深冷轧制制备纳米6061铝合金的热稳定性评估

Trans. Nonferrous Met. Soc. China 27(2017) 754−762

Thermal stability evaluation of

nanostructured Al6061 alloy produced by cryorolling

M. ABBASI-BAHARANCHI, F. KARIMZADEH, M. H. ENAYATI

Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

Received 2 January 2016; accepted 12 October 2016

Abstract: Grain growth of nanostructured Al6061 produced by cryorolling and aging process was investigated during isothermal

heat treatment in 100−500 °C temperature range. Transmission electron microscopy (TEM) observations demonstrate that after

cryorolling and aging at 130 °C for 30 h, the microstructure contains 61 nm grains with dispersed 50−150 nm precipitates and

0.248% lattice strain. In addition, an increase in tensile strength up to 362 MPa because of formation of fine strengthening

precipitation and nano-sized grains was observed. Thermal stability investigation within 100−500 °C temperature range showed

release of lattice strain, dissolution of precipitates and grain growth. According to the X-ray diffraction (XRD) analysis, Mg2Si

precipitates disappeared after annealing at temperatures higher than 300 °C. According to the results, due to the limited grain growth

up to 200 °C, there would be little decrease in mechanical properties, but within 300−500 °C range, the grain growth, dissolution of

strengthening precipitates and decrease in mechanical properties are remarkable. The activation energies for grain growth were

calculated to be 203.3 kJ/mol for annealing at 100−200 °C and 166.34 kJ/mol for annealing at 300−500 °C. The effect of

precipitation dissolution on Al lattice parameter, displacement of Al6061 (111) XRD peak and Portevin−LeChatelier (PLC) effect on

stress−strain curves is also discussed.

Key words: mechanical characterization; X-ray diffraction; aluminium alloy; bulk deformation; grain growth; grain refinement

1 Introduction

In recent years, ultra-fine grained/nanostructured

(UFG/NS) materials have received a great attention due

to their superb mechanical properties[1−4]. However,

structural changes, including grain growth and phase

transformations as a result of increasing in working

temperature will affect attractive properties of the

mentioned materials. Generally, severe plastic

deformation (SPD) processes are used to produce bulk

UFG/NS materials. Recently, UFG/NS materials

developed by equal channel angular pressing (ECAP),

accumulative roll bonding (ARB), friction stir processing

(FSP), high pressure torsion (HPT) and cryorolling (CR)

have shown high strength and hardness as compared to

coarse-grained structures [5]. Highly unstable

microstructure containing non-equilibrium grains and

high dislocation density are the main characteristics of

severe plastic deformed materials. During recent years,

for aluminum alloys cryorolling process followed by

optimum heat treatment is introduced as an effective

process to develop bulk UFG/NS Al alloys. The excellent

mechanical properties during cryorolling is obtained

from suppression of dynamic recovery due to

deformation at cryorolling temperature, which follows by

optimal heat treatment and hence stable UFG/NS

materials would be produced, which are suitable for

structural and thermomechanical applications. DAS

et al [6] have reported the improved tensile strength and

impact toughness of the cryorolled Al7075 alloy due to

grain refinement, and ultra-fine grain formation by

multiple cryorolling passes. According to their results,

the yield strength and impact toughness of the cryorolled

Al7075 alloy up to 70% thickness reduction have

increased by 108% and 60% respectively compared to

the starting material. CHENG et al [7] have reported

simultaneous increases in strength, work-hardening

ability and ductility of cryorolled Al2024 alloy. The

effect of deformation at the cryogenic temperatures

during rolling of Al6061 and Al6063 was investigated by

RAO et al [8,9]. According to the results, the pre-CR

solid solution treatment combined with post-CR

aging treatment for Al6061 alloy results in enhancing

Corresponding author: M. ABBASI-BAHARANCHI; Tel: +98-3133915744; Fax: +98-3133912752; E-mail: @

DOI:

10.1016/S1003-6326(17)60086-4