Microstructural development and mechanical properties in wrought Mg-Zn-RE alloys

Magnesium-zinc alloys with and without rare earth metals were examined. Particles form when rare earth metals are present and these affect the development of the internal structures in the alloys. Finer, more numerous and more uniformly distributed particles result in alloys with the best combination of high strength and ductility.

Compositional effects on the restoration behaviour in Mg-Zn-Re alloys

Additions of rare earth elements to magnesium alloys are qualitatively reported in the literature to retard recrystallisation. However, their effect in the presence of other (non-rare earth) alloy additions has not been systematically shown nor has the effect been quantified. The microstructural restoration following the hot deformation of Mg-xZn-yRE (x = 2.5 and 5 wt.%, y = 0 and 1 wt.%, and RE = Gd and Y) alloys has been studied using double hit compression testing and microscopy. It was found that, in the absence of rare earth additions, increases in zinc level had a negligible influence on the kinetics of restoration and the microstructure developed both during extrusion and throughout double hit testing. Adding rare earth elements to Mg-Zn alloys was found to retard restoration of the microstructure and maintain finer recrystallised grains. However, in the Mg-Zn-RE alloys, increasing the zinc concentration from 2.5 wt.% to 5 wt.% accelerated the restoration process, most likely due to a depletion of rare earth elements from solid solution and modification of the particles present in the matrix.

Effect of alloying and extrusion temperature on the microstructure and mechanical properties of Mg-Zn and Mg-Zn-RE alloys

Extruded Mg-Zn-RE alloys have been shown to exhibit excellent combinations of yield strength and ductility, but it is not completely clear how adding rare earth metals to Mg-Zn alters the microstructure and affects the mechanical properties. Microstructural changes and the resulting mechanical properties from changes in composition and extrusion temperature have been investigated for Mg-. x Zn-. y RE (. x=2.5 and 5. wt.%, y=0 and 1. wt. %, and RE=Gd and Y) alloys. Adding RE to Mg-Zn increased the strength and reduced the ductility, while increasing the zinc concentration in the Mg-Zn-RE alloys had the reverse effect.

Microstructures, mechanical and corrosion properties and biocompatibility of as extruded Mg-Mn-Zn-Nd alloys for biomedical applications

Extruded Mg-1Mn-2Zn-xNd alloys (x=0.5, 1.0, 1.5 mass %) have been developed for their potential use as biomaterials. The extrusion on the alloys was performed at temperature of 623K with an extrusion ratio of 14.7 under an average extrusion speed of 4mm/s. The microstructure, mechanical property, corrosion behavior and biocompatibility of the extruded Mg-Mn-Zn-Nd alloys have been investigated in this study. The microstructure was examined using X-ray diffraction analysis and optical microscopy. The mechanical properties were determined from uniaxial tensile and compressive tests. The corrosion behavior was investigated using electrochemical measurement. The biocompatibility was evaluated using osteoblast-like SaOS2 cells. The experimental results indicate that all extruded Mg-1Mn-2Zn-xNd alloys are composed of both α phase of Mg and a compound of Mg7Zn3 with very fine microstructures, and show good ductility and much higher mechanical strength than that of cast pure Mg and natural bone. The tensile strength and elongation of the extruded alloys increase with an increase in neodymium content. Their compressive strength does not change significantly with an increase in neodymium content. The extruded alloys show good biocompatibility and much higher corrosion resistance than that of cast pure Mg. The extruded Mg-1Mn-2Zn-1.0Nd alloy shows a great potential for biomedical applications due to the combination of enhanced mechanical properties, high corrosion resistance and good biocompatibility.

Effects of Mg17Sr2 Phase on the bio-corrosion behavior of Mg-Zr-Sr alloys

The effect of the second phase Mg17Sr2 on the biocorrosion behavior of Mg5ZrxSr (x = 0, 2, 5 wt%) alloys before and after solution treatment was investigated. Electrochemical impedance spectroscopy, cathodic polarization and hydrogen evolution were used to evaluate the biocorrosion of Mg5ZrxSr. We found that Mg17Sr2 precipitated on boundary zones and enhanced the galvanic effect, leading to a severer corrosion of the Mg matrix adjacent to Mg17Sr2. The corrosion subsequently spread gradually from the regions adjacent to the Mg17Sr2 to the central Mg matrix. However, a high volume fraction of Mg17Sr2 could also form a continuous network, isolate the Mg matrix and act as a barrier of corrosion.

Method for determining reaction rate of mild steel containers during melting of magnesium-aluminium alloys and effect of aluminium content on directionally solidified microstructures

A magnesium alloy of eutectic composition (33 wt-'%Al) was directionally solidified in mild steel tubes at two growth rates, 32 and 580 mum s(-1,) in a temperature gradient between 10 and 20 K mm(-1). After directional solidification, the composition of each specimen varied dramatically, from 32'%Al in the region that had remained solid to 18%Al (32 mum s(-1) specimen) and 13%Al (580 mum s(-1) specimen) at the plane that had been quenched from the eutectic temperature. As the aluminium content decreased, the microstructure contained an increasing volume fraction of primary magnesium dendrites and the eutectic morphology gradually changed from lamellar to partially divorced. The reduction in aluminium content was caused by the growth of an Al-Fe phase ahead of the Mg-Al growth front. Most of the growth of the Al-Fe phase occurred during the remelting period before directional solidification. The thickness of the Al-Fe phase increased with increased temperature and time of contact with the molten Mg-Al alloy. (C) 2003 Maney Publishing.

Influence of grain size on hot working stresses and microstructures in Mg-3A1-1Zn

The influence of the grain size on the deformation of Mg–3Al–1Zn was examined in compression at 300 °C. At low strains the flow stress increases with increasing grain size. This is interpreted in terms of dynamic recrystallization. Empirical models of dynamic recrystallization are developed and employed to generate a microstructure map.

Observation of deformation modes in Mg-3Al-1Zn

Since magnesium alloys are the lightest metallic materials, they are very attractive for automotive and aerospace industries. The main problem of these alloys is limited ductility due to a shortage of independent slip systems. In order to improve the formability in these alloys, an understanding of the deformation modes is required. In the present work, different slip systems were investigated in rolled Mg-3Al-IZn by means of in situ tensile tests in the SEM. These permitted electron backscatter diffraction (EBSD) and electron backscatter diffraction imaging (QBSD) to be carried out during the test. The results show that non-basal slip systems are active at room temperature.

Microstructural features of rolled mg-3Al-1Zn

The microstructures of hot- and cold-rolled Mg-3Al-1Zn (AZ31) are examined using scanning electron and optical microscopy. It is shown that the microstructures following multipass hot rolling and annealing are more uniform than those formed by heavy single pass rolling and annealing. The importance of twins in producing intragranular recrystallization is evident, although the most dominant nucleation site is grain boundaries. The cold-rolled structure after a rolling reduction of 15 pct is dominated by the presence of deformation twins. Twin trace analysis suggests that approximately two thirds of the twins are a form of “c-axis compression” twin. A number of “c-axis tension” twins were also observed and additional in-situ scanning electron microscopy experiments were performed to confirm earlier observations that suggest these twins can form after deformation, during unloading.