Investigation On Microstructure And Mechanical Properties Of Mg-Mn Series Alloys

Mg-Mn series alloys with extremely low cost have been being developed as a high performance magnesium alloys because of their excellent corrosion resistance, good creep behavior, ultra-high damping capacity and outstanding weldability. However, the alloys are not considered as structural applications due to the extremely low solid soluability of Mn in α-Mg matrix, poor grain refinement effect and strong basal texture after the extrusion process.In the present work,the influence of Mn addition on microstructure and mechanical properties of both as-cast and as-extruded Mg-Mn alloys have been investigated by means of the optical microscope(OM), scanning electron microscope(SEM), energy dispersive spectrometer(EDS), X-ray diffraction(XRD), electron back-scattered diffraction(EBSD), transmission electron microscope(TEM) and mechanical properties test. The grain refinement mechanism, dynamically recrystallization behaviors and strengthening mechanism in both as-cast and as-extruded Mg-Mn alloys have also been discussed in this paper, aiming at optimizing the composition of the alloy. Based on the results of Mg-Mn series alloys discussed above, effect of extrusion temperatures on microstructure and mechanical properties of as-extruded Mg-1Mn alloy have been investigated. The dynamically recrystallization behaviors, strengthening mechanism, flow behaviors and constitutive equations of the alloy have been discussed for the purpose of optimizing the extrusion parameters of M1 alloy. Due to the excellent performace of the as-extruded M1 alloy at room temperature, the influence of alloying elements, Al and Y, on microstructure and mechanical properties of Mg-1Mn alloy have been investigated as well.The grain refinement mechanism, dynamically recrystallization behavior and strengthening mechanism have also been investigated in the present work. The contents of this dissertation are list as follows:① The phase constitution ofas-cast Mg-Mn alloys is α-Mg matrix and α-Mn precipitate. With the increasing of Mn content, microstructures ofas-cast alloys have been refined, and the quantity of α-Mn precipitate increases obviously. The yield strength of the alloys have been enhanced with the inceasing of Mn. In addition, yield strength, ultimate tensile strength and tensile elongation of as-cast Mg-3Mn alloy are 34 MPa, 98 MPa and 5.6%, respectively. Grain refinement and precipitation hardening can be considered as the main factor of improving the strength of the as-cast alloys.② Mg-Mn series alloys subjected to the low extrusion temperature show the fully recrystallization and extremely refined microstructure in the present work. With the increasing of Mn content, average grain sizes of the alloys decrease obviously, resulting in the improvement on yield strength. The results reveal that the alloy with only 1wt.% Mn addition shows the intensified basal texture, which is unfavorable to activate the basal slip, leading to the higher yield strength(204MPa). In addition, due to the extremely refined microstructure of the alloy, the critical resolved shear stress(CRSS) of prismatic slip decreases obviously, which is beneficial to activate the prismatic slip accommodating basal slip, causing the superior ductility(38.8%). When the alloy containing 3 wt.%, the quantity of precipitates increases obviously, and the microstructure has been significantly refined. The alloy shows the high yield strength(213MPa) in this paper. Additionally, the alloy shows the weakened basal texture due to the particle stimulated nucleation(PSN) effect, and basal slip can be easily activated. The alloy shows the high ductility with the maximum elongation property of 29.9%. Therefore, Mg-1Mn alloy exhibites the excellent mechanical properties at room temperature.③The morphology and orientation relationship between α-Mn precipitate and α-Mg matrix have also been determined. In the present work, a great number of finer spherical α-Mn precipitates lying in the basal planes of Mg matrix, and the orientation relationship between α-Mn precipitate and α-Mg matrix is(0001)Mg //(111)Mn, [21 1 0]Mg // [011]Mn.④ With the increasing of extrusion temperatures, Mg-1Mn alloy shows the coarsened microstructure. The alloy shows the weakened basal texture because of more α-Mn precipitates precipitating in the alloy. The alloy subjected to low extrusion temperature shows the extremely refined microstructure and excellent mechanical properties. The yield strength, ultimate tensile strength and tensile elongation of the alloy extruded at the temperature of 250℃ are 204 MPa, 234 MPa and 38.8%, respectively, while those properties of the alloy extruded at the temperature of 300℃ are 191 MPa, 224 MPa and 32.4%, respectively. The alloy subjected to high extrusion temperature shows the coarsened microstructure and poor mechanical properties. The yield strength, ultimate tensile strength and tensile elongation of the alloy extruded at the temperature of 350℃ are 154 MPa, 248 MPa and 11.7%, respectively, while these properties of the alloy extruded at the temperature of 400℃are 185 MPa, 234 MPa and 9.6%, respectively.⑤The hot simulation has been conducted on the alloy to investigate the hot deformation behavior. The material constants of the alloy has been calculated and the flow stress equation has been established in the present work.⑥ The phase constitution of the as-cast Mg-x Al-1Mn(x=1, 3, 6 and 9) series alloys varies obviously with the increasing of Al content.The dominant phase constitution of AM11 alloy is α-Mg matrix, Al8Mn5 and Al11Mn4 precipitates. The phase constitution of AM31, AM61 and AM91 alloys are α-Mg matrix, Al4 Mn and Mg17Al12 precipitates. And with the increasing of Al content, the quantity of Mg17Al12 precipitate increases obviously, and microstructure has been effectively refined.The yield strength of the as-cast alloys have been significantly improved, and AM91 alloy shows the highest yield strength of 123 MPa in the present work. Precipitation hardening and grain refinement can be recommended as the main reasons on improving the strength of the alloy.⑦ In the as-extruded Mg-x Al-1Mn(x=1, 3, 6 and 9) alloys, when Al content lower than 6 wt.%, the alloys show the un-recrystallization microstructure. AM11 alloy with high volume of un-recrystallization microstructureshows the strong basal texture, which is unfavorable to activate the basal slip, resulting in the improvement of yield strength. In addition, the decreasing of the grain size of recrystallization reduces the CRSS of prismatic slip of the alloy, leading to the enhancement of ductility. The yield strength, ultimate tensile strength and elongation of AM11 alloy are 250 MPa, 287 MPa and 21.4%, respectively. Grain refinement and texture strengthening can be attributed to the dominant reasons on improving the mechanical properties.When Al content higher than 6 wt.%,the alloys show the fully recrystallization microstructure due to the PSN effect of Mg17Al12 precipitates.With the increasing of Al content, the average grain size of recrystallization grains have been refined obviously, and the basal textures have been weakened as well. The yield strength, ultimate tensile strength and elongation of AM91 alloy are 205 MPa, 317 MPa and 16.1%, respectively.The enhancement of yield strength can be attributed to the grain refinement strengthening and precipitation strengthening. ⑧ In the present work, the main phase constitutions of the as-extruded Mg-1Mn-x Y(x=0.2, 0.5 and 1.0) alloys are α-Mg matrix, α-Mn and Mg24Y5 precipitates. With the increasing of Y content, the quantity of Mg24Y5 phase increases obviously and the grain size of the alloys decreases significantly. The alloy shows the strong basal texture when alloying about 0.5 wt.% Y, while the alloy containing 1.0 wt.% Y shows the extremely weakened basal texture. Mg-1Mn-0.5Y alloy shows the highest strength and good ductility at room temperature in this paper. The yield strength, ultimate tensile strength and tensile elongation of the alloy are 311 MPa, 321 MPa and 11.7%, respectively. The improvement of the strength can be attributed to the texture strengthening and grain refinement.