Impact Toughness,Thermal Stability And Electrical Conductivity Of Bulk Ultrafine-grained Cu And CuCrZr Alloy

Nanostructured(NS)and uktrafine grained(UFG)materials possess some superior physical,chemical and mechanical properties as well as promising industrial applications.Therefore,the investigations related with the NS materials are becoming hotspots of materials research in recent three decades.In recent years,more and more attentions have been payed to the industrial applications of bulk NS materials which require the NS materials possess excellent stability etc.properties under service environments.For instance,the service under impact loading requires the NS materials have high impact toughness,i.e.the ability of a material to resist crack propagation at high strain rates.However,most of literature reported work on impact properties of UFG materials are limited to measure the value of impact energy/toughness,seldom tp explore the strength and microstructure evolutions during impact process.The use of a material in elevated temperatures needs termal stability,and the UFG/NS material,due to their high volume fraction of grain boundary(GB),are instable at high temperatures.Alloying elements were usually introduced into pure UFG/NS materials to lower the grain growth energy and enhance the thermal stability.For pure NS materials,how to design a stable nanostructure is still a challenge.As a promising engineering material for contact wires for high-speed railways,CuCrZr alloys need more attractive combination of ultimate tensile strength,electrical conductivity and thermal stability to endure the service conditions of high-speed trains.However,high strength and high electrical conductivity are often mutually exclusive in materials strengthened by conventional methods,such as aging,or grain refinement.Based on above issues,in this dissertation equal channel angular pressing(ECAP)was employed to produce UFG microstructure in typical f.c.c.metals,pure Cu and CuCrZr alloy at room temperature.The Charpy impact behavior and thermal stability of pure Cu with different microstructure was studied.We quantitatively calculated the impact induced temperature rise and thermodynamics and kinetic parameters of recrystallization in theory.UFG CuCrZr alloy with a superior combination of high strength and high electrical conductivity was obtained by heat treatment.Insight understanding on the mechanisms for strengthening,thermal stability and electrical conductivity was provided by deeply structure characterization.The main conclusions can be drawn:(1)UFG Cu samples were produced by ECAP process for 2 and 16 passes with Bc route.For ECAP-2 sample,elongated/lamella grain structure was observed and the thickness was measured to be about 230 nm.The lamella structure was surrounded by low-angle GBs(LAGBs)with high proportion of 89%.For ECAP-16 sample,micro-grains were fully refined to be equi-axed with grain size measured to be about 270 nm.Grains were uniform and more than 55%GBs were detected to be high-angle GBs.(2)Both the UFG Cu samples have comparable impact toughness of about 48 J/cm2,which is almost comparable with that of the CG Cu samples:55 J/cm2.Compared with the tensile curves under quasi-static strain rate,high strain rate was found to enhance the strain hardening capability of the UFG Cu due to the suppression of dislocation dynamic recovery.EBSD mapping revealed that the CG Cu sample underwent large plastic deformation mediated by dislocation slip in near-crack region,which produced elongated grains and subgrain structure,while the UFG Cu samples formed cracks at the GBs and triple junctions due to limited plasticity and dislocation activity.Along the crack,recrystallized refined grains in the ECAP-2 Cu and large grown grains in the ECAP-16 Cu were found although the high temperature rises were close for both samples.(3)The hardness and yield strength measurements indicate that ECAP-2 Cu exhibits a higher thermal stability than ECAP-16 Cu under isochronal and isothermal annealing.The EBSD and TEM analyses verified that the recrystallization process of the ECAP-2 Cu was retarded to higher annealing temperature(295?)or longer annealing(10 h)time compared with that of the ECAP-16 Cu(240?,0.6h).The stored energy of the ECAP-2 Cu(0.4 J/g)is calculated to be smaller than that of the ECAP-16 Cu(0.66 J/g)due to its low GB energy of LAGBs and low boundary area per unit volume.The activation energy for recrystallization of the ECAP-2 Cu(94 kJ/mol)is measured to be larger than that of the ECAP-16(72 kJ/mol)due to the low mobility of the LAGBs.(4)A multiscale architectured structure,nanotwinned ultrafine grains surrounded by nano-precipitates at grain boundaries,was developed in a bulk CuCrZr alloy prepared by aging treatment(at 460? for 1.5 h)following ECAP.A superior combination of high strength(676 MPa),high electrical conductivity(73%IACS)and good thermal stability was obtained,which avoided the trade-off among these important properties of electric conductive materials.This provides insight understanding on the mechanisms for strengthening,thermal stability and electrical conductivity,and could help the development of high-performance electrical conductors.