Elevated temperature Mg-Al-Sr: Creep resistance,mechanical properties,and microstructure

Mg-Al-Sr-based alloys (AJ alloys) have shown superior creep performance and tensile strength at temperatures as high as 175° with stresses up to 70 MPa. Mg-6Al-2.4Sr (AJ62x) exhibits an optimum combination of creep resistance and excellent castability, while AJ62Lx (strontium <2.1) has better ductility than other AJ formulations. The AJ alloy microstructure is characterized by the Al₄Sr-α(Mg) lamellar phase that forms at the interdendritic/grain boundary region of the primary magne sium matrix. Mg-5Al-2Sr (AJ52x) contains a ternary phase that was tentatively named Al₃Mg₁₃Sr. When the strontium level is low in AJ62x, the volume fraction of Al₄Sr is reduced, the aluminum supersaturation of the magnesium primary phase increases, and Mg₁₇Al₁₂ forms. In this article, a mechanism is proposed whereby the creep resistance decreases with the strontium level but the tensile strength and ductility increase. For more information, contact Eric Baril, Noranda, Noranda Technologies Centre, 240 Hymus Pointe-Claire (Montréal), Québec, H9R 1G5 Canada; (514) 630-9347; fax (514) 630-9379; e-mail eric.baril@ntc.noranda.com.     

Magnesium technology 2002, part I: Primary production,environmental issues,high-temperature alloys

Conclusion It is expected that the research in high-temperature alloy development will reach a critical mass in about five years. This will not only raise the materials science of magnesium to a higher level but will also increase the confidence of the industry in magnesium as a structural performance material. Currently, applications are being actively pursued in United States Council for Automotive Research power train programs and European Council for Automotive R&D engine block programs to develop the technology for elevated-temperature magnesium. More research is still needed in this interesting materials field.     

Magnesium global development: Outcomes from the TMS 2007 annual meeting

The magnesium industry is experiencing dramatic growth, due in part to the demands of the automotive industry to improve fuel economy and emissions. As a result, the TMS annual magnesium symposium is one of the fastest-growing subject areas at TMS. In addition, TMS is recognized as the global light metals community’s preeminent forum for the presentation of magnesium science and technological issues. As a focal point of the 2007 TMS Annual Meeting in Orlando, Florida, the TMS Magnesium Committee held its first plenary session with expert representatives from around the world. In order to address the surge in magnesium technology in recent years, the TMS Magnesium Globalization plenary session included a variety of subjects that ranged from economic to technical.     

Texture Evolution and Twinning During the Expansion of Hot Extruded AZ31 + Sr Seamless Tubes

Seamless tubes of AZ31, AZ31 + 0.4, and 0.8 wt pctSr were extruded at elevated temperatures. By compressing pure copper inserts inside the tubes, the extruded tubes were expanded at room and elevated temperatures [373 K and 473 K (100 °C and 200 °C)]. Microstructural examinations reveal the formation of twining in the as-extruded and expanded tubes. The amount of twinning decreased with increasing level of Sr in the expanded microstructures as a result of grain refinement and of decreasing Al in solution that facilitates dislocation motion. During expansion at room temperature, AZ31 shows higher elongation and lower strength than the alloys containing Sr. At 473 K (200 °C), compared to the lower temperatures, the Sr containing alloys exhibit lower flow stress and no fracture in the strain range investigated (40 pct reduction in cylinder height). The textures of the extruded alloys contain two main components named as RD (c-axis parallel to the radial direction) and HD (c-axis parallel to the hoop direction) based on their orientation with the sample coordinates. During expansion, extension twinning in the HD grains reorients the lattice to strengthen the RD and form a new ED (c-axis parallel to the extrusion direction) component. By increasing the temperature or level of Sr, the ED component is weakened due to the decrease in twinning. During expansion, the RD grains undergo contraction and double twining which reduce the overall texture strength.     

The effect of strontium (Sr) on the ignition temperature of magnesium (Mg): a look at the pre-ignition stage of Mg–6 wt% Sr

The effect of strontium (Sr) addition on the ignition and oxidation behavior of magnesium has been investigated. Continuous heating experiments carried out in dry air-flow reveal that the ignition temperature (T _i) is raised from 640 °C (of pure Mg) to up to 854 °C at 6 wt% Sr addition. The oxidation behavior of Sr containing alloys was investigated during (i) isothermal oxidation experiments above the liquidus temperature (~640 °C) and (ii) during pre-ignition heating to 700 °C. The change in the ignition temperature of various alloy compositions is related to the amount of Sr that can be segregated to the surface and to the activity of the elements in the surface in the stages prior to ignition.     

Study on edge cracking and texture evolution during 150 °C rolling of magnesium alloys: The effects of axial ratio and grain size

Pure Mg, AZ31 (Mg-3 wt%Al-1 wt%Zn) and experimental alloys, Mg-1 wt%Zn-1 wt%In, Mg-2 wt%Li-1 wt%Zn, Mg-2 wt%Li-1 wt%Zn-1 wt%In were rolled at 150 °C to 0.3 and 0.55 strain. The samples rolled to 0.3 strain were subsequently annealed for 10 min at 400 °C. The texture was evaluated in rolled and in rolled/annealed conditions. The axial ratio (c/a) of the alloys strongly influenced edge cracking during rolling (expressed as cracking index, I_C), which was explained via the influence of c/a on the twinning mode. The as-cast grain size did not correlate to I_C (edge cracking). Texture intensity was strongly influenced by the as-cast grain size (which was attributed to plastic compatibility at grain boundaries), showed weak correlation to the lattice parameter a, but did not depend on c/a. The Mg-2 wt%Li-1 wt%Zn alloy, with fine grain size and small c/a and a, exhibited the optimum combination of weak texture and crack-free rolling at 150 °C.     

Effect of pre-deformation anneal on the microstructure and texture evolution of Mg–3Al–1Zn–0.7Sr alloy during hot extrusion

The effect of pre-deformation annealing on the microstructure and texture of an AZ31 + 0.74 wt% Sr alloy has been investigated. As-cast samples as well as three samples that have been annealed at 400 °C for 10, 30, and 120 min were extruded at 300 °C. Results indicate that annealing transforms the bulky non-equilibrium Al–Mg–Sr precipitates to stable Al₄Sr spheroids. As the extent of this transformation increases before extrusion, there is seen an increase in the amount of uniformly dispersed intermetallic stringers in the extruded material. Texture measurements reveal the alignment of basal poles with the compression axis (perpendicular to the circular cross section of the extruded bar) and the formation of the basal ring texture in all the samples. However, an increase in the duration of the pre-deformation anneal switches the plane facing the extrusion direction from first order prismatic (10-10) to second order prismatic planes (11-20). Annealing decreases the Al solute concentration in Mg and lowers the lattice resistance against dislocation movement. Consequently, the more favorable (0002)[11-20] slip system is activated in grains that see low basal resolved shear stress (τ). As a result, those grains work harden and are consumed by dynamic recrystallization (DRX). However, the (0002)[-1100] slip system with high τ still avoids basal dislocation movement. Hence, the grains with high τ_{(0002)[-1100]}, which need to move dislocations in the (0002)[-1100] system to fulfill the strain compatibility conditions across the microstructure would be prevented from work hardening and DRX. This specific orientation has a (11-20) plane facing the extrusion direction.