Evolution of microstructure and hardness during artificial aging of an ultrafine-grained Al-Zn-Mg-Zr alloy processed by
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Evolution of microstructure and hardness during artificial aging of an ultrafine-grained Al-Zn-MgZr alloy processed by high pressure torsion } Gubicza1,*, Moustafa El-Tahawy2, Ja´nos L. La´ba´r1,3, Elena V. Bobruk4, Jeno Maxim Yu Murashkin4, Ruslan Z. Valiev4, and Nguyen Q. Chinh1 1
Department of Materials Physics, Eötvös Loránd University, P.O.B. 32, Budapest H-1518, Hungary Department of Physics, Faculty of Science, Tanta University, Tanta 31527, Egypt 3 Institute for Technical Physics and Materials Science, Centre for Energy Research, Budapest, Hungary 4 Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, 12 K. Marx Str, Ufa, Russia 450008 2
Received: 13 May 2020
ABSTRACT
Accepted: 24 August 2020
An ultrafine-grained (UFG) Al-4.8%Zn-1.2%Mg-0.14%Zr (wt%) alloy was processed by high pressure torsion (HPT) technique and then aged at 120 and 170 °C for 2 h. The changes in the microstructure due to this artificial aging were studied by X-ray diffraction and transmission electron microscopy. It was found that the HPT-processed alloy has a small grain size of about 200 nm and a high dislocation density of about 8 9 1014 m-2. The majority of precipitates after HPT are Guinier–Preston (GP) zones with a size of * 2 nm, and only a few large particles were formed at the grain boundaries. Annealing at 120 and 170 °C for 2 h resulted in the formation of stable MgZn2 precipitates from a part of the GP zones. It was found that for the higher temperature the fraction of the MgZn2 phase was larger and the dislocation density in the Al matrix was lower. The changes in the precipitates and the dislocation density due to aging were correlated to the hardness evolution. It was found that the majority of hardness reduction during aging was caused by the annihilation of dislocations and some grain growth at 170 °C. The aging effect on the microstructure and the hardness of the HPT-processed specimen was compared to that observed for the UFG sample processed by equal-channel angular pressing. It was revealed that in the HPT sample less secondary phase particles formed in the grain boundaries, and the higher amount of precipitates in the grain interiors resulted in a higher hardness even after aging.
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The Author(s) 2020
Handling Editor: P. Nash.
Address correspondence to E-mail: [email protected]
https://doi.org/10.1007/s10853-020-05264-4
J Mater Sci
Introduction Severe plastic deformation (SPD) is an effective and extensively studied way to produce ultrafine-grained (UFG) metals and alloys in the bulk form [1]. The two most frequently used SPD methods are the equalchannel angular pressing (ECAP) and the high pressure torsion (HPT) [2, 3]. These techniques yield very hard materials due to the high dislocation density and the small grain size developed during SPD [1, 4]. Additional hardening can be achieved by post-SPD annealing due to segregation of solutes to lattice defects (e.g., to dislocations, stacking faults and grain boundaries) [5–8], annihilation of mobile dislocations [9],
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