Materials Science Forum Vols. 747-748

Abstract: Magnesium alloys have a good application prospect in the fields of bone implants and cardiovascular stents due to their excellent properties, such as close density and elastic modulus to those of nature bone, high specific strength and rigidity, biodegradation and biocompatibility. In this paper, the feasibility of the use of the forged Mg-Li-X alloys (Mg-Li-Al-Zn-Ca-Sr) as biodegradable metals was investigated by immersion tests in the Hanks solution and skin-tissue implantation tests. The methods of SEM-EDS and XRD were used to research the corrosion morphology and corrosion products of the forged Mg-Li-X alloys after the in-vitro and in-vivo tests. At last, the in-vitro cytotoxicity was investigated by the MTT tests on L929 cells. Results showed that corrosion mechanism of the forged Mg-Li alloys was pitting and the surfaces were covered by Mg (OH)₂,CaCO₃ and Ca (H₂PO₄)₂ . During the 5 weeks implantation period, the rats survived from the operation, which indicates that the elements and the compounds formed are non-toxic to the rats. The forged Mg-Li alloy showed Grade 0~1 cytotoxicity, which manifests the forged Mg-Li alloy has good biocompatibility and eligible toxicity for implant applications.

Abstract: In order to achieve the composite of the AZ91 magnesium alloy and pure aluminum sheet, Zn-Sn-Al solder as compound agent, electric arc plating was employed to spray the brazing solder on the surface of AZ91 and pure Al sheets. The sheets were then subjected to heat treatment at 420 for 10min before rolling lamination at a reduction of 10%. The as-rolled sheets were annealed at 390 for 4hours. The microstructure, phase constitution and bonding strength of the composite interface were tested and analyzed by using SEM and energy dispersive scope (EDS), x-ray diffraction and tensile test. Experimental results showed that, under the atmospheric environment, AZ91/Al composite can be successfully fabricated with Zn-Sn-Al solder by brazing and hot-rolling, and the shearing strength can reach 22.46MPa. After annealing for 4 hours, a diffusion layer composed mainly of Al and Zn elements was formed near Al matrix, and the intermetallic phases were MgZn₂ and Mg₂Sn which was the same to the phases at the interface of AZ91/Al composite without annealing, but the amount was relatively higher. The interfacial fracture of composite was mainly due to the existence of intermetallic phases.

Abstract: In view of low strength and poor corrosion resistance of Mg alloys, a Mg-12Al-0.7Si alloy was designed, fabricated and subjected to equal channel angular pressing (ECAP) in order to refine the microstructure. Microstructure observation and electrochemical performance test were conducted to investigate the influence of the microstructural variation subjected to multi-pass ECAP processing on the corrosion behavior of the alloy. The results showed that both α-Mg matrix and β-Mg₁₇Al₁₂ of the alloy were significantly refined after processing for different passes (2,4,6,8) through route BC, and the 4-pass ECAPed alloy in 3.5% NaCl solution presents the lowest weight loss, lower corrosion current and higher corrosion potential in the polarization curves. The reason for high corrosion resistance of 4-pass ECAPed alloy and the effects of grain size of the matrix and the particle size, distribution of second phase and dynamic precipitates on corrosion behavior of the alloy were discussed.

Abstract: Normal and submerged friction stir processing (SFSP) were conducted to AZ91 magnesium alloy plates with 6mm in thickness, and influence of processing speed (ν) on microstructures and mechanical properties of the experimental materials was investigated. The results revealed that fine and equiaxed grains were observed in the stirred zone (SZ). As the processing speed increased from 60mm/min to 150mm/min, the average grain size in the SZ of normal FSP material decreased. However, the grain size of the SFSP specimens first increased with the processing rate increasing from 60mm/min to 120mm/min, and then decreased when the processing rate increased to 150mm/min. Microstructure of the SFSP specimen was much finer compared with the normal FSP one, and the grain size of α-Mg was about 1.2µm when the processing speed was 60mm/min during SFSP. Because of much finer microstructure of SFSP, the microhardness, tensile strength and elongation were all improved. SEM fracture observation showed that fine dimples and tearing edges could be observed on SFSP specimen which showing good ductility. In addition, high temperature tensile tests showed that SFSP AZ91 alloys exhibited excellent superplasticity at high strain rate, with an elongation of 1202% at 623 K with a strain rate of 3x10-3s-1. The present study demonstrated that SFSP possesses great potential in preparing fine-grained materials.

Abstract: The influence of trace strontium (Sr) addition on the microstructure and mechanical properties of Mg-8Al-1Nd-0.5Zn was investigated with OM, SEM, and XRD etc. The results show that the lamellar eutectics and divorced β-Mg₁₇Al₁₂ were reduced or refined with trace Sr addition. Among the as-cast Mg-8Al-1Nd-0.5Zn-xSr alloys, the Mg-8Al-1Nd-0.5Zn-0.05Sr alloy exhibited the best mechanical properties in which the tensile strength, the yield strength reached to 244.9 MPa and 111.7 MPa respectively. In addition, with the increase of Sr addition, the ductility was improved and it was observed that the number of cleavage steps and secondary cracks decreased on the fracture surfaces of tensile samples. It was also observed that the fractures occurred in the coarse β-Mg₁₇Al₁₂ phase instead of the Mg/Mg₁₇Al₁₂ interface or Al₁₁Nd₃ phase.

Abstract: The Mg-6Zn-2Si alloy was processed by equal channel angular pressing (ECAP) for 4 passes and 8 passes at 573K, and the microstructure and mechanical properties of the alloy before and after ECAP were studied. The results show that Chinese script type interphase of Mg₂Si was crushed into dispersed particles, and significant grain refinement was also introduced to the matrix phase (α-Mg) and Mg₅₁Zn₂₀ phase after 4 passes of ECAP. The yield strength was increased by 180%, elongation by 140% and tensile strength by 75%. The microstructure and mechanical properties remained reasonably constant between 4 and 8 passes of ECAP. The mechanism of improvement on microstructure and mechanical properties of the experimental alloy by subjecting ECAP was also investigated.

Abstract: The effects of micro-arc oxidation (MAO) on corrosion resistance of Mg-Gd-Y-Zr magnesium alloy have been studied. The Mg-Gd-Y-Zr with thicker oxide coating presented higher corrosion resistance in simulated body fluid (SBF). The corrosion rates were measured by several methods including hydrogen evolution volume measurement, weight-loss method and determination of pH as an auxiliary reference. The surfaces of specimens were observed by SEM and white light confocal microscopy before and after the corrosion. Besides the effects of MAO, the pitting resulted from breakage of oxide coating played a notable role as well. This provided a new direction towards the enhancement of corrosion resistance of magnesium alloy.

Abstract: The microstructure and mechanical properties of AZ81-4%Gd magnesium alloy sheet after aging at 150, 200, 250, 300 were analyzed and tested by x-ray diffraction,scanning electron microscope,energy dispersive spectrometer and hardness tester. The results demonstrated that β-Mg₁₇Al₁₂ phases were precipitated in two patterns, those were continuous precipitation and discontinuous precipitation during aging below 250 and the morphology of β-Mg₁₇Al₁₂ phase was different. The β-Mg₁₇Al₁₂ phases were precipitated in the pattern of continuous during aging at 300. The analysis showed that the precipitation pattern and morphology of β phases were related to the mechanism of nucleation and growth. The alloy hardness achieved the highest value after aging at 250 and 300 for 24 hours, the hardness of the alloy was the highest after aging at 150 and 200 for 32 hours. In addition, the observations on microstructure and the testing on hardness revealed that the best aging temperature is 200 for the AZ81-4%Gd alloys within the scope of the study.

Abstract: The effect of heat treatment on vacuum die-casting (VDC) AT72 magnesium alloy was studied. The optimal process of heat treatment was obtained. The result shows that the alloy was composed of α-Mg, Mg₁₇Al₁₂ and Mg₂Sn. After solution treatment at 686K for 24h, Mg₁₇Al₁₂ completely dissolved in α-Mg matrix. With the aging treatment following solution treatment, Mg₁₇Al₁₂ kept precipitating in the matrix and along grain boundary. Moreover, Mg₂Sn distributed along the grain boundary did not disappear after solution treatment at 686K. This indicates that Mg2Sn phase exhibits very high thermal stability. The heat treatment process was optimized with solution at 686K for 24h plus ageing at 473K for 18h, in the condition of which AT72 magnesium alloy exhibits a maximum hardness with value of 90.8Hv. The successful application of heat treatment for AT72 magnesium alloy could be attributed to the elimination of the air bubble in the casing through VDC. However, the porosity in the cast couldnt be efficiently eliminated by VDC, which result in the growth of shrinkage pore.

Abstract: AZ80/Al composite plate was fabricated by means of friction stir processing (FSP) aimed at the improvement of corrosion resistance of magnesium alloy. The cross-section microstructure, surface morphology and corrosion resistance of the Al composite layer were investigated. The experiment results indicated that a dense composite Al layer with superfine and uniform grains was formed, and a few amount of intermetallic compounds existed in the area of Mg/Al interface. The bonding strength of AZ80 magnesium alloy substrate and 1060 pure Al layer was proved to be high which was resulted from the metallurgical bonding of FSP. Microhardness measurement showed the continuous changing of microhardness values from the outmost surface of composite Al layer to the magnesium alloy substrate. Results of electrochemical corrosion test of the composite plate in 5 wt.% NaCl solution showed the better protection effect of the composite Al layer on the magnesium alloy in a corrosion medium. Almost the same corrosion level on the whole corrosion surface was observed which indicated the highly uniform microstructure of the composite layer. It was also proved that the plain arches on the outmost surface of the composite Al layer had no influence on the corrosion resistance of composite Al layer.