V. Raman

Structure-property correlation of Zr-base alloys

Zirconium alloys, because of their unique combination of high strength, good corrosion resistance in water and low capture cross-section for thermal neutrons, have become attractive for use as structural materials in the nuclear industry. Presently, Zircaloy-2 and Zircaloy-4 find wide application as fuel and pressure tube materials for water cooled power reactors. In order to understand how the various alloying elements of Zircaloy, namely Sn, Fe, Cr and Ni, affect the mechanical properties, a programme has been initiated to evolve a correlation between chemistry, microstructure and mechanical properties of Zr-alloy containing various amounts of Sn, Fe and Cr. In the present investigation, mechanical properties of Zr-alloys with various addition of Sn, Fe and Cr have been determined at 300 K and 573 K in various metallurgical conditions such as recrystallised annealed, β-quenched, tempered and α-annealed conditions. The study revealed that the reduced tin content dit not affect the mechanical properties as the reduced tin leads to formation of fine precipitates. The mechanical properties were also not altered drastically with the low level of iron and chromium concentrations studied. Cold work and α-annealing after β-quenching resulted in the growth and redistribution of second phase particles. Metallographie studies showed that particle distribution was not uniform. A TEM investigation of the alloys has also been undertaken to study the details of microstructure developed during various heat-treated conditions. It has been found that the β-quenched samples exhibit the most uniform microstructure consisting of acicular alpha phase with lath boundary enriched by solute element and fine intermetallic particle formation. The observed microstructural features together with the mechanical properties data have been compared with the available mechanical properties cum microstructure of Zircaloy.

Stress distribution and hydride orientation in ZrNbCu garter spring under complex loading

Abstract A garter spring of the tight fit design in a PHWR experience three type of loads: an axial load due to spring tension, a compressive load due to coolant tube resting on it and a torus moment due to bending. The influence of these, on the hydride orientation in Zr2.5Nb0.5Cu garter springs has been studied by stress analysis and out-of-pile simulation experiments. The stress analysis showed that the magnitude of the stress is maximum at the surface and it decreases rapidly towards the core of the spring wire of rectangular cross section. This trend has also been observed in the experimental results. The preferred orientation of hydrides was found to be confined to the surface up to a depth of 300 μm. A random distribution of hydrides was observed at the central region where the stress was evidently below the threshold value for reorientation. Further a map of the planes of maximum principal stresses at the surface layer of the spring helix, constructed by stress analysis was found to be consistent with the observed hydride distribution. The diametrical compressive load did not show much influence on the hydride morphology in the range of 0–4 kg/turn under the axial load of 1 kg. It was also seen that the axial load of 1 kg imposed by the tight fit design did not significantly alter the hydride distribution which is primarily governed by torus bending.

Precipitation of Zr3 al in a Zr-4.6 wt% Al martensite

The discontinuous mode of precipitation of the equilibrium Zr3Al phase in a supersaturated Zr-4.6 wt% Al martensite has been examined by transmission electron microscopy. The orientation relationship between the matrix and the precipitate phases has been established. The structure of the α-Zr(Al)Zr3Al interfaces has been studied.

Influence of microstructure of the mechanical properties of a Zr-4.6 wt.% Al alloy

Abstract The influence of microstructure on the room temperature mechanical properties of a Zr-4.6 wt.% Al alloy was investigated. Quenching from the beta phase produced a significant solid solution hardening. On aging the alloy at low temperatures for short periods aluminium rejection from the solid solution occurred and a fine dispersion of a metastable Zr 3 Al phase (DO 19 structure) formed. The strengthening caused by the presence of these ordered particles was found to more than compensate the softening brought about by decreasing supersaturation. The high strength corresponding to this structure could be explained in terms of the contributions from the coherency strains associated with and the state of order within the metastable particles. Aging at these temperatures for longer periods or at higher temperatures gave rise to a lamellar distribution of the α-zirconium (aluminium) and the equilibrium Zr 3 Al (Ll 2 structure) phases. The strength associated with this lamellar structure was found to be appreciably lower and to be strongly dependent on the interlamellar spacing. Investigations of the fracture surfaces showed that the modes of fracture associated with these different microstructures were different. An attempt was made to rationalize the observed strength properties in terms of existing theoretical models.

Tempering of a beta quenched Zr-1.9 wt % Cr alloy

The morphology and the substructure characterizing the martensite produced on beta quenching a Zr-1.9 wt % Cr alloy have been examined by optical and transmission electron microscopy. The structural changes that occur in the martensite structure on tempering at various temperatures in the α+ZrCr2 phase field have also been investigated.

Structure and strength of dilute zirconium-copper martensites

The morphology, the substructure and the strength of dilute zirconium-copper martensites were examined. The martensites were of the dislocated lath type. The precipitation of an intermetallic phase at the lath boundaries could not be suppressed even by rapid beta-quenching. The strength of these martensites was substantially enhanced with increasing copper additions. This increased strength appeared to be due both to solid solution hardening and the distribution of precipitates at the lath boundaries. The work-hardening behaviour of these martensites could be analysed in terms of Ashbys theory of the deformation of plastically inhomogeneous materials.

Tempering of a beta quenched Zr-1.9wt % Cr alloy: Part 3 Tensile properties

The room temperature tensile properites of a Zr-1.9 wt % Cr alloy have been examined in the beta quenched as well as in tempered conditions and the results have been rationalized in terms of possible hardening mechanisms. The appearance of the fracture surfaces has been correlated to the microstructures developed in the alloy after various heat-treatments.