Development and Characterization of Low-Density Ca-Based Bulk Metallic Glasses: An Overview
O.N. SENKOV
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D.B. MIRACLE
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V. KEPPENS
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P.K. LIAW
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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)