Microstructure And Properties Of Cu-Cr-Nb-Ce Alloy Fabricated By Selective Laser Melting

As a key material in aerospace and nuclear energy fields,Cu-Cr-Nb alloy has excellent strength,low cycle fatigue properties,creep properties,electrical and thermal conductivity and high-temperature stability.To meet the demand for high-performance copper alloys at the application end,it is a challenge to develop Cu-Cr-Nb alloys by regulating the size and distribution of Cr₂Nb phase and improving its comprehensive performance.In this paper,the Cu-Cr-Nb-Ce alloy was fabricated by selective laser melting（SLM）,using Ce as the trace addition element.The distribution uniformity of alloying elements is improved and its solid solubility in copper matrix is increased.By heat treatment process,alloying elements are uniformly precipitated to form fine and dispersive second phase,which can realize the synchronous improvement of alloy strength and electrical conductivity.The structural evolution of the second phase during aging and the growth mechanism of the second phase under high temperature were investigated.It mainly includes the following contents and results:（1）In the range of 270 W-360 W,the relative density of forming parts increases with the increase of laser power,and the highest relative density reaches 99.2%.At low power,the defects are mainly irregular lack of fusion defects,and at high power,the defects are mainly spherical pores.The microstructure of XY surface of high relative density forming parts is composed of long columnar crystals in the center of molten pool and equiaxed fine crystals distributed along the boundary of molten pool.The overall morphology of XZ surface is columnar crystal along the building direction.The second phase in the formed part is dispersed and evenly distributed in the matrix,Cr₂Nb phase is the C14 structure of HCP,and Cr phase is GP zone or FCC.The hardness of the forming part is 126 HV,the electrical conductivity is 18.5%IACS,the ultimate tensile strength is 403MPa and the elongation is 19.9%.（2）After direct aging at 550℃for 20 min,the alloy has excellent comprehensive properties,with microhardness of 232 HV,tensile strength of 680 MPa,elongation of 15.9%and conductivity of 71.6%IACS.After direct aging treatment,the precipitation sequence of Cr phase is:supersaturated solid solution→FCC GP zone→FCC Cr phase→BCC N-W Cr phase→BCC K-S Cr phase.The Cr₂Nb phase precipitated during aging is FCC structure,and the crystal structure of HCP Cr₂Nb formed during SLM has not changed.（3）After heat treatment at 700℃for 10 h,50 h and 100 h,the average size of Cr₂Nb phase of as-SLMed Cu-Cr-Nb-Ce alloy is 42.7 nm,51.7 nm and 57.5 nm respectively,and the microhardness decreases to 121 HV,113HV and 112 HV respectively.After high-temperature heat treatment at1000℃for 10 h,50 h and 100 h,the average size of Cr₂Nb phase is 348.6nm,524.9 nm and 589.4 nm respectively,and the microhardness decreases to 84 HV,79 HV and 75 HV respectively.The high-temperature heat-treated specimens maintain the overall grain morphology of the formed parts,and the grain size of the 700℃specimens have no obvious change,while the grain size of the 1000℃heat-treated specimens for 10 h,50 h and 100 h increase by 11.2%,21.7%and 30.4%respectively compared with the formed parts.After 10 h high-temperature heat treatment,the electrical conductivity of 700℃and 1000℃high-temperature heat-treated specimens increase to 79.2%IACS and 54.6%IACS respectively,whereas no significant progress is observed with extension of the heat treatment time.