Materials for engineering, 3rd Edition - (Malestrom)

Glasses and ceramics 157
In recent years, successful efforts have been made to increase the critical
casting thickness and bulk metallic glasses have now been developed. The
key criteria for slow crystallization kinetics and, thus, a stabilized supercooled
liquid state and high glass-forming ability include:
(a) multi-component alloys of three or more elements;
(b) atomic radius mismatch between elements greater than 12% leading to
a higher packing density and smaller free volume in the liquid state than
with pure metals and requiring a greater volume increase for crystallization;
(c) negative heat of mixing between the main elements, increasing the energy
barrier at the liquid–solid interface and decreasing the atomic diffusivity,
which in turn increases the melt viscosity to three orders of magnitude
greater than binary alloys;
(d) an alloy composition close to a deep eutectic forms a liquid stable at low
temperatures.
Slower crystallization allows a decreased critical cooling rate, enabling
bulk metallic glass (BMG) formation and fabrication by conventional casting
techniques. Vitreloy is a commercially produced Zr-based BMG whose
properties are compared with some crystalline alloys in Table 4.5. Zr-based
glasses have similar densities but high Young’s modulus (96 GPa) and elastic
strain to failure limit compared with crystalline alloys. Less energy absorption
and greater re-release of elastic energy (i.e. low damping capacity) makes
the material suitable for applications such as sporting equipment.
Above T g in the supercooled liquid regime, Zr-based BMG remains stable
against crystallization, so that it is malleable at ~400°C – behaving more
like a thermosetting polymer than a metal. This allows shaping and forming
by thermoplastic processing as easily and cheaply as polymers, with a critical
casting thickness of up to 10 cm.
Figure 4.15 illustrates the relative improvement in properties obtained
with BMG in comparison with other engineering materials.
At present, the principal areas of application of BMG are sports and
luxury goods. The first application was as golf club heads, where BMG is
twice as hard and four times as elastic as Ti drivers, so that 99% of the
Table 4.5 Properties of Vitreloy compared to some crystalline alloys
Properties Zr-based BMG Al alloys Ti alloys Steels
Density Mg m –3 6.1 2.6 – 2.9 4.3– 5.1 7.8
Tensile yield strength 1.9 0.10 – 0.63 0.18– 1.32 0.50– 1.60
σ y (GPa)
Elastic strain limit 2% ~0.5% ~0.5% ~0.5%
Fracture toughness 20–140 23 – 45 55– 115 50– 154
K Ic (MPa m 1/2 )