N. Saibaba

Mechanical properties and microstructural evolution of ultrafine grained zircaloy-4 processed through multiaxial forging at cryogenic temperature

Abstract The mechanical properties and microstructural evolution of zircaloy-4 subjected to cumulative strains of 1.48, 2.96, 4.44 and 5.91 through multiaxial forging (MAF) at cryogenic temperature (77 K) were investigated. The mechanical properties of the MAF treated alloy were measured through universal tensile testing and Vickers hardness testing equipment. The zircaloy-4 deformed up to a cumulative strain of 5.91 showed improvement in both ultimate tensile strength and hardness from 474 MPa to 717 MPa and from HV 190 to HV 238, respectively, as compared with the as-received alloy. However, there was a noticeable decrement in ductility (from 18% to 3.5%) due to the low strain hardening ability of deformed zircaloy-4. The improvement in strength and hardness of the deformed alloy is attributed to the grain size effect and higher dislocation density generated during multiaxial forging. The microstructural evolutions of deformed samples were characterized by optical microscopy and transmission electron microscopy (TEM). The evolved microstructure at a cumulative strain of 5.91 obtained after MAF up to 12 cycles depicted the formation of ultrafine grains with an average size of 150–250 nm.

Microstructural Studies of Heat Treated Zr-2.5Nb Alloy for Pressure Tube Applications

Zr-2.5Nb alloy is used as pressure tube material in pressurized heavy water reactors (PHWR). Generally, these pressure tubes are used in the cold drawn condition. Heat treated Zr-2.5Nb alloy pressure tubes are used in reaktor bolshoy moshchnosti kanalniy and FUGEN type of reactors. In recent times, there has been a greater interest toward increasing the life of pressure tubes in advanced reactors. In the present work, fabrication parameters were optimized to manufacture heat treated Zr-2.5Nb alloy tube. A quenching dilatometer study was performed to establish the continuous cooling transus temperature for the alloy used in this study. Heat treatment under controlled condition in a dilatometer was performed to study microstructure at different soaking temperatures and cooling rates. In the dilatometer, during gas quenching, quenching rates were varied from 0.06 to 100°C/s to assess the effect of cooling rate on resulting microstructures. Soaking temperature and cooling rates were varied to obtain martensitic microstructure with appropriate volume fractions of primary α. On the basis of the results obtained during controlled heat treatments performed in the quenching dilatometer, 883°C was selected as the soaking temperature and water as the quenching medium for the α + β quenching operation for large dimension tubes. The α + β quenched microstructures, consisting of fine martensite phase along with 20%–25% primary α volume fraction, were used for further cold deformation and subsequent aging below the recrystallization temperature. Aging at 540°C produced fully recovered and tempered structure consisting of βNb of equilibrium composition. Mechanical properties of the finished heat treated pressure tube (aged at 515°C/24 h) produced with the present route were similar to the cold work pressure tube. Tube produced with 540°C/24 h aging exhibited substantially higher yield strength value at reactor operating temperature (300°C). The bulk texture at different stages of fabrication was evaluated. The volume fraction of the primary α phase significantly controls the final texture.

Evaluating Fracture Toughness of Rolled Zircaloy-2 at Different Temperatures Using XFEM

Fracture toughness and mechanical properties of the zircaloy-2 processed by rolling at different temperatures have been investigated, and simulations have been performed using extended finite element method (XFEM). The solutionized alloy was rolled at different temperatures for different thickness reductions (25–85%). Fracture toughness has been investigated by compact tension test. The improved fracture toughness of the rolled zircaloy-2 samples is due to high dislocation density. SEM image of the fractured surface shows the reduction in dimple sizes with the increase in dislocation density due to the formation of microvoids as a result of severe strain induced during rolling. Compact tension test, edge crack, center crack and three-point bend specimen simulations have been performed by XFEM. In XFEM, the cracks are not a part of finite element mesh and are modeled by adding enrichment function in the standard finite element displacement approximation. The XFEM results obtained for compact tension test have been found to be in good agreement with the experiment.

Influence of Sn on Deformation Mechanisms During Room Temperature Compression of Binary Zr–Sn Alloys

Role of Sn on the deformation mechanisms of Zr was investigated using in situ neutron diffraction and complementary electron microscopy techniques. Binary Zr-Sn alloys having fully recrystallized microstructure and typical rolling texture were subjected to in situ loading and diffraction experiments along the rolling direction of the sample. Significant twinning activity was observed and the twins were observed to be {101 ̅2}〈101 ̅1〉 type tensile twins. Critical stress for the twin nucleation and the extent of twinning were found to be strongly influenced by the Sn content. Critical plastic strain for the nucleation of twining, however, was observed to be weakly dependent on the Sn content. Results indicate significant plastic slip activity to be a necessary condition for the onset of twinning. http://mc04.manuscriptcentral.com/astm-stp STP: Selected Technical Papers

Microstructural Studies of Zr-2.5Nb and Zircaloy-2 Pressure Tubes Irradiated in Indian Pressurized Heavy Water Reactors

The Indian PHWR uses Zr-2.5% Nb pressure tubes and its in-reactor performance mainly irradiation creep and growth depends strongly on its microstructure. A detailed microstructural examination was carried out on unirradiated pressure tubes off-cuts and an irradiated pressure tube S-07 of KAPS-2 (operated for 8 effective full power years (EFPYs)), Microstructural characterization was carried out using transmission electron microscopy. Microstructual observation of un-irradiated off-cuts shows the lamellar morphology of the -Zr along with the -phase present as stingers between two alpha laths as well as fine and coarse beta globules. The size of -Zr lamellae was found to be in the range from 0.17 to 0.2 m, 1.8 to 2.4 m and 1.7 to 2.8 m in the radial, circumferential and axial direction respectively (aspect ratio of 1:7:8). TEM-EDS analysis showed composition of the  phase tin the range of 15-50 wt%Nb. The irradiated pressure tube samples obtained from 13 locations were showing average alpha grain width, grain length and aspect ratio in the range of 0.17-0.27 micron, 1.7-2.3 micron and 7.1-8.5 respectively. Extensive modification in beta morphology could be seen at the high flux and high temperature regions. The  phase was observed to have globulised completely in many regions. They were present at the interface of -Zr laths as well as within the lath. The Nb concentration of the  phase appeared to have increased as the volume fraction had reduced. The microstructure details of irradiated and un-irradiated pressure tubes obtained in this study is expected to help in modeling the dimensional change occurring during irradiation in reactor.

Texture and Microstructure Development during Swaging and Annealing Process of Fabrication of Zircaloy-4 Rod Products

In this study a systematic characterization of the microstructural and textural evolution in each thermo-mechanical processing step of Zircaloy-4 rod fabrication has been carried out. The possible micro-mechanisms leading to the observed microstructural evolutions have been discussed. The thermo-mechanical steps followed resulted in a completely recrystallized microstructure and retention of the hot-extruded texture in the finished product.

Role of activation energies of individual phases in two-phase range on constitutive equation of Zr–2.5Nb–0.5Cu alloy

Abstract Dominant phase during hot deformation in the two-phase region of Zr–2.5Nb–0.5Cu (ZNC) alloy was studied using activation energy calculation of individual phases. Thermo-mechanical compression tests were performed on a two-phase ZNC alloy in the temperature range of 700–925 °C and strain rate range of 10 −2 –10 s −1 . Flow stress data of the single phase were extrapolated in the two-phase range to calculate flow stress data of individual phases. Activation energies of individual phases were then calculated using calculated flow stress data in the two-phase range. Comparison of activation energies revealed that α phase is the dominant phase (deformation controlling phase) in the two-phase range. Constitutive equations were also developed on the basis of the deformation temperature range (or according to phases present) using a sine-hyperbolic type constitutive equation. The statistical analysis revealed that the constitutive equation developed for a particular phase showed good agreement with the experimental results in terms of correlation coefficient ( R ) and average absolute relative error (AARE).

Hot Deformation Behaviour and Microstructural Evaluation of Zr-1Nb Alloy

In nuclear water reactors, zirconium alloys are extensively used as fuel cladding material and in other structural applications. Uniaxial hot compression tests were performed to understand the deformation behavior of Zr-1Nb alloy. Therefore, hot compression tests were performed in the temperature range of 700-1050°C, which envelopes α-phase, (α+β) phase, and β-phase. True stress-strain curves, processing maps, microstructural observation and kinetic analysis were used to discuss the deformation behavior of Zr-1Nb alloy. Deformation at a strain rate of 10-2 s-1 reveals softening at lower temperatures and steady state behavior at higher temperatures. Processing map also reveals domain of high efficiency at 10-2 s-1 strain rate for a wide range of deformation temperatures. The flow softening and high power dissipation efficiency predicts dynamic recrystallization or dynamic recovery during the hot deformation of studied alloy.

Determination of Instability in Zr-2.5Nb-0.5Cu Using Lyapunov Function

Hot workability of Zr-2.5Nb-0.5Cu alloy has been investigated by means of hot compression test using Gleeble-3800®, in the temperature and strain rate range of 700 to 925°C and 0.01-10s-1, respectively. Deformation behavior was characterized in terms of flow instability using peak stress with the help of Lyapunov Function. The true stress-strain curves shows that softening occurs at all lower temperature and for entire strain rates of deformation. The instable flow was suggested by negative m value at deformation condition of 700°C (5 and 10 s-1), while s value at 925°C (10 s-1). The combined result of rate of change of m and s with respect to log strain rate suggest that the deformation condition ranges from 725-780°C (10-2- 10-1 s-1) and 700°C (1-10 s-1) representing safe domain for stable flow.

Experimental evaluation of mechanical properties and fracture-fatigue simulation of cryo– and room–temperature–rolled zircaloy–2

In this study, the mechanical properties of mercury–quenched, cryorolled (CR) and room–temperature–rolled (RTR) zircaloy–2 have been examined by performing the tensile and hardness tests. The effect of cryorolling and room temperature rolling on the fracture properties has been evaluated by performing two–dimensional (2D) quasi–static crack growth simulations using finite element approach under plane stress condition. J–integral and internal energy of the mercury–quenched, CR and RTR zircalloy–2 are evaluated and compared with each other. The S–N curves for mercury–quenched, CR and RTR zircaloy–2 are obtained through finite element simulations. After performing the fracture and fatigue simulations, it is found that 85% cryorolled zircaloy–2 possesses better fracture and fatigue behaviour when compared with mercury–quenched and 85% RTR zircaloy–2.