Study of ideal strength and electronic structure in transition-metal compounds and alloys.

摘要

First principle total energy methods are employed to study the ideal strengths and electronic structures in two metallic systems: transition-metal aluminides (FeAl, CoAl, and NiAl) and Ti-V binary alloys.; The ideal strength calculation resolves the substantial difference between FeAl and NiAl: Under ⟨100⟩ tension, FeAl shows unique weakness while both CoAl and NiAl have high ideal strengths. Under ⟨111⟩ shear, NiAl has its ideal shear strength dictated by crystallographic symmetry whereas the instability of FeAl is found to be associated with the weakness in ⟨100⟩ tension. The electronic structure study reveals that the unique weakness of FeAl along ⟨100⟩ direction is due to the instability introduced by the filling of antibonding d states. The elastic stability analysis further demonstrates that under the ⟨100⟩ stress FeAl simply fails by tension while NiAl fails by shear. The calculation explains the experimentally observed {lcub}100{rcub} preferred cleavage that appears in FeAl only.; Ti-V alloys are adopted to study the intrinsic strength of the complex alloy "gum metal." The predicted ideal strength of Ti75V25 , which shares the same valence electron concentration as in "gum metal," are in good agreement with the measured strength of the bulk alloy. The low intrinsic strength originates from the nearly vanishing C 11--C12 as the alloy's composition draws close to the BCC-HCP structure instability. Accounting for the pinning of the dislocation by alloying additions, our study explains the absence of the dislocation activity observed in experiment, and confirms that ideal strength dominates plastic deformation in "gum metal". Three conditions, i.e ., a nearly zero C11--C 12, a sufficient number of impurity clusters, and a low C 44, are suggested as criteria that should be met in order to exhibit "gum metal-like" behavior.