Structural properties and stability of metastable phases in the Zr-Nb system: Part I. Systematics of quenching-and-aging experiments

An experimental study is presented of the effect of an isothermal treatment ("aging") at 773 K upon the structural properties of three metastable phas es formed by quenching Zr-Nb alloys from 1273 K, viz., hcp (alpha), bcc (be ta), and Omega. Using neutron-diffraction (ND) measurements and Rietveld an alysis, the lattice parameters (LPs) and the constitution of Zr-Nb alloys w ith up to 18 at. pet Nb are determined. By combining these data with the LP vs at. pet Nb relations previously determined by us on quenched alloys, an analysis is performed of the composition changes and phase reactions occur ing upon aging. The present results open up the possibility of using quench ing-and-aging experiments to gain insight into the alpha + beta metastable equilibrium in the Zr-rich side of the Zr-Nb system.