Development of High Strength and Corrosion Resistant Magnesium Alloys Using Severe Plastic Deformation Process

Abstract

In the present work, several Mg alloys as well as pure Mg were produced through different processes including casting, extrusion, screw rolling (SR) and multi directional forging (MDF). The mechanical properties and corrosion behavior of the extruded materials were studied and materials with the best mechanical properties and corrosion behavior were subjected to the SR and MDF. The related processing conditions of SR and MDF were changed and mechanical properties and corrosion behavior of the produced materials were analyzed. The microstructure was characterized by optical microscopy (OM), x-ray diffraction (XRD) and scanning electron microscope (SEM). The mechanical properties including tensile and compression properties were measured using a R&B, an Instron universal testing system and hardness vickers. The electrochemical, hydrogen evolution and mass loss techniques in a 3.5 wt. % NaCl solution saturated with Mg(OH)2 were used for corrosion studies. Moreover, the scanning kelvin probe force microscopy (SKPFM) was utilized to measure the volta-potential of the matrix and various intermetallic phases. ZAXM4211 alloy was subjected to both processes of SR and MDF at different temperatures. It was observed that increasing the screw rolling temperature from 220 to 340 oC results in decreasing the yield strength, while it results in decreasing the corrosion rate up to 300 oC and increasing it from 300 to 340 oC. Moreover, it was observed that increasing the screw rolling temperature results in the dissolution of intermetallics and increase in the grain size. Then, several alloys like ZAXM4211, XM11, ZAXM4411, ZTXM4411 were also subjected to MDF at different temperatures and same trend as SR were obtained. It was also confirmed from the MDF results that, both the mechanical and corrosion properties were improved after MDF. However, increase in MDF temperature decreased the yield strength and increased the corrosion resistance. Yield strength reduction at higher temperatures is due to increase in grain size while corrosion rate behavior is under control of two parameters including reduction of second phases (galvanic cells) and grain size. A very low corrosion rate material was obtained from XM11 alloy MDFed at 300oC. The corrosion rate of this material was 0.26 mm/y which is even less than that of high purity Mg (0.39 mm/y). Finally to understand the corrosion rate improvement, all the corrosion rate data which was obtained in this study as well as some available literature were quantitatively analyzed and an experimental formula was proposed to model the corrosion rate using the microstructure parameters. These parameters were related to the composition, grain size, and area fraction of the second phases and volta-potential difference of the second phase relative to the matrix. The experimental data was in a good agreement with the data of the proposed model.