Microstructures and mechanical properties of high strength Mg-Zn-Mn alloy

ZHANG Ding-fei(张丁非)^{1, 2}, SHI Guo-liang(石国梁)², DAI Qing-wei(戴庆伟)²,

YUAN Wei(袁 炜)², DUAN Hong-ling(段红玲)²

1. National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China;

2. College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China

Received 12 June 2008; accepted 5 September 2008

Abstract: The microstructures and mechanical properties of a new Mg-6%Zn-1%Mn (mass fraction) wrought magnesium alloy were studied, which could be extruded smoothly at 310-330 ℃ with a complete dynamic recrystallization. After solution treatment one and two-step aging techniques were used. All as-aged microstructures contained two types of dispersed phases: β' phases and pure α-Mn particles. The two-step aging had a better strengthening effect than the traditional one-step aging, and the strength value achieved by the two-step aging could reach that of the ZK60 wrought magnesium alloy. The outstanding precipitation strengthening effect of the alloy should be attribute to the GP zones, diffusive solute-rich zones and some metastable phases formed during the first step aging that provide more effective nuclei for Mg-Zn strengthening phases during the second step aging.

Key words: high strength; Mg-Zn-Mn alloy; microstructures; mechanical properties

1 Introduction

With combination of low density, high specific strength and stiffness, high damping capacity, outstanding electromagnetic shielding characteristic and good recycle ability[1-2], magnesium alloys have been honored “the green engineering and structural materials of 21st century”. Although die-casting magnesium alloys are now wildly used, there is a lack of competitive wrought magnesium alloy products, such as sheet materials and forged products, which are much needed for numerous weight-sensitive applications[3]. Wrought magnesium alloys can achieve higher strength and better ductility after heat treatment than die-casting magnesium products[4-6].

Most magnesium alloys are based on the Mg-Zn and Mg-Al systems, both of which are age hardenable. As for the Mg-Al systems, a high Al content contributes to high yield strength and ultimate tensile strength at ambient temperature and grain refinement, but leads to poor creep resistance. The latter can be attributed to discontinuous precipitation and coarsening of Mg₁₇Al₁₂. As for the Mg-Zn system, an obvious solid solution strengthening effect can be achieved and the Mg-Zn phase is more stable than Mg₁₇Al₁₂ at elevated temperature, which result in higher mechanical properties at ambient and elevated temperatures, so the Mg-Zn series alloys commonly are more suitable for making high strength structural components than the Mg-Al series alloys[7-9]. Adding Zr, Y etc. into Mg-Zn series alloys will remarkably elevate their strength and toughness values, however, on the other hand, the addition greatly increases the costs of the alloys and raises difficulties in smelting and the possibility of hot tearing in the casting process[1]. A new Mg-6%Zn- 1%Mn (mass fraction) wrought magnesium alloy has been developed in this work with high zinc content based on the system of a commonly used ZM21 wrought magnesium alloy. This alloy exhibits some excellent industrial application properties such as low cost (without rare earth elements), low extrusion temperature and high strength. In this work, the extrusion behavior is described and the effects of different heat treatments on the microstructures and mechanical properties of the extruded alloy are compared.

2 Experimental

Ingots with a nominal composition of Mg-6%Zn- 1%Mn (mass fraction) were cast in semi-continuous anner, and the chemical composition measured by the atomic absorption spectrophotometer is listed in Table 1.

The ingots (Ф112 mm×200 mm) were homogenized at 310 ℃ for 24 h and then extruded on an extruder at 800 t. The extrusion parameters are listed in Table 2.

Table 1 Chemical composition of investigated alloy (mass fraction, %)

Table 2 Parameters of extrusion process

The specimens with 120-140 mm in length were cut from the extruded bar with 15 mm in diameter. Covered by graphite powder, all the specimens were solution treated (T4) followed by water quenching, then peak-aging (T6) and two-step aging were utilized. Tensile properties were measured using tensile specimens with gauge length of 50 mm and gauge diameter of 5mm at a strain rate of 2×10^-3 s^-1 at room temperature. The microstructures and phase components were characterized by an optical microscope and a scanning electron microscope (SEM, TESCAN VEGA ⅡLMU) with an energy dispersive spectrometer (EDS, OXFORD INCA). For metallographic observation, the specimens were mechanically polished and etched with a dilute solution of 4% nitric acid in alcohol.

3 Results and Discussion

3.1 Extrusion behavior

The dynamic recrystallization (DRX) temperature is the selection basis for hot working temperatures, since the DRX process yields a good workability and microstructure free from defects and instabilities[10]. The typical extrusion temperature range for magnesium alloys is 330-400 ℃, however the experimental ingots could be smoothly extruded at 310 ℃, which is classified as low-temperature extrusion. The extrusion temperatures of commonly used magnesium alloys are listed in Table 3.

The extrusion temperature of the new alloy was   90 ℃ lower than that of its counterpart ZM21 in the same series. From the preliminary analysis it is concluded that the high zinc content will decrease the DRX temperature.

Table 3 Extrusion temperature of commonly used magnesium alloys[11-14]