Nanocrystallization in Cu-Zr-Al-Sm Bulk Metallic Glasses

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MOTIVATED by the potential technological benefits of designing new advanced engineering and structural materials, efforts have been given to develop bulk metallic systems with high glass forming ability (GFA) to attain new properties or enhance their functionality. Among these systems, Cu-Zr binary alloys are particularly promising due to their wide composition range of GFA, starting from 35 to 70 at. pct Cu in alloys and appreciable compressive plasticity which prevents any catastrophic failures. We have previously investigated the glass formation, kinetics, and phase transformation mechanisms during devitrification in Cu-Zr binary systems.[1,2] Our previous studies revealed several details concerning crystallization transformations involving the phases, the kinetics of the crystallization of individual phases under isothermal and constant heating conditions, and structural dynamics associated with devitrification in Cu-Zr binary systems. Recent studies have reported that the GFA of Cu-Zr system increases

FATIH SIKAN and BENGISU YASAR are with the Department of Metallurgical and Materials Engineering, Middle East Technical University, 06800, Ankara, Turkey. ILKAY KALAY is with the Department of Materials Science and Engineering, Cankaya University, 06790, Ankara, Turkey. Contact e-mail: [email protected] Manuscript submitted July 7, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

significantly with an addition of third element, especially the transition metals (TM), such as Al, Ti, Ni, or Ag.[3–5] Among ternary Cu-Zr-TM systems, Cu-Zr-Al alloys have attracted much attention due to their high GFA and superior mechanical properties. However, since the GFA of Cu-Zr-Al BMGs is lower than the GFA of Beor Pd-containing BMGs, the efforts are given to increase the GFA of Cu-Zr-Al BMGs.[6] Recent studies indicated an increase in GFA of quaternary Cu-Zr-Al-X [X = Ag, Ce, Pr, Tb, Gd] alloys prepared by the addition of fourth element.[7,8] Besides the addition of transition metals to Cu-Zr-Al alloys, it has been reported that addition of rare-earth elements (RE) results in remarkable enhancement in GFA and mechanical properties of Cu-Zr-Al alloys by increasing the stability of supercooled liquid and reduced glass transition temperature (Trg). For instance, it has been shown that Lu and Dy rare earths increase both GFA and plasticity in Cu47Zr45Al8 alloy.[9] Deng et al. reported that the amorphous Cu-Zr-Al (Cu47Zr45Al8)98Y4 rods up to 25 mm diameter and (Cu47Zr45Al8)97Lu3 with a plastic strain of 4.65 pct can be obtained with a minor Y or Lu addition.[9] The effective applicability of BMG alloys is hindered by their limited plasticity and ductility.[10] Therefore, in the literature, a great effort has given to enhance mechanical properties especially the ductility of these alloys.[11–13] One way of introducing ductility into this system is to generate nanocrystals embedded in the amorphous matrix through controlled devitrification. Jiang et al.

showed that Cu46Zr47Al7 bulk metallic glass composite with a distribution of