| In order to develop biodegradable magnesium alloy with practical application significance, this paper chose Mn, Ca and Sr as alloying elements to design and melt Mg-Mn-Ca series and Mg-Mn-Ca-Sr series biological magnesium alloy. The effects of alloying elements and T4 treatment on microstructure, phase composition and mechanical properties and corrosion resistance of alloys were investigated. The effects of casting defects on the mechanical properties and corrosion resistance of magnesium alloys was studied for improving the properties of materials. By calculating the safety degradation rate of magnesium alloy and safety alloy element content, the design standards for biodegradable magnesium al oy was discussed. The results prove that:1. As-cast Mg-2Mn-x Ca is mainly composed of α-Mg substrate, Mg2 Ca and β-Mn phases. Mg2 Ca phase mainly arranged along the grain boundary and formed eutectic with α-Mg. The β-Mn phase is randomly distributed in the matrix and formed peritectic with α-Mg. With the content increasement of Ca, the strength of alloy increased significantly, but the elongation decreased. T4 treatment can effectively improve the mechanical properties and corrosion resistance of Mg-2Mn-x Ca alloys. Through T4 treatment, Mg2 Ca phase completely dissolved into the α-Mg matrix. The grain boundaries are refined, which obviously reduced the fracture tendency of alloy and increased the elongation. The β-Mn phase uniformly distributed in the form of small rod on α-Mg matrix and represented dissolve-reprecipitation phenomenon. The solid-solution strengthening effectt and natural aging strengthening effect together significantly improved the tensile strength and elongation of T4 treated alloys. The corrosion mechanism of Mg-2Mn-x Ca alloy is mainly controlled by the form, quantity and distribution of the second phase. Connected with α-Mg matrix, Mg2 Ca acted as anode was prior corroded in galvanic corrosion reaction. T4 treated Mg-2Mn-1.0Ca alloy has the best comprehensive performance, whose hydrogen corrosion rate is 0.4 mm/y, yield strength is 70.69 MPa and tensile strength and elongation are 150.61 MPa and 8.77%, respectively.2. As-cast Ca Mg-1.5Mn-1Ca-x Sr alloy is mainly composed of α-Mg matrix, Mg2 Ca, β-Mn, Mg17Sr2 and Ca-Sr phases. The Mg2 Ca,Mg17Sr2 and Ca-Sr phases are mainly arranged along the grain boundaries and form eutectic with α-Mg. The β-Mn phase is randomly distributed in the matrix and formed peritectic with α-Mg. The addition of Sr obviously refined the grain and improved the strength of alloy, but reduced the elongation. T4 treatment could effectively increase the mechanical and corrosion properties of Mg-1.5Mn-1Ca-x Sr alloy. Through T4 treatment, Mg2 Ca phase completely dissolved into α-Mg matrix which not only reduced the fracture tendency, but also improved the elongation of alloy. At the same time, the sits of micro-cell reaction reduced, which would improve the corrosion resistance. The β-Mn phase uniformly distributed in the form of small rod on α-Mg matrix and represented dissolve-reprecipitation phenomenon, which could avoid serious localized corrosion caused by Mn segregation. The solid-solution strengthening effect, second phase strengthening effect and natural aging strengthening effect together significantly improved the tensile strength and elongation of T4 treated alloy. The corrosion mechanism of Mg-1.5Mn-1Ca-xSr is also controlled by the form, quantity and distribution of the second phase in alloys. Compared with Mg-1.5Mn-1Ca alloy, Mg-1.5Mn-1Ca-1Sr alloy has smaller grains and larger amount of second phase. The distribution of second phase is denser and supports the function of corrosion barrier which inhibit the corrosion process. Therefore, the addition of Sr made the dissolve of anode phase of Mg-1.5Mn-1Ca-1Sr alloy more uniform. T4 treated Mg-1.5Mn-1Ca-1Sr alloy has the best comprehensive performance, whose hydrogen corrosion rate is 0.27 mm/y, yield strength is 76 MPa and tensile strength and elongation are 180 MPa and 14.5%, respectively.3. The corrosion oxide film of Mg-Mn-Ca-Sr alloy mainly contains the elements of Ca, Mn, Mg, C l and O, which provided that Ca and Mn had participated in the formation of corrosion film and would improve the corrosion resistance of alloy. Combined with XRD analysis, it was confirmed that the main components of the corrosion film is Mg(OH)2. The corrosion mechanisms of magnesium alloy under the condition of the saline soak are mainly filiform corrosion and pit. To filiform corrosion, the corrosion path keeps continuous extension on metal surface. The corrosion depth is generally between 2μm-10μm, and displayed uniform corrosion on themacro-observation. The casting defects have mainly three effects on the corrosion behavior of magnesium alloys. ① The irregular morphology of casting defects will influence corrosion product film and cause the breakage of the corrosion product film, which will make the matrix alloy to stay in contact with the corrosive medium and unable to inhibit the corrosion process. ② Casting defects will cause the morphology change of alloys. The formation of surface curvature will increase the surface energy, thus increase the adsorption driving force of the surface, which make the area contains casting defect are more likely to corrode by liquid corrosion. ③ The formation of a special surface morphology such as the genera tion of crack morphology will also promote the occurrence of crevice corrosion.4. HPDC AM50 alloy is mainly composed of α-Mg matrix,Mg17Al12 and AlMn phase. Due to the different solidification rates, the surface of alloy shows fine grains, but the grain size of core is larger. The average grain size of HPDC AM50 alloy is about 30μm. The casting defects mainly arranged along the clearance of dendrite and mixed with the intergranular second phase. The size of defects is generally 100μm or less. According to Weibull model, the m value of HPDC AM50 alloy could be calculated: m(UTS): 12.03, m(YS): 15.88, m(EI): 3.86. The smaller of m value, the smaller the volatility of alloy. HPDC AM50 alloy has volatile mechanical properties because of the affect of casting defects. The material fracture preferred occurred in the areas where contain large casting defects. The relation between material elongation(e) and fracture porosity(f)(in the biggest defects plane) could be expressed as: e = e0 [1-f] n.5. ABAQUS finite element analysis software and Johnson-Cook material and failure model was used to simulate the mechanical properties and fracture behavior HPDC AM50 alloy. When materials were subjected to applied loading stress, the defect tip in the vertical direction of stress suffered the most serious stress concentration. So when material is stressed, fractures are usually occurred in the place where has a higher stress concentration like the place where defect is large or gathered or in a thin strips form(shrinkage). With the increase of load, the crack in the extension process would prefer extend along the area that has larger equivalent stress field. Therefore, the cracks in different planes will interconnected and finally form the fracture surface. NDT methods can effectively predict the defect features such as size, shape and location. Mathematical model and finite element analysis method could be applied to predict fracture behavior of materials in advance.6. Put forward a calculation method for acceptable magnesium alloy degradation rate in human body. By using the magnesium alloy plate model in this paper, the highest acceptable magnesium alloy degradation rate in human body could be calculated, which is 0.0282 mg/cm·h. Under this degradation rate, it could be ensured that the degradation products generated through the corrosion of magnesium alloy corrosion will not have harmful effects on organisms. Al, Zn, Sr, Ca and Mn can be used as healthy alloying elements in biodegradable magnesium alloys. The appropriate adding scope of each element could be determined according to different implant time. The degradation law of mechanical properties of Mg alloys in the process of corrosion can be effectively predicted by applying the rate of weight loss rate or the fraction of defects on the cross section, by using the formula: σ = σ0(1-x) n, in which x represents the rate of weight loss rate or the fraction of defects on the cross section, the fitting values of σ0 and n are 275.73 and 6.4 respectively. The decrease of mechanical performance of materials in servicing could be divided into two stages: in the first stage, the mechanical properties of material are mainly affected by the depth of localized corrosion. In the second phase, the localized corrosion has penetrated through the whole material and formed defects in the cross-section and the mechanical properties of the material are mainly affected by the defects in the cross-section perpendicular to the stress direction. |
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