Elsevier

Materials Science and Engineering: A

Volume 651, 10 January 2016, Pages 786-794
Materials Science and Engineering: A

Superplasticity of a nano-grained Mg–Gd–Y–Zr alloy processed by high-pressure torsion

https://doi.org/10.1016/j.msea.2015.10.094Get rights and content

Abstract

While most of the reports on Mg–Gd–Y–Zr alloys report superplasticity after extrusion or friction stir processing, it is important to investigate superplasticity in these alloys after other severe plastic deformation processes having greater grain refinement capability. Accordingly, superplasticity was studied in an Mg–9Gd–4Y–0.4Zr (GW94) alloy after different high-pressure torsion (HPT) conditions. The HPT was performed at room temperature under an applied pressure of 6.0 GPa for up to 16 turns. TEM microstructural characterization revealed that the grain size was reduced from an initial value of ~8.6 μm in the extruded condition to ~95±10 and ~85±10 nm after 8 and 16 turns, respectively. A shear punch testing method was used for evaluation of superplasticity at 573, 623, 673 and 723 K. Maximum strain rate sensitivities of ~0.51±0.05 and ~0.48±0.05 were obtained at 623 K for the material processed through 16 and 8 turns, respectively. This strain rate sensitivity and an activation energy of ~100±5 kJ mol–1 suggests the occurrence of grain boundary sliding in the superplastic region.

Introduction

Magnesium alloys, as the lightest structural materials, have been the subject of many researches in recent years because of their numerous advantages such as low density and high specific strength. Despite these advantages, Mg alloys suffer from poor room temperature formability because of the limited slip systems in their hexagonal close-packed (hcp) structure [1]. In order to overcome these limitations, attempts have been made to enhance their formability through the use of superplastic deformation which will permit the fabrication of light structural components having complex shapes [2], [3], [4] . Excellent thermal stability was reported recently in a series of Mg–Gd alloys and this permitted the occurrence of extensive superplasticity in these alloys [5]. Nevertheless, most of the investigations of the Mg–Gd alloys reported superplasticity after simple extrusion [5], [6] or friction stir processing (FSP) [7], [8], [9] and the minimum grain sizes attained by these methods were of the order of ∼3 μm [8]. This suggests that it may be advantageous to evaluate the occurrence of superplasticity in these alloys after processing using severe plastic deformation (SPD) processes where there is a capability of achieving an even greater level of grain refinement.

It is now well established that processing by high-pressure torsion (HPT) provides the capability of producing materials with extremely fine grain sizes which are suitable for achieving superplastic flow [10]. There are now several reports of superplastic flow in metals processed using SPD techniques [11], [12] and Table 1 provides a comprehensive summary of the investigations reported to date for superplastic flow in metals processed by HPT [13], [14], [15], [16], [17], [18], [19]: in this table, the HPT processing conditions are listed in columns 2–4, the resultant grain sizes are listed in column 5 and the superplastic testing conditions are given in columns 6–8 where data are included only when the maximum elongations exceed the critical requirement of a tensile elongation of at least 400% for superplastic flow [20]. It is readily apparent from inspection of Table 1 that all of these materials exhibit excellent superplastic properties with a maximum elongation of 1330% reported for an Mg–8Li alloy [19]. Nevertheless, there is only a single report of superplasticity in an Mg–10Gd alloy [17] and thus it is important to fully investigate superplasticity of the HPT-processed Mg–Gd alloys.

Although tensile testing is the conventional standard procedure for delineating superplastic flow, other testing methods are now available for measuring the strain rate sensitivity (SRS) and thus providing an indirect method for identifying the possible occurrence of superplasticity. For example, shear punch testing (SPT) was recently introduced as a suitable technique for measuring SRS in different materials processed by SPD and a summary of the results available to date is given in Table 2 [6], [21], [22], [23], [24]. It is important to note also that the SPT-tensile correlation was already validated several years ago [25]. The advantages of studying superplasticity by SPT were reviewed earlier [21] and it includes the requirement for using only very small amounts of material as is readily produced using HPT processing. Thus, the objective of this research was to use SPT to examine the possible occurrence of superplasticity in an Mg–Gd–Y–Zr alloy after processing through different HPT conditions. It should be noted also that SPT has the advantage of being conducted locally and thus on selected positions of the radius of HPT discs.

Section snippets

Experimental material and procedures

An Mg–9 wt% Gd–4 wt%Y–0.4 wt% Zr (GW94) alloy was prepared from high purity Mg and Mg–30Gd, Mg–30Y and Mg–30Zr master alloys which were melted in an electric furnace under a covering flux. The molten material was poured into a steel die preheated to 573 K using a tilt-casting system to minimize casting defects and any melt turbulence. Extrusion was conducted to a diameter of 10 mm using an extrusion ratio of 19:1 at a temperature of 673 K and a graphite lubricant.

Thin slices with thicknesses of ~1.2 

Microstructural evolution during HPT processing

When a thin disk is processed by HPT under an applied pressure, the equivalent von Mises strain, ε, imposed on the disk by torsional straining is given by the relationship [29]ε=2πNrh3where r is the radial distance from the center of the disk and h is the initial thickness of the sample. Eq. (2) shows that the strain varies across the disk. Variations of strain, microstructure and hardness both across and within the HPT discs are summarized elsewhere [10]. Since the shear punch tests were

Grain and precipitate refinement during HPT processing

Microstructural characterization revealed that the microstructure of the material was severely twinned in the initial stages of deformation after 0.5 and 2 HPT turns. This matches an earlier report of the microstructure of a ZK60 magnesium alloy after HPT [15]. In low temperature deformation in hcp metals, deformation twinning occurs at the early stage of deformation and serves as an additional deformation mechanism to dislocation slip. Additionally, it has been reported that in magnesium

Summary and conclusions

  • 1.

    A GW94 alloy with an initial grain size of ~8.6 μm was processed by HPT at room temperature to produce refined grain sizes of ~95±10 and ~85±10 nm after 8 and 16 turns, respectively. The average grain size of 85±10 nm is the smallest grain size reported to date for an Mg–Gd alloy.

  • 2.

    Shear punch testing was used to investigate the potential for achieving superplasticity in this alloy. A maximum strain rate sensitivity of m≈0.51±0.05 was achieved after 16 turns when testing at 623 K whereas there was

Acknowledgements

This work was supported in part by the European Research Council under ERC Grant Agreement no. 267464-SPDMETALS (YH and TGL).

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