Microstructures And Properties Of Nanotwinned CuCrZr Alloy Strengthened By Precipitates

Nanotwinned materials have attracted wide attentions over the past twenty years owing to their high hardness and strength,excellent electrical conductivity,as well as outstanding thermal stability.Precipitation strengthening is usually strengthening method in metal materials,which is often used in Al alloys,Cu alloys,Mg alloys,Ni alloys and other precipitation-strengthened alloys to improve their strength.However,the current researches of nanotwinned materials mainly focus on single-phase materials,and related researches on nanotwinned strengthening in precipitation-strengthened alloys are rather limited.How to achieve the effective superposition of nanotwinned strengthening and precipitation strengthening in metal materials is not clear.Meanwhile,the effects of nano-scaled grain boundaries of nanotwinned materials on precipitation strengthening and the effects of precipitates on the thermal stability of nanotwinned materials are also unclear.Therefore,the purpose of this study is to discuss the three issues mentioned above.In this work,we systematically investigated the microstructures of the solid solution treated Cu-1Cr-0.1Zr alloy after dynamic plastic deformation at liquid nitrogen temperature,and the effects of different aging treatment processes on the microstructures and performances were analyzed emphatically.In addition,the effects of precipitates on the thermal stability of nanostructures were investigated.Furthermore,the effects of grain boundaries with different scales on the aging behaviors and precipitation strengthening mechanisms in the CuCrZr alloy were studied systematically.The main results are as follows:1.A mixed nanostructure composed of a high density of nanotwins and nanograins was prepared in a solid solution treated CuCrZr alloy by means of dynamic plastic deformation at liquid nitrogen temperature with ?=2.0(LNT-DPD).The microstructures of the LNT-DPD sample were composed of nanograins with an average transverse size of-39 nm,nanotwin bundles with a twin/matrix lamella average thickness of?20 nm as well as a small number of dislocation structures.Among them,the volume fraction of nanograins was?62%,the volume fraction of nanotwins was?23%,and the volume fraction of dislocation structure was?15%.In the peak-aged LNT-DPD sample aged at 400? for 3 hours,most of the nanostructures were retained,and a mass of nano-scaled precipitates were distributed on the grain boundaries.Precipitates were successfully introduced into the nanotwinned sample,and the precipitation strengthening was effectively superimposed on the basis of the nanotwinned strengthening.The nanotwinned CuCrZr sample with precipitates had a tensile strength of?832 MPa and an electric conductivity of?71.2%IACS.The strength-conductivity combination is obviously better than those of other samples in the literatures.Such combined strengthening effect from nanotwins and precipitates provides a new way to prepare high-strength and high-conductivity CuCrZr alloys.2.A pure Cu sample,a pre-aged CuCrZr sample at 600? for 2 hours and a solid solution treated CuCrZr sample at 1000? were subjected to dynamic plastic deformation at liquid nitrogen temperature(LNT-DPD,?=2.0)to prepare three groups of nanotwinned samples.The average transverse sizes of lamellar nanograins in three groups of nanotwinned samples were?79 nm,?75 nm,and?64 nm,respectively.The precipitates with an average size of?5.5 nm were distributed homogeneously inside the grains in the pre-aged DPD CuCrZr sample(P-interior CuCrZr).After the solid solution treated DPD CuCrZr sample was aged at 400? for 10 min,an aged sample(P-GB CuCrZr)was achieved and the precipitates were distributed on the grain boundaries with an average size of?1 nm.Then,the DPD Cu(NS Cu),the P-interior CuCrZr sample and the P-GB CuCrZr sample were annealed at different temperatures.The onset temperature of grain growth in the NS Cu was 80?,and it was 250? in the P-interior CuCrZr.The enhanced thermal stability of the P-interior CuCrZr sample was derived from the pinning force provided by the uniformly distributed precipitates inside the grains,which hinder the movement of the grain boundaries.Furthermore,the P-GB CuCrZr sample had excellent thermal stability,and the nanostructures remained stable at 400?.The precipitates in the P-GB CuCrZr sample were located on the grain boundaries.The special distribution of precipitates greatly improved the pinning force and the thermal stability of nanostructures.It provides a new way for preparing nanostructured materials with good thermal stability,and it is of great significance for the high temperature application of nanostructured materials.3.Ultrafine-grained sample(UFG)with an average transverse size of?266 nm was prepared in a solid solution treated coarse-grain CuCrZr alloy(CG)by means of dynamic plastic deformation at room temperature.A mixed nanostructured sample(NS)composed of nanotwins and nanograins was prepared in a solid solution treated CG by means of dynamic plastic deformation at liquid nitrogen temperature.Appropriate aging treatments were performed respectively in the three groups of solid solution treated samples(CG,UFG,NS),and three groups of peak-aged samples with similar volume fractions of precipitates(?0.79%)were obtained.In the aged CG sample,the precipitates with an average size of?3.7 nm were distributed homogeneously inside the grains,which contributed to the precipitation strengthening of?190 MPa.In the aged UFG sample,the precipitates with an average size of?2.6 nm were distributed homogeneously inside the grains.A high density of dislocations in the UFG sample provided a large number of nucleation sites for the precipitates,which made the precipitation more uniform and reduced the precipitate size.The precipitates contributed to the precipitation strengthening of?87 MPa.The precipitation strengthening was significantly weakened than that in the aged CG sample.The dislocations introduced by RT-DPD and the precipitates introduced by aging treatment strengthen the materials by hindering the movement of dislocations,but a large number of precipitates coincided with the positions of the dislocations,which resulted in the decreased superposition of dislocation strengthening and precipitation strengthening.Thereby the precipitation strengthening exhibited in the aged UFG sample was weakened.In the aged NS sample,the precipitates were located on the grain boundaries of nanotwins and nanograins.The precipitates were dispersed on the grain boundaries and the precipitate size was reduced,with an average size of?3.3 nm.After considering the softening of the deformed structures after aging treatment,the precipitation strengthening was?246 MPa.Obviously,the enhanced precipitation strengthening was obtained in the aged NS sample than that in the aged UFG sample.Moreover,the special distribution of precipitates reduced the precipitate spacing and improved the strength for dislocation bypassing,the higher precipitation strengthening was obtained in the aged NS sample than that in the aged CG sample.The aging behaviors and precipitation strengthening mechanisms in the aged UFG sample and the aged NS sample are of great significance for understanding the effects of grain boundaries with different scales and the high density of dislocations on precipitation strengthening.This work makes it possible through precipitation-strengthened nanostructures to obtain a higher strength,and is of great significance for the development of ultra-high-strength metal materials.