Development and Characterization of Low-Density Ca-Based Bulk Metallic Glasses: An Overview

Metallurgical and Materials Transactions A, Oct 2007

Ca-based bulk metallic glasses (BMGs) have unique properties and represent a new seventh group of BMGs. Many of them have excellent GFA, which can be related to their efficient atomic packing, low onset driving force for crystallization, and high viscosity (high relaxation time) of the supercooled liquid. The Ca-based glasses have the lowest density and elastic moduli among all BMGs discovered to date. Unfortunately, as many other glasses, Ca-based BMGs are brittle below the glass transition temperature, and they also have marginal oxidation and corrosion resistance. The latter can be improved by proper selection of alloying elements. In this article, we review recent work on the development of low-density Ca-based BMGs and discuss the effect of alloy composition on the thermal, physical, and chemical properties of these glasses.

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Development and Characterization of Low-Density Ca-Based Bulk Metallic Glasses: An Overview

O.N. SENKOV 0 D.B. MIRACLE 0 V. KEPPENS 0 P.K. LIAW 0 0 O.N. SENKOV, Senior Scientist, is with UES, Inc., Dayton, OH 45432 . Contact Ca-based bulk metallic glasses (BMGs) have unique properties and represent a new seventh group of BMGs. Many of them have excellent GFA, which can be related to their efficient atomic packing, low onset driving force for crystallization, and high viscosity (high relaxation time) of the supercooled liquid. The Ca-based glasses have the lowest density and elastic moduli among all BMGs discovered to date. Unfortunately, as many other glasses, Ca-based BMGs are brittle below the glass transition temperature, and they also have marginal oxidation and corrosion resistance. The latter can be improved by proper selection of alloying elements. In this article, we review recent work on the development of low-density Ca-based BMGs and discuss the effect of alloy composition on the thermal, physical, and chemical properties of these glasses. I. INTRODUCTION: CLASSIFICATION AND MAIN FEATURES OF Ca-BASED BULK METALLIC GLASSES NEW metallic alloys with exceptional glass forming ability (GFA), which can remain amorphous after solidification at relatively low cooling rates typical of conventional metal mold casting, open new opportunities both for fundamental studies and technological advances. High thermal stability, exceptional mechanical and physical properties, as well as the ability to be plastically formed in the supercooled liquid state make bulk metallic glasses (BMGs) attractive for engineering applications. Over the last two decades, a number of BMGs based on Pd, Zr, rare earth metals, Fe, Cu, Ni, Mg, and some other elements have been produced, and different methods have been proposed to guide the discovery of new glasses with better GFA.[14] The Ca-based BMGs are a relatively new class of amorphous alloys. The first Ca-based BMGs were reported by Amiya and Inoue in 2002.[5,6] They produced two ternary glasses, Ca57Mg19Cu24 and Ca60Mg20Ag20, with a maximum diameter of 4 mm and a quaternary Ca60Mg20Ag10Cu10 with a maximum diameter of 7 mm. During the following years, a number of Ca-Mg-Zn, Ca-Mg-Cu, Ca-Mg-Al, Ca-Al-Cu, Ca-MgZn-Cu, Ca-Mg-Al-Zn, Ca-Mg-Al-Cu, Ca-Y-Mg-Cu, Ca-Mg-Al-Zn-Cu, Ca-Y-Mg-Zn-Cu, and Ca-Mg-AlAg-Cu BMGs with thicknesses up to 10 mm were reported by Senkov et al.[4,712] Approximately, at the same time, Park and Kim[13] produced a Ca65Mg15Zn20 alloy, which was fully amorphous in up to 15-mm diameter cross sections. They also developed several other Ca-Mg-Zn amorphous alloys.[14] Guo et al.[15] reported on Ca-Al-Cu, Ca-Al-Ag, and Ca-Al-Mg amorphous alloys with a maximum diameter of 2 mm and Ca-Mg-Al-Cu and Ca-Mg-Al-Ag glasses with a maximum diameter of 4 mm. All of these Ca-based BMGs can be described by the formula[4,7] CaAY,LnBMg,SnCAl,Ag; Ga,ZnDCu,NiE with A = 40 to 70, B = 0 to 30, C = 0 to 30, D = 0 to 35, E = 0 to 35, and A + B + C + D + E = 100. A strong topological basis[4,1620] exists for the compositions represented in Eq. [1], and a structural model has recently been developed following these background developments.[21] The breadth of Eq. [1] and the results just cited indicate that many Ca-based alloys are good glass formers. A graphical presentation of Eq. [1] is shown in Figure 1, where concentrations of the elements are plotted vs their atomic radius ratios Ri (i.e., the atomic radius of an element divided by the atomic radius of Ca). The elements with the same Ri (within 2 pct deviation) are combined in the same group, and their sum concentration range is shown as solid bars. It can be seen from Figure 1 that Ri has discrete values, which correspond to densely packed solute-centered clusters with solvent atom coordination numbers of 8, 9, 10, and 12 at Ri = 0.62, 0.71, 0.80, and 0.90, respectively,[17] and the elements in each group have specific concentration ranges, which are necessary for efficient atomic packing.[10,16,21,22] Such atomic-concentration arrangement of the alloying elements in the Ca-based BMGs, which provides efficient atomic cluster packing,[17,22] may be one of the reasons for their good GFA. Fig. 1Atomic size ratio vs concentration of elements in Ca-based BMGs. Atomic size distribution plots for two ternary alloys, CaMg-Zn (dotted line) and Ca-Mg-Cu (solid line), and their structural designations[10,21] are also shown here. Typically, no more than three of the four possible solute sizes are present in any given metallic glass. Recently, Takeuchi and Inoue conducted a classification of BMGs.[23] According to their classification, which is shown in Figure 2, Ca-based glasses represent a new seventh group of BMGs, which consists of simple alkaline metals (Ca and Mg) and late transition metals (e.g., Ag, Cu, Zn, and Ni). However, it is well established now and represented by Eq. [1] that Cabased BMGs may also contain Al-, Ga-, Y-, and Ln-group metals. Therefore, the diagram in Figure 2 should be modified by adding two arrows, w (...truncated)


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O.N. Senkov, D.B. Miracle, V. Keppens, P.K. Liaw. Development and Characterization of Low-Density Ca-Based Bulk Metallic Glasses: An Overview, Metallurgical and Materials Transactions A, 2007, pp. 1888-1900, Volume 39, Issue 8, DOI: 10.1007/s11661-007-9334-z