FATIGUE AND FRACTURE OF NANOCRYSATALLINE METALS

Abstract

Magnesium alloys are attracting engineers for their practical applications to the industrial use because of its light weight and high specific strength. In this work, fatigue- fracture properties, as they are the most important mechanical property of any material for its structural use, have been examined for Magnesium ZE41 alloy processed under different conditions. Magnesium ZE41 alloy is mainly used in the aviation industry, so fatigue and fracture properties are very crucial for the material to be used for these applications. Tensile strength and Vickers Hardness of the alloys was examined for different processed samples. Fracture toughness of the processed and unprocessed alloy was find out by conducting 3-point bend test. The experimental results were verified by performing finite element simulations, using ANSYS software. The simulation results were found quite similar to the experimental results. Fracture mechanics simulations were carried out for edge cracked, center cracked and double edge cracked specimen to find out the effect of processing on the alloy. There was a clear increment in the fracture strength of the alloy after high strain rolling at high temperature. The strength of the alloy showed maximum increment of more than 200% for 3-pass forging followed by 70% rolling condition. The ductility of the alloy showed maximum increment of about 400% for 6-pass forging followed by 70% rolling condition. Microstructural examination of the processed alloy showed the alloy possessed an ultrafine grained structure after 3-pass forging followed by 70% rolling with grain size of approximately 800nm and also after 6-pass of forging with grain size of approximately 700nm. TEM observations were carried out for the submicron level feature of the processed alloy. Fractrography of the broken tensile samples were carried out using SEM, showed that the mechanism of fracture after application of high strain deformation was mainly grain boundary sliding. Fatigue simulations were carried out using ANSYS software for different condition processed alloys. There was a clear improvement in the fatigue life of the alloy because of the increase in both strength and ductility of the alloy.