More demanding performance expectations for total joint arthroplasty are driving the development of alternative bearing materials. Oxidized zirconium was developed as an alternative to cobalt-chromium alloy for knee and hip femoral components in order to reduce wear of the polyethylene counterface and to address the needs of metal-sensitive patients. Oxidation in high temperature air transforms the metallic Zr-2.5Nb alloy surface into a stable, durable, low-friction oxide ceramic without creating the risk for brittle fracture associated with monolithic ceramic components. This presentation reviews aspects of this technology with a historical perspective, including standards for the zirconium alloy, non-medical applications for oxidized zirconium, and previous orthopaedic applications for zirconium. Manufacturing processes for oxidized zirconium components are described, beginning with refining of the zirconium from beach sand, to producing the alloy ingot and bar, to fabricating the component shape, and finally to oxidizing the surface and burnishing it to a smooth finish. Conditions are described for producing the oxide with excellent integrity, which is nominally 5 µm thick and predominantly monoclinic phase. The metal and oxide microstructures are characterized and related to the mechanical properties of the components and durability of the oxide. Laboratory hip and knee simulator tests are reviewed, which indicate that oxidized zirconium components reduce wear of the polyethylene counterface by 40–90 % depending on test conditions. As evidenced by promising early clinical experience, oxidized zirconium components have characteristics that provide an alternative to conventional cobalt-chromium components with an interchangeable surgical technique, while providing the potential for superior performance.
Trace elements can reduce the fracture resistance of Zr-2.5Nb pressure tubes. The effects of hydrogen as hydrides and oxygen as an alloy-strengthening agent are well known, but the contributions of carbon, phosphorus, chlorine, and segregated oxygen have only recently been recognized. Carbides and phosphides are brittle particles, while chlorine segregates to form planes of weakness that produce fissures on the fracture face of test specimens. A high density of fissures is associated with low toughness. With long hold times in the (α + β) region, oxygen partitions into the α-grains; such grains are hard and, if they survive fabrication, may reduce the toughness of the finished tube. Through a cooperative program involving AECL and the manufacturers, a series of manufacturing innovations and controls has been introduced that minimizes these harmful effects. Hydrogen is present in the zirconium sponge as water, can be absorbed at each stage of tube fabrication, and needs to be carefully controlled, particularly during ingot breakdown and subsequent forging. Hydrogen concentrations in finished tubes have been reduced by a factor of three through the optimization of manufacturing processes and the implementation of new technology. Multiple vacuum arc melting, use of selected raw materials, and intermediate ingot surface conditioning have resulted in much improved fracture toughness through the reduction of chlorine and phosphorus concentrations. Optimum distribution of oxygen may be achieved through changes to the extrusion process cycle. An understanding of the Zr-2.5Nb-C phase diagram, particularly the solubility of carbon at low concentrations, has resulted in the specification of a lower carbon concentration.
Flat mill products of Zircaloy-4 and controlled-chemistry Zircaloy-2 were processed using different rolling schedules in order to generate different textures and other metallurgical conditions. Tubular products of three heats of controlled-chemistry Zircaloy-2 were also processed using various thermo-mechanical parameters and different tube reduction schedules. Samples were taken for 500 and 520°C steam autoclave tests and texture and microstructure examination. Intermetallic precipitates were analyzed for mean diameters and particle size distributions using optical high magnification photographs. Matrix concentrations of Fe, Cr, and Ni were analyzed by electron microprobe with automated stage control. Results showed that the nodular corrosion resistance for controlled-chemistry Zircaloy-2 was excellent, while that for Zircaloy-4 was rather poor. The Zircaloy-2 tubeshell may be annealed up to 663°C/2h without detriment to the nodular corrosion resistance of the final size tubing if other fabrication variables are properly controlled. The optimum process for tube reducing is a high Q ratio and high percent of reduction in area. Neither β quench nor α + β treatment after tube extrusion is required to produce cladding tube that is free of nodules in 500 or 520°C autoclave tests. Low basal pole densities are generally related to high corrosion weight gains. The accumulated annealing parameter, ΣA, can generally predict the nodular corrosion behavior of the material, but it cannot account for the higher corrosion weight gain at an intermediate stage and lower weight gain at the final size. The most significant correlation is established between variation in concentrations of iron, chrome, and nickel in Zircaloy-2 and nodular corrosion behavior of the material. A depletion of these elements in the matrix or a large spacing between intermetallic precipitates causes poor nodular corrosion resistance. The positive effect of cold work on the nodular corrosion resistance is attributed to increasing the yield strength of the substrate.
This paper describes the corrosion behavior and the ZrO2 microstructure of Zircaloy-4 (Zry-4) cladding tubes that were intermediate annealed at various temperatures. The corrosion behavior of the cladding tubes was studied by autoclave tests performed under 633 K water condition and 673 K steam condition. A TEM examination shows that the microstructure of ZrO2 formed on the Zry-4 matrix consisted of both the columnar structure and the equiaxed grain. The grain size of the columnar grain was approximately 30 by 200 nm, while that of the equiaxed grain was less than 20 nm. The equiaxed grain was dominantly observed near lateral cracks and around intermetallic compounds that were incorporated into the ZrO2 film. An analysis of the HR-SEM images indicated that the equiaxed grain to columnar grain volume ratio increased with increasing weight gain, especially after the first transition. The equiaxed grain to the columnar grain volume fraction decreased with increasing annealing temperature, which corresponded to decreasing weight gain. It was suggested that grain boundary diffusion of oxygen ions was accelerated by grain-size change of the oxide owing to the ZrO2 microstructure transformation from the large columnar grains to the fine equiaxed grains. The ZrO2 microstructure transformation might be caused dominantly by the oxidation of the intermetallic precipitates. The intermetallic precipitates were fine and uniformly distributed in the low-temperature TREX annealed Zry-4. This resulted in high-temperature TREX annealing being beneficial for improving corrosion resistance of the Zry-4 tube in PWR environments.
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