Effect of stoichiometry in the HfCo2 C15 laves phase
- Dan J.
- Michael I.
In binary Laves phases (AB2) that do exhibit a homogeneity range, the ability to accommodate nonstoichiometry is related to the atomic size requirements for the topologically close-packed structure. Experimentally, based upon a hard sphere model, Laves phases with a metallic atom size ratio RA/RB close to the ideal value of -1.225 exhibit homogeneity ranges. Interestingly, the enthalpy of formation for Laves phases also display the largest values at the ideal ratio. For example, Campbell plots are empirical investigations of enthalpy of formation as a function of atom size ratio. Within an elemental class of AB2 Laves phases (Le,, systematic variation of the A element), a maximum value in enthalpy is observed. To interpret this result, Embedded Atom Methods (EAM) using Lennard-Jones potentials have been used to correlatc atomic size to enthalpy of formation (and therefore homogeneity range), thus providing insight into geometric trends that define Laves phases. In addition, constitutional defect types (mostly anti-site substitution) can be interpreted. With this information, a model system can be explored. The C15 phase, Hl'Coz, has a radius ratio of RA{approx}/R=c {approx}1. 26 and a reported range of solubility of 9 at.%. The solubility limits and constitutional defects have been investigated, and with this information, the elastic and mechanical properties as a function composition were defined. The results indicate that anti-site substitution is the governing defect mechanism on both sides of stoichiometry. The elastic and mechanical properties show a maximum at stoichiometry (unlike most intermetallics), with the brittle behavior decreasing with increasing Co content. The properties are effectively described in terms of the geometric/electronic structure models developed for Laves phase intermetallics. Support from DOE-OBES is gratefully acknowledged.
- Research Organization:
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 975721
- Report Number(s):
- LA-UR-01-4908; TRN: US201018%%809
- Resource Relation:
- Conference: Submitted to: Nonstoichiometric Ceramics and Intermatallics Conference II, Ciocco Hotel and Conference Center, Castelvecchio Pascoil, Italy, September 30-October 5, 2001
- Country of Publication:
- United States
- Language:
- English
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