Oxidation Properties of a Zirconium-2.7 w/o Niobium Alloy in the Temperature Range 650°–1000°C

Abstract

A B S T R A C TThe reaction kinetics and structures of the alloy/oxide composite system were determined for a Zr-2.7 weight per cent Nb alloy exposed at t e m p e r atures over the range 650~176 in oxygen at 1 atm pressure. These kinetics may be represented by a parabolic-linear relationship. The zirconia scales contained small amounts of the intramolecular oxide, 6ZrO2 9 Nb2Os. An investigation was carried out on the morphological development of the col u m n a r scale and the underlying layer of oxygen stabilized ~-Zr plates in the $-Zr alloy during parabolic oxidation. The sequence of the reaction product layers was consistent with predictions from the Z r -N b -O phase diagram.Investigations on the oxidation properties of Z r -N b alloys containing 2-3 w / o (weight per cent) Nb at elevated temperatures have been only carried out to a limited extent (1-5). The reaction kinetics over the temperature range 300~176 are characterized initially by continuously decreasing rates followed by transitions to more rapid approximately constant rates. At temperatures less than the a + ~ eutectoid point at 610~ a martensitic alloy oxidizes more rapidly than the Widmanstatten-structured alloy (2, 3). The zirconia scales may contain small amounts of the intramolecular oxide, 6ZrO2-Nb205 (2-5). At t e mperatures higher than 900~ parabolic oxidation of a E-structured alloy gives rise to the formation of an oxygen stabilized columnar zone of a-Zr plates beneath the scale (4, 5). The objectives of this investigation, accordingly, were to determine the temperature dependence of the oxidation kinetics for the alloy in the a + ~ and ~-Zr structures and to examine the m o rphological development of the scale and a-Zr columnar layer in the ~-Zr alloy. ExperimentalA t h e r m o g r a v i m e t r i c technique was employed to m e asure the oxidation rates of alloy plates and spheres. The assembly which has been described contained an Ainsworth, Type AU-1, balance for continuously recording weight gains up to 400 mg to a sensitivity of 0.02 mg (6). Test temperatures w e r e controlled to •176 A specimen was lowered into the reaction zone and held for 15 min to ensure t e m p e r a t u r e equilibration after the assembly had been flushed with argon purified of oxygen and water vapor. The assembly was subsequently pumped down to less than 10 -4 Torr and oxygen, 99.99% pure, then admitted to a pressure of 760 Tort. At the termination of a run, the specimen was quenched by rapidly raising it into the air-cooled portion of the assembly.The material was a Zr-2.7 w / o Nb alloy received as a 2 m m thick sheet and as a bar. Analyses for i m p u r i -* E l e c t r o c h e m i c a l Society A c t i v e M e m b e r . K e y w o r d s : z i r c o n i u m -n i o b i u m alloys, o x i d a t i o n kinetics, o x i d e scale, alloy microstructures.ties have been presented, the residual oxygen content in the alloy being 1100 ppm (2). Plates prepared from the sheet, 2 • 1 • 0.2 cm, were wet abraded on 220 to 600-grit silicon carbide papers, polished on 6 ...

Laser surface modification of metallic biomaterials

“…23,24 For orthopedic application, oxide scale thicknesses of ~5 μm are generally achieved on Zr and Zr-Nb alloys after several hours of oxidation in dry air between 600 and 700ºC. 36,37 In addition, high-temperature oxidation has been found to develop undesirable voids in the oxide scale, 38 lowering its stability during service. 36,37 In addition, high-temperature oxidation has been found to develop undesirable voids in the oxide scale, 38 lowering its stability during service.…”

Oxidation Microstructures and Interfaces in the Oxidized Zirconium Knee

“…The Nb‐depleted α‐Zr phase is stabilized by the incorporated oxygen and can dissolve up to 28.5 wt% O. Indeed, it was proposed some time ago 48 that diffusion of oxygen into Zr–2.7 wt% Nb alloy leads to the formation of an α‐Zr+β‐Zr+oxide compound microstructure predicted by the Zr–Nb–O ternary phase diagram. At lower temperature, metastably retained β‐Zr can decompose according to the scheme 49 where the β‐Zr(enr) is enriched to more than 20 wt% Nb.…”

“…The Nbdepleted a-Zr phase is stabilized by the incorporated oxygen and can dissolve up to 28.5 wt% O. Indeed, it was proposed some time ago 48 where the b-Zr(enr) is enriched to more than 20 wt% Nb. The formation of nanocrystalline o-Zr grains in a Nb-enriched matrix, and their subsequent oxidation to m-ZrO 2 more quickly than a-Zr, 50 has been previously reported for 4001C steam oxidation, 42,43 but is unlikely to be the precise mechanism responsible for the Nb segregation at and above 5931C, because o-Zr is stable only below 5321C.…”