Microstructure Of AZ91 Magnesium Alloy Research Articles

Effect of homogenization heat treatment on the microstructure of AZ91 magnesium alloy at different temperatures and ageing times

Magnesium (Mg) alloys are promising biodegradable implant materials. If successful, they do not require a second surgiabscal operation for their removal. These alloys possessed microstructure, which contained alpha (α-Mg) phase, eutectic (α + β) phase and intermetallic or beta (β-Mg17Al12) phase. The bio-corrosion rate of any intermetallic alloy is related to its β phase volume fraction (VF). However, this β phase contributes galvanic corrosion which hinders the clinical application of Mg alloys. Thus, this study is focused to reduce the VF of the β phase and select the appropriate temperature for Mg homogenization heat treatment (HHT). HHT was carried out on Mg AZ91 alloy samples by using a high-temperature rotary furnace at 350 °C for 10 h, 370 °C for 10 h, 390 °C for 5 h and10h, 410 °C for 5 h and10h and 430 °C for 5 and 10 h. The influence of varying temperature with time was investigated on the microstructure and microhardness of Mg AZ91 alloy by using a field emission scanning electron microscope equipped with an energy-dispersive spectroscope and image Digimizer. The results revealed that the higher the HHT or the longer the homogenization heat dwelling time, the more the homogenous distribution of the large β phase over the microstructure, hence containing little or without a β phase. The microhardness increased with an increase in temperature and ageing time with the peak hardness at 410 °C and 430 °C for 10 h.

Effect of Ca and rotation speed on microstructure and solidification parameters of AZ91 magnesium alloy produced by semi-solid casting through rotating container process

Microstructure of AZ91 magnesium alloy was modified by using semi-solid rotating container process (RCP) and addition of 1 wt.% Ca. Thermal analysis (CA-CCTA) was performed during semi-solid process in order to determine the relationship between microstructural modification and RCP parameters. The container containing molten AZ91 alloy was rotated by rotation speed of 0, 150, 180 and 210 r/min at 640−580 °C. The cooling curves were plotted to determine the solidification characteristics, solid fraction and dendrite coherency point (DCP). The results indicated that the cooling rate increased with increasing the rotation speed significantly and led to an increase in maximum solidification range of 15.9 and 6.4 °C for AZ91 and AZ91−1Ca (wt.%), respectively. Applying shear force to the slurry at 180 r/min minimized the solid fraction and delayed appearance of DCP. It affected the microstructure characteristics such as grain size and circularity factor. The rotation of 180 r/min at 585 °C resulted in the globular microstructure with a circularity factor of 0.72 and a grain size of 73.4 μm. The addition of 1 wt.% Ca to AZ91 in the optimum condition of RCP increased the circularity factor to 0.81 and reduced the grain size to 53.2 μm.

Microstructure evolution and corrosion mechanism of AZ91 magnesium alloy in chloride environments for commercial purposes prepared by Ohno continuous casting

Billets of commercially used AZ91 magnesium alloy were prepared by Ohno continuous casting (OCC) process. Microstructure of AZ91 magnesium alloy was observed by performing optical microscope images. In addition to microstructure analysis, effect of OCC process comparing to the conventional continuous casting on corrosion performance of AZ91 in solutions consisting of 0.01, 0.1, 1M NaCl was evaluated by using potentiodynamic polarization. In addition, immersion test in 0.1M NaCl was used to reveal the corrosion mechanism. Results revealed that chloride ion concentration affected corrosion kinetics and thermodynamics. SEM images of corroded surfaces after potentiodynamic polarization supported the influence. Differences in microstructure play an important role on the corrosion kinetics especially at higher salt concentrations (1M NaCl) what is proven in this study.

Investigation on the effect of Sn on solidification and microstructure of AZ91 magnesium alloy using cooling curve thermal analysis

Thermal analysis technique has been used to study the effect of Sn addition on solidification characteristics and microstructure of AZ91 magnesium alloy. Solidification parameters have been extracted from cooling curves, first and second derivative curves. Different ranges of Sn between 0.1 to 1.0 wt%. have been added to the alloy. The results illustrated that the alloy containing 0.5 wt.% Sn displayed the best solidification behavior such as high nucleation temperature (TN.α), low recalescence undercooling (ΔTR), low dendrite coherency temperature and more solid fraction at dendrite coherency point. Microstructure of the alloys has been studied through optical microscopy (OM) and scanning electron microscopy (SEM). The phases were characterized by use of X-ray diffraction (XRD). Microstructural analysis showed that by adding Sn up to 0.5 wt.% into AZ91 alloy, refinement of α-Mg primary dendrites and β-Mg17Al12 took place due to the formation of new Mg2Sn intermetallic compound.

Effects of cerium on as-cast microstructure of AZ91 magnesium alloy under different solidification rates

In order to research the different effects of Ce on as-cast microstructure of AZ91 magnesium alloy under different solidification rates, the die-cast samples with different diameters and different Ce contents were prepared, and some characteristics of as-cast microstructure were analyzed. The results showed that Ce could refine grain size, decrease the fraction of eutectic β-Mg17Al12 phase, form Al4Ce phase and decrease solid solution of Al in the α-Mg matrix, but the above behaviors of Ce would be weakened under higher solidification rate. The essential reason for Ce to affect as-cast microstructure was that the Ce and Al concentrated in the liquid phase in front of solid/liquid interface during solidification because Ce itself is difficult to solid solute in α-Mg matrix and inhibits solid solution of Al in α-Mg matrix. However, the dynamic condition of concentration of Ce, Al would be changed when solidification rate varied, resulting in different influence extents of Ce on as-cast microstructure under different solidification rates.

Refinement mechanism of in-situ Al4C3 particles on AZ91 magnesium alloys

The effects of SrCO3 on the microstructure of AZ91 magnesium alloy were investigated by OM, SEM, EDS, XRD and DTA analyses. The results show that AZ91 magnesium alloy with 1.0% SrCO3 addition has the best refining effect at 730 ° for 20 min, the average size of the α-Mg grain in AZ91 matrix alloy is reduced from about 91 μm to 58 μm, with reduction of about 36%. Based on the analysis results of EDS, binding energy, and Gibbs free energy, it is evident that the grain is refined because of the generation of A14C3 particles which can be used as the crystal nucleus of the magnesium alloy when SrCO3 is added. Owing to the generation of A14C3 in AZ91 magnesium alloy, the grain boundary is pined and the grain growth is inhibited. In the accession to the Al4C3, small subcooling temperature difference leads to the formation of fine grain microstructure in alloy melt. Fine grain microstructure in AZ91 alloy melt with the addition of SrCO3 can be obtained on the condition of lower undercooling temperature according to the DTA analysis results.

Effects of Hot Extrusion and Heat Treatment on Mechanical Properties and Microstructures of AZ91 Magnesium Alloy

The effects of heat treatment on the microstructure, tensile property and fracture behavior of as-extruded AZ91 magnesium alloy were studied by OM and SEM. The results show that the grain of as-cast AZ91 alloy is refined by extruding and dynamic recrystallization, and the mechanical properties increase obviously. The ductility is significantly enhanced after solution treatment of the as-extruded AZ91 alloy, tensile strength is almost the same before and hardness is significantly reduced after solution treatment and artificial aging treatment. The tensile strength reduced and the ductility is significantly enhanced of as-extruded AZ91 magnesium alloy after annealing processes. The fracture surface of as-extruded AZ91 magnesium alloy has the mixture of ductile and brittle characteristic. But after T6 or annealing treatment, its dimple number increases evidently.

Effects of Hot Extrusion and Heat Treatment on Mechanical Properties and Microstructures of AZ91 Magnesium Alloy

The effects of heat treatment on the microstructure, tensile property and fracture behavior of as-extruded AZ91 magnesium alloy were studied with OM and SEM. The results show that the grains of as-cast AZ91 alloy are refined by hot extrusion due to dynamic recrystallization, and the mechanical properties are improved obviously. The ductility is significantly enhanced after solution treatment of the as-extruded AZ91 alloy, tensile strength is almost the same as before, and hardness is significantly reduced after solution treatment and artificial aging treatment. The tensile strength reduced and the ductility is significantly enhanced of as-extruded after annealing processes. The fracture surface of as-extruded AZ91 magnesium alloy has the mixed modes of ductile and brittle characteristics. But after T6 or annealing treatment, the dimple number increases evidently.

Refining microstructure of AZ91 magnesium alloy by introducing limited angular oscillation during initial stage of solidification

Abstract Rheocasting by applying limited angular oscillation was reported for AZ91 magnesium alloy, in which the molten metal was rapidly cooled and vigorously mixed during the initial stage of solidification. Microstructure and mechanical properties of treated AZ91 alloy were investigated. Experimental results indicate that the present rheocasting could give rise to significant microstructure refinement and spherical structure evolution. The hardness of the soft α-Mg phase was dependent upon the grain size and followed the Hall-Petch equation. The hardness of microstructure rheocast under oscillation intensity above the level of 0.26 rad/s 2 can be predicted by a rule of mixtures (equal strain treatment). Simultaneous increase in both strength and ductility of AZ91 alloy has been obtained. The refinement of α-Mg phase combined with the discontinuity and refined β-Mg 17 Al 12 phase is thought to be the main reasons for the improvement of mechanical properties. Possible mechanisms responsible for microstructure refinement were discussed.

Effects of Erbium Addition on the Corrosion Resistance and Microstructure of AZ91 Magnesium Alloy

The microstructures and corrosion resistance of AZ91 and AZ91+0.5 wt.% erbium (Er) magnesium alloys were studied, respectively. The results show that the Er addition in scrap AZ91 magnesium alloy can improve the corrosion resistance, markedly. The discontinuous precipitation phase (DPP) for Mg17Al12 was retarded and the amount of DPP was decreased by 41% due to the formation of Al8ErMn4 phase during solidification. The amount of continuous precipitation phase (CPP) in grains was decreased by 8% because of the formation of Al7ErMn5 phase during solidification.

Non-flux purification behavior of AZ91 magnesium alloy

The effects of non-flux purification techniques on the mechanical properties and microstructure of AZ91 magnesium alloy were investigated by ICP, OM, XRD and SEM. The results show that Ar spraying with high flow rate could remove non-metallic inclusions and improve the mechanical properties of AZ91. The alloy obtains the best properties after argon spraying for 30 min at the melt temperature of 740 °C. The ceramic foam filter (CFF) could effectively improve the ultimate tensile strength and elongation of AZ91 alloy, especially the elongation, which increase with increasing pores per inch (ppi) and the thickness of CFF. Non-flux purification does not change the microstructure of AZ91 alloy. However, filtration has a certain effect on the fracture pattern of AZ91 alloy. To improve the mechanical properties effectively, both filtration and gas spraying should be utilized together.

Deformed Microstructure of AZ91 Magnesium Alloy Impacted by Projectiles with Velocities of 2-3 km/s

Deformed Microstructure of AZ91 Magnesium Alloy Impacted by Projectiles with Velocities of 2-3 km/s

Refinement role of electromagnetic stirring and calcium in AZ91 magnesium alloy

Effects of calcium addition and electromagnetic stirring on the microstructure of AZ91 magnesium alloy and refinement mechanism were investigated. The results show that calcium addition ranging from 0.1wt% to 0.3 wt% does not lead to formation of any new phases but cause the refi nement of as-cast microstructure.However, combined calcium alloying and electromagnetic stirring significantly decrease the grain size, change the morphologies of the β-Mg17Al12 phases,and reduce their volume percentage. The minimum grain size of AZ91 alloy is obtained in the case of the addition of 0.2 wt%Ca with exciting voltage of 100 V. The microstructural refi nement is attributed to the increase of the degree of undercooling and nucleation temperature of primary α-Mg phases on the basis of DTA analysis results.

Effect of melting technique on the microstructure and mechanical properties of AZ91 commercial magnesium alloys

The microstructure of AZ91 magnesium alloy refined by thermal rate treatment technique (TRT), and the β-phase is more dispersive and uniform. Though the superheating microstructure is finer than microstructure with TRT technique, TRT technique can reduce the casting defects such as hot tears and shot run, more suits the practical requirement. The microstructure of AZ91 alloy keeps some characteristics of the high temperature melt after TRT. On the other hand, the microstructure of AZ91 magnesium alloy containing excess element Fe could not be refined and even be coarsened after thermal rate treatment, which brings on the decrease of mechanical properties as the results of interdendritic shrinkage microporosity and coarse β-phases. These different effects of TRT on AZ91 magnesium alloys can be related to the presence of non-equilibrium condition in the mixed melt structure through mixing the high superheating temperature and low temperature melts in TRT technique.