Tensile and fatigue behavior of AZ91D magnesium alloy

Recently magnesium alloys, owing to their low density and excellent specific strength have been attractive to designers as an advanced material for coping with energy conservation and environmental pollution regulations. Much of the recent developments of magnesium alloys and processes have been driven by the requirements of the aerospace industry, although some successful ones have found application in other high performance applications particularly those in the automotive area [1, 2]. However, low fatigue strength under service conditions has been an important factor in limiting the use of magnesium alloys in highly stressed designs. As there have been few publications on the fatigue behaviors of magnesium alloys and magnesium composite materials, it is the aim of this study to establish some microstructure-property relationship for AZ91D, which is one of the highest strength magnesium alloys. AZ91D alloy was subjected to solution heattreatment at 380 ◦C for 8 h and then at 420 ◦C for 24 h in argon atmosphere followed by water quench at 25 ◦C. The solution treated alloy was aged at 200 ◦C for 24 h in a vacuum oven. Uniaxial tensile test was carried out at an initial strain rate of 1× 10−4/s using cylindrical specimens with 6 mm gauge diameter and 30 mm gauge length on an Instron machine equipped with a computer data acquisition system [3]. The microhardness was measured for each heat treatment condition using a Vickers microhardness tester under a 200-g load and 15 s duration. Fatigue life of each of the microstructures was then characterized using cylindrical tension-compression type specimens at a fixed frequency of 25 Hz and load ratio (R) of 0.1 using a servo hydraulic universal testing machine of 25 kN capacity in the laboratory environment. The fatigue crack growth rates were studied using 1/2 compact tension (CT) specimens as described in ASTM standard E64793 [4]. All specimens were optically polished using 0.5-micron diamond particles before testing. Fatigue crack growth rates as a function of the alternating stress intensity factor range (1K = Kmax − Kmin) were determined on the fabricated specimens under load control mode in a servo-hydraulic machine. These tests were conducted in the laboratory air at room temperature (25 ◦C) using a sine-wave cyclic frequency of 10 Hz and a load ratio of R= 0.1. The crack extensions in the test specimens were monitored using a stereo microscope on optically polished specimen surfaces. The major emphasis in these studies was laid on crack path morphology and determining the 1Kth values. In order to achieve these values, all tests were started with an initial 1K ≈ 5 MPa√m, and load-shedding was done in steps keeping the load-ratio of R= 0.1. The tests were terminated at the 1K value where no physical crack growth could be monitored for 1 million cycles of loading. Fracture surfaces of all tested specimens were gold coated immediately after the tests. The coated fracture surfaces were examined using SEM. The optical micrographs of as cast, solution treated and aged microstructures are shown in Fig. 1. In the as-cast state, the β phase (Mg17Al12) can be seen at the grain boundary and eutectic α phase exists at the adjacent to β phase as shown in Fig. 1a. In the solution treated condition, Fig. 1b showed that someβ phase still remains at the grain boundaries without dissolving. In the solution and aged condition, β phase precipitates again at the grain boundaries as shown in Fig. 1c. Tensile properties and hardness of AZ91D alloy in three different heat treatment conditions are shown in Table I. It can be seen from the table that the hardness, tensile, and yield strength of the material are significantly improved after heat treatment. However, the elongation to failure is lowest in the case of solution and aged condition material. The differences in the fracture behaviors between the as-cast and the heat treated AZ91D alloy can be seen from the SEM fractographs in Fig. 2a through c. The absence of radial zone in the fracture surface of aged specimen as shown in Fig. 2c indicates that crack initiation takes place in several locations and such cracks grow and join to form the fibrous zone and also intergranular propagation of the cracks mainly from the defects. This is in agreement with the low elongation of the aged specimens. The fractographs, Fig. 2a and b, of as-cast and solution treated specimens show predominantly cleavage fracture. The S-N data obtained from the fatigue life test (Fig. 3) show that the solution treated material has improved fatigue strength at any given stress level as compared to as-cast or aged materials.