An elevated temperature Mg–Dy–Zn alloy with long period stacking ordered phase by extrusion

doi:10.1016/j.msea.2011.01.065

Materials Science and Engineering: A

Volume 528, Issues 10–11, 25 April 2011, Pages 3609-3614

https://doi.org/10.1016/j.msea.2011.01.065 Get rights and content

Abstract

A Mg–2Dy–0.5Zn (at.%) alloy was prepared by casting, heat treating at 525 °C for 10 h, extruding at 360 °C and ageing at 180 °C for 90 h. Microstructure characterization revealed that this alloy consists of the fine-grained α-Mg matrix composed nearly of the 14H long period stacking ordered (LPSO) phase and a small amount of (Mg, Zn)_xDy precipitations. Tensile tests showed that the yield and ultimate tensile strengths of this alloy can reach 245 MPa and 260 MPa at 300 °C, which are only slightly lower than those at room temperature (287 MPa and 321 MPa), the elongation of this alloy is 36% at 300 °C, which is far higher than that at room temperature (11.6%). The high volume fraction of the 14H LPSO phase in the fine-grained α-Mg grains and the high thermal stability of the microstructure during elevated temperature deformation are responsible for the excellent elevated temperature properties observed for this alloy.

Research highlights

► A Mg–2Dy–0.5Zn (at.%) alloy was prepared by extrusion and ageing treatment. ► This alloy exhibits the excellent tensile strengths at 300 °C, i.e., σ_0.2 = 245 MPa; σ_b = 260 MPa. ► The excellent strengths are attributed to the high volume fraction of LPSO phase.

Introduction

Magnesium alloys, as high performance structural materials, have been used in automotive, aerospace and electronic industries due to their low density, high specific strength, good damping and ease of recycling. However, poor strengths of Mg alloys at elevated temperature (e.g. above 200 °C) limit their application. Therefore, many attempts have been made to develop new Mg alloy systems for various elevated temperature application. Kawamura et al. [1] developed a new nanocrystalline Mg₉₇Zn₁Y₂ alloy by powder metallurgy process. This alloy exhibits a yield strength of ∼600 MPa and an elongation of ∼5% at room temperature. Besides ultra-fine grain structure, the superior mechanical properties of this alloy originate from the presence of long period stacking ordered (LPSO) phase. The further investigations demonstrated that the presence of LPSO phases in Mg alloys can increase the critical resolved shear stress for the basal plane slip and hence can result in the activation of the non-basal plane slip [2] and LPSO phases can exhibit high thermal stability even at temperatures above 400 °C [3], [4]. These investigations indicated that LPSO phases have a potential role in improving the mechanical properties of Mg alloys both at room temperature and elevated temperatures.

LPSO phases have various types, e.g. 10H, 14H, 18R and 24R-types [5], and can form mainly in Mg–Zn(Ni,Cu)–RE alloys (RE represents rare earth elements) during casting [6], [7], [8], [9], [10] and can transform and develop during subsequent heat treatment [11], [12], [13] and ageing treatment [14], [15], [16]. The mechanical properties of Mg–Zn–RE alloys containing LPSO phases can be thus improved by altering the type, morphology and amount of LPSO phases through appropriate designs of casting and subsequent treatments. Furthermore, hot extrusion is an effective way in evaluating strengths and improving ductility of Mg alloys due to its structural refinement role. For Mg–Zn–RE alloys containing LPSO phases, hot extrusion can lead to a significant improvement in the strengths and ductility through the effective refinements of LPSO phases and α-Mg grains [13], [17], [18]. The investigation of Yamasaki et al. [13] showed that the yield strength and elongation of the extruded Mg_96.5Zn₁Gd_2.5 (at.%) alloy containing LPSO phases can reach 345 MPa and 6.9% due to highly dispersed LPSO phases and fine α-Mg grains caused by hot extrusion. Similar result was also reported by Yoshimoto et al. [17] for the extruded Mg₉₆Zn₂Y₂ alloy containing LPSO phases, which exhibited yield strength of 390 MPa and an elongation of 5% at room temperature. The recent experiments revealed that the presence of LPSO phases can also improve greatly the elevated temperature strengths of the extruded Mg₉₇Y₂Cu₁ alloy [7] and the rolled Mg_90.5Ni_3.25Y_6.25 alloy [8], the yield and ultimate tensile strengths of these two alloys remain similar levels in a temperature range from room temperature (RT) to 200 °C, but drop rapidly above this temperature [8].

Recently, a criteria for the formation of LPSO phases in Mg–Zn–RE alloys was proposed by Kawamura and Yamasaki [18], i.e. RE elements should have a hcp structure at room temperature, a large solid solubility limit above 3.75 at.% in binary Mg–RE alloys and an atomic size larger than that of Mg by 8.4–1.9%. According to Kawamura and Yamasaki, Dy can satisfy well this criteria and can promote more effectively the formation of LPSO phases in Mg–Zn–RE alloys than other RE elements used commonly, such as Y and Gd, because of its smaller difference in atomic size with that of Mg and higher solid solubility limit in Mg. However, the investigation about the formation of LPSO phases in Mg–Zn–Dy alloys and the effect of LPSO phases on the elevated temperature mechanical properties, especially at temperatures above 200 °C, have been reported less. In this study, an extruded Mg–2Dy–0.5Zn alloy (at.%) containing the high volume fraction of LPSO phase was prepared through adjusting the processes of casting and subsequent heat treatment and ageing treatment. The mechanical properties of this alloy in a temperature range from room temperature to 300 °C were measured, which were then connected with its microstructure.

Section snippets

Experimental

Mg–2Dy–0.5Zn (at.%) alloy was prepared from pure Mg and Zn and Mg–(20 wt.%)Dy master alloy in a graphite crucible under an anti-oxidizing flux. The melts were homogenized at 750 °C for 0.5 h and then were poured into a water-cooling mould of a diameter of 85 mm and a length of 350 mm at 720 °C. The ingots were homogenized at 525 °C for 10 h and were machined into the round bars with a diameter of 80 mm. The bars were extruded by an extrusion ratio of 17 at 360 °C and then were aged at 180 °C for 99 h.

Microstructures

Fig. 1(a) shows the optical microstructure of the as-cast Mg–2Dy–0.5Zn alloy. Lots of secondary phases can be observed at the grain boundaries and in the grain interior of α-Mg matrix. The XRD analysis in Fig. 2 reveals that these secondary phases are mainly the Mg₁₂ZnDy phase with a 18R LPSO structure. Such 18R LPSO phase has been commonly observed in the as-cast Mg₉₇Zn₁Dy₂ alloy [18] and Mg₉₇Zn₁Y₂ alloy [6]. In order to obtain a microstructure containing the high content or high volume

Conclusion

We have examined the microstructure and mechanical properties of the extruded Mg–2Dy–0.5Zn alloy. The alloy consists of the fine-grained α-Mg matrix composed nearly of the 14H LPSO phase and a small amount of (Mg, Zn)_xDy precipitations. The yield and ultimate tensile strengths of the alloy can reach 245 MPa and 260 MPa at 300 °C, which are slightly lower than those at room temperature (287 MPa and 321 MPa); the elongation of this alloy is 36% at 300 °C, which is far higher than that at room

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (no. 50771049). The authors would like to express their thanks to Q. Zhang, D. Zhou, W. Zhang, S. Li and D. Liu for their assistance in some experiments.

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An elevated temperature Mg–Dy–Zn alloy with long period stacking ordered phase by extrusion

Abstract

Research highlights

Introduction

Section snippets

Experimental

Microstructures

Conclusion

Acknowledgements

Mater. Sci. Eng. A

Scripta Mater.

Scripta Mater.

Scripta Mater.

J. Alloys Compd.

J. Alloys Compd.

Scripta Mater.

Mater. Sci. Eng. A

Acta Mater.

Mater. Chem. Phys.

Scripta Mater.

Acta Mater.

Acta Mater.

Mater. Sci. Eng. A