Growth kinetics of cellular precipitation in a Mg–8.5Al–0.5Zn–0.2Mn (wt.%) alloy

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Abstract

Microstructural evolution and growth kinetics were studied in an isothermally aged Mg–8.5Al–0.5Zn–0.2Mn (wt.%) alloy by means of X-ray diffraction, scanning electron microscopy, Vickers hardness measurements and transmission electron microscopy. Specimens were solution-treated and then aged at 373, 473 and 573 K for different time period. The characterization results indicated the presence of both continuous and discontinuous precipitations of the Mg17Al12-γ phase in a Mg-rich matrix. The discontinuous or cellular precipitation was present with a lamellar structure, and the growth kinetics was evaluated using the Johnson–Mehl–Avrami–Kolmogorov equation analysis, which gives a time exponent close to 1. This value confirms that cellular precipitation takes place on the saturation sites corresponding to grain boundaries. In addition, the activation energy for cellular precipitation was determined to be about 64.6 kJ mol−1. This also indicates a grain boundary diffusion process. The variation of cellular spacing with temperature follows the behavior expected by Turnbull theory. The highest hardness peak corresponded to the lowest aging temperature and it is associated with a fine continuous precipitation; while the lowest hardness peak was detected at the highest aging temperature and it is attributed to the rapid coarsening process of both precipitations.

Research highlights

▶ The growth kinetics of lamellar spacing follows the behavior predicted by Turnbull theory. ▶ The growth kinetics of cellular precipitation is a process controlled by grain boundary diffusion. ▶ The presence of two types of morphology for cellular precipitation depends on the aging temperature. ▶ The highest hardness peak is associated to a fine continuous precipitation at the lowest temperature. ▶ The lowest hardness is attributed to the fast coarsening process of both precipitations.

Introduction

Mg–Al–Zn alloys have become one of the most important light alloys with a wide range of applications in the automotive industry. This is attributed to the best combination of castability, mechanical strength and ductility [1], [2], [3]. The AZ series of magnesium alloys is mainly based on the Mg–Al binary alloys system. According to the equilibrium Mg–Al alloy phase diagram, the equilibrium phases are the hcp Mg-rich α phase and Mg17Al12-γ phase with a complex bcc structure [4]. During the aging process of the Mg–Al based alloys, two types of precipitation are present. That is, discontinuous precipitation takes place on grain boundaries. One of these, intergranular precipitations occurs forming a lamellar structure and it is also known as cellular precipitation [5]. Additionally, continuous precipitation takes place in an intragranular manner and it exhibits more complicated morphologies and orientation relationships than the cellular precipitation [5], [6], [7]. It has been shown in several works [5], [6], [7] that these alloys have a poor response to precipitation hardening, compared with precipitation-hardenable Al alloys. Furthermore, the aging hardness is strongly influenced by the morphology, the size and the distribution density of Mg17Al12 precipitates [7]. Besides, it has been reported that both discontinuous and continuous precipitations have effect on the hardness of these alloys. Thus, the purpose of the present work is to study the mechanism and growth kinetics of cellular precipitation in a Mg–8.5Al–0.5Zn–0.2Mn (wt.%) alloy aged isothermally at 373, 473 and 573 K for different time period.

Section snippets

Experimental procedure

A Mg–8.5Al–0.5Zn–0.2Mn (wt.%) alloy was melted using pure metallic elements under a protective argon atmosphere. Table 1 shows the chemical analysis corresponding to this alloy. Specimens of 10 mm × 10 mm × 10 mm were cut from the ingot and encapsulated in a Pyrex tube under an argon atmosphere. These were homogenized at 703 K for 3 days and subsequently water-quenched. Homogenized and solution-treated specimens were aged at 373, 473 and 573 K for different times. The heat-treated specimens were

Microstructural characterization

The X-ray diffraction patterns of the specimens in the conditions of solution-treated and aged at 573 K for 540 ks (150 h) are shown in Fig. 1. A single-phase is confirmed in the solution-treated specimen, while the appearance of XRD peaks corresponding to the Mg17Al12-γ phase are evident in the XRD pattern of the specimen aged at 573 K for 540 ks. No other phases were detected. The presence of these phases for each case is in agreement with the equilibrium Mg–Al phase diagram [8].

Fig. 2(a)–(i)

Conclusions

Microstructural evolution and growth kinetics were studied in an isothermally aged Mg–8.5Al–0.5Zn–0.2Mn (wt.%) alloy and the growth kinetics of cellular precipitation was evaluated using the Johnson–Mehl–Avrami–Kolmogorov equation analysis, which gives a time exponent close to 1. This value confirms that cellular precipitation takes place on the saturation sites corresponding to grain boundaries. Additionally, the activation energy for the cellular precipitation was determined to be about 64.6 kJ

Acknowledgements

The authors acknowledge the financial support from SIP-PIFI-IPN and CONACYT.

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