Mechanical properties and microstructure of a Mg alloy AZ31 prepared by equal-channel angular pressing

doi:10.1016/j.msea.2006.01.174

Materials Science and Engineering: A

Volume 462, Issues 1–2, 25 July 2007, Pages 116-120

https://doi.org/10.1016/j.msea.2006.01.174 Get rights and content

Abstract

A squeeze cast magnesium–aluminium–zinc alloy AZ31 was subjected to equal-channel angular pressing (ECAP) at 200 °C up to four passes following route B_c. The grain size was reduced by a factor of about 100–200 through ECAP processing. The mechanical properties of as cast (non-pressed) and the ECAP-processed material were examined using compression tests from room temperature to 300 °C. ECAP pressing resulted in a significant increase of the room temperature yield strength of the alloy. At elevated temperatures (200 and 300 °C) an inverse dependence was found, i.e. the yield stress in the ECAP-processed specimens was lower than in the cast material. A recrystallized structure was formed only after four passes of ECAP.

Dynamic recrystallization occurred during the deformation at elevated temperatures (300 °C). Coarser recrystallized grains were formed in the specimen that had undergone four passes of ECAP. The strain imposed by ECAP influences the heterogeneity of the final microstructure of specimens after compression tests.

Introduction

For over 20 years, equal-channel angular pressing (ECAP) appears to be one of the most popular and efficient methods for the production of bulk ultra fine grained (UFG) materials. This technique allows achieving effective strengthening with many metals and alloys. Because of their low density (ρ_Mg = 1.74 g/cm³) and high specific strength, magnesium alloys are of interest for many technical applications, particularly in the field of the automotive industry. ECAP could enhance not only the mechanical properties of the Mg alloys, but also their ability to be formed at high temperatures. The positive influence of ECAP on magnesium alloys was demonstrated by many authors [1], [2], [3], [4]. An ECAP die has two intersecting channels with the same cross-sectional dimensions that allow subjecting a billet to multiple pressing. Therefore, a very high shear strain can be accumulated in the billet. Several processing routes may be applied differing in the rotation of the billet between individual passes of ECAP. The final structure and mechanical properties strongly depend on the applied route of pressing. The use of route B_c (where the specimens are rotated by 90° in the same sense between the consecutive passes) is known to produce the most pronounced grain refinement. Several studies report the use of ECAP for extruded AZ31 alloys [5], [6], [7]. This article presents some new results on the mechanical and microstructural properties of a squeeze cast AZ31 after route B_c ECAP. The objective of the investigation was two-fold. First, to investigate the effect of ECAP on the mechanical response of the alloy. Second, to study the underlying microstructure evolution of the material associated with ECAP and subsequent uniaxial compression.

Section snippets

Experimental

Specimens produced from a squeeze cast AZ31 magnesium alloy (3 wt% Al, 0.8 wt% Zn, 0.2 wt% Mn) with dimensions 10 mm × 10 mm × 60 mm and an initial grain size of about 380 μm were severely deformed by ECAP to an equivalent strain of about 1 and 4 at 200 °C. Route B_c was used for ECAP processing. The ECAP die was manufactured from a general tool steel (X38CrMoV51) which was hardened to 48–49 HRC. A split die design was used, i.e. the die consists of two parts, the entire channel being contained within

Mechanical properties

The temperature dependence of the yield stress of an as cast specimen and specimens after one and four passes of ECAP is shown in Fig. 1. ECAP pressing resulted in significant increase of the yield stress as measured by compression tests. The as-received alloy (after squeeze-casting) deformed at room temperature (RT) had a yield stress of 65 MPa. After one ECAP pass the yield stress increased to 120 MPa, whereas after four passes the yield stress reached 210 MPa, which is about three times larger

Discussion

The microstructural changes occurring in the alloy during ECAP pressing and the following deformation are a direct consequence of dislocation density changes and the rearrangement of dislocations. The following simple qualitative model may explain these changes. In the initial stage of ECAP many dislocations are generated and accumulated in the coarse grained material. In the initial stage of deformation the dislocations are homogeneously distributed within grains (Fig. 2a). With subsequent

Conclusions

The microstructure evolution and the mechanical properties of AZ31 alloy after ECAP were examined using transmission electron microscopy and subsequent compression tests. The following conclusions can be drawn from this investigation:

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The grain size was reduced by a factor of about 100–200 through ECAP processing.
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ECAP deformation resulted in a significant increase of the yield stress of the alloy deformed at room temperature, while at elevated temperatures (200 and 300 °C) the inverse dependence

Acknowledgements

This work was financially supported by the research program MSM 0021620834 of Ministry of Education of the CR (M.J.) and the DFG under grant Es 74/13.

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Mechanical properties and microstructure of a Mg alloy AZ31 prepared by equal-channel angular pressing

Abstract

Introduction

Section snippets

Experimental

Mechanical properties

Discussion

Conclusions

Acknowledgements

Scripta Mater.

Scripta Mater.

Acta Mater.

Scripta Mater.

Acta Mater.

Scripta Mater.