Materials Science Forum Vols. 546-549

Abstract: The microstructure, mechanical properties and creep behavior of Mg-Gd-Y-Zr alloys at different temperatures and stresses have been investigated. Results show that the tensile properties and creep resistance at T6 condition is much higher than that at as-cast condition. The creep resistance of Mg-Gd-Y-Zr alloy at 250°C/80MPa is markedly better than that at 300°C/50MPa, indicating that temperature makes more effects on the creep resistance than the stress.

Abstract: Compact AM50 alloy components were cast by Low Pressure Die Casting (LPDC) process. The microstructure and mechanical properties of cast components were investigated under as-cast and heat treated states. It was found that the microstructure of LPDC AM50 is composed of α-Mg and second phases - Mg17Al12 and Al8Mn5. Compared with Gravity die casting, LPDC AM50 alloy had much coarser grains and higher density, with smaller sizes and less content of second phases. The density of AM50 alloy by LPDC process was ρ=1.7836g/cm3, with increase of 0.45% based on Gravity die casting and much more increase compared with high pressure die casting. The as-cast mechanical properties by LPDC process were: σ0.2=57.8Mpa, σb=192.3Mpa, δ=8.7%. These of Gravity die casting were: σ0.2=53Mpa, σb=173.4Mpa, δ=8.1%. UTS in LPDC increased about 20MPa, with better YTS and Elongation. Compared with that of high pressure die cast AM50, the YTS of LPDC was much lower, with comparable UTS and Elongation. The mechanical properties of the heat treated AM50 alloy were still in the same level of as-cast state. AM50 alloy by LPDC process is not necessary subjected to tempering treatment.

Abstract: Tensile and compressive creep properties of Mg-5wt.%Al-1wt.%Sr alloy produced by gravity casting were investigated in this paper. Creep tests were carried out in the temperature range from 125 °C to 200 °C and stress range from 35 to 85MPa. The second creep rate in tension is significant different from that in compression, indicating that coarse-grained Mg-Al-Sr alloy exhibits tension/compression asymmetric behavior. Moreover, the activation energies and stress exponent in tension and compression are not the same, which suggest that creep mechanisms in tension and compression are different.

Abstract: Mg-10wt%Gd-3wt%Y alloy was cast in a step-like mould, which provided five different cooling rates. The dentrite morphology and hardness of the as-cast alloy from the surface to the center was investigated and the influence of the cooling rates on these was analyzed. It was indicated that there were two different trends for the hardness distribution: in the section of the step castings with the two slowest cooling rates, the hardness decreased with the increase of depth; while in the other three step castings the hardness increased with the increase of depth. Also it was founded that the hardness could be influenced by the grain boundary, dentrite morphology and dentrite arm spacing (DAS) in the alloy. At last, according to the standard deviation curves of the hardness, the chilled depths were about 5mm in 1st step and 10mm in 2nd step. And the other steps were fully chilled.

Abstract: In present work, Si and Sr elements were added into AZ91 alloy and cast directly into test samples using permanent mold. Mechanical properties of the samples at room temperature were evaluated by tensile test and the microstructure was analyzed. The results show that β-phase (Mg17Al12) of AZ91 alloy decreases with the addition of Si element and Mg2Si phase forms at the same time. Irregular Mg2Si phase precipitates preferentially at the grain boundaries at a low silicon content level. With the increase of the Si content, Mg2Si phase shows a complicated “Chinese- script” shape distributed at the grain boundary which leads to a lower ultimate tensile strength. Subsequently, the addition of Sr element has a remarkable effect on the form and distribution of Mg2Si phase of AZ91-Si alloys.

Abstract: The influence of Al-10.5%Sr master alloy, which is much cheaper than Mg-Sr master alloys, on the as-cast microstructure of the AZ31 alloy was investigated. The research results revealed that the Al-10.5%Sr master alloy produced obvious modification of the as-cast microstructure of the AZ31 alloy, and the modification efficiency increased with the holding time from 0min to 60min and the amount of Sr from 0.01% to 0.1%. Moreover, the results also showed that the Al-10.5%Sr master alloys of different states had different modification efficiency on the as-cast microstructure of the AZ31 alloy. The Al-10.5%Sr master alloys in extrusion deformation state and rapid solidification state had better modification efficiency than the Al-10.5%Sr master alloys received and in heat treatment state, which could be related to the microstructure of the Al-10.5%Sr master alloys with different states.

Abstract: The Mg-8Zn-8Al-4RE (RE = mischmetal, mass%) magnesium alloy was prepared by using casting method. The microstructure and mechanical properties of as-cast alloy, solid solution alloy and aged alloy samples have been investigated. Optical microscopy, X-ray diffractometery and scanning electron microscope attached energy spectrometer were used to characterize the microstructure and phase composition for the alloy. Net shaped τ-Mg32(Al,Zn)49 phase was obtained at the grain boundary, and needle-like or blocky Al11RE3 phase disperses in grain boundary and α-Mg matrix. The τ-Mg32(Al,Zn)49 phase disappeared during solution treatment and a new phase of Al2CeZn2 formed during subsequent age treatment. The mechanical properties were performed by universal testing machine at room temperature, 150 °C and 200 °C, separately. The ultimate tensile strength of as-cast alloy is lower compared to an age treatment alloy at 200 °C for 12h. The strengths decreased with enhancing test temperature, but elongation has not been effect by age treatment.

Abstract: The effects of the Al content on the as-cast microstructure and mechanical properties of Mg-xAl-0.7Si based magnesium alloys which contained 1%Zn, 0.25%Mn, 0.4%Sb and 0.25%RE were investigated by optical microscopy(OM), scanning electron microscopy(SEM), X-ray diffraction(XRD) analysis and tensile testing. The results indicated that the effects of the Al content from 6% to 8% on the alloy phase types of the experimental alloys were not obvious, and the as-cast microstructure of the experimental alloys with different Al content were composed mainly of the α-Mg matrix, Mg17Al12 phase, Mg2Si phase and Mg3Sb2 phase. The Chinese script morphology of the Mg2Si phase was very obvious in the experimental alloy with 6%Al. With the increase of Al content from 6% to 8%, the yield strength of the experimental Mg-xA1-0.7Si based alloys at room temperature increased, but the yield strength of the experimental Mg-xA1-0.7Si based alloys at 150 °C decreased.

Abstract: Rare earth was added in foundry magnesium alloy in vacuum melting. Analysis and measurement showed that the quantity of inclusions was decreased and the mechanical properties were improved. The size of crystal grain was decreased by 59%, the yield strength was increased by 38% and elongation percentage was increased more than 7 times. On the basis of this experiment, the approaches of improving the mechanical properties of magnesium alloy were brought up.

Abstract: Tensile tests were performed on T4-treated specimens of GN62K (Mg-6Gd-2Nd-Zr(wt%)) and GN112K (Mg-11Gd-2Nd-Zr) alloys at room temperature and on T6-treated ones from room temperature to 300°C. The fracture behavior was investigated by using SEM images of fracture surfaces together with optical metallographs adjacent to the surface. Quasi-cleavage fracture was observed in the fracture surfaces of both alloys in T4-treated condition, and it seems that RE content has no significant effect on the fracture mechanism in T4-treated alloys. As for T6 treated condition, cracking was in a manner of transgranular cleavage and/or quasi-cleavage fracture in GN62K alloy but intergranular fracture in GN112K alloy, which implies that the fracture mechanism in T6-treated Mg-Gd-Nd-Zr alloys can be altered by the amount of RE addition. This phenomenon is interpreted in association with the relative strength of grain bulk and boundary.