Structural properties and stability of metastable phases in the Zr-Nb system: Part II. Aging of bcc (β) alloys and assessment of β-Zr

The lattice parameters (LPs) of the bcc (β) phase occurring metastably in a series of Zr-rich Zr-Nb alloys have been determined at and above room temperature (TR) using neutron diffraction techniques. LP values at T = TR were determined in alloys with Nb contents in the range 8.4 ≤ at. pct Nb ≤ 23 using both standard and high-resolution neutron diffractometry. These results support the applicability of Vegard’s law at TR. In addition, a neutron thermodiffractometric technique is used to establish the LP vs T relation of the β phase in two Zr-Nb alloys with 18 and 23 at. pct Nb up to 700 K. Such high-temperature data are used in a generalized Vegard’s law approach to gain insight into the LP vs T relation of the β phase in pure Zr, which is metastable in this temperature range. With these new LP values, an assessment is performed of the thermal expansion coefficient of β-Zr covering both stable and metastable states. As a direct application of the present findings, the results of an aging experiment are discussed, which provide information on the composition of the β phase taking part in the bcc-hcp equilibrium in the Zr-Nb phase diagram at 900 K. Such composition has been the subject of contradictory reports in recent literature. The composition value arrived at in the present analysis agrees well with the results of a thermodynamic calculation of the Zr-Nb phase diagram (A. Fernández Guillermet: Z. Metallkd., 1991, vol. 82, p. 478)