Fracture behavior of AZ91 magnesium alloy
Introduction
AZ91 alloy is the most widely used die casting magnesium alloy, offering good combination of castability, corrosion resistance and mechanical properties [1], [2]. Because of the hexagonal-close-packed (h.c.p.) crystal structure of magnesium, its restriction of slip systems doesn't allow much deformation, which leads to the limitation of deformability and work strengthening [3], [4]. The grain boundaries are the principal obstacles in AZ91 magnesium alloy. Thus the alloy follows the Hall–Petch relationship [4], [5]. The failure of magnesium alloys usually assumes brittle fracture, in which cleavage and quasi-cleavage are the most common fracture modes. The cleavage in magnesium is usually along (0001) crystal plane.
For AZ91 magnesium alloy, the main strengthening intermetallic is Mg17Al12 phase. The body-centered cubic (b.c.c.) structure of Mg17Al12 is incompatible with the h.c.p. structure of magnesium matrix, which leads to the fragility of the Mg/Mg17Al12 interface. In addition, Mg17Al12 itself is relatively soft and has poor strength [6]. Therefore, microcracks tend to initiate in the Mg/Mg17Al12 interface and even in the Mg17Al12 particles. The volume and morphology of Mg17Al12 greatly influence the mechanical properties of AZ91 alloy.
The current evaluation systems for fracture modes (such as cleavage, quasi-cleavage and microvoid coalescence etc.) are based on the study of ferrous alloys, which may be unsuitable for non-ferrous metals, especially the h.c.p. magnesium. The fracture surface morphologies of magnesium alloy are rather complex and easily influenced by the changing of alloying elements and experimental conditions. However, little has been known about the fracture behavior of magnesium alloys until now. The present work investigated the fracture behavior of AZ91 alloy in tensile and impact tests.
Section snippets
Experimental
The composition of the studied alloy is shown in Table 1. The as-cast specimens for tensile and impact tests were cast in permanent moulds. Tensile test bars were fabricated to a gauge length of 50 mm and a gauge diameter of 10 mm, and the tensile tests were carried out at a strain rate of 1 mm/min in a SHIMADZU AG-100KNA materials test machine. The impact test specimens were in the size of 10×10×55 mm without any notch. The fracture surface and metallographic sections perpendicular to the
Tensile fracture behavior
Fig. 1 shows the optical result for the vertical section of the test bar ruptured by tensile test. As can be seen in Fig. 1, in the as-cast AZ91 alloy, irregular Mg17Al12 particles distribute discontinuously in the matrix. The crack propagation primarily occurs near the Mg17Al12 particles and grain boundaries, especially in the Mg/Mg17Al12 interface. The cracks converged into a long crack “river”, which connected with the main crack and existed as secondary crack. The deep and developed
Conclusions
The results of optical microscopy and SEM revealed that AZ91 magnesium alloy usually failed by brittle fracture with little deformation. Cleavage is the principal fracture mode and sometimes the alloy is deformed by mechanical twinning. The interface of Mg/Mg17Al12 often acts as the crack initiation source and Mg17Al12 is not an ideal strengthening phase.
References (9)
- et al.
Scr. Mater.
(1998) - et al.
Scr. Metall.
(1989) - et al.
Acta Mater.
(1999) - et al.
Acta Mater.
(1997)