Fabrication And Investigations On Microstructure And Mechanical Properties Of Ultrafine-grained Cu-Mn Alloys

As the integration of integrated circuits increases,interconnects become thinner and narrower,the traditional pure aluminum and pure copper sputtering targets cannot satisfy the process technology of metal interconnect material,and alloying has become mainstream of the target development.Copper-manganese(Cu-Mn)alloy has a selfdiffusion effect and can form a self-diffusion barrier layer,which is increasingly used in the integrated circuit manufacturing industry.Rolling and supplemented recrystallization annealing is the traditional preparation process of the Cu-Mn alloy sputtering target,but the grain refinement effect is limited by this method and the mechanical properties significantly decline after heat treatment,which cannot meet the sputtering requirements of high-quality integrated circuits.In the present work,a Cu-Mn alloy with a Mn content of 5 at% is used as the base metal,and pure copper was used as the contrast material.Ultrafine-grained(UFG)Cu-Mn alloys were prepared by two severe plastic deformation(SPD)processes,equal channel angular pressing(ECAP)and friction stir processing(FSP),respectively.The effects of deformation mechanisms and process parameters on the microstructure and properties of the UFG Cu-Mn alloys were studied while the microstructure modification was realized,and the differences in material characteristics between these two production processes are compared to provide a reference for industrial production.The main results of the present study are as follows:(1)The addition of Mn elements promoted the dislocation accumulation during the deformation of ECAP,the formation of shear bands and the local stress concentration,which induced deformation twinning and accelerated the grain size reduction of Cu-Mn alloy in ECAP deformation,resulting in a better grain refinement than that of pure copper.After four passes of deformation,the grain sizes of Cu-5Mn alloy and pure copper reached260 nm and 570 nm in the cross section,respectively.(2)Under the combination effects of solid solution strengthening,grain refinement,twinning strengthening and dislocation strengthening,a significant increase in the hardness and strength of the Cu-5Mn alloy is achieved after ECAP treatment.After four passes of processing,the hardness values at the cross-section and the longitudinal section are similar,but the strength of the longitudinal section sample is slightly higher,which can reach up to 559 MPa.(3)The refinement of microstructure in the processed zone of FSP Cu-5Mn alloy mainly relies on dynamically recrystallization.The ultrafine-grains were prepared by using a columnar threaded stirring pin at 300 rpm with additional water cooling(WC),with an average grain diameter of about 460 nm and a large number of annealed twin boundaries.FSP Cu-5Mn alloy can reach a high hardness value of 166 HV and a strength of 503 MPa,which is better than that of the cold rolled Cu-5Mn alloy and the plasticity is simultaneously improved.(4)A spherical stirring pin without shoulder was used for multiple-pass FSP at a low rotation rate of 300 rpm.The grain sizes of the processed and transition zones of the 300rpm(WC)FSP 6-pass lap sample were similar,with an average grain size of about 238 nm,an ultimate tensile strength of 617 MPa,and an average hardness value of about 209 HV.(5)Compared with the non-equilibrium ultrafine grained structure consisting of most low-angle grain boundaries obtained by ECAP processing,the processed zone of FSP Cu-5Mn alloy has an equiaxed ultrafine grained structure with low dislocation density,weak texture and dominance of high-angle grain boundaries(HAGBs),which exhibits more uniform microstructure.