Combined Theoretical and Experimental Study of Transition Metal Laves Phases

Intermetallics based on transition metal and rare earth Laves phases have attractive technological and physical properties with a high potential for a wide variety of applications based on hydrogen storage capacity, magnetism, superconductivity, high structural and phase stability, high strength, good oxidation resistance, and excellent creep properties. Although good knowledge exists about the basic structural principles of Laves phases, many questions remain, in particular concepts on structure-property relations for taylored alloy design are still not well developed. The objective of the project is to study physical properties of the ordered Laves phase compounds XM2 (X=Zr, Hf, Nb, Ta, and M= Cr, Mn, Fe, Co ) by combining theoretical and experimental efforts. The experimental part will provide data on the thermal stability and crystal structure, information about the defect structure and site occupation as well as on the extent of the homogeneity region of the compounds. These results will be summarized in a state- of-the-art phase diagram of the Laves phase region, replacing older inconsistent data. Theory will apply an ab initio density functional package for calculating ground state properties of the three basic structure types of Laves phases. The equilibrium volume and coordinates will be computed for nonmagnetic, antiferro-magnetic and ferromagnetic spin phases. Energies of formation will be derived in combination with calorimetric experiments. Elastic constants will be calculated. For a realistic phase diagram the ab initio method will be applied to obtain defect formation energies and vibrational properties for selected interesting cases. The synergy in the combined experimental and theoretical efforts is manifold employing a knowledge-driven research approach: (a) the state-of- the-art experiments will provide reliable phase diagrams for the Laves-phase region which can be checked on the basis of the ab initio data; (b) the experiment will provide details on phase stability and magnetic ground state accompanied by ab inito calculations to analyze and interpret the results; (c) the ab initio approach will be applied for searching for metastable and low temperature stable phases which could not be resolved by experiments; (d) the degree of instability derived from the ab initio data may provide clues for phase stabilization via alloying which could be checked by experiment; (e) unexpected ab initio results may call for specific experiments to clarify points of discrepancies and vice versa.(f) due to the very close interaction of theory and experiment, the results will provide a consistent materials properties data base for future technological exploitations.

Intermetallics based on transition metal and rare earth Laves phases have attractive technological and physical properties with a high potential for a wide variety of applications based on hydrogen storage capacity, magnetism, superconductivity, high structural and phase stability, high strength, good oxidation resistance, and excellent creep properties. Although good knowledge exists about the basic structural principles of Laves phases, many questions remain, in particular concepts on structure-property relations for taylored alloy design are still not well developed. The objective of the project is to study physical properties of the ordered Laves phase compounds XM2 (X=Zr, Hf, Nb, Ta, and M= Cr, Mn, Fe, Co ) by combining theoretical and experimental efforts. The experimental part will provide data on the thermal stability and crystal structure, information about the defect structure and site occupation as well as on the extent of the homogeneity region of the compounds. These results will be summarized in a state- of-the-art phase diagram of the Laves phase region, replacing older inconsistent data. Theory will apply an ab initio density functional package for calculating ground state properties of the three basic structure types of Laves phases. The equilibrium volume and coordinates will be computed for nonmagnetic, antiferro-magnetic and ferromagnetic spin phases. Energies of formation will be derived in combination with calorimetric experiments. Elastic constants will be calculated. For a realistic phase diagram the ab initio method will be applied to obtain defect formation energies and vibrational properties for selected interesting cases. The synergy in the combined experimental and theoretical efforts is manifold employing a knowledge-driven research approach: (a) the state-of- the-art experiments will provide reliable phase diagrams for the Laves-phase region which can be checked on the basis of the ab initio data; (b) the experiment will provide details on phase stability and magnetic ground state accompanied by ab inito calculations to analyze and interpret the results; (c) the ab initio approach will be applied for searching for metastable and low temperature stable phases which could not be resolved by experiments; (d) the degree of instability derived from the ab initio data may provide clues for phase stabilization via alloying which could be checked by experiment; (e) unexpected ab initio results may call for specific experiments to clarify points of discrepancies and vice versa.(f) due to the very close interaction of theory and experiment, the results will provide a consistent materials properties data base for future technological exploitations.