Tensile behavior of Zirconium-2 alloy rolled at low temperature
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https://www.eduzhai.net American Journal of M aterials Science 2012, 2(5): 138-141 DOI: 10.5923/j.materials.20120205.01 Tensile Behavior of Cryorolled Zircaloy-2 P. Aditya Rama Kamalanath1,*, Apu Sarkar2 1Department of metallurgical and materials engineering, NIT Warangal, Warangal, 506004, India 2M echanical metallurgy section, BARC, M umbai, 400094, India Abstract Zircaloy-2 is ma inly used in nuclear technology, as cladding of fuel rods in nuclear reactors, especially water reactors (BWRs). Hence high strength of Zircaloy-2 is of prime importance. This investigation deals with the effect of cryorolling on Zircaloy-2 by co mparing d ifferent tensile properties. For this analysis, four samp les with various degrees of cryorolling are taken and tensile tests are conducted on these samples. The obtained results are analyzed and the optimu m degree of cryorolling of Zircaloy-2 is obtained. The cryorolling imp roved the mechanical properties of the material as the dislocations are entangled near the grain boundaries and also due to decrease in the grain size. The microstructure of the sample is analysed by optical microscope, before and after cryorolling and the grain structure analysis is done. Keywords Zircaloy-2, Cryoro lling, Entanglement of Dislocations, Dynamic Recovery, Degree of Cryorolling 1. Introduction 2. Experimental Procedure Zirconiu m has very low absorption cross-section of therma l neutrons, high hardness, and ductility and corrosion resistance. Hence its alloys are mainly used in nuclear reactors for the cladding of fuel rods. Zircloy-2 is one such alloy wh ich is main ly used in boiling water reactors (BW R). In the recent past, water reactors of h igher capacity are being developed .In the late 1990s GE Hitachi (GEH) and Toshiba has produced advanced boiling water reactor (ABWR). The standard ABWR plant design has a net output of about 1350 MWe (3926 MWth). Various tests are being conducted on zircaloy-2 at such high burn-up, and while the zircaloy-2 cladding has had a very good track record of safe use in nuclear reactors, the material becomes susceptible to failure over long times disowning to its strength aspects for the above ABWRs at such high burn-up. As a result, fuel rods are often taken out of service even though they may have a substantial amount of fuel remaining to produce energy . So methods which increase the strength of zircaloy-2 without decreasing its ductility and corrosion resistance are being explored. Cryorolling, deformation at cryogenic temperature is proved to be effective method for increasing the yield strength and tensile strength for various Al alloys . So this technique is imp lemented on zircaloy-2. Also optimu m degree of cryorolling for zircaloy-2 is also found in this in v es tig atio n . * Corresponding author: firstname.lastname@example.org (P. Aditya Rama Kamalanath) Published online at https://www.eduzhai.net Copyright © 2012 Scientific & Academic Publishing. All Rights Reserved Process of cryorolling: The samples are dipped in LN2(liquid nitrogen) for 10min before first pass and 2 min for each pass, sample was found to attain nearly -1600 C. The process is controlled by microprocessors in order to avoid thermal shocks and also damage to the components. Here in cryorolling as the material cools its mo lecular structure contracts and hence there is entanglement of dislocations near the grain boundaries . The samples are cryorolled up to three degrees of ro lling (leaving the annealed sample) .One up to 20%; another to 50%: and the last one up to 70% of cryorolling. The standard tensile rectangular flat specimens are prepared according to ASTM E8 for the four samples . Then the material is tested on Instron model 1185 Screw driven Universal Testing Machine and the testing data is supervised by blue-hill software to get the required data of the material. Table 1. The specifications of the samples used for the tensile testing Sample Annealed 20% cryorolled 50% cryorolled 70% cryorolled T hickness (mm) 4.02 3.69 1.99 1.31 Width(mm) 4.36 Cross-sectional area(mm2) 17.53 4.12 15.20 4.11 8.19 4.08 5.34 Gauge length(mm) 10 10 10 10 Initially when the sample is loaded on the upper grip there won’t be any load on the samp le but that recorded on the blue hill window is that of the load due to the grip. It is therefore 139 American Journal of M aterials Science 2012, 2(5): 138-141 set to zero in order to balance that load. The sample is then fixed to the lower grip. The test is then started with a strain rate of 10-3 s-1. The crosshead speed i.e. the speed by which the crosshead moves is determined fro m the strain rate. Strain rate = crosshead speed/gauge length which gives the crosshead speed as 0.01 mm/s. The test is started and the load- elongation data along with its graph are obtained which are further processed in order to evaluate the mechanical p roperties . Then graphs are simulated using the data obtained for both annealed sample and cryorolled samp le using ORIGIN PRO s o ftware. 3. Results and Discussion On co mparing the Engineering stress-strain curves for the four samples . So this makes the machine to deform elastically. Therefore in the true stress –strain cure it includes even the elasticity of the machine wh ich is to be corrected. The slope of the elastic region gives the combined modulus of elasticity. This divided by the instantaneous stress gives the strain in e lastic region wh ich is subtracted fro m the true strain to get the corrected true strain and similarly corrected true stress was found out and the graphs were deduced. 3.2. Strain Hardening Curve The strain hardening exponent (n) depicts the relat ive amount a meta l strain hardens before undergoing fracture. It was found by fitting a curve of the form y=a + b xc as σ=σ0+Kεn to the corrected True stress and strain curve and getting the value of c. The curve fitted plots are indicated by the red colour line obtained Figure 1. The engineering stress-strain curves of the four samples compared Figure 2. The flow curve fitted to the corrected true stress-strain curve of annealed sample This shows that the Yield stress and the Ultimate tensile stress of the sample increase with the % of cryorolling. 3.1. The Values Obtaine d fr om the Gr aph: Table 2. The Yield stress, Ultimate Tensile stress and the elongation of the four cryorolled samples Yield st ress Ult imate Tensile st ress eu Annealed sample 381 MP a 523.26 Mpa 20% cryorolled sample 496.5 Mpa 609.5 Mpa 50% cryorolled sample 668.9 Mpa 753.5 Mpa 70% cryorolled sample 732.3 Mpa 795.9 Mpa 0.23 0.106 0.083 0.07 The true stress-strain curves have been drawn fro m the Engineering stress-strain curves up to the onset of necking. When the specimen is loaded in the testing mach ine the load is transmitted to the specimen as well as to the machine. Figure 3. The flow curve fitted to the corrected true stress-strain curve of 70% cryorolled sample It is found that the value of n is found to decrease from the annealed sample to 70% cryorolled samp le indicating that the mean free path of the dislocations has decreased with cryorolling due to their increasing density by the previously produced dislocations and newly produced one . P. Aditya Rama Kamalanath et al.: Tensile Behavior of Cryorolled Zircaloy-2 140 Table 3. The values of st rain hardening exponent of the four samples Annealed 20% 50% 70% Sample cryorolled cryorolled cryorolled n 0.28 0.04 0.019 0.004 3.3. Work Hardening These curves are drawn between θ (dσ/dε) and the corresponding corrected true stress (σ) and true strain (ε) and the Work hardening behaviour of the specimen is indicated with the help of these curves. More the steepness of the curve, more being the recovery of the dislocations. They are obtained as: Fi gure 6. Opt ical microscope Image of 70% cryorolled sample There is a noteworthy decrease in the grain size fro m annealed sample to 70% cryorolled sample i.e. with the increased amount of rolling . 4. Conclusions We observe that with the increasing amount of cryorolling there is a significant increase in the Y.S and U.T.S at the cost of its ductility. An optimu m degree of cryorolling is obtained between 20%-50% o f cryorolling Figure 4. T he work hardening curves of the four samples compared These curves show that there is a decrease in the dynamic recovery pace with the % of cryorolling . 3.4. Microstructure Analysis The microstructures of annealed sample and that of the 70%cryorolled are observed under the microscope after undergoing a set of metallographic polishing operations. Both the samples are cut and mounted using a Bakelite resin and the moulds were polished on Sic papers of grade fro m 800 to 2400 papers on STRUERS polisher. Final polishing is done with help of 3 µm and1 µm diamond part icles suspended in solution. The samples are then etched in a chemical solution containing 45ml HNO3+45ml H2O+.10ml HF as the etchant . Figure 5. Optical microscope Image of Annealed sample Fi gure 7. The Y.S, U.T.S, % elongat ion of the four samples compared Due to cryorolling, we get a) Fine grain size and b) More dislocation density c) Suppression of dynamic recovery. a) Fi ne grain size: Normally fo r annealed sample the dislocations are present within the grain and the grain boundaries. When some stress is applied, the dislocations move along one grain to another. In this process, when it comes through another grain, it encounters a barrier due to the misorientation of the crystallographic texture fro m one grain to another. Thus some addit ional force is required to move the dislocations across the barrier. No w due to cryorolling, since the grain size is reduced, there is an increase in the number of grains and overall grain boundary and therefore the size of the overall barriers for the dislocations increases and more force is required for the 141 American Journal of M aterials Science 2012, 2(5): 138-141 dislocations to cross the barrier which in turn increases the strength of the material. b) More dislocati on density: Due to rolling, quite a large number o f d islocations are produced. These dislocations get entangled between the grain boundary which impedes their motion and the strength gets increased. With the increasing extent of cryoro lling, more amount of dislocations get piled up within the grain boundaries and the sample starts to fracture after quite so me t ime with increasing stress. Thus the ductility gets decreased with the extent of cryorolling at the cost of its strength. c) Suppression of dynamic recovery: There is suppression of dynamic recovery as in cryogenic temperature, the total internal energy of the ato ms decreases as it is a function of te mperature of the materia l. So the atoms kinetic energy decreases which results in the suppression of dynamic recovery . Thus cryorolling has been found effective in increasing the mechanical properties of the Zircaloy-2 samp le if the optimu m amount of rolling is been chosen to have both enough strength and ductility. It is found that the rolling ductility decreases steeply fro m 10-20% of rolling and after which much decrease has not been observed. Thus depending on the ductility and strength preferred the degree of rolling is to be chosen. Due to the very fine grain size obtained there has been only a petite decrease in ductility with the increase in strength. 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