High-pressure torsion processing of AZ91 magnesium alloy

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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ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

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