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9-Constitutional+Defects+in+Compounds

9-Constitutional+Defects+in+Compounds - Constitutional...

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110 Constitutional defects in intermetallic compounds Many intermetallic alloys are not line compounds; rather they show a wide range of composition, even if they are still described using the molecular compound terminology of A x B x , as if the material were always stoichiometric. When the compositions deviate from the stoichiometric composition they do so by the introduction of what are called constitutional defects, that is, defects that accommodate constitutional or compositional changes. Consider the simple example of NiAl, an intermetallic alloy having the CsCl or B2 crystal structure. As shown in the phase diagram below, the NiAl phase can exist over a wide range of compositions from about 45 atomic % Ni to 60 atomic % Ni. We may ask how such compositional changes can occur in a compound with a definite structure. The answer is that constitutional defects are responsible for the different compositions. Ni rich alloys: Ni atoms reside on the Al sites. These are called anti-site defects or anti-structure defects Ni poor (Al rich ) alloys: Ni vacancies are present on the Ni sub-lattice Much research on the structural and physical properties of this phase has led to this knowledge.
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