MECHANICAL BEHAVIOUR OF UFG ZIRCALOY-2 PROCESSED BY CRYOROLLING: EXPERIMENTS & SIMULATION

Abstract

Zircaloy-2 is used for fabricating core components in nuclear power reactors. The alloy in the form of tubes i) 60 mm diameter with a wall thickness of 1 to 2 mm is used as Calandria tubes in Pressurised Heavy Water Reactor (PWR). ii) Less than 30 mm diameter is used as a fuel cladding in Boiling Water Reactor (BWR). These components are exposed to temperature of 400oC to the inner surface and outer surface temperature of 280 to 350oC. Almost 90% of the zirconium produced is used in nuclear plants and it cannot be recycled due to high cost factor. It is very essential to improve the mechanical and corrosion resistance properties of the Zr alloys, for enhancing service life of the structural components, through new thermo mechanical processing routes without altering its chemical composition. Ultrafine and nanostructured bulk materials produced by SPD techniques exhibit enhanced mechanical and functional properties as compared to bulk alloys as reported in the literature. The reduced grain size enhances the yield strength as per Hall-Petch relation. Strengthening is also possible by other structural features such as dislocations, subgrain boundaries, nanotwins and solute atoms. The sum of all these structural features are responsible for strengthening mechanisms. . The performance of structural components in nuclear power reactors depends on tensile, creep and corrosion resistance properties. These properties are heavily influenced by microstructure of the material fabricated through different thermo mechanical treatments. SPD processing along with apt post heat treatment could produce multimodal microstructures in the materials, which can provide simultaneous improvement in strength and ductility. Cryorolling is one of the novel deformation processing techniques used widely to produce ultrafine and nanostructures in the pure metals and alloys. In this technique, dynamic recovery is suppressed to accumulate high density of dislocation in the materials during processing at liquid nitrogen temperature. The literature on the influence of cryorolling and cross rolling on the mechanical behaviour of zircaloy-2 is limited. Therefore, the present work has been focused on producing ultrafine and nanocrystalline zircaloy-2, with improved mechanical properties, from its coarse grained alloy using the various thermomechanical processing such as cryorolling, room temperature rolling, room temperature cross rolling and cryo cross rolling. The objectives of the present work was to investigate (i) Mechanical behaviour and microstructural characteristics of ultrafine grained zircaloy-2 processed by cryorolling; (ii) ii Mechanical and microstructural evolution of ultrafine grained zircaloy-2 produced by room temperature rolling; (iii) Texture and mechanical behavior of zircaloy-2 rolled at different temperature; (iv) Development of ultrafine grained zircaloy-2 by room temperature cross rolling and cryo cross rolling; (v) Microstructure and mechanical behavior of room temperature rolled zircaloy-2 after water and mercury quenching; (vi) Experimental evaluation of mechanical properties and fracture-fatigue simulation of cryo and room temperature rolled zircaloy-2; (vii) Experimental and simulation study on the fracture toughness of zircaloy-2 processed by rolling and cross rolling at different temperatures A chapter wise summary of the thesis is given below. Chapter 1 highlights a brief introduction to the material used in the present investigation, deformation mechanism of the material used, various SPD processes used to produce UFG materials, applications of UFG materials in various fields and pros and cons associated with the SPD techniques. The literature relevant to the present research work on ultrafine grained zircaloy-2 is critically reviewed in Chapter 2. A brief description of the properties and application of zirconium alloys as well as the effect of alloying elements on mechanical and corrosion behaviour are made. Deformation mechanism and mechanical properties of UFG Zr and its alloy is reviewed. It forms a strong basis to formulate the key objectives pertaining to the development of ultrafine grained zircaloy-2 with improved strength and ductility as compared to their commercially available bulk counterpart. All the related experimental techniques and procedures employed in the present work are outlined in the Chapter 3. The methodology for characterizations and mechanical behaviour of UFG materials are discussed. The simulation is performed by using FEM and XFEM software package Abaqus. In Chapter 4, Section 4.1 describes the following experimental investigations performed on zircaloy-2. The mechanical properties and microstructural characteristics of ultrafine grained zircaloy-2 processed by cryorolling (CR) were investigated. The solutionised zircaloy-2 was rolled at liquid nitrogen temperature (77K) with different thickness reductions (25% to 85%). The dislocation density ‹ρ› in the cryorolled zircaloy-2 increases with increasing true strain due to the suppression of dynamic recovery. The CR 85% alloy showed hardness and yield strength values of 282 HV and 891 MPa, respectively. The annealed CR 85% alloy showed higher ductility (9.5% and 11.2%) in rolling and transverse direction, respectively, as compared to CR 85% alloy. iii In section 4.2, the effect of deformation strain at room temperature on the microstructural and mechanical properties of zircaloy-2 is discussed. The deformed alloy reveals the misorientation of incidental grain boundaries (IDBs) due to large plastic strain induced in the sample. The hardness of the alloy after 85% room temperature rolling (RTR) is found to be 269 HV, while the tensile strength is 679MPa and 697 MPa, in the rolling and transverse direction, respectively. The deformed alloy subjected to annealing at 400o C for 30 minutes sample shows increase in ductility (6% and 7.2% in rolling and transverse direction respectively) due to the annihilation of dislocations as evident from the TEM study. In section 4.3, the microstructural characteristics, texture, and mechanical properties of the zircaloy-2 processed by CR and RTR are described. Texture results show the activation of basal slip at higher strains in room temperature rolled zircaloy-2. In cryorolled zircaloy-2, only activation of prism slip is observed. Grain refinement, sub-structures, and texture in the deformed alloy contribute for the improvement in mechanical properties. Effect of change in strain path by cross rolling at 300 K up to a true strain of 1.89 and at 77K upto 0.69 true strain has been studied and reported in section 4.4. The texture and mechanical properties of cross rolled zircaloy-2 at 77K and 300 K temperature are discussed. The fragmentation of near basal grains due to change in strain path is evident from the EBSD micrographs. 1012 extension twins are observed initially up to 25% reduction, with the further reduction in thickness, near basal grains are oriented towards the normal direction. These basal grains are fragmentated due to changes in strain path upon room temperature cross-rolling as observed from KAM and EBSD images. TEM results of the room temperature cross rolled sample confirm the formation of ultrafine and nanograins in the alloy due to orientation of incidental dislocation boundaries (IDB) in the direction of macroscopic plastic flow and postannealing treatment of the deformed alloy. A tensile strength of 991 MPa with 7.5% ductility is observed in the 85% room temperature cross rolled alloy. Annealing at 673K for 30 minutes recrystallizes the deformed microstructure and forms ultrafine grains. Evolution of deformed microstructure occurs by the activation of prismatic slip, 1122 contraction and 1012 extension twin at 77K. Probability of basal slip <a> at 77K is also observed by Taylor and Schmid factor analysis. The deformation at 300K occurs by prismatic, basal <a> and Pyramidal <c+a> slip predicted by texture images. Stored energy and dislocation density in the deformed alloy is calculated with the help of KAM. iv In section 4.5, the effect of water and mercury quenching on the microstructural and mechanical behavior of room temperature rolled zircaloy-2 is discussed. Solution treatment of zircaloy-2 at 1073K, followed by quenching in mercury and water has been made prior to rolling. Different reduction from 25% to 85% of the quenched alloy was given for characterizing their microstructures and mechanical properties. Rolling reduction accumulates high dislocation density inside the materials, thereby enhancing the mechanical strength. Initial deformation has occurred by the activation of extension twinning as can be seen from EBSD microstructure. By optimizing the annealing temperature (400o C for 30 minutes), ultrafine grains are produced in 85% room temperature rolled zircaloy-2. The mechanical properties of ultrafine grained zircaloy- 2 is found to be better than the coarse grained zircaloy-2. Chapter 5 describes the results of compact tension test, FEM and XFEM simulation performed for zircaloy-2. Section 5.1 describes Finite element method used to study the 2-D quasi static crack analysis. J-integral and internal energy of the processed alloys are evaluated and compared with each other. Fatigue simulations are performed by using ANSYS software to find the S-N curve of mercury quenched, CR and RTR zircaloy-2. In section 5.2, the fracture toughness of zircaloy-2 processed under rolling conditions is discussed. The rolled sample shows better crack arresting capabilities due to larger back stress produced during loading. Xtended finite element simulation is performed by using Abaqus software package to evaluate the fracture toughness of the alloy. CT results obtained after XFEM simulation show a good match for undeformed samples (MQ), while for severely deformed samples, the values obtained are less because dislocations are not modeled in the simulation. The conclusions of the present investigation and scope of the future work are reported in