Hot deformation behavior and microstructure evolution of ZK21 magnesium alloy

https://doi.org/10.1016/j.msea.2010.03.016Get rights and content

Abstract

The hot compression deformation behavior and microstructure evolution of the homogenized ZK21 magnesium alloy were investigated in the temperature range of 250–400 °C and at the strain rate range of 0.1–50 s−1. The dynamic recrystallization (DRX) extent at the grain boundaries and twins played a determinant role on the hot workability of ZK21. At the lower temperature range (250–300 °C) and the lower strain rate (<1 s−1), DRX could hardly occur and microcracks emerged at the grain boundaries due to the large stress concentration. At the higher temperature (350–400 °C) and the lower strain rate (≤1 s−1), DRX developed mainly at the grain boundaries. At all the temperature range (250–400 °C) and the higher strain rate (≥10 s−1), DRX developed extensively at the grain boundaries and twins, resulting in a more homogeneous microstructure than the other conditions. The hot deformation characteristics of ZK21 exhibited an abnormal relationship with the strain rate, namely the hot workability increased with the strain rate increasing. However, the DRX grain size was almost the same with the temperature increasing at the strain rate of 10 s−1, while that increased obviously at the strain rate of 50 s−1. Therefore, hot deformation at the strain rate of 10 s−1 around and at the temperature range of 250–400 °C was desirable and feasible for the ZK21 alloy.

Introduction

Magnesium alloys have attracted a great deal of attention from automobile, aviation, electronic and communication industries, due to their excellent specific strength and stiffness, machinability, dimensional stability and recycling capability [1], [2]. However, conventional magnesium alloys have poor formability at room temperature owing to the limited slip systems in the hexagonal close packed (hcp) lattice. Therefore, it is necessary for magnesium alloys to be deformed at warm temperatures (225 °C), at which the non-basal slip systems can be activated. The absence of the related data on the forming technological factors is still existent, which limits the widespread use of wrought magnesium alloys. Therefore, it is necessary to investigate the basic forming data of magnesium alloys [3].

The Mg–Al–Zn alloys and the Mg–Zn–Zr alloys are the most widely used magnesium alloys. To date, there are some researches on the hot and warm deformation behaviors of the Mg–Al–Zn alloys [4], [5], [6], [7], [8], [9], while there are only a few studies on the warm deformation behaviors of the Mg–Zn–Zr alloys [9], [10], [11] and the related composites [12]. The Mg–Zn–Zr alloys have great potentials for the development of low cost high strength wrought magnesium alloys since they can exhibit a considerable age hardening response and achieve good grain refinement by zirconium and good application prospects in automobiles, electronics, etc. [13], [14]. However, they exhibit inferior hot working ability to the Mg–Al alloys such as AZ31 and AM60. Plastic processing at a fairly high strain rate and rather a low temperature is desirable for the fabrication of wrought Mg–Zn–Zr alloy products. Nevertheless, the investigations on the high strain rate deformation behaviors of magnesium alloys are very sparse. In the paper, the hot compression deformation behavior especially the high strain rate deformation and microstructure evolution of ZK21 magnesium alloy were investigated to provide a useful guide for the selection of the hot working processing (including forging and rolling) parameters.

Section snippets

Experimental

Hot compression tests were conducted on ZK21 with the chemical compositions of Mg–2.3Zn–0.45Zr (wt.%). The as-cast billets were homogenized at 330 °C for 12 h followed by water quenching. Cylindrical specimens with the diameter of 10 mm and the height of 15 mm were machined from the homogenized billets and both ends of each specimen were recessed to the depth of 0.2 mm to entrap the lubricant.

Compression tests were carried out using a computer servo-controlled Gleeble 1500 machine at the strain rate

Hot compression flow behaviors

The true stress–true strain curves of the homogenized ZK21 alloy hot compressed at the various strain rates and temperatures are shown in Fig. 1. At the strain rate lower than 1 s−1, the curves were characteristic of initial work hardening followed by the steady-state deformation at the relatively large strain. At the strain rate higher than 10 s−1, work hardening was more rapid and discontinuous yielding occurred at the true strain less than 0.05 and then continuous softening appeared after the

Conclusions

The hot deformation behavior and microstructure evolution of the homogenized ZK21 alloy were investigated and the conclusions were listed as the following.

  • 1.

    At the lower temperature (250–300 °C) and the lower strain rate (<1 s−1), DRX could hardly occur and microcracks generated at grain boundaries due to the stress concentration. At the higher temperature (350–400 °C) and the lower strain rate (≤1 s−1), DRX was developed mainly at the grain boundaries. At the higher strain rate (≥10 s−1), DRX was

Acknowledgements

The research described in this paper was supported by the Doctoral Program of Higher Education of China (20070532087) and the Natural Science Foundation Project of China (50844034), the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-06-0701).

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