Inverse Hall–Petch Behaviour in an AZ91 Alloy and in an AZ91–Al₂O₃ Composite Consolidated by High‐Pressure Torsion

Abstract

High-pressure torsion (HPT) is a significant procedure for achieving substantial grain refinement but it may be used also to consolidate metallic particles to form bulk samples or composites where two (or more) different phases are mixed and consolidated. Herein, the consolidation of particles of the magnesium AZ91 alloy and a composite with an AZ91 matrix combined with 1% alumina powder is investigated. The results show that it is possible to fully consolidate this alloy after a large number of turns. As a consequence of the severe plastic deformation, the grain structure is significantly refined, with average grain sizes of %116 and %98 nm in the unreinforced alloy after 20 or 50 HPT turns and %76 nm in the composite after 50 HPT turns, respectively. This grain refinement is associated with a decrease in hardness and an increase in the strain rate sensitivity due to the onset of a grain boundary diffusion-assisted creep mechanism at room temperature. The results are consistent with the theoretical prediction of a breakdown in the Hall-Petch relationship at very small grain sizes.

The Effect of Ultragrain Refinement on the Strength and Strain Rate Sensitivity of a ZK60 Magnesium Alloy

“…metals. [39] It is important to note that the inverse Hall-Petch effect takes place at grain sizes in the range of a few micrometers in pure Mg, whereas such effects are only observed at grain sizes in the range of %100 nm in AZ91 [16] and in ZK60 (current experiments) alloys. Grain refinement softening, also called inverse Hall-Petch effect, has been reported for grain sizes smaller than Figure 5.…”

“…A similar effect is observed in the AZ91 magnesium alloy that also displays a saturation in hardness evolution after a few turns of HPT [23] but also displays continuous grain refinement with increasing number of turns of HPT. [16] The saturation in hardness evolution in these magnesium alloys, which misleads us to the idea of saturation in grain refinement, is actually attributed to a breakdown in Hall-Petch effect as explained later. The well-known Hall-Petch behavior is based on empirical observations that the strength of metallic materials, σ, is related to the grain size, d, through a relationship shown in Equation (1).…”

“…This agrees with a recent report of continuous grain refinement in magnesium alloy AZ91 with increasing number of turns in HPT. [16] Small particles, with tens of nanometers in size, were observed throughout the structure and are attributed to Mg-Zn intermetallics. The average grain sizes were estimated as %160 AE 60 nm and %100 AE 10 nm for the samples processed to 5 and 50 turns of HPT, respectively.…”

“…[8][9][10]14,15] A recent paper has shown that a breakdown in Hall-Petch relation also takes place in a magnesium alloy AZ91 and inverse Hall-Petch effect was reported at grain sizes smaller than %120 nm in samples of this alloy consolidated using a large number of turns of HPT. [16] These effects were also associated with enhanced strain rate sensitivity in the AZ91 alloy, although not as remarkable as in pure Mg. An increase in strain rate sensitivity has also been reported in ZK60 alloy after HPT processing. [17] However, previous papers did not report evidence of a breakdown in Hall-Petch relation in ZK60 [18] or ZKX600 [19] alloys in the ultrafine-grained range.…”

“…In contrast, grain refinement saturation was not observed in the AZ91 alloy up to 50 turns of HPT. [16] Thus, the present work has multiple objectives. It aims to provide more experimental evidence of the relationship between grain size and hardness for the ZK60 alloy in the ultrafinegrained (<1 μm) range.…”