Solid-solution Aging And Electronic Structure Of Mg-5wt%Sn Alloy

The most commonly used alloys are those based on the Mg-Al system,principally AZ91 and AM50/60. However, at elevated temperatures, an evident creepproblem does not allow them to be used in automotive applications such as:gearboxes, engine blokes, etc. This phenomenon stems from Al₁₂Mg₁₇ phase inMg-Al system alloys. The Al₁₂Mg₁₇ phase becomes soft when the operationtemperature is beyond 120℃. Concomitantly, other alloy systems are beinginvestigated to find the novel candidates of creep-resistant magnesium alloys. Inrecent years, a new family of Al-free Mg-Sn alloys are been developed. Themaximum solidification temperature range for Mg-Sn binary alloys is about 67℃,which is much narrower compared with 136℃and 283℃for Mg-Al and Mg-Znbinary alloys respectively. As a result, the casting defects such as dispersed shrinkageand hot tearing in Mg-Sn alloys are less severe than in Mg-Al and Mg-Zn alloys. Thesolubility of Sn inα-Mg solid solution drops sharply from 14.85wt% at the eutectictransformation temperature 561℃to 0.45wt% at 200℃. This provides a fundamentalbasis for improving the mechanical properties of these alloys through solid-solutionaging treatment. The Mg₂Sn precipitate (FCC, a=0.676 nm, point groupm(?)m) inMg-Sn alloys has a high melting temperature (～771.5℃), which is higher than that ofMg₁₇Al₁₂ (462℃) in Mg-Al alloys and MgZn (347℃) in Mg-Zn alloys. Recentlyafter 2000 there has been a renewed global interest in these alloys which are believedto have potential for applications at elevated temperatures. However, only very limited research achievements have been gained on this subject. Therefore, systemicand ingoing research is necessary about the Mg-Sn alloys.This thesis presents a systemic study of microstructure, tensile properties, andcreep behavior of as-cast Mg-(1～10)wt%Sn alloys, which shows that theMg-5wt%Sn alloy offers good comprehensive properties. Based on the results, themechanism of solid-solution aging in Mg-5%Sn alloy was deeply researched. Tounderstand the mechanism of solid-solution aging, the model of electronic structures,which is depended on the EET theory, was put forward and calculated. Additionally,the effect of Di (a Nd-Pr mixed rare-earth metal) on the microstructure and propertiesof Mg-5%Sn alloy were investigated. The primary conclusions were listed asfollows:1. During the solid-solution process of Mg-5%Sn alloy at 480℃, the secondarydendrite disappears quickly and the eutectic Mg₂Sn phase dissolves into theα-Mgmatrix rapidly. A few devoiced eutectic Mg₂Sn phase with larger sizes dissolvesslowly. Finally, a phenomenon like recrystallization process occurs in thesupersaturation solid solution ofα-Mg phase.2. The supersaturated solid solution of Mg-5%Sn alloy decomposes duringaging, the continuous inhomogeneous precipitation happens along grain boundaryand the continuous homogeneous precipitation within grain. The precipitation orderis SSSS(supersaturation solid solution→β′(metastable)→β(stable) at 160℃～240℃, SSSS→βat 280℃。3. The plate-and lath-shaped precipitate morphologies within the grains occurduring ageing of SSSS Mg-5%Sn alloy. TEM analysis reveals that the precipitateshave the orientation relationships of(110)_p//(0001)_m, <001>_p//<11(?)0>_m at aging of160℃for 720h, and EDX analysis suggests that the atomic concentration of Sn inprecipitation phase is among 5-7%, which is much poorer than that of equilibriumMg₂Sn phase, and HREM analysis indicates the interface of the precipitates withmatrix is coherent. When aged at 240℃for 16h, the precipitates have the orientationrelationships of (11 1)_p//(0001)_m, <110>_p//<11(?)0>_m, and the atomic concentrationof Sn in lath-shaped is 20.51%, and the interface with matrix is semi-coherent; Further the atomic concentration of Sn in plate-shaped is 20.51%, and the interfacewith matrix is non-coherent.4. The plate-shaped precipitates along the grain boundaries occur duringageing of SSSS Mg-5%Sn alloy, which has the same orientation relationships as theprecipitation phase within grains, and EDX analysis indicates that the atomicconcentration of Sn in precipitation phase is 20.51% at 160℃for 720h and 32.26%at 240℃for 16h, respectively, and the interfaces between precipitation phases andmatrix are coherent at 160℃, but non-coherent at 240℃.5. Precipitate free zones (PFZ) were found adjacent to the boundary duringaging of SSSS Mg-5%Sn alloy. The width of PFZ increases with the aging time andaging temperature rising, it decreases if the aging time further delaying. The effect ofaging temperature on the width of PFZ is far more than that of aging time. The PFZforming is probably decided by a solute depletion mechanism in this experiment.6. Based on EET principle, the models for the valence electron structures ofthe precipitation phase phases in SSSS Mg-5%Sn alloy were established. Thecomputation result shows the covalent bond between metastableβ′phase andequilibriumβphase is stronger than that among the atoms in the matrix. The bondstrength of the strongest bond inβ′metastable phase is lower than that inequilibriumβphase. The electronic density between the matrix and the precipitateβ′phase is continuous, but it is discontinuous forβphase. These computationresults are helpful to understand the solid-solution aging process of Mg-5%Sn alloy.6. The addition of Di improves the properties of Mg-5%Sn alloy both at roomtemperature and elevated temperature, especially the latter. The tensile properties ofMg-5%Sn-2%Di are slightly worse than those of AE42 at room temperature, butbetter than those of AE42 at150℃and 175℃. The indentation creep experimentsindicate that Mg-5wt%Sn-2wt%Di alloy has significantly better creep resistancethan AE42. The creep rate of Mg-5wt%Sn-2wt%Di is obviously lower than that ofAE42. It could be reasonably deduced that the innovative Mg-Sn-RE system alloyshave highly potential for applications at elevated temperatures.