Composition-Dependent Collapse of β {111} Planes Leading to ω Precipitation in Ti-Nb Alloys: A First-Principles Study

The effects of Nb content on the collapse of β {111} planes leading to ω phase precipitation were investigated in binary Ti-Nb alloys by first-principles exact muffin-tin orbitals-coherent potential approximation (EMTO-CPA) method. In accordance with the accepted mechanism of the beta to omega transformation occurring due to the collapse of the bcc {111} planes, the total energies of β Ti-Nb binary alloys containing multiple states corresponding to different Nb concentration ranging from 20at.% to 30at.% were calculated. The results indicated that the total energies for the same Nb content are not monotonically decreasing with the z (z denotes the degree of collapse of β {111} planes) value increasing, but keeping an energy barrier to cross. The energy barrier increases gradually along with increasing Nb content. The density of states (DOS) was given to elucidate the changes of electronic structure during the collapse of β {111} planes.

Ordered Omega Derivatives in (Zr3Al)-Nb Alloys

ABSTRACTVarious kinds of phase transformations, viz., spinodal decomposition, omega transformation, precipitation reactions and martensitic transformation can be induced in ternary (Zr3Al) -Nb alloys in conditions far removed from equilibrium. Transformation sequences in alloys containing 3% niobium are described and rationalized in terms of some basic tendencies such as phase separation and chemical ordering in the β (bcc) phase and displacive omega and β to α (hcp) transformations. Microstructures of rapidly solidified alloy showed a distribution of cuboidal (D88 phase) particles in the β matrix. The periodic arrangement of these particles along the <100>β directions was indicative of a spinodal transformation which preceded their formation. The β → D88 transformation could be accomplished by the superimposition of three processes, namely, chemical ordering, lattice collapse akin to ω transformation and vacancy ordering. During isothermal aging the D88 phase transformed into the B82 phase. The observed lattice correspondence and transformation morphology suggested that the D88 to B82 structural change involved the replacement of structural vacancies in the former by zirconium atoms without any reconstitution of the lattice. The evolution of the equilibrium Zr3Al (L12 structure) phase during prolonged aging were also studied.