Superplastic behaviour of a Zr-2.5 Wt% Nb pressure tube alloy
TL;DR: In this article, the authors reported two activation energy (Q) values for superplastic deformation of Zr-2.5 wt% Nb alloy at test temperatures of 600 C and above.
Abstract: The current fabrication route of Zr-2.5 wt% Nb pressure tubes develops a two phase structure of strongly textured, fine, elongated [alpha]-Zr grain and a grain-boundary network of [beta]-Zr phase. This network of [beta]-phase causes the microstructure to be stable even at high temperature, since grain-growth requires long-range diffusion of Nb. Recently three modified fabrication routes have been developed with a view to achieve reduced in-reactor dimensional changes in pressure tubes. During tensile testing of specimens machined from a Zr-2.5 wt% Nb pressure tube, manufactured by a fabrication route, broadly similar to the modified route-2, large elongations (> 100%) were obtained at test temperatures of 600 C and above. these observations indicated the possible occurrence of superplasticity in this alloy during high temperature deformation. Nuttall reported two activation energy (Q) values for superplastic deformation of Zr-2.5 wt% Nb alloy. The higher value can be matched with that for lattice diffusion in [alpha]-Zr at relatively lower temperature, whereas the lower value is comparable with the activation energy for lattice diffusion in [beta]-Zr at higher temperature. The abnormality in the observed values of Q in Zr-2.5 wt% Nb alloy was attributed to the variation in strain-rate sensitivity, composition and volume fraction of [alpha]more » and [beta]-phases with temperatures. Further studies on superplasticity in Zr-2.5 wt% Nb alloy was thus felt necessary and led to the present investigation.« less
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15 Feb 2005-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the effect of grain size on superplastic deformation in Zr −2.5% Nb was studied by constructing processing maps, which depict the variation of strain rate sensitivity with temperature and strain rate.
Abstract: The effect of grain size on superplastic deformation in Zr–2.5 wt.%Nb was studied by constructing processing maps (which depict the variation of strain rate sensitivity with temperature and strain rate) in the temperature range of 650–830 °C and strain rate range of 5 × 10 −6 to 2 × 10 −3 s −1 . The occurrence of superplastic domain with respect to temperature and strain rate was identified for three grain sizes (4, 10 and 16 μm). The 4 μm grain size material exhibited a domain centered around 800 °C and 10 −4 s −1 , exhibiting a ductility of 700%. With increasing grain size the domain shifts to higher temperatures and lower strain rates. A detailed characterization of deformed microstructure revealed equiaxed grain structure (curved boundaries) with considerable grain growth within the superplastic domain. Creep equation was used to evaluate the parameters of superplastic deformation, which resulted in activation energy of 125 kJ/mol and a grain size exponent of 1.6. The accommodation mechanism for superplastic deformation was deduced to be either non-conservative jog motion or grain boundary migration, with the rate controlling step for both being the grain boundary diffusion of Zr and Nb in β phase.
31 citations
TL;DR: In this article, the formation of duplex microstructure in Zr-2.3Nb alloys at a temperature range of 850-925°C, its plastic behaviour at various strain rates and the accommodation of each phase in each phase during the tensile deformation process were investigated thoroughly.
31 citations
TL;DR: In this paper, the authors used 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.
17 citations
TL;DR: In this article, a Zr-2.5 wt% Nb pressure tube alloy was deformed at strain rates (e ) of 3.2 × 10−4−2 s−1 and test temperatures (T) of 625-800°C to investigate the nature of stress-strain curves and the optimum condition for superplasticity.
13 citations
TL;DR: In this article, the effect of microstructural evolution on superplastic deformation parameters, such as the nature of σ-e plots, strain-rate sensitivity parameter, and activation energy, were studied for unstable and thermally stable microstructures of a Zr-2.5 wt pct Nb pressure-tube alloy.
Abstract: The effect of microstructural evolution on superplastic deformation parameters, such as the nature of σ-e plots, strain-rate sensitivity parameter, and activation energy, were studied for unstable and thermally stable microstructures of a Zr-2.5 wt pct Nb pressure-tube alloy. Two types of differential strain-rate tests (increasing temperature (IT) and decreasing temperature (DT), in the temperature range of 610 °C to 810 °C at 20 °C intervals) were conducted within a strain-rate range of 10−5 to 10−3 s−1. Single specimens were used to obtain the σ-e plots for all the test temperatures in the aforementioned temperature range. The effect of orientation (with respect to the axial direction of the tube) on the superplastic deformation parameters was also studied. The microstructural evolution was studied along the three orthogonal planes of the tube by water quenching underformed samples in the beginning of differential strain-rate tests at each test temperature. The observed apparent activation-energy values associated with deformation were in the two distinct ranges of 287 to 326 and 151 to 211 kJ/mole. In the temperature range of 730 °C to 810 °C, the apparent activation-energy value depended on the direction of approach of the test temperature. The mechanisms of superplastic deformation in this alloy were found to be dislocation climb—controlled creep in region III and grain-boundary sliding accommodated by grain-boundary diffusion or lattice diffusion in the α or β phases in region II. Based on the observed microstructural features, a model to explain the σ-e plots and apparent activation energy has been proposed.
13 citations
References
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TL;DR: In this article, the high temperature steady state deformation behavior of PbSn eutectic alloy was investigated over wide ranges of grain size (9.7 − 32.0 μm), temperature (298 − 443 K) and strain rate (10 −7 − 10 −2 s −1 ), after stability of microstructure and strength had been achieved through prior tensile deformation.
149 citations
01 Apr 1972-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this paper, the authors considered various deformation mechanisms for superplastic deformation and showed that grain boundary sliding is the major deformation mode, but the sliding rate should be governed by the lesser rate of dislocation creep within the grains.
Abstract: Under various conditions of stress and temperature various deformation mechanisms could be rate-controlling for superplastic deformation. In general at low stresses diffusion creep should be rate-controlling. At temperatures between approximately 40 and 65 pct of the absolute melting point grain boundary diffusion should be the dominant diffusion path while at higher temperatures volume diffusion should dominate. At intermediate stresses, grain boundary sliding should be the major deformation mode, but the sliding rate should be governed by the lesser rate of dislocation creep within the grains. At temperatures between 40 and 65 pct of the melting point, the rate of dislocation creep should be controlled by dislocation pipe diffusion, while at higher temperatures volume diffusion should be ratecontrolling. At high stresses the superplastic effect of unusually large tensile extensibility should diminish due to the greater possibility of work-hardening processes such as dislocation cell, tangle, and pile-up formation.
120 citations
TL;DR: In this article, a production route can be specified to achieve the desirable texture of basal plane normals oriented radially in the tube, and the texture developed is controlled by the direction of major strain in the final stages of fabrication.
73 citations
IBM1
TL;DR: The deformation behavior of superplastic Zn-Al alloy of eutectoid composition (40.6 at.% Al) has been investigated by metallography, X-ray diffraction, and creep deformation as mentioned in this paper.
48 citations