چكيده به لاتين
Thermomechanical treatments are usually performed on Magnesium alloys in order to obtain a suitable microstructure and enhance flexibility and strength. Therefore, in this research, hot extrusion was conducted at 370ᵒC, 400ᵒC, and 420ᵒC on Mg-3Zn-1Ca alloy to achieve the goals mentioned above. For this purpose, microstructures of extruded samples and its alteration were investigated after the natural aging process by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). As the results show, extrusion temperature enhancement not only would cause completion of dynamic recrystallization but also would increase mean grain size from 14 to 26 micrometers. Microstructural studies after thermomechanical operation exhibit that increasing the extrusion temperature results in instability of the structure, why so the more the extrusion temperature, the more the precipitation in the structure after the process. So that, extruding at 370ᵒC, 400ᵒC, and 420ᵒC increased the precipitation by the amount of 6%, 18 %, and 123%, respectively compared to the structure before natural aging. Results of the hardness test show that by increasing the extrusion temperature, the hardness of the samples decreases, due to the enhancement in mean grain size. Additionally, the tensile test expresses that the maximum ultimate tensile strength and the yield strength of 256.51 and 144 MPa, respectively, occurred at the sample extruded at 370ᵒC. These results indicate the fact that by increasing the extrusion temperature, the ultimate tensile strength and the yield strength decline. The mechanical properties decrease maybe is due to enhancement in mean grain size and also the inappropriate distribution of secondary phase by increasing the extrusion temperature. Finally, performing fatigue tests at the conditions of R=-1 and F=20 revealed that the sample possessing the maximum ultimate tensile strength and the yield strength has the highest fatigue limit the largest safe area for utilizing.
