Wenjiang Ding

Effects of rare earths on the microstructure, properties and fracture behavior of Mg–Al alloys

Abstract AZ91–xRE and Mg–6Al–xRE magnesium alloys were studied, where x is 0, 1, 2 and 3% (in weight percent, wt.%), respectively. Influence of rear earths (RE) on the microstructure was investigated. Fine morphology could be achieved by high cooling rate. By casting fluidity spiral specimens, fluidities of the alloys were achieved. The hardness and microhardness of the alloys was tested. RE improved fluidity and hardness. By casting specimens in permanent mold, tensile properties of the alloys with different RE additions at ambient and elevated temperatures were studied. RE had little effect on ambient temperature tensile strength of AZ91 alloy but greatly improved that of Mg–6Al alloy and high temperature tensile properties of both alloys. The fracture behavior of the alloys, which was changed by RE and high temperature, was examined by scanning electron microscopy (SEM) and optical microscopy. Fracture of the alloys is predominantly brittle cleavage or/and quasi-cleavage failure.

Tensile properties of extruded ZK60-RE alloys

Abstract ZK60–RE alloys were made by melting ZK60 alloy and cerium-rich rare earth (RE) metal in an electric furnace. The content of RE is 0, 0.5, 1, 1.5, 2, 3 wt.% RE, respectively. The influence of RE on microstructure and tensile mechanical properties of ZK60 magnesium alloys was studied. The results showed that cerium-rich misch metal (MM) had an obvious effect of reducing the grain size of the as-extruded ZK60 alloys. The ultimate and yield strengths of extruded ZK60–RE alloys were improved by the addition of RE. The ultimate and yield strength increased with increasing RE content in the range investigated. It is also found that the ultimate strength and yield strength decrease with increasing extrusion temperature. The improvement of tensile properties of ZK60–RE alloys was due to the extrusion texture and the fine microstructure induced by RE additions.

Microstructure, mechanical properties, biocorrosion behavior, and cytotoxicity of as-extruded Mg–Nd–Zn–Zr alloy with different extrusion ratios

Recently, commercial magnesium (Mg) alloys containing Al (such as AZ31 and AZ91) or Y (such as WE43) have been studied extensively for biomedical applications. However, these Mg alloys were developed as structural materials, not as biomaterials. In this study, a patented Mg-Nd-Zn-Zr (denoted as JDBM) alloy was investigated as a biomedical material. The microstructure, mechanical properties, biocorrosion behavior, and cytotoxicity of the alloy extruded at 320 °C with extrusion ratios of 8 and 25 were studied. The results show that the lower extrusion ratio results in finer grains and higher strength, but lower elongation, while the higher extrusion ratio results in coarser grains and lower strength, but higher elongation. The biocorrosion behavior of the alloy was investigated by hydrogen evolution and mass loss tests in simulated body fluid (SBF). The results show that the alloy extruded with lower extrusion ratio exhibits better corrosion resistance. The corrosion mode of the alloy is uniform corrosion, which is favorable for biomedical applications. Aging treatment on the as-extruded alloy improves the strength and decreases the elongation at room temperature, and has a small positive influence on the corrosion resistance in SBF. The cytotoxicity test indicates that the as-extruded JDBM alloy meets the requirement of cell toxicity.

In vitro degradation behavior and biocompatibility of Mg–Nd–Zn–Zr alloy by hydrofluoric acid treatment

In this paper, Mg-Nd-Zn-Zr alloy (denoted as JDBM) coated with hydrofluoric acid (HF) chemical conversion film (MgF2) was researched as a potential biodegradable cardiovascular stent material. The microstructures, in vitro degradation and biocompatibility were investigated. The field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) showed that a compact MgF2 film was formed on the surface of JDBM. The corrosion rate decreased in artificial plasma from 0.337 to 0.253 mm·y(-1) and the electrochemical measurement demonstrated that the corrosion resistance of JDBM alloy could be obviously improved due to the protective MgF2 film on the surface of the substrate. Meanwhile, the hemolysis ratio of JDBM decreased from 52.0% to 10.1% and the cytotoxicity met the requirement of cellular application after HF treatment. In addition, JDBM and MgF2 film showed good anti-platelet adhesion, which is a very favorable property for implant material in contact with blood directly.

Behavior of surface oxidation on molten Mg–9Al–0.5Zn–0.3Be alloy

Abstract Excellent ignition proof performance was obtained during the melting of Mg–9Al–0.5Zn–0.3Be alloy directly in the atmosphere. XRD and AES analysis indicated that the oxide film on the surface of the molten Mg–9Al–0.5Zn–0.3Be alloy exhibited a duplex structure, which was in agreement with the result of thermodynamic analysis. The outer layer of the oxide film mainly consists of MgO, which grows according to the parabolic law. The inner layer is a mixture of MgO and BeO, and its growth follows a linear law approximately. This inner layer acts as a barrier to reduce the outward diffusion of Mg 2+ , which leads to the excellent ignition proof performance. An oxide model was established to describe the oxidation procedure on the surface of the molten Mg–9Al–0.5Zn–0.3Be alloy.

An understanding of the hot tearing mechanism in AZ91 magnesium alloy

A new method of investigating the hot tearing behavior of magnesium alloys is developed. The results of measurement, by EDAX and optical microcopy revealed that the end-solidifying temperature was around the eutectic temperature under practical solidification conditions, because of a quantity of eutectic caused by dendritic segregation. The temperature at which hot tearing occurred most probably in the AZ91 alloy was the practical end-solidifying temperature, which was 424.2 °C in the present solidification conditions. The appearance of eutectic was the inducement for hot tearing of AZ91 alloy.

STUDY ON IGNITION PROOF MAGNESIUM ALLOY WITH BERYLLIUM AND RARE EARTH ADDITIONS

School of Material Science & Engineering, Shanghai Jiao Tong University,Shanghai, 200030, People’s Republic of China(Received January 12, 2000)(Accepted in revised form May 2, 2000)Keywords: Beryllium; Oxidation; Rare earth; Magnesium alloyIntroductionApplications of magnesium alloys are expanding in many industries due to their lightweight property(1,2). However, magnesium is prone to severe oxidation or even burning at high temperature. Fluxesor protective gases have to be used to prevent the ignition of magnesium alloys during melting. Bothmethods cause problems such as environmental pollution and complication of the equipment. So it isnecessary to investigate a better ignition-proof method to melt magnesium alloys.In the past years, a lot of studies have been done on the oxidation resistance of magnesium alloysby the means of alloying. SAKAMOTO et al. (3–8) studied the oxidation of Mg-Ca alloys. The resultsindicated that the ignition temperature could be increased by 250°C after 5wt.%Ca was added into puremagnesium. However, the improvement of the ignition proof performance was achieved at the cost ofthe decreasing of the tensile properties. Beryllium addition can also improve the oxidation resistance ofmagnesium alloys (9–11). However, the ignition of the alloys can’t be prevented completely and highberyllium concentration will deteriorate the mechanical properties. Generally, the allowed maximumcontent of beryllium in magnesium alloy is 0.01wt% in permanent mold casting and 0.002wt% in sandmold casting.In the present study, excellent ignition proof performance was achieved when the beryllium additionwas up to 0.1wt% and the magnesium alloy could be melt without any flux and protective gas, but thetensile properties decreased greatly. After RE addition, the tensile properties of the Be-containing alloycould be increased to be close to that of AZ91 magnesium alloy.ExperimentalMaterials PreparationThe main composition of AZ91 alloy and Ignition Proof Magnesium Alloys(IPMA) are shown in Table1. All the alloys were melt in a steel crucible by an electric resistance furnace. For the IPMA alloys,pure magnesium was melt, then Al-5.3wt%Be, pure zinc and rare earths(RE) were added to obtain thestudied composition. Rare earth was added in the form of cerium rich misch metal, whose chemicalcomposition is shown in Table 2.

Enhanced biocorrosion resistance and biocompatibility of degradable Mg-Nd-Zn-Zr alloy by brushite coating.

To further improve the corrosion resistance and biocompatibility of Mg-Nd-Zn-Zr alloy (JDBM), a biodegradable calcium phosphate coating (Ca-P coating) with high bonding strength was developed using a novel chemical deposition method. The main composition of the Ca-P coating was brushite (CaHPO4·2H2O). The bonding strength between the coating and the JDBM substrate was measured to be over 10 MPa, and the thickness of the coating layer was about 10-30 μm. The in vitro corrosion tests indicated that the Ca-P treatment improved the corrosion resistance of JDBM alloy in Hanks solution. Ca-P treatment significantly reduced the hemolysis rate of JDBM alloy from 48% to 0.68%, and induced no toxicity to MC3T3-E1 cells. The in vivo implantation experiment in New Zealands rabbit tibia showed that the degradation rate was reduced obviously by the Ca-P treatment and less gas was produced from Ca-P treated JDBM bone plates and screws in early stage of the implantation, and at least 10weeks degradation time can be prolonged by the present coating techniques. Both Ca-P treated and untreated JDBM Mg alloy induced bone growth. The primary results indicate that the present Ca-P treatment is a promising technique for the degradable Mg-based biomaterials for orthopedic applications.

Structural and mechanical properties of Mg17Al12 and Mg24Y5 from first-principles calculations

Using density functional theory within GGA approximation, the structural and elastic properties of two important phases (Mg17Al12 and Mg24Y5) in Mg-based alloys have been studied. The obtained equilibrium structural parameters for both phases agree very well with experimental data. The calculated negative cohesive energy and formation energy show that both cubic precipitates have strong structural stability as well as good alloying ability. Three independent single-crystal elastic constants (C11, C12 and C44) at zero pressure as well as polycrystalline mechanical parameters such as bulk modulus B, shear modulus G, Youngs modulus E, Poissons ratio ν and anisotropy value A for both phases have been calculated. The mechanical properties of the cubic phases such as ductility and tenacity are further analysed and discussed.

Effects of Zn and RE additions on the solidification behavior of Mg–9Al magnesium alloy

Abstract Effects of Zn and RE additions on the solidification behavior of Mg–9Al alloy were studied. Zinc (Zn) and rare earths (RE) elements were added to a maximum quantity of 1.2 and 1.6wt.%, respectively. Cooling curves were obtained under commercial production conditions. Hot tearing susceptibility was investigated using crack-ring molds. Microstructure was analyzed by metallographic examination and electron probe micro-analysis. Zinc additions decreased the end-solidifying temperature, promoted the precipitation of Mg 17 Al 12 in grain boundaries and increased the hot-tearing susceptibility coefficient (HSC). Al 4 RE phase precipitation slowed down the temperature decrease at the initial stage of solidification. RE additions had little effect on HSC of Mg–9Al with low or no Zn content, while when Zn content was exceeded 0.8 wt.%, RE additions decreased HSC of Mg–9Al alloys distinctly.