Microstructures and mechanical properties of ultrafine grain

Microstructures and mechanical properties of ultra?ne grained 7075Al alloy processed by ECAP and their evolutions during annealing

Y.H.Zhao

a,*

,X.Z.Liao a ,Z.Jin b ,R.Z.Valiev c ,Y.T.Zhu

a,*

a Materials Science and Technology Division,Los Alamos National Laboratory,MS G755,MST-STC,Los Alamos,NM 87545,USA

b

Alcoa Technical Center,Alcoa Center,PA 15069,USA

c

Institute of Physics of Advanced Materials,Ufa State Aviation Technical University,12K.Marx Street,450000Ufa,Russian Federation

Received 8June 2004;accepted 9June 2004

Available online 2July 2004

Abstract

Microstructures and mechanical properties of equal-channel angular pressing (ECAP)processed and naturally aged ultra?ne grained (UFG)and coarse grained (CG)7075Al alloys as well as their evolutions during annealing were investigated.After the same natural aging,the tensile yield strength,ultimate strength,and microhardness of the UFG samples were 103%,35%,and 48%higher,respectively,than those of the CG samples,because of higher densities of Guinier–Preston (G–P)zones and dislocations in the UFG sample.Upon annealing,the microhardness of the UFG sample decreased gradually,while a hardening peak appeared for the CG sample.The peak was caused by the precipitation hardening of the metastable g 0phase.For the UFG sample,the precipitation hardening was overcompensated by the signi?cant decrease of microstrain (dislocation density)upon annealing,resulting in a lack of precipitation hardening peak.Di?erential scanning calorimetry indicates that the ECAP process only accelerated the phase precipitations,but did not change the sequence of phase precipitation.This study shows that severe plastic deformation has the potential to signi?cantly improve the mechanical properties of age-hardening Al alloys.ó2004Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.

Keywords:Severe plastic deformation;Aluminum alloy;Aging hardening;Microstructures;Mechanical properties

1.Introduction

Ultra?ne grained (UFG)metals and alloys processed by severe plastic deformation (SPD)techniques have been reported to have superior mechanical properties,such as high strength with good ductility and excellent superplasticity at lower temperature and higher strain rates [1–3].Among all the SPD techniques,equal channel angular pressing (ECAP)is especially attractive because it can economically produce bulk UFG mate-rials that are 100%dense,contamination free and large enough for real structural applications.[1,2,4,5].

In the last decade,many metals and alloys,including Al and its alloys,Cu,Ni,Ti and its alloys,and steels,

have been successfully processed by the ECAP tech-nique.For Al alloys,most ECAP e?ort has focused on work strengthening Al–Mg alloys [6–9].Little attention has been paid to precipitate strengthening Al–Zn–Mg 7000series alloys [10,11],which are widely used for high strength structural applications such as aircrafts and sporting goods.Cold working,which usually improve the strength of metals and alloys,has been found to be ine?ective in improving the strength of 7000series Al alloys [12].It is of great interest to investigate if the strengthening e?ect observed in ECAP processed UFG materials can be added to the precipitation hardening e?ect in the 7000series Al alloys.If these two strengthening e?ects can be made additive,it will be possible to signi?cantly improve the strength of 7000Al alloys,making them much more attractive in high strength structural applications.

It is the objective of this study to investigate the e?ect of ECAP processing on the mechanical properties and

*Corresponding authors.Tel.:+1-505-665-6774(Y.H.Zhao),+1-505-667-4029(Y.T.Zhu);fax:+1-505-667-2264(Y.T.Zhu).

E-mail addresses:zhao@b4a938afdd3383c4bb4cd2c8 (Y.H.Zhao),yzhu@b4a938afdd3383c4bb4cd2c8 (Y.T.Zhu).

1359-6454/$30.00ó2004Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.

doi:10.1016/j.actamat.2004.06.017

Acta Materialia 52(2004)

4589–4599

b4a938afdd3383c4bb4cd2c8

precipitation kinetics of the Al–Zn–Mg–Cu7075alloy, which is one of the7000series Al alloys.The alloy was processed by ECAP immediately after solution treat-ment to produce a UFG structure,and then naturally aged at room temperature for one month.The micro-structures and mechanical properties of the aged UFG 7075Al alloy and their evolutions during non-isother-mal annealing were investigated.The results demon-strate that ECAP processing can enhance the precipitation kinetics and add signi?cant strengthening e?ect to the precipitation hardening in the7075Al alloy.

2.Experimental materials and procedures

2.1.Sample preparation

Commercial7075Al alloy rods with a diameter of 20mm were treated at480°C for5h to form a solid solution with an initial grain size of approximately40 l m,and then quenched to room temperature.The Al alloy contains(in wt%)5.60%Zn,2.50%Mg,1.60%Cu, 0.50%Fe,0.40%Si,0.30%Mn,0.23%Cr,and0.20%Ti.

The ECAP processing was carried out immediately after quenching using a die with an intersecting channel angle of90°and an outerarc angle of45°.This die con?guration imposes an e?ective strain of approxi-mately one per ECAP pass[13].The sample was pro-cessed for two passes by route B c in which the work piece was rotated90°along its longitudinal axis after the ?rst pass.The ECAP-processed UFG and initial solu-tion treated coarse grained(CG)7075Al alloys were naturally aged at room temperature for one month,then were cut into small pieces in an orientation perpendic-ular to the pressing direction.The surfaces of these thin pieces were polished for subsequent characterizations.

2.2.Thermal analysis

Thermal analysis was performed in a Perkin–Elmer di?erential scanning calorimeter(DSC-7).Polished alloy disks with a diameter of5mm were sea …… 此处隐藏：30324字，全部文档内容请下载后查看。喜欢就下载吧 ……