Severely plastically deformed AZ80 magnesium alloy: microstructure and mechanical properties

Abstract

In this study the evolution of microstructure and the mechanical properties of an AZ80 magnesium alloy were investigated. It examined prepared samples of AZ80 magnesium alloy before and after processing by severe plastic deformation (SPD) using the High-Pressure Torsion (HPT) technique.

An AZ80 magnesium alloy with a chemical composition of Mg-8.7% Al-0.5% Zn was processed using HPT. The processing was conducted at room temperature, 296 K, and at the elevated temperature of 473 K under quasi-constrained conditions, using an imposed pressure of 6.0 GPa at a speed of one revolution per minute (rpm) through different numbers of turns: 1/4, 1, 3, 5 and 10. Processing magnesium alloy by HPT produced excellent grain refinement in the alloy, and it prevented the samples from developing cracks and segmentation at ambient temperature better than the other popular technique of SPD, for instance Equal-Channel Angular Pressing (ECAP).

The initial microstructure and the microstructural development after HPT processing were subsequently examined by optical microscopy (OM), scanning electron microscopy (SEM) and Transmission Electron microscopy (TEM). Microstructural investigations for the as-received condition showed an average grain size of ~25 m. Optical microscopy images revealed microstructural evolution at both room and elevated temperature after the HPT process. The small proportion of refined grains at the edges expanded towards the disc centre with consecutive increasing numbers of revolutions. The TEM images demonstrate an evolution toward homogeneity at increasing numbers of revolutions. The final average grain size after 10 turns when the alloy was processed at room temperature was ~200 nm and ~330 nm when the alloy was processed by HPT at 473 K. The selected area electron diffraction (SAED) images of HPT samples after 10 revolutions show a fully developed ring at room temperature, indicating a microstructure with high angles of misorientation grain boundaries, and a less developed ring at 473 K. Microstructural observation through the disc thickness demonstrates more heterogeneity in the vertical than the radial direction.

Vickers microhardness (Hv) values were taken along the disc diameter (radial direction) and over the total surface of the discs (colour-coded contour mapping). The results of Vickers microhardness (Hv) measurements along the diameters of the discs verify the heterogeneity of HPT deformation at lower numbers of turns. In the samples, the microhardness values increased rapidly at the edges of the disc, while the centres showed a lower value, and this large difference confirms the heterogeneity of HPT deformation in the early stages. With further straining samples showed a significant increase in microhardness values from the edges towards the disc centre. The microhardness values of samples processed by 5 and 10 turns showed a reasonable homogeneity across the disc diameter, with an average value of ~120 Hv when AZ80 was processed at room temperature and an average value of ~110 Hv when processed at 473 K.

Likewise, three selected discs processed by HPT for 1, 3 and 10 turns at both 296 K and 473 K were sectioned vertically across their diameter to be tested by (OM) and Vickers microhardness (Hv) through their thickness (axial direction). The results of (OM) and Vickers microhardness (Hv) confirmed the high heterogeneity in the axial direction than the radial direction.

Subsequent to the HPT process at room temperature, tensile specimens were cut from the processed discs and pulled in tension to failure at different tensile test temperatures (473, 523 and 573 K) and strain rates of (1.4×10-4 s-1, 1.4×10-3 s-1, 1.4×10-2 s-1 and 1.4×10-1 s-1). The superplasticity of AZ80 magnesium alloy was confirmed for the first time (to the author’s knowledge) at a maximum elongation of 645% when the alloy was pulled in tension to failure at 573 K using strain rate of 1.4×10-4 s-1. Moreover, the alloy exhibited a lower temperature superplasticity when it attained 423% at 473 K. Despite this superplasticity, AZ80 magnesium alloy does not show the predicted behaviour of increasing ductility with increased imposed strain during HPT process and decreased average grain size. The maximum elongation was reached in a sample processed by HPT for one turn, in which a smaller average grain size and the homogenous microstructure were not achieved.

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Identifiers

ORCID for Terence G. Langdon: orcid.org/0000-0003-3541-9250

ORCID for Yi Huang: orcid.org/0000-0001-9259-8123

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