Study Of Severe Plastic Deformation’s Effect On The Microstructure, Strength And Plasticity Of Magnesium Alloy

The application on structure materials of hexagonal close crystal structuremagnesium alloy is limited due to its low ductility and strength. Grainrefinement reasonable control for morphology and distribution of strengtheningphase are effective ways to improve the mechanical properties of the alloy.Angle extrusion technology is a kind of serve plasticity deformation technology,which can refine microstructure and improve mechanical properties. Currentlywidely used equal channel anger pressing refining technology is difficult to beindustrialized due to its high cost and low efficiency. In order to further improvethe strength and toughness of the magnesium alloy, a new type of magnesiumalloy composite extrusion method-unequal channel anger pressing(UCAP)waspresented which organically combined the traditional extrusion and the serveplastic deformation ECAP (equal channel anger pressing). In this study,Mg-7.5Al-0.85Zn and Mg-6Zn-1.2Y-0.6Zr magnesium alloys as serve plasticdeformation materials were processed by ECAP technology and UCAPtechnology to further enhance the strength and plasticity. Microstructureevolution and dynamic recrystallization mechanisms of the as-cast, ECAPed andUCAPed magnesium alloys were studied thoroughly by Optical microscopy(OM), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energyspectrum analysis (EDS), Transmission electron microscopy (TEM) andDifferential scanning calorimetry (DSC). The mechanical properties andmechanism of these magnesium alloys in different conditions were discussed bymechanical test and Gleeble1500thermal simulator. Hot deformation behaviorand deformation mechanism in different strain rate and different temperaturewere systematically studied and the constitutive equation of the two magnesiumalloys was put forward. The extrusion forces, stress and strain distributionduring UCAP processing were investigated by numerical simulation techniques.The correctness of the simulation results through the experimental results wasverified and ECAP and UCAP experiment mold with preheating device weredesigned and manufactured. The relations between microstructure evolution and mechanical properties of the as-cast and ECAPed, UCAPed magnesium alloywere discussed. The main results have been obtained as follows:1. High temperature compressive deformation behavior under differentdeformation conditions (deformation temperature, strain rate and strain capacity,etc.) of as-cast Mg-7.5Al-0.85Zn and Mg-6Zn-1.2Y-0.6Zr alloy weresystematically analyzed and compared by high temperature compressive testing.Flow stress of two kinds of alloys presented a different form with the differentdeformation temperature and strain rate, and presented a different dynamicrecrystallization characteristic. True stress was very sensitive to the change oftemperature and strain rate, which was the character as the positive strain ratesensitive material. The stress of the curve increased with the increase of strainand then reduced. The maximum value of flow stress and its correspondingstrain value decreased with increasing of deformation temperature or decreasingof strain rate. The constitutive equations of plastic deformation for the twoalloys at elevated temperatures were obtained. The deformationactivation energy of Mg-6Zn-1.2Y-0.6Zr alloy presented the tendency ofincrease to242KJ/mol and to181KJ/mol for as-cast Mg-7.5Al-0.85Zn alloyunder the same condition. The main reason was that elements Y could reduce themagnesium alloy’s stacking energy and hinder the gathered dislocation andincrease climbing tendency of the high temperature dislocation. The optimal hotworking condition was the temperature in the range of350～400℃and strainrate less than0.05s-1.2. The microstructure observation results of as-cast Mg-6Zn-1.2Y-0.6Zralloy in different deformation condition showed that the deformationmechanisms were mainly discontinuous dynamic recrystallization. Theelongated deformation microstructure in Mg-6Zn-1.2Y-0.6Zr alloy at lowtemperature and small deformation stage appeared; the twin played a leadingrole. The jagged characteristics of geometric dynamic recrystallization wereformed in internal grains; slip dislocation played a key role during hightemperature.3. The distribution of stress and strain and extrusion force of Mg-6Zn-1.2Y-0.6Zr magnesium alloy during the UCAP process under differenttemperature, strain rate and extrusion ratio were simulated by deform software.Extrusion ratio and extrusion temperature were the key factors in UCAPdeformation process, which controlled actual extrusion pressure and distributionof stress and strain easily.The distribution of stress and strain mostlyconcentrated at the angular of the extrusion dies. The effective stress, effectivestrain and extrusion load became larger as the extrusion ratio. The extrusionforce and effective stress reduced with the increase of extrusion temperature, butthe effective strain had little changes. It was found that the results of simulatedUCAP process could certainly guide the UCAP actual process.4. The influence on the microstructure and mechanical properties of theECAPed and UCAPed alloys were analyzed and compared. ForMg-7.5Al-0.85Zn magnesium alloy, ECAP had significant grain refinement.With the increasing of number of ECAP passes, mean grains became finer;especially the most effective ECAP pass in the first pass, but the microstructurewas obviously nonuniform. Remelted Mg-7.5Al-0.85Zn alloy with initial grainsize of about145μm can be ECAPed at285℃without crack occurrence. Themost finest mean grain size1.5-3.0μm and the best microstructure uniformitywas obtained after four ECAP passes while grain coarsening occurred after morethan4passes associated with an increase in high angle grain boundary.Grainrefinement mechanism during ECAP can be described as dynamicrecrystallization with combination of the dissolution of the β-Mg17Al12phase atmatrix boundary interface and pure shear stress generated by ECAP. β-Mg17Al12phase inhibited the rate of the dynamic recrystallization and the brokenβ-Mg17Al12phase during ECAP precipitated in the matrix, which led to the finemicrostructure and improved tensile strength and elongation.The fracturemorphology changed from the quasi-cleavage fractures of as-cast alloy todimple-like fracture characteristics. Compared with as-cast Mg-7.5Al-0.85Znalloy, the tensile strength of ECAPed alloys increased by42%, namely, from180MPa to306MPa while the elongation of the ECAPed alloys increased from4.7%to15.8%and the hardness for four passes was the highest value, which was142HL, much higher than91HL of as-cast alloy, which increased by36%.5. UCAP also has significant refining grain effect with lower temperaturethough only1pass was pressed. Extrusion temperature became the easilycontrolled key factor during UCAP deformation process. Mg-7.5Al-0.85Zn alloycould obtain the non crack UCAPed alloy without remelting. The strength andelongation of Mg-7.5Al-0.85Zn magnesium alloy got improvement as the grainsbecame smaller. When the extrusion temperature was250℃, the average grainsize was4.5μm, strength changed to350MPa, prolongation rate reached14.1%,the strength and toughness of magnesium alloy were more effectively improvedthan ECAPed4passes alloy. Grain refinement mechanism of Mg-7.5Al-0.85Znalloy during UCAP can be described as grain fragmentation in the pure shearstress and extrusion and with the dissolution of the β-Mg17Al12phase at matrixboundary interface and generation by UCAP whole dynamic recrystallization.Under the effect of shear stress, new crystal nucleus can be emerged in theincreased density dislocation high-energy area and then arranged into dislocationboundaries when dislocation continuously moved and nailed up interphase andtransformed to the new grain, therefore the grains could be refined.6. The effect on the microstructure and mechanical properties of UCAP edMg-6Zn-1.2Y-0.6Zr alloy was different from UCAPed Mg-7.5Al-0.85Zn,Mg-6Zn-1.2Y-0.6Zr alloy grains also had different degree of refinement duringUCAP process, but its refinement mechanism can be described as mechanicalshear mechanism of shear extrusion stress, which was related to the secondphase-high temperature resistant I phase and fragile hard brittle w phasegrain.The tensile tests at room temperature showed lower yield strength andelongation rate to failure of the UCAPed Mg-6Zn-1.2Y-0.6Zr alloy as comparedwith the UCAPed Mg-7.5Al-0.85Zn alloy.The Mg-6Zn-1.2Y-0.6Zr UCAPedalloy showed the cleavage fracture mode, while the fracture surface ofMg-6Zn-1.2Y-0.6Zr UCAP+ECAPed alloy exhibited the some tougheningdimples.The obvious recrystallization occurred in the Mg-6Zn-1.2Y-0.6Zr alloyleads to the large average grain size as well as a number of I phase and W phaseparticles. Tensile tests showed that the Mg-6Zn-1.2Y-0.6Zr alloy exhibits higher yield and ultimate tensile strengths, but lower elongation to failure at roomtemperature. In contrast, the Mg-6Zn-1.2Y-0.6Zr alloy showed higher strengthsand ductility at200℃and300℃, respectively. These differences in the tensilemechanical properties between the two alloys are from the differentcontributions of the strengthening due to the grain refinement and theprecipitation and dispersion strengthening effects generated by the second phaseparticles. The average grain size of the UCAPed+ECAPed Mg-6Zn-1.2Y-0.6Zralloy was finer, which was1.5μm, the second phase particles were dispersed,and performance were obtained with tensile strength of350MPa and theelongation of18%.