School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan, China.

Email: am_lab@yeah.net

Copyright © 2013 Xiaoping Luo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Received January 16^th, 2013; revised March 1^st, 2013; accepted April 3^rd, 2013

Keywords: Mg-Zn-Y-Zr Magnesium Alloy; Unequal Channel Angular Pressing; Microstructure; Mechanical Property

ABSTRACT

A new unequal channel angular pressing (UCAP) procedure is proposed for ultrafine-grained metals and alloys. The microstructures and mechanical properties of Mg-5.8Zn-1.2Y-0.7Zr alloys subjected to unequal channel angular pressing (UCAP) are investigated. It is found that the optimum condition in UCAPed alloy is obtained at 523 K with a largest elongation to failure of 13.1% and ultimate tensile strength of ~400 MPa. Microstructural observations show that the grain size is refined to ~1.0 µm during UCAP. The probable mechanisms for these high mechanical properties are attributed to grain size and destroyed secondary phase strengthening effects and fine precipitates formed during pressing at high temperature by severe shear and plastic deformation.

1. Introduction

During the last decade, equal-channel angular pressing (ECAP) has attracted considerable attention as a widely known procedure for the fabrication of ultrafine-grained metals and alloys [1-3]. As is known, the mechanical and physical properties of metallic materials are determined by several factors, and the average grain size of the material generally plays a very significant role. To convert a coarse-grained solid into a material with ultrafine grains by ECAP, metals were subjected to large deformation under high applied pressures in attempts to improve their mechanical properties. However, the ECAP process as currently used in the laboratory is labor-intensive, low efficiency and high in production cost because it requires many extrusion passes to achieve the desired grain refinement. Accordingly, it is now recognized that any extensive industrial application will require the development of some form of efficient continuous processing technique. Therefore, in this paper we propose a new procedure, unequal channel angular pressing (UACP).

Magnesium-based alloys, being biomedical metallic materials, have great potential for application in bone fixed parts due to their high strength-to-density ratio and biocompatibility [4-6]. However, applications of the closepacked hexagonal structure of magnesium alloys are still limited because of low mechanical properties [7,8]. Therefore, extensive efforts have been focused on alloying and plastic processing to develop magnesium alloys with good mechanical properties. Among many possibilities, addition of Y element can provide with high strength and small amount of Zr may provide finer solidified microstructure in the Mg-Zn alloy, as reported effects of yttrium on the microstructure and mechanical properties of Mg-Zn-Y-Zr alloys, etc. [9-11].

In recent years, the interest in Mg-Zn-Y-Zr alloys containing quasicrystal I-phase [12-14] as the secondary phase, which formed upon solidification and has many good properties such as high hardness, thermal stability, etc., has been growing, because these alloys posses improved yield and ultimate tensile strengths after going through thermomechanical processing [15-17] such as hot compression, hot extrusion, etc. In these studies, mechanical properties at room temperature, superplastic behaviors at high temperatures, precipitation behavior during thermo-mechanical treatment, etc., have been investtigated.

Up to now there have been limited studies addressing the changes on microstructural and mechanical properties of Mg-Zn-Y-Zr alloys during UACP processes. To improve the strength and ductility of Mg-Zn-Y-Zr alloys further in the present investigation, UCAP processing is carried out on Mg-5.8Zn-1.2Y-0.7Zr magnesium alloy at different temperature.

2. Experimental

The materials used in the present study were prepared from pure magnesium (99.9%), pure Zn (99.99%), Mg- 25%Y and Mg-33%Zr master alloys by electric crucible melting under the protection of a mixed gas atmosphere of 99.5% CO₂ and 0.5% SF₆ (vol%). The melted Mg- 5.8Zn-1.2Y-0.7Zr alloy was cast into a permanent mould and machined to dimensions of 40 × 40 × 20 mm using spark cutting and then pressed in an UCAP die (as shown in Figure 1) before being extruded into rods, 10 × 10 mm, at different temperatures (473 - 673 K) with an extrusion ratio of 16. The microstructures of both the ascast and UCAP processed alloys were observed with an optical microscope (VHX-600E) and scanning electron microscope (Hitachi S-4800) and a JEM-2010 transmission electron microscope (TEM). Phase analyses were performed with a PANalytica X’ X-ray diffractometer (XRD). The samples were ground and polished using standard metallography procedures, and the untreated alloys were etched with a mixture of 1 mL nitric acid and 24 mL ethylene glycol. UCAP processed alloys were etched with an etchant comprising 1 mL oxalic acid, 1 mL nitric acid and 98 mL water. TEM samples were prepared by slicing plates perpendicular to the long axis of the billet. Final thinning was performed by twin-jet electropolishing in a solution of 10% HClO₄ and 90% C₂H₅OH. Tensile tests were carried out on an MTS universal testing machine at a nominal strain rate of 0.4 mm/s at room temperature, dog-bone shape tensile specimens with cross-section of 4 × 2 mm and gauge length of 32 mm machined from the UCAP processed samples, with their tensile axes lying parallel to the pressing direction.

3. Results and Discussion

3.1. Microstructures

Figure 2 shows SEM images of the Mg-5.8Zn-1.2Y- 0.7Zr cast alloy. As seen in Figure 2(a), there are two distinct regions present, which appear as white and grey in the image. Figures 2(a₁) and (a₂) are the EDS analysis results at test points 1 (grey region) and 2 (white region), respectively in Figure 2(a). Test point 1 is grey α-Mg solid solution matrix, as shown by its very high Mg content, and small Zn content; this implies that not much Zn was dissolved into the Mg during melting. The EDS analysis also shows that the white region (Figure 2(a₂), test point 2) contains three elements. The enlarged image of

the grain boundary, Figure 2(b), shows that the grain boundary has a layered microstructure. The EDS spectra of test points 1 and 2 in Figure 2(b) show that the microstructure is a mixture of α-Mg matrix and some ternary intermetallic compounds. A small amount of Zn has been dissolved into the α-Mg interstitial matrix solution, and the Y has segregated in the ternary compounds. The Mg- 5.8Zn-1.2Y-0.7Zr as-cast alloy consists of α-Mg, with coarse grains, ~110 µm, forming a net-like structure, and the secondary phase or phases at the grain boundaries. Based on XRD analysis (as shown in Figure 3) and microstructure observation, it can be determined that the secondary phases in the as-cast Mg-5.8Zn-1.2Y-0.7Zr alloy are Mg₃Zn₆Y (I-phase) and Mg₃Zn₃Y₂ (W-phase).

Figure 4 shows the microstructure of Mg-5.8Zn-1.2Y- 0.7Zr alloy after UCAP of three orthogonal surfaces representing the X, Y, and Z planes as defined in Figure 1. Inspection of the X plane in Figure 4(a) shows that the grains which were initially equiaxed have become elongated along the Y direction and flattened in the Z direction. In addition, slip is visible within the elongated grains lying approximately parallel to the Z direction. On the Z