MATERIALS SCIENCE MATERIALS SCIENCE & ENGINEERING ENGINEERING Anandh Subramaniam & Kantesh Balani Materials Science and Engineering (MSE) Indian Institute of Technology, Kanpur- 208016 Email: [email protected], URL: home.iitk.ac.in/~anandh AN INTRODUCTORY E-BOOK AN INTRODUCTORY E-BOOK Part of http://home.iitk.ac.in/~anandh/E-book.htm A Learner’s Guide A Learner’s Guide
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Size Factor compounds: (i) Laves phases (ii) Frank-Kasper PhasesD
These phases have a formula: AB2
Laves phases can be regarded as Tetrahedrally Close Packed (TCP)* structures with an ideal ratio of the radii (rA/rB) = (3/2)1/2 ~1.225 [or usually rA/rB (1.1, 1.6)]
If rA/rB = 1.225 then a high packing density is achieved with the chemical formula AB2 with a average coordination number of 13.3
There are more than 1400 members belonging to the ‘Laves family’ Many ternary and multinary representatives of the Laves phases have been reported with
excess of A or B elements. Some ternary Laves phases are known in systems with no corresponding binary Laves phases.
The range of existence of the three phases (C15, C36, C14) in ternary Laves phases is influenced by the e/a ratio
D(i) Laves Phases
* Also called Topologically Close Packed structures?
Laves phases containing transition metals as components have interesting Physical and mechanical properties. Engineering materials based on Laves phases are being developed for: High temperature applications
(for use in turbine blade fine precipitates of Laves phases is shown to improve fatigue strength)
Hydrogen storage applications (in nickel-metal hydride batteries)
MgZn2 (Laves)Lattice parameter(s) a = 5.18 Å, c = 8.52 Å
Icosahedral coordination around W atoms Local icosahedral symmetry is destroyed in the long range packing Note that icosahedral symmetry is not found in crystals This phase is closely related to quasicrystals