Texture evolution in two phase Zr-2.5 wt-%Nb through modified route

Abstract: Both in situ tension and compression tests have been carried out on textured Zr–2.5Nb plate material at room temperature. Deformation along all the three principle plate directions has been studied and the evolution of interphase and intergranular strains along the loading and the principle Poisson's directions has been investigated by neutron diffraction. The evolution of interphase and intergranular strain was determined by the relative phase properties, crystal properties and texture distribution. The average phase behaviors are similar during tension and compression, where the β-phase in this material is stronger than the α-phase. The asymmetric yielding of the α-{0 0 0 2} grain family results in a relatively large intergranular strain in the loading direction during compression and different dependence of strength during tension and compression on texture. The combination of the thermal residual stress and the asymmetric CRSS in the 〈 c 〉 axis gives the {0 0 0 2} grain family a higher strength in compression than in tension.

Abstract: Threshold stress, σth, for reorientation of hydrides in cold worked and stress-relieved (CWSR) Zr–2.5Nb pressure tube material was determined in the temperature range of 523–673 K. Using tapered gage tensile specimen, mean value of σth was experimentally determined by two methods, half thickness method and area compensation method. The difference between local values of σth measured across the thickness of the tube and the mean σth values yielded the residual stress variation across the tube thickness. It was observed that both the mean threshold stress and residual stress decrease with increase in reorientation temperature. Also, the maximum value of residual stresses was observed near the midsection of the tube.

Abstract: Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr–2.5Nb alloy. Inherent variability in the process parameters during the fabrication stages of pressure tube and also along the length of component have their effect on micro-structural and texture properties of the material, which in turn affect its strength parameters (yield strength and ultimate tensile strength) and flow characteristics. Data of tensile tests carried out in the temperature range from room temperature to 800 °C using the samples taken out from a single pressure tube have been used to develop correlations for characterizing the strength parameters’ variation as a function of axial location along length of the tube and the test temperature. Applicability of Ramberg–Osgood, Holloman and Voce’s correlations for defining the post yield behaviour of the material has been investigated. Effect of strain rate change on the deformation behaviour has also been studied.

Abstract: The properties and performance of Zr-2.5 Nb alloys are strongly influenced by their crystallographic texture. As in similar Ti alloys, the texture evolution during hot-processing depends on the complex interactions between the α and β phases and involves deformation, annealing and phase transformation. Although the effect of temperature and deformation has been studied for extruded tube in this alloy, there is no data for texture development during rolling. There is some rolling data for Ti-64 (Ti–6Al–4V), but it is usually for just one of the phases and for a limited set of temperatures. We carried out hot-rolling trials from 700 °C–900 °C to reductions of 50%, 75% and 87.5% and found that the texture in both phases strengthens sharply before the β-transus and when both phases are present in similar amounts. At this point, the texture in α is a strong 0002 | | TD and the texture in β a strong { 001 } 〈 110 〉 rotated cube component. The results suggest there might be a synergistic effect between the two components, which includes dynamic phase transformation. The texture evolution towards stable α { 11 2 ¯ 0 } 〈 10 1 ¯ 0 〉 or { 11 2 ¯ 1 } 〈 10 1 ¯ 0 〉 crystallographic components and their final intensity depend on the starting texture. Texture was measured using electron-backscatter diffraction (EBSD) over large areas, with a β reconstruction software used to determine the high temperature β orientations. The texture development in Zr-2.5Nb appears similar to that reported for rolled Ti-64 at temperatures with equivalent phase fractions, although it is difficult to compare the two because of the lack of a titanium dataset as detailed as the one presented here.

Abstract: We report the development of intergranular and interphase constraints in textured Zircaloy-2, Zr-2.5Nb, and Excel alloy during room temperature tension and compression loading in two or three directions relative to the parent texture. Neutron diffraction was used to track the lattice strain development in the α-phase (all alloys) and β-phase (Zr-2.5Nb and Excel) in three principal directions relative to the parent texture. Zircaloy-2 at room temperature is essentially single phase hcp αZr. The active deformation mechanisms appear to be, in order of increasing critical resolved shear stress, prism (a) slip, basal (a) slip, tensile twinning and pyramidal (c+a) slip. No compressive twinning was observed. Combined with intergranular constraints due to prior thermal treatment, these mechanisms result in substantial asymmetry in the yield stress and lattice strain development (compression versus tension). In Zr-2.5 Nb and Excel, the α-phase appears to deform by the same slip mechanisms as Zircaloy-2, and similar assymmetry of the yield stress and lattice strain development is observed. However, the existence of tensile twinning is not clearly evidenced. The β-phase also deforms by slip, but the critical resolved shear stress is much higher than that for the slip mechanisms in the α-phase, leading to the development of very large interphase constraints in the plastic deformation regime. This is attributed to a combination of solution strengthening of the β-phase (by Nb and, in Excel, Mo) and by grain size.

Abstract: Practical problems of the reproduction of the orientation distribution function (ODF) from pole figures (PFs) using discrete methods (with special attention paid to Imhof's method,) are analysed A new variant of the iteration method for the determination of the ODF in arbitrarily defined size of cells in the orientation space, is suggested The special case, in which the centro-symmetrical equivalency of the PFs is induced by the elements of proper rotations in crystal symmetry, is considered Es werden praktische Probleme der Reproduktion der Orientierungsverteilungsfunktion (OVF) aus Polfiguren (PFn) bei Anwendung diskreter Methoden (bei besonderer Beachtung der Imhof Methode) analysiert Eine neue Variante des Iterationsverfahrens fur die Bestimmung des OVF mit einer Wahlfreiheit der Zellengrose im Orientierungsraum wird vorgeschlagen Es wird ein Spezialfall betrachtet, in dem Elemente der eigentlichen Drehungen in der Kristallsymmetrie eine zentrosymmetrische Gleichwertigkeit in den PFn induzieren

Abstract: Zirconium-2.5 wt % niobium, furnace cooled or air cooled from the (α + β) phase field increases in hardness when aged in the temperature range 300 to 500 °C. Electron microscope studies revealed a fine two phase structure of α zirconium grains surrounded by a network of a second phase identified by X-ray diffraction to be cubic β-zirconium containing about 20 wt % Nb. When the β-zirconium phase is aged in this temperature range it transforms to enriched-β and ω phases with a corresponding increase in hardness and strength. From this it is inferred that the hardness and strength increases observed in the two phase alloy are due to the age hardening of the β phase. This composite structure of ductile α grains in a hard strong network may be the reason for the good in-reactor creep resistance of extruded and coldworked pressure tubes.

Abstract: Microstructural changes occurring during the fabrication of Zr-2.5 pct Nb alloy pressure tubes by a modified route, involving hot extrusion followed by two pilgering operations with an intermediate annealing step, have been examined in detail. In the conventional fabrication route, the hot extrusion step is followed by a single cold drawing operation in which the cold work to the extent of 25 pct is imparted to the material for achieving the required mechanical properties. Tensile properties obtained at each stage of fabrication have been evaluated and compared between the two processes. The main aim of this work has been to produce a microstructure and texture which are known to yield a lower irradiation growth. Additionally, suitable annealing conditions have been optimized for the intermediate annealing which annihilates the cold work introduced by the first cold pilgering operation without disturbing the two-phase elongated microstructure. This elongated α+ β I microstructure is required for obtaining the desired level of strength at 310 °C. The final microstructure and the crystallographic texture of the finished pressure tube have been compared with those reported for the conventionally processed material.

Abstract: A method employing a diverging x‐ray beam is described for the determination of preferred orientation in flat transmission samples with a Geiger counter x‐ray spectrometer. It is shown that for a certain range of sample thickness the counting rate is independent of the rotation of the samples, and therefore, no correction formula is required.