Microstructure Evolution And Mechanical Behavior Of Mg-Gd-Y System Alloy Produced By Severe Plastic Deformation

As a lightweight structural metal material, magnesium alloys have widely potential applications for use in national defense, aerospace, automotive and electronic industries. However, the application of Mg alloys has been substantially hindered by their relatively low strength and limited ductility at room temperature. It is very important to investigate the toughening of magnesium alloys for promoting their applications. Owing to absent of allotropic transformation, mechanical properties of magnesium alloys cannot be enhanced by phase transformation. Recently, the most effective ways for strength enhancement are solid solution strengthening, grain refinement and precipitation hardening. Combining these mechanisms, high performance rare earth magnesium alloys are prepared by a novel super plastic deformation technique, repeated upsetting(RU), and followed by aging treatment in this study. Systematic studies of microstructure evolution, mechanical properties changing and strengthen mechanism interaction during the deformation and annealing are carried out to develop the toughening technology of magnesium alloys. In this way,some key issues in fabrication of high performance rare earth magnesium alloys are solved. The detailed researches are as followed:The deformation behavior of as-extruded Mg-Gd-Y(GW103K) alloy is investigated by compression test with Gleeble-3500 thermal simulator at temperature of 648-723 K and strain rate of 0.01-5 s-1. It is found that the flow stress is sensitive to deformation temperature and strain rate for GW103 K alloy. At any strain rate, the peak stress increases along with the decrease of temperature suggesting that the working hardening effect is significant at low temperatures. On the other hand, the peak stress increases along with the increase of strain rate at a given temperature indicating that the release of work hardening factors like dislocation accumulation is more effective during slower deformation processes. Base on the results of flow stress, we proposed a hyperbolic sine constitutive equation in which the determined average activation energy is 229.5 k J/mol. Processing maps describing the variation of power dissipation efficiency is constructed as a function of temperature and strain rate, which exhibit a domain of dynamic recrystallization(DRX) occurring at temperature of 420-450 °Cand strain rate of 0.01-0.1s-1corresponding to the optimum hot working window. The instability zones of flow behavior are also recognized from the maps.Finite element simulation was carried out to investigate deformation temperature and passes effect on material flow, stress and strain evolution. Flow field results show that the deformation of RU can be divided into two components of pure shear and uniform extend, which leads to a turbulent flow during deformation. Stress results show that stress concentration occurs at edges of the samples during the beginning stage of deformation. The sample processed by a single pass of RU exhibits a wave-type distribution along extrusion direction. As the number of passes increase, the flow strain shows normal distribution. The minimum strain of the sample reaches ~3.7, indicating no dead zone existed in RU deformation.After pre-extrusion and 4 passes of RU at 350 °C, initial grain size reduces from139μm to less than 1μm, forming a homogenous and ultra-fined grain structure. As the deformation passes increasing from 1 to 4, the maximum texture intensity gradually decreases from 7.6 to 2.0. The grain size of the samples deformed at 350 °C, 400 °C and 450 °C for 4 passes are ~ 1μm, 4 ~ 5μm and 10 ~ 20μm, respectively. As temperature decreases from 450 °C to 400 °C and 350 °C, the maximum pole density slightly increases from 2.0 to 2.2 and 2.5. Effective grain refinement is achieved by producing with route A, which also induces many shear bands into the microstructure.However, the samples deformed after 4 pass of Route B did not show equivalent refinement. Many coarse grains of 5~10μm retained, while no obvious shear bands are found in the microstructure.Mechanical properties of the samples with different parameters were investigated.After 4 passes of RU processing, the yield strength, ultimate strength and uniform elongation improved 150%, 52% and 110%, respectively, which reached 250 MPa,350MPa and 12%. The homogeneity of mechanical properties are also improved with the increase of RU passes. The range of micro hardness changes from 650~850MPa to850~950MPa. After 4 passes of RU processing, the yield strength is improved to 271 MPa in tension and 262 MPa in compression without showing the yield strength asymmetry. As the temperature increase from 350 °C to 400 °C and 450 °C, the yield strength decrease from 250 MPa to 210 MPa and 190 MPa, which is similar to the trend of ultimate strength. Elongation improve from 12% to 18.5% and 24% with the temperature increase from 350 °C to 400 °C and 450 °C. The elongation of the sample produced by route B is larger than route A, while the strength of route B is lower.Therefore, route B is suitable for fabricating the sample with better ductility, while routeA is better to produce high performance GW103 K alloy.Mechanical properties can be further improved by annealing after RU. The ultimate strength improves from 350 MPa to 450 MPa after peak aged, while the elongation decreases to ~3.3 %. A previously unobserved metastable phase(βT) is discovered to coexist with reported β″ and β′ metastable phases under peak aging conditions. The βT phase has an orthorhombic crystal structure with lattice parameters of a = 2· a Mg ≈ 0.64 nm, b = 8· d(1010)Mg ≈ 3.33 nm, c = c Mg ≈ 0.52 nm, with a composition of Mg5RE(RE= Gd and Y). It can be seen that the orientation relationship between aβT precipitate and the matrix is:(100)Tb//(1120)aand [001]Tb//[0001]a. According to our observations, a precipitation sequence of S.S.S.S â†' β″ â†' βT â†' β′ â†' β1 â†' β is proposed, where some phases may coexist under some aging conditions.Ag addition is found to significantly affect segregations a high-energy interfaces.Ag-assisted segregation at{10 12}twin boundaries(TBII)and lamellar grain boundaries(LGB)exhibits a new periodic spinal-shaped structure that is different from the single-lined segregation in the alloy without Ag.The segregation consists of Gdand Ag-rich columns.It appears that high Ag content in the spinal-shaped segregation induces fcc-like cells structures.