The relationship between microstructure and creep resistance in die-cast magnesium–rare earth alloys (original) (raw)
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Microstructural analysis of the creep resistance of die-cast Mg–4Al–2RE alloy
Scripta Materialia, 2008
The microstructure and microstructural stability of die-cast AE42 (Mg-4Al-2RE) alloy were investigated by transmission electron microscopy. It is shown that the formation of Mg 17 Al 12 after ageing at 200°C is not due to the decomposition of A1 11 RE 3 as reported in the literature, but, rather, is associated with the supersaturation of Al solute in the a-Mg matrix. The level of Al solute retained in the a-Mg matrix after die-casting is suggested to be an important factor in influencing creep resistance.
Materials Science and Engineering: A, 2009
The effect of Al content and Si addition on the microstructural and creep properties of Mg-Al-RE alloys was investigated in this study. The steady state creep rates were specified and it was found that the creep behavior of the alloy, which is dependent on the stability of the near grain boundary microstructure, was improved by the RE and Si addition. For the AZ91 alloy, the results indicate a mixed mode of creep behavior, with some grain boundary effects contributing to the overall behavior. However for the RE and Si added samples, sliding of grain boundaries was greatly suppressed and the dislocation climb controlled creep was the dominant deformation mechanism. Analysis of creep rates also showed that the Si addition resulted in formation of Mg 2 Si particles (in Chinese Script form) which have a high thermal stability. After Si addition the steady state creep rates were decreased and the creep resistance was improved. This was due to formation of Mg 2 Si particles which change the deformation mechanism at elevated temperatures. Addition of cerium rich misch metal to AZ91 alloy resulted in formation of needle shape particles, which also had a very high thermal stability, providing increased creep resistance and superior mechanical properties compared to AZ91 magnesium alloy. As a result, the grain boundaries were less susceptible for grain boundary sliding at high temperatures. By decreasing the Al content of the alloy having 2 wt.% RE from 9 to 4 wt.%, the steady state creep rate was also decreased compared to AZ91 + 2% RE alloy. The fracture mechanism was also investigated and it was observed that although the Si addition improves the creep resistance, it can make the alloy brittle at ambient temperature.
Materials Science and Engineering: A, 2015
The relationship between the as-cast microstructure and creep behaviour of the heat-resistant MRI230D Mg alloy produced by two different casting technologies is investigated. The alloy in both ingot-casting (IC) and high pressure die-casting (HPDC) conditions consists of α-Mg, C36 ((Mg,Al) 2 Ca), Al-Mn and Sn-Mg-Ca rich phases. However, the HPDC alloy resulted in relatively finer grain size and higher volume fraction of finer, denser network of eutectic C36 phase in the as-cast microstructure as compared to that of the IC alloy. The superior creep resistance exhibited by the HPDC alloy at all the stress levels and temperatures employed in the present investigation was attributed to the more effective dispersion strengthening effect caused by the presence of finer and denser network of the C36 phase. The increased amount of the eutectic C36 phase was the only change observed in the microstructures of both alloys following creep tests.
Journal of Magnesium and Alloys, 2014
Previous investigations indicate that the creep resistance of magnesium alloys is proportional to the stability of precipitated intermetallic phases at grain boundaries. These stable intermetallic phases were considered to be effective to suppress the deformation by grain boundary sliding, leading to the improvement of creep properties. Based on this point, adding the alloying elements to form the stable intermetallics with high melting point became a popular way to develop the new creep resistant magnesium alloys. The present investigation, however, shows that the creep properties of binary MgeSn alloy are still poor even though the addition of Sn possibly results in the precipitation of thermal stable Mg 2 Sn at grain boundaries. That means other possible mechanisms function to affect the creep response. It is finally found that the poor creep resistance is attributed to the segregation of Sn at dendritic and grain boundaries. Based on this observation, new approaches to improve the creep resistance are suggested for magnesium alloys because most currently magnesium alloys have the commonality with the MgeSn alloys.
Creep resistance in magnesium alloys and their composites
International Journal of Materials & Product Technology, 2003
A considerable improvement in the creep properties of magnesium alloys can be potentially achieved by non-metallic reinforcement (metal matrix compositesÐMMCs). This paper provides a comparative report on experiments which were conducted on the two representative magnesium alloys (AZ91 and QE22) and their composites in order to evaluate the creep properties and to clarify the direct and indirect strengthening effects of reinforcement in creep. A comparison between the creep characteristics of squeeze-cast AZ91 and QE22 magnesium alloys reinforced with 20 vol.% Al 2 O 3 short fibres and unreinforced matrix alloys shows that the creep resistance of the reinforced materials is considerably improved compared to the monolithic alloys. By contrast, the investigations of the creep behaviour of a particulate QE22-15 vol.% SiC composite prepared by powder metallurgy and the squeeze-cast hybrid QE22 matrix composites have revealed no substantial increase in the creep strength of the composite compared to the matrix alloy and AZ91 matrix composites. These results indicate a paramount importance of the choice of the composite matrix alloy and the reinforcement used.
Creep behavior of pure magnesium and Mg–Al alloys in active environments
Materials Science and Engineering: A, 2005
Environment-enhanced creep, which we have called "corrosion creep" (CC), was investigated in pure Mg and die-cast AZ91D, AM50 and AS21 alloys in a borate buffer solution and 3.5% NaCl at room temperature. In contrast to the data in the air demonstrating only the first stage of creep process, in corrosive solutions, secondary and tertiary creep due to the plasticization effect of the solution was observed. The lifetime of pure Mg increases by one order of magnitude in the buffer solution in comparison with that in 3.5% NaCl. Elongation-to-fracture in the former was approximately twice as high as that in NaCl, probably, due to hydrogen embrittlement in the latter. In corrosive solutions, the creep life and elongation-to-fracture of Mg alloys decreases with increasing aluminum content from 2.3 to 8.4% in AS21, AM50 and AZ91D alloys. Cracking and final creep-rupture of pure Mg originate in a transcrystalline manner in comparison with intercrystalline fracture of the alloys. Cracks are observed in pure Mg at the primary creep stage, but their depth and amount are relatively small. The effect of environment on the creep behavior of magnesium is connected, mainly, with plasticization of metal assisted by chemical reactions. Also, anodic dissolution of Mg is enhanced by creep stress.
CREEP BEHAVIOUR OF Mg–BASED ALLOYS AND THEIR SELECTED COMPOSITES
The creep resistance of Mg -based alloys is rather limited at temperatures above 423 K. Recently, new families of precipitation and/or dispersion strengthened magnesium alloys are being developed for elevated temperature applications. A further considerable improvement in the creep properties of magnesium alloys can be potentionally achieved by short fibre reinforcement [metal matrix composites -MMCs]. This work reports some experimental results obtained in an investigation of the high temperature creep behaviour [423 -473 K] of selected fibre-reinforced matrix composites and their unreinforced matrix alloys. The objective of present paper is a further attempt to clarify the factors limiting the creep strengthening and plasticity in short fibre reinforced magnesium alloys.
Roles of Nd and Mn in a new creep-resistant magnesium alloy
Materials Science and Engineering: A, 2020
Modification of the recently developed creep-resistant Mg-3Gd-2Ca (wt.%) magnesium alloy using Nd and Mn remarkably improved the creep resistance at both 180 � C and 210 � C. The modified Mg-2Gd-1Nd-2Ca-0.5Mn alloy after solid solution treatment exhibited outstanding creep resistance that is superior to the commercial creep-resistant Mg alloy, EV31, but contained less RE addition. The microstructural observations revealed that partial replacement of Gd with 1 wt% Nd did not enhance the effect of dynamic precipitates significantly. But further analysis by atom probe tomography verified the stronger co-segregation between Nd solute atoms and other solute atoms than that of Gd and Ca in the Mg solid solutions, leading to a higher solid solution strengthening effect on the creep resistance. The addition of 0.5 wt% Mn led to the formation of polygonal-shape α-Mn precipitates, which served as heterogeneous nucleants for dynamic precipitates, refining their size and increasing the number densities. As a result, the creep properties of newly developed Mg alloys were strengthened by a combination of improved solid solution strengthening by Nd and increased precipitation hardening by Mn addition.