Qu Dong Wang

Microstructure refinement of Mg–Al–Zn–Si alloys

Magnesium alloys containing Mg2Si particles, as a promising cheap heat-resistant magnesium alloy for automobile engine applications, are attracting more and more attention of both material scientists and design engineers. Refinement of the Chinese script Mg2Si particle is a key for using this alloy in sand casting or permanent mould casting. In the present work, the refinement effect of antimony(Sb) or calcium(Ca) on the microstructure was investigated. The study shows that Sb or Ca addition promotes the formation of fine polygonal type Mg2Si particles by providing the nucleation sites. Moreover, the grain sizes of Sb or Ca modified Mg–Al–Zn–Si alloys are much finer than that of base alloy. The effect of modification and refinement with Sb addition is more effective than that with Ca addition. Such improved microstructure of the modified alloys results in large improvement in tensile properties and toughness as compared to the base alloy.

Fracture behavior of AZ91 magnesium alloy

Abstract Optical microscopy and scanning electron microscopy (SEM) were used to examine the fracture behavior of AZ91 magnesium alloy ruptured by tensile and impact tests. AZ91 alloy generally revealed features of brittle fracture and cleavage is the principal fracture mode. The fracture behavior for tensile test is different from that for impact test due to different load forms. The fracture morphologies are different in different areas of the impact specimen. The Mg/Mg 17 Al 12 interface often acted as the crack initiation source.

Superplasticity and grain boundary sliding in rolled AZ91 magnesium alloy at high strain rates

Abstract The superplastic deformation characteristics and microstructure evolution of the rolled AZ91 magnesium alloys at temperatures ranging from 623 to 698 K (0.67–0.76 Tm) and at the high strain rates ranging from 10−3 to 1 s−1 were investigated with the methods of OM, SEM and TEM. An excellent superplasticity with the maximum elongation to failure of 455% was obtained at 623 K and the strain rate of 10−3 s−1 in the rolled AZ91 magnesium alloys and its strain rate sensitivity m is high, up to 0.64. The dominant deformation mechanism in high strain rate superplasticity is still grain boundary sliding (GBS), which was studied systematically in this study. The dislocation creep controlled by grain boundary diffusion was considered the main accommodation mechanism, which was observed in this study.

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.

Behavior of Mg–6Al–xSi alloys during solution heat treatment at 420°C

Abstract The effect of solution heat treatment at 420°C on the morphology of Mg 2 Si particles in Mg–Al–Si alloys was investigated. The Mg 2 Si particles tended to be spherodized during the treatment due to the diffusion of Si atoms along the Mg 2 Si/Mg interface, which is benefit the mechanical properties of the alloys.

Creep and Fracture Behavior of Peak-Aged Mg-11Y-5Gd-2Zn-0.5Zr (wt pct)

The tensile-creep and creep-fracture behavior of peak-aged Mg-11Y-5Gd-2Zn-0.5Zr (wtxa0pct) (WGZ1152) was investigated at temperatures between 523xa0K (250xa0°C) to 598xa0K (325xa0°C) (0.58 to 0.66Tm) and stresses between 30xa0MPa to 140xa0MPa. The minimum creep rate of the alloy was almost two orders of magnitude lower than that for WE54-T6 and was similar to that for HZ32-T5. The creep behavior exhibited an extended tertiary creep stage, which was believed to be associated with precipitate coarsening. The creep stress exponent value was 4.5, suggesting that dislocation creep was the rate-controlling mechanism during secondary creep. At Txa0=xa0573xa0K (300xa0°C), basal slip was the dominant deformation mode. The activation energy for creep (Qavgxa0=xa0221xa0±xa020xa0kJ/mol) was higher than that for self-diffusion in magnesium and was believed to be associated with the presence of second-phase particles as well as the activation of nonbasal slip and cross slip. This finding was consistent with the slip-trace analysis and surface deformation observations, which revealed that the nonbasal slip was active. The minimum creep rate and time-to-fracture followed the original and modified Monkman-Grant relationships. The microcracks and cavities nucleated preferentially at grain boundaries and at the interface between the matrix phase and the second phase. In-situ creep experiments highlighted the intergranular cracking evolution.

Development of microstructure in solution-heat-treated Mg-5Al-xCa alloys

The development of the microstructure in Mg-Al-Ca alloys was investigated both in the as-cast condition and after solution heat treatment at 415 °C for 0.25 -120 h, using secondary electron imaging, electron probe microanalysis and X-ray diffraction analysis in combination with light microscopy and image analysis. The main intermetallic compound in the Ca-containing alloys is Al 2 Ca, and the formation of the Mg 17 Al 12 phase is suppressed in the presence of Ca. The decomposition of Al 2 Ca during the solution treatment is more difficult than that of Mg 17 Al 12 , and undissolved Ca was still present in the form of stable Al 2 Ca even after 120 h. With increasing treatment time, the coarse Al 2 Ca phase, which was continuously distributed like a network around the α-Mg grains in the as-cast state, underwent thinning and necking in the initial stage. Then it became disconnected and fragmentized, and finally the fragmentized Al 2 Ca particles were spheroidized and fined due to the diffusion of Ca and Al along the Al 2 Ca/Mg interface. As a result, the continuous network-like Al 2 Ca phase turned into a dispersion of spherical particles. The fine Al 2 Ca particles were homogeneously distributed in the matrix, which can be very beneficial to creep resistance and mechanical properties of Ca-containing alloys.

Mechanical Properties and Creep Behavior of Mg–Al–Ca Alloys

This paper investigates the microstructure, mechanical properties and creep behavior of Mg–Al–Ca alloys with different Ca content. SEM and EDAX analyses show that the dominant second phase in the as-cast Mg–Al–Ca alloys is Al2Ca, which distributes at the grain boundaries and disperses in the grain interior as well. Both the elevated tensile strength and the creep resistance of Mg–Al–Ca alloys obviously increased with increasing Ca at high temperature. TEM analyses reveal that finer Al2Ca particles with an average size of 0.02 µm precipitated dynamically during the creep process. Selected area electron diffraction (SAD) patterns show that the dynamic Al2Ca precipitates have a coherent interface with matrix as (0110) Mg // (220) Al2Ca, [2110] Mg // [112] Al2Ca. The strengthening mechanism of Mg–Al–Ca alloys at elevated temperature was discussed.

Influence of Thermal and Thermo-Mechanical Processing on the Creep Resistance of Mg-10Gd-3Y-0,4Zr Alloy

Alloy Mg-10Gd-3Y-0,4Zr in as-cast, as-extruded, cast-T6 (peak aged) and extruded-T5 (peak aged) state was tensile creep tested at 200, 250 and 300 °C and stress 50, 80 and 120 MPa. Comparison of minimal creep rate shows that alloy Mg-10Gd-3Y-0,4Zr in cast-T6 conditions is characterized by an excellent creep resistance, which is higher than that of commercially available Mg-alloys. Creep resistance of as-cast, as-extruded and extruded-T5 alloy Mg-10Gd-3Y-0,4Zr is lower. Cavity nucleation is heavily affected by the amount of secondary phases on the grain boundaries and also by the initial grain size of the microstructure. After extrusion and in the extruded-T5 conditions creep cavitation was not observed, whereas in the as-cast and cast-T6 conditions creep cavitation occurred on the high fraction of grain boundaries.

Microstructure and Mechanical Properties of Mg-Gd-Sm-Zr Alloy

Microstructure and mechanical properties of three kinds of Mg-Gd-Sm-Zr alloys have been analyzed in this paper. Results exhibit that the microstructure of as-cast Mg-Gd-Sm-Zr alloy contains α-Mg and eutectic compounds which are mainly comprised of most Mg5Gd-base phases and a few Mg41Sm5-base phases by EDX and XRD analysis. Ultimate tensile strength and yield strength of the alloys can be significantly improved after T6 treatment. Mechanical properties of studied alloys in T6 condition are better than that of WE54-T6 alloy.