K.V. Mani Krishna

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.

A study of morphological and compositional evolution of nanoprecipitates in the Zr–Nb system and their transformational behavior

The metastable transformational behavior (both martensitic and omega) along with compositional and morphological evolution of bcc β precipitates, dispersed in the hcp α matrix of a Zr–1 wt% Nb alloy, were studied as a function of temperature and time. The evolution of the chemical composition of the β phase suggested preference towards metastable compositions having Nb content higher than the equilibrium value. Thermodynamic analysis showed that the metastable chemical compositions are the driving force for the nucleation of such β precipitates. The β to martensite transformation was observed to be possible only if β precipitate size exceeded a critical value of 160 nm. Micromechanical modeling was performed to estimate the critical size of β precipitate required to induce martensite transformation and the model predictions were in close agreement with the experimental observations. The omega transformation, on the other hand, showed less size dependence.

Orientation sensitivity of focused ion beam damage in pure zirconium: direct experimental observations and molecular dynamics simulations

A high-purity predominantly single crystalline zirconium was subjected to controlled focused ion beam (FIB) damage. Damage estimates were obtained from electron backscattered diffraction (EBSD) and nano-indentation measurements on exactly the same area/orientation. The damage kinetics, between different crystallographic orientations, differed by one order of magnitude and a clear hierarchy of orientation sensitive ion damage emerged. Use of a simple geometric approach, linear density of atoms and corresponding scattering cross-sections to impinging gallium ions, could differentiate between extreme damage kinetics; but failed when such differences were relatively minor. Numerically intensive molecular dynamics (MD) simulations, on the other hand, were more effective. However, MD simulations or direct EBSD observations failed to justify anisotropic irradiation hardening (AIH): 3–8 times more hardening for near basal. Though explanation for AIH is indirect, evidence and rationalization for orientation-sensitive radiation damage appears clear and statistically reproducible.

High-purity Zirconium under Niobium ion implantation: possibility of a dynamic precipitation?

High-purity (6N purity) Zirconium was subjected to different Niobium ion (Nb+) fluences in a particle accelerator. Grazing incidence X-ray diffraction confirmed subsurface phase transformation. While an approximate scaling was noted between Nb+ fluence and quantum of phase transformation, the sample subjected to the highest ion irradiation also showed significant subsurface shear residual stresses (τ13). Molecular dynamics simulations, considering momentum transfer, revealed a drop in τ13 beyond a critical displacements per atom or Nb+ fluence. High-resolution cross-sectional transmission electron microscopy (HRXTEM) confirmed formation of bcc (body-centred cubic) β phase and also linked τ13 with such transformation. HRXTEM revealed, at about 100 nm depth, presence of 10–15% β with 2–15 nm size. The β particles and the surrounding α also had significant microscopic shear strains. The dynamic nature of the Nb implantation is expected to create fluctuations in temperature, Nb concentration and relative lattice damage. Such fluctuation, on the other hand, is/was hypothesized to dynamically alter the critical nuclei size: a clear possibility of ‘dynamic precipitation’.

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

Grain boundary energy and relative ion damage: experimental observation and molecular dynamics simulation

In large-grained (>1 mm grain size), high-purity (200 ppm of oxygen as major impurity), single-phase Zirconium: a combination of thermal grooving and molecular statics (MS) enabled measurements of γGB (grain boundary energy). Controlled focused ion beam damage, with Ga+ (galium) ions, provided a clear scaling between γGB and damage kinetics. The latter was obtained through direct observations on apparent grain boundary width by high-resolution electron backscattered diffraction. MS simulations were also used to create tilt boundaries of different γGB. Molecular dynamics, on the other hand, simulated grain boundary damage through Ga+ ion implantation. Simulations, capturing the momentum transfer, reproduced a qualitatively similar trend of γGB dependence of experimental ion damage.

Orientation-Dependent Tensile Flow Behavior of Zircaloy-4 at Room Temperature

Present work describes a correlation between texture and sample orientation-based flow parameters of three different zircaloy-4 sheet materials. The three different sheet materials [slab route sheet (SRS), tube route sheet (TRS) and low-oxygen sheet (LOS)] corresponded to three different routes of fabrication and hence represented as variation in starting condition. All the three materials exhibited the presence of moderate texture. The intensity is more in TRS samples in comparison with that of the SRS and LOS. This in turn resulted in moderate values of anisotropy parameters. The strength parameters and elongation values have increased and decreased with increase in strain rate, respectively. The flow behavior of the alloys followed typical Holloman equation. The instantaneous work-hardening rate curves of the present alloys exhibited all the three typical regimes (i.e., regime I, regime II and regime III).