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High-pressure torsion processing of AZ91 magnesium alloy

High-pressure torsion processing of AZ91 magnesium alloy
High-pressure torsion processing of AZ91 magnesium alloy
AZ91 magnesium alloy has been successfully processed at room temperature by high–pressure torsion as well as at elevated temperatures. Ultrafine grains and nano–sized particles of ?–phase have developed with increasing number of turns. The hydrostatic pressure, the geometry of the processing zone and the unidirectional nature of torsional straining during the HPT processing have facilitated processing of AZ91 alloy at room temperature. Extensive grain refinement and twinning segmentation of the coarse grains have been observed in the microstructures processed at room temperature and elevated temperatures, respectively. The twins have been observed at all processing temperatures during processing and their distribution was proportional to the processing temperature and number of turns. The morphology and distribution of the ?–phase have altered during processing, with fragmentation of coarse clusters of the ?–phase into nano–sized particles and the alignment of these particles in the direction of torsional strain being observed. Microstructural homogeneity has gradually developed at a relatively low number of turns using the lower processing temperature and continued with increasing number of turns. A significant improvement in the strength of the alloy has been found after HPT processing at all processing temperatures. The dislocation density has developed significantly for the alloy processed at room temperature rather than at elevated temperatures with increasing number of turns. An experimental Hall–Petch relationship has emphasized a significant dependence of the strength on grain size for the alloy processed at room temperature. The high–strain rate superplasticity, low–temperature superplasticity, and thermal stability of the processed alloy have been observed and attributed to the ultrafine–grained microstructures produced by HPT at room temperature and the dispersion of nano–sized ?–phase particles. Grain–boundary sliding was the main deformation mechanism during the high–strain rate superplasticity regime. Glide–dislocation creep accommodated by grain–boundary sliding was the deformation mechanism operating during the low–temperature superplasticity regime. At high temperature and slow strain rate grain–boundary sliding was accommodated by a diffusion creep mechanism.
Al-Zubaydi, Ahmed
ab0946e5-c193-4f29-82b7-a06423f496f5
Al-Zubaydi, Ahmed
ab0946e5-c193-4f29-82b7-a06423f496f5
Reed, Philippa
8b79d87f-3288-4167-bcfc-c1de4b93ce17

Al-Zubaydi, Ahmed (2015) High-pressure torsion processing of AZ91 magnesium alloy. University of Southampton, Faculty of Engineering and the Environment, Doctoral Thesis, 318pp.

Record type: Thesis (Doctoral)

Abstract

AZ91 magnesium alloy has been successfully processed at room temperature by high–pressure torsion as well as at elevated temperatures. Ultrafine grains and nano–sized particles of ?–phase have developed with increasing number of turns. The hydrostatic pressure, the geometry of the processing zone and the unidirectional nature of torsional straining during the HPT processing have facilitated processing of AZ91 alloy at room temperature. Extensive grain refinement and twinning segmentation of the coarse grains have been observed in the microstructures processed at room temperature and elevated temperatures, respectively. The twins have been observed at all processing temperatures during processing and their distribution was proportional to the processing temperature and number of turns. The morphology and distribution of the ?–phase have altered during processing, with fragmentation of coarse clusters of the ?–phase into nano–sized particles and the alignment of these particles in the direction of torsional strain being observed. Microstructural homogeneity has gradually developed at a relatively low number of turns using the lower processing temperature and continued with increasing number of turns. A significant improvement in the strength of the alloy has been found after HPT processing at all processing temperatures. The dislocation density has developed significantly for the alloy processed at room temperature rather than at elevated temperatures with increasing number of turns. An experimental Hall–Petch relationship has emphasized a significant dependence of the strength on grain size for the alloy processed at room temperature. The high–strain rate superplasticity, low–temperature superplasticity, and thermal stability of the processed alloy have been observed and attributed to the ultrafine–grained microstructures produced by HPT at room temperature and the dispersion of nano–sized ?–phase particles. Grain–boundary sliding was the main deformation mechanism during the high–strain rate superplasticity regime. Glide–dislocation creep accommodated by grain–boundary sliding was the deformation mechanism operating during the low–temperature superplasticity regime. At high temperature and slow strain rate grain–boundary sliding was accommodated by a diffusion creep mechanism.

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Published date: 1 November 2015
Organisations: University of Southampton, Engineering Science Unit

Identifiers

Local EPrints ID: 386345
URI: http://eprints.soton.ac.uk/id/eprint/386345
PURE UUID: e7b36374-a0c1-4bef-b169-fe16f6013f66
ORCID for Philippa Reed: ORCID iD orcid.org/0000-0002-2258-0347

Catalogue record

Date deposited: 10 Feb 2016 14:15
Last modified: 06 Jun 2018 13:09

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