（2?20）_Cu//（2?20）_Cr,[112]_Cu//[110]_Cr.Under the two different orientations,the Cr-rich phase of the FCC structure is prevented from dislocating by the Orowan mechanism.The Orowan mechanism impedes dislocation movement and thus increases the strength of the alloy.In addition,the shift in orientation causes the Cr-rich phase to have a different degree of mismatch with the Cu matrix along different grain orientations,leading to the formation of ellipsoidal precipitated phase particles.When the aging time exceeds 8 h,the FCC Cr-rich phase gradually transforms into a BCC Cr-rich phase with an N-W orientation relationship.As the ageing time progressed,Co remained uniformly distributed in the Cr-rich phase,and the concentration in the FCC Cr-rich phase remained relatively low,below 7 at.%.In this paper,Cu-Cr-Sc（Co）alloy plates were prepared by the"casting-hot rolling-solid solution-cold rolling-aging"process,and the mechanical properties of the Cu-Cr alloy were improved by introducing dislocations and increasing the Cr and Sc content.The results show that pre-deformation before ageing can introduce a high density of dislocations in the Cu-Cr-Sc（Co）alloy,resulting in a strong dislocation strengthening effect.The hardness of the Cu-Cr-Sc（Co）alloy is less affected by the increase in Cr and Co content,but the strength of the Cu-Cr alloy is improved by the addition of Sc.The Cu-0.65Cr-0.20Sc alloy has a hardness of 188.0HV0.5 and a tensile strength of 587 MPa after deformation heat treatment.The tensile strength reached 587 MPa and the electrical conductivity reached 73.6%IACS,which has a good overall performance.