Microstructure And Deformation Properties Of Nanoparticle-fine Grain (NPFG) Synergistically Reinforced Copper Alloys

The attainment of both high strength and high ductility is a vital requirement for most of structural materials,unfortunately these properties are generally mutually exclusive.Here,copper microstructure was engineered via alloying and casting process.Two groups of the cast samples including(Ⅰ)Cu-10Sn-2Zn and Cu-10Sn-2Zn-1.5Fe-0.5Co alloys;and(Ⅱ)pure Cu,Cu-(1.0,1.5,2.0,3.0)Fe-0.5Co and Cu-1.5Fe-0.1Sn alloys were fabricated and characterized by OM,SEM.TEM and tensile test.For group(Ⅰ),numerous iron-rich nanoparticles(NP)with different crystallographic characteristics were dispersed within fine copper grains(20-40 μm.FG).deemed as NPFG structure,in Cu-10Sn-2Zn-1.5Fe-0.5Co alloy.Brittle tin-rich δ phase in NPFG alloy was greatly reduced comparing to that in Cu-10Sn-2Zn alloy.Coupled with DSC and APT methods and Thermo-Calc software,formation mechanism of NPFG structure was interpreted as the interactions between the nanoparticle and copper melt during solidification,and δ phase reduction was explained by nanoparticle-wall mechanism.Mullins-Sekerka’s instability theory was applied to evaluate the morphology evolution of iron-rich crystal in Cu melt from the stable sphere to petal-like unstable shape.NPFG alloy possessed improved tensile strength of 463MPa,yield strength 215MPa and total elongation of 28%,respectively,at room temperature,in contrast to Cu-10Sn-2Zn alloys.This suggest that the synergistic effect of refined micrometer-sized grains and dispersed nanoparticles in regard to strengthening can be applicable to achieve excellent combination of high strength and high ductility.With respect to group(Ⅱ),iron-rich nanoparticle grew in size with increasing Fe.Meantime,the nanoparticle underwent morphological evolution from sphere to cuboid,to petal with size increment.Fe,Co doping provoked the columnar to equiaxed transition(CET)and grain refinement,and minor Sn can enhance refinement efficiency.NPFG structure achieved in Cu-(1.5～3.0)Fe-0.5Co and Cu-1.5Fe-0.1 Sn alloys gave enhanced tensile strength,yield strength,as well as uniform and total elongation,almost double values on all these properties compare to coarse-grained(CG)pure Cu.Coupled with Thermo-Calc calculation and phase-field simulation,precipitation behavior,transformations of crystal structure and morphology were investigated.Grain refinement was attributed to heterogeneous nucleation effect of iron-rich nanoparticles.Combined with in-situ TEM tensile test,the dependences of mechanical properties on the Fe,Co,Sn doping were discussed in terms of the microstructural evolution and quantitatively described by using strengthening models.The results indicate an optimum balance of strength and ductility is achieved by designing a microstructure containing fine grains,intragranular smaller spherical nanoparticles,and minor solution element with higher misfit and growth restriction effect.This work provides a guideline for engineering NPFG microstructure to architecture simultaneously strong and ductile bulk metals.