Metallurgical factors controlling the phase stability and mechanical behavior of transition-metal Laves alloy systems

Author

Jiahong Zhu

Date of Award

8-1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

Peter K. Liaw

Committee Members

C. R. Brooks, R. A. Buchanan, D. C. Joy

Abstract

Recent interest in developing ultrahigh temperature intermetallics for structural use has prompted a series of research on Laves-phase based alloys. Alloys based on the NbCr₂ Laves phase are shown to be the most promising, since NbCr₂ has a high melting point of 1730°C, good creep resistance, high strength, and excellent oxidation resistance. However, the room-temperature fracture toughness of Laves phases is poor. There is a pressing need to (1) understand the scientific principles controlling the phase stability and deformation behavior of the Laves-phase materials, and (2) investigate the ways to toughen these materials so that their applications can be widened. In this dissertation, the phase stability in transition-metal NbCr₂-based Laves phases was first studied following a phase diagram survey. An electron concentration rule for the phase stability of NbCr₂-based Laves phase systems was proposed, and the critical electron concentrations for controlling the C15/C14/C15 phase transitions were identified. Pseudobinary Nb(Cr,X)₂ (X = Fe or Co) systems with various X contents were investigated to determine the homogeneity ranges of various Laves phases, which was then correlated with the electron concentration factor, e/a. The experimentally determined critical e/a values corresponding to the C15/C14/C15 phase transitions were close to those from the phase diagram survey: at e/a < 5.77, the C15 structure was stabilized; at 5.85 < e/a < 7.5, the C14 structure was stable; and at e/a > 7.56, the C15 structure was stabilized again. No intermediate structure was detected between the C15/C14 structures. As the Fe or Co content increased, the volume of a 24-atom unit cell decreased linearly for both C15 and C14 structures, while the Vickers hardness increased over the whole C15 and C14 ranges. The fracture toughness was low for all the alloys, but the fracture toughness of the C15 structure was greater than that of the C14 structure for the two ternary systems. No improvement in fracture toughness for alloys near the C15 and C14 phase boundaries and in the C15/C14 two-phase regions was observed.

The stability of the Laves phases was also investigated from the consideration of the bonding characteristics and the enthalpy of formation of the Laves phases. A survey on the enthalpies of formation of Laves phases reveals that there are metallic, covalent and ionic bonds, or a mixed metallic-covalent-ionic bond, in Laves phases. A thermodynamic interpretation is offered for the first time to explain the geometric principle for Laves-phase formation. As the deviation from the ideal size ratio increases, the maximum negative enthalpy of formation decreases linearly, which is assumed to be due to the elastic strain energy expended in compressing the atoms. At the atomic size ratios, R_a/R_b, of 1.03 and 1.65, the enthalpy of formation reaches zero. Further deviation in the R_a/R_b ratio will lead to the enthalpy of formation being positive. Thus, the free energy of formation becomes positive, due to the negligible entropy-of-formation term. Consequently, Laves phases can only be stabilized in certain atomic size ratios.

The constitutional defect mechanisms on both sides of stoichiometry in the binary NbCr₂, NbCo₂ and NbFe₂ Laves-phase systems were found to be the anti-site substitution. For the NbCr₂ alloys, thermal vacancies exhibiting a maximum at the stoichiometric composition were observed in samples quenched from 1400°C. These vacancies could be completely eliminated by annealing at 1000°C.

Anti-site hardening was found in the NbCr₂, NbCo₂ and NbFe₂ Laves-phase systems. The presence of thermal vacancies in the NbCr₂ alloys, on the other hand, was found to soften the Laves phases. Both the anti-site defects and thermal vacancies do not significantly affect the fracture toughness of the Laves phases.

Recommended Citation

Zhu, Jiahong, "Metallurgical factors controlling the phase stability and mechanical behavior of transition-metal Laves alloy systems. " PhD diss., University of Tennessee, 1998.
https://trace.tennessee.edu/utk_graddiss/9415