Evolution of microstructure in AZ91 alloy processed by high-pressure torsion

Abstract

An investigation has been conducted on AZ91 magnesium alloy processed in high-pressure torsion (HPT) at 296, 423 and 473 K for different numbers of turns. The microstructure has altered significantly after processing at all processing temperatures. Extensive grain refinement has been observed in the alloy processed at 296 K with apparent grain sizes reduced down to 35 nm. Segmentation of coarse grains by twinning has been observed in the alloy processed at 423 K and 473 K with average apparent grain sizes of 180 nm and 250 nm. Substantial homogeneity in microhardness has been observed in the alloy processed at 296 K compared to that found at 423 K and 473 K. The ultrafine-grained AZ91 alloy exhibited a significant dependence of the yield strength on grain size as shown by the microhardness measurements, and it obeys the expected Hall-Petch relationship. The alloying elements, fraction of nano-sized particles of b-phase, and the dominance of basal slip and pyramidal modes have additional effects on the strengthening of the alloy processed at 296 K.

The Effect of High‐Pressure Torsion on Microstructure, Hardness and Corrosion Behavior for Pure Magnesium and Different Magnesium Alloys

“…The AZ91 and the ZK60 alloys present a broader hardening stage up to strains of >20. Previous reports described strain hardening up to strains of ≈20 in ZK60 and a broad strain hardening in the AZ91 alloy . It is known that hardening is effective in preventing deformation localization and this agrees with the higher homogeneity of the hardness distribution in these alloys compared to the AZ31 alloy.…”

Processing Magnesium and Its Alloys by High‐Pressure Torsion: An Overview

“…Figure shows the appearance of specimens of a Mg‐9% Al alloy processed by 5 turns of HPT at 423 K and pulled in tension at 473 K at different strain rates, where the elongations over 400% confirm the occurrence of superplasticity . A maximum strain‐rate sensitivity of ≈0.52 and elongations over 1000% were also reported in an AZ91 alloy …”

“…Later it was shown that this heterogeneity is not observed in aluminum processed using similar procedures and this demonstrates that the tendency for heterogeneity in the hardness distribution is associated specifically with magnesium. A recent report showed that a homogeneity along both the radial direction and the through‐thickness direction was achieved in an AZ91 alloy after 10 turns of HPT …”

Evolution of microstructure and mechanical properties of an as-cast Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy processed by high pressure torsion

“…It has been reported that in HPT-processed AZ31 and AZ91 alloys the dislocation density, as measured by positron lifetime spectrometry and XRD, increased gradually to about ~1.2×10 14 m -2 and ~2×10 14 m -2 , respectively, after processing for the first several turns [35,36], whilst the model in [37] predicts a dislocation density of 1.4× 10 14 m -2 for pure Mg processed by HPT to a saturated hardness. In the present alloy, the dislocation density increases dramatically at the early stage, and after 10 and 16 turns, the saturated dislocation density reaches ~3.2×10 14 m -2 .…”