Study On Solidification Behavior And Grain Refining Mechanism Of Sand-cast Mg-Gd-Y Alloys

Compared with materials with fcc or bcc structure, magnesium alloys with hcp structure have higher Taylor indexes and Hall-Petch coefficients. It means that grain refinement is a remarkable way to reinforce magnesium alloys. Mg-Gd-Y alloys have shown strong application in aerospace and national defense field, due to the novel high strength and heat resistant properties. These alloys are used to manufacture some special aerospace components with huge size and complex structure, only be formed through sand casting process. The slow cooling rate increases the grain size, reduces strength and limits the application of Mg-Gd-Y alloys. Based on development of new solidification technologies, the traditional sand-cast processing shows a new life.In this dissertation, influences of Gd, Y, Zr, cooling rate, pouring temperature and pulsed electric current(PEC) on the solidification behavior, mechanical properties and the grain refining mechanism of sand-cast Mg-Gd-Y alloys were studied, by using computer-aided cooling curve analysis(CA-CCA) system, optical microscope(OM), X-ray diffraction(XRD), scanning electron microscope(SEM) with Energy-dispersive X-ray spectroscopy(EDS) and mechanical property test machine. A co-treated method for refining α-Mg grains was developed by combining adding Zr and using PEC, with grain refining mechanism and solute immigration formulated, which was applied to form an Mg-Gd-Y alloy sand casting.Effects of alloying elements, casting parameters and PEC on the α-Mg undercooling temperature(ΔTN) of sand-cast Mg-Gd-Y alloys were identified. The influence of solidification condition on α-Mg nucleation temperature(Tα, N) of sand-cast Mg-Gd-Y alloys was clarified. Zr has the strongest effect on ΔTN, among Gd, Y and Zr. ΔTN of sand-cast Mg-Gd-Y alloys raises by 7.9 ℃ with Gd content increasing from 0 to 12.7 wt. %, 3.4℃ with Y content increasing from 0 to 5.6 wt. %, and 10.4℃ with Zr content increasing from 0 to 0.70 wt. %. Increase in Gd content and Y content decrease Tα,N by increasing the constitutional supercooling of the Mg-Gd-Y melt. Increase in Zr content increase Tα, N by reducing liquid-solid phase transition resistance, based on the “peritectic reaction” theory. PEC decreases the critical α-Mg nucleus size and undercooling for the critical α-Mg nucleus forming. Forming probability of α-Mg nuclei from atom clusters was increased by static electricity and Lorentz force caused by PEC.The relationship between the average α-Mg grain size(d) and total alloying elements content(CTotal=x+y) of sand-cast Mg-x Gd-y Y-0.45 Zr alloys is formulated d=45.3+106.6exp(-CTotal/5.8). The optimum alloy element content of sand-casting Mg-Gd-Y alloys was optimized. It was discovered that Zr solute could transform into Zr particles in sand cast process, when Zr content was under 0.45 wt. %(solubility of Zr in Mg alloy). Grain refining mechanism of Gd, Y and Zr was uncovered. The relationship between d and Gd content(CGd) of sand-cast Mg-x Gd-3Y-0.45 Zr alloys is formulated d=52.5+59.0exp(-CGd/3.8). The relationship between d and Y content(CY) of sand-cast Mg-10Gd-y Y-0.45 Zr alloys is formulated d=45.1+20.7exp(-CY/5.1). Increase in Gd content and Y content decrease the α-Mg nucleation rate by increasing the constitutional supercooling of the melt. Sand-cast Mg-10.2Gd-3.5Y-0.45 Zr has the optimum strength and ductility with tensile properties(σ0.2=142MPa, σb=220MPa and δ=4.0%). The relationships between d and Zr content(CZr) of sand-cast Mg-10Gd-3Y-z Zr alloys are formulated d1=351.0-658.2CZr,(CZr﹤0.45 wt. %) and d2=79.9-46.7CZr,(CZr﹥0.45 wt. %). When Zr content is over 0.45 wt. %, there is dissolved Zr(Zr S), precipitated Zr(Zr P1) and undissolved Zr(Zr P2) in Mg-Gd-Y melts. All Zr in these three status can refine α-Mg grains with different grain refining mechanism: Zr S and Zr P1(before precipitation) is for the constitutional supercooling, Zr P1(after precipitation) is for the curvature undercooling, and Zr P2 is for the heterogeneous nucleation which also decreases the critical undercooling for forming of α-Mg nuclei(ΔTCritical).Sand-cast Mg-10Gd-3Y-0.61 Zr has the optimum strength and ductility with tensile properties(σ0.2=153MPa, σb=224MPa and δ=7.3%). Some undissolved Zr reinforces the strength of sand-cast Mg-Gd-Y alloys as a strengthening phase in the particle manner.Relationships between casting parameter, PEC and the average α-Mg grain size(d) of sand-cast Mg-Gd-Y alloys was formulated. The optimum cooling rate, pouring temperature and PEC parameter(current peak I, frequency f and duty ratio P) of sand-casting Mg-Gd-Y alloys was optimized. The grain refining ability of PEC was discovered. Influences of undercooling, overheating and PEC disturbance on the α-Mg nucleation rate and solute immigration were uncovered. With increase in cooling rate(CR) from 1.4 oC/s to 10.5 oC/s, d of sand-cast Mg-10Gd-3Y-0.45 Zr alloys decreased from 59 μm to 39 μm and the relationship between CR and d is formulated d=39.4+72.2exp(-CR/0.9). Increase in cooling rate increase both nucleation rate( I*) and crystalline growth velocity( u) of the α-Mg grain. The effect of cooling rate on the α-Mg nucleation rate is stronger than it on crystalline growth velocity of the α-Mg grain. With increase in pouring temperature(PT) from 680 oC to 750 oC, d of sand-cast Mg-10Gd-3Y-0.45 Zr alloys increased from 44 μm to 71 μm. When the melt was poured at 780 oC, d decreased to 46 μm. The relationship between PT and d is formulated d=–5444+15.1PT-0.01PT2. Liquid/liquid transformation caused by overheating increases the atomic diffusion coefficient ratio(DL2/DL1), resulting increase in the α-Mg nucleation rate. Eutectics with net shape and island shape, casting shrinkage produced at low pouring temperature and oxidation with solute concentration generated at high pouring temperature reduces the mechanical properties of the alloy. With cooling rate ranging from 2.1 oC /s to 3.5oC /s, sand-cast Mg-10Gd-3Y-0.45 Zr alloy poured at 750 oC has the optimum strength and ductility. During the solidification process, PEC treatment refines the microstructure of sand-cast Mg-10Gd-3Y alloys. The grain refining efficiency of PEC appeals the threshold with the PEC parameters(I=100 A, f=500 Hz and P=60%). PEC enhances α-Mg nucleation rate by reducing α-Mg nucleation barrier, and releases solute concentration in α-Mg grains by increasing the solute distribution coefficient(0k). The extra Joule heat could re-melt the formed α-Mg nucleus.Co-treatment method for refining grain was developed, combining adding Zr and using PEC, with grain refining mechanism uncovered and a modified coefficient added into Stocks-Einstein equation. The model of free nuclei force and solute immigration was built up. It was discovered that the grain refining ability of co-treatment method increased with decrease in cooling rate. Co-treatment can further refine α-Mg grains and enhance the α-Mg nucleation undercooling of sand-cast Mg-10Gd-3Y alloys with Zr in all status(ZrS, Zr P1 and Zr P2). Based on the thermodynamic analysis, co-treatment increases the change of Gibbs free energy(ΔGv), the collision frequency of atomic clusters and the amount of the critical α-Mg atomic clusters, but reduces the α-Mg nucleation barrier, the electrostatic repulsion potential(UR) of atomic cluster and the critical α-Mg nucleus size(r0), resulting increase in the α-Mg nucleation rate(I*). Based on the kinetic analysis, most of free α-Mg nuclei are from the surface of the melt. PEC accelerates the drop of free α-Mg nucleus.The grain refining efficiency of co-treatment increased from 21.5% to 26.1%, when the cooling rate decreased from 1.32 oC /s to 0.97 oC /s. On condition of co-treatment, the viscosity of solute atoms(Gd, Y and Zr S) increases at the interface between solid and liquid, causing the diffusion coefficient(*iD) of solutes decreased, with the actual immigration rate of solutes equation:(0＜a＜1).Effects of the co-treatment on microstructure and mechanical properties of Mg-Gd-Y alloys were explored in the trial produce of a sand-cast Mg-10Gd-3Y alloy casting. Compared with the alloy treated by adding Zr only, co-treatment further refined α-Mg grains, resisted the casting shrinkage and increased the density of sand-cast Mg-10Gd-3Y alloys, based on results of the application experiment. The co-treated alloy in T6 condition show higher strength and ductility, attributed to refined α-Mg grains and β phases along the α-Mg grain boundaries with its increased amount.