Mg-Al-Ca In-Situ Composites with a Refined Eutectic Structure and Their Compressive Properties
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OVER the past decade, Mg alloys have become more and more attractive as structural materials due to their light weight, low cost, and recyclability. However, their strength is often inadequate for many engineering applications. Among the currently developed Mg alloy families, Mg-Al-Ca–based alloys exhibit advantages in several aspects such as high creep resistance and super light weight due to the very low densities of the constituent elements (qMg = 1.74 g/cm3, qAl = 2.70 g/cm3, and qCa = 1.54 g/cm3). In addition, the Ca element in the alloys serves to protect the melt surface from fast oxidation, resulting in less slag formation, during processing. However, the yield strength of the Mg-Al-Ca alloys remains at a low level around 160 to 250 MPa at ambient temperature below 450 K.[1] A dramatic strength enhancement has not been possible, based on the conventional strengthening approaches such as grain refinement, solid solution, and precipitation. It is therefore important to develop novel microstructures using new alloy design methods or fabrication techniques to improve their mechanical properties.[2,3] As is well known, the fracture strength of amorphous alloys, without dislocations and grain boundaries in their structure, can be much higher than that of the crystalline counterpart. A number of new Mg alloys with high glass-forming ability have been discovered. The critical size of the glass formation is no longer a LING-LING SHI, Graduate Student, and JIAN XU, Professor, are with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, P.R. China. Contact e-mail: [email protected] EVAN MA, Professor, is with the Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, MD 21218. Manuscript submitted November 13, 2007. Article published online March 7, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
bottleneck for the application of such bulk metallic glasses (BMGs).[4–7] For the Mg-Cu-RE (RE = rare earth) BMGs fabricated using copper mold casting, the fracture strength of around 1188 MPa can be reached under compressive loading,[8] which is at least a factor of 3 higher than conventional polycrystalline Mg alloys. Unfortunately, plastic deformation in a uniformly glassy structure is concentrated into highly localized shear bands, leading to catastrophic failure and very limited plastic deformability. Such brittleness for Mg-based BMGs is attributed to their intrinsically low Poisson’s ratio (m ~ 0.31).[8–10] On the other hand, an alternative route to good strength and plasticity has been demonstrated in recent years, via the formation of in-situ composites consisting of nanocrystalline eutectic matrix and ductile dendrites in several multicomponent alloy systems. Such an approach in Ti-based alloys, for example, led to high fracture strength of 2400 MPa together with the plastic strain up to 14.5 pct.[11,12] It was suggested that deformation occurred partially through dislocation movement in micrometer-si
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