Abstract
This article investigates the influences of heat-treating on the microstructures and the high cycle fatigue (HCF) properties of AZ91 and AZE911 (AZ91 + 1%RE) magnesium alloys. For such an objective, AZ91 and AZE911 alloys were used after solution treatment at 415°C for 5 hours aged at 215°C for 3 hours and at 215°C for 5 hours, respectively. To investigate the HCF behavior, a rotational bending fatigue test was performed with stress ratio (R) −1 and frequency of 100 Hz at room temperature. Optical microscopy (OM) study demonstrates that heat treatment leads to a reduction in grain size and transformation of continuous and discontinuous precipitations into needle-shaped precipitations, which are located on the grains of the alpha phase. Scanning electron microscopy (SEM) showed both marks of quasicleavage and cleavage on the fracture surface of specimens. These planes indicated the brittle behavior of the fracture. Moreover, in heat-treated specimens, the size of cleavage patterns was smaller, and microcracks were shorter. These behaviors affected the strength of the material and the fatigue lifetime. The results of mechanical tests show a negligible influence of heat-treating on the HCF behaviors of AZ91-T6 and AZE911-T6 alloys. Stress-lifetime curves (S-N) show an increase in fatigue strength in 3.8 × 105 fatigue cycles, from 95 MPa to 125 MPa for the AZ91-T6 alloy and from 125 MPa to 155 MPa for the AZE911-T6 alloy, after heat treatment.
Highlights
Research ArticleAhmad Yousefi Parchin Oliya, Mohammad Azadi ,2 Mohammad Sadegh Aghareb Parast, and Mehdi Mokhtarishirazabad
Magnesium alloys are lightweight materials that are used in the automotive and aerospace industries due to their high strength-to-weight ratio and good vibration damping [1] and proper thermal conductivity [2,3]
Heat-treating led to two distinct influences on the microstructure of AZ91 and AZE911 alloys. e first effect was the transformation of continuous and discontinuous β phase (Mg17Al12) precipitations at the grain boundary to needle-shaped precipitations on α-Mg phase grains, which was the most effective factor in increasing the hardness. e second effect was the reduction of grain size from 300 μm to 130 μm for the AZ91 alloy and from 230 μm to 90 μm for the AZE911 alloy, which caused a significant increase in the number of grain boundaries as well as the number of grains, which was the main reason for increasing the strength limit. erefore, logical agreements could be seen compared to the literature for these microstructural changes by heat-treating [8,18,25,26]
Summary
Ahmad Yousefi Parchin Oliya, Mohammad Azadi ,2 Mohammad Sadegh Aghareb Parast, and Mehdi Mokhtarishirazabad. Is article investigates the influences of heat-treating on the microstructures and the high cycle fatigue (HCF) properties of AZ91 and AZE911 (AZ91 + 1%RE) magnesium alloys. For such an objective, AZ91 and AZE911 alloys were used after solution treatment at 415°C for 5 hours aged at 215°C for 3 hours and at 215°C for 5 hours, respectively. E results of mechanical tests show a negligible influence of heat-treating on the HCF behaviors of AZ91-T6 and AZE911-T6 alloys. Stress-lifetime curves (S-N) show an increase in fatigue strength in 3.8 × 105 fatigue cycles, from 95 MPa to 125 MPa for the AZ91-T6 alloy and from 125 MPa to 155 MPa for the AZE911-T6 alloy, after heat treatment
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