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Role and effectiveness of plastic deformation and stress relief in zirconium: origin of residual stress
(Indian Institute of Technology Bombay, 2014-05) Kumar, Gulshan
Industrially produced Zircaloy-4 clads were subjected to pressurized water in standard closed end burst test and shown to possess different ductilities. The latter was not determined by microstructure and crystallographic texture: but was shown to have a clear scaling with hydrostatic component (Ph) of the measured tri-axial residual stress matrix. The study then tried to bring out the origin of residual stresses during plastic deformation and stress relief annealing. Zircaloy-4 sheets were cold rolled to different percentages of deformation 20%, 40% and 60% and were well electro polished before undergoing measurement in X-ray diffraction and electron backscattered diffraction (EBSD) for measuring residual stress and plastic strain. A clear gradient of elastic and plastic strains were predicted through finite clement simulation. Such predictions were verified against experimental data on microtexture and bulk crystallographic texture. Microfocused X-ray also provided clear distinction in residual stress developments between different features of the deformed microstructures. For example deformed Zirconium grains were classified as fragmenting and non-fragmenting. The latter, mostly basal, had strongest signatures of residual stresses. During recovery, microstructure dependent stress-relief was established. Initial recovery or stress relief was primarily through reductions in orientation gradients in non-fragmenting grains. Only during the latter stages, recrystallization and stronger orientation sensitive stress-relief was noted for the fragmenting grains
Orientation dependent mechanical properties of commercially pure (cp) titanium
(2014-12) Kumar, Gulshan
The present investigation is an attempt at correlating the crystallographic orientation and mechanical properties of hexagonal commercially pure titanium (cp-titanium). Annealed cp-titanium sheets are subjected to tensile deformation along the rolling direction, along 45° to the rolling direction and along 90° to the rolling direction respectively. Crystallographic textures and mechanical properties of these cp-titanium samples are investigated in the present study. The hardness of different grains/orientations is estimated through nanoindentation, grain average misorientation, orientation estimated elastic stiffness and Taylor factor measurements. It is observed that the hardness of the grains close to basal orientation is higher compared to non-basal orientations. It is further observed that the estimated bulk mechanical properties of cp-titanium have a direct relationship with the volume fraction of basal grains/orientations.
Influence of non-covalent modification of multiwalled carbon nanotubes on the crystallization behaviour of binary blends of polypropylene and polyamide 6
(RSC, 2014-12) Kumar, Gulshan
Blends of polypropylene (PP) and polyamide 6 (PA6) with multiwalled carbon nanotubes (MWNTs) were prepared using different processing strategies in a twin-screw micro-compounder. The effect of MWNTs on the crystallization behaviour of the PP phase and the PA6 phase of the blend has been investigated through non-isothermal crystallization studies by differential scanning calorimetric analysis. Furthermore, the effect of the addition of the compatibilizer (PP-g-MA) and the modification of MWNTs (m-MWNTs) with a non-covalent organic modifier (Li-salt of 6 amino hexanoic acid, Li–AHA) has also been studied in context to the crystallization behaviour of the PP and PA6 phase in the blend. The crystallization studies have indicated a significant increase in bulk crystallization temperature of the PP phase in the blend in the presence of MWNTs. Moreover, the formation of ‘trans-lamellar crystalline’ structure consisting of PA6 ‘trans-crystalline lamellae’ on MWNTs surface was facilitated in the case of blends prepared via ‘protocol 2’ as compared to the corresponding blends prepared via ‘protocol 1’. Wide angle X-ray diffraction analysis has showed the existence of a β-polymorph of the PP phase due to incorporation of the PA6 phase in the blend. Addition of MWNTs in the blends has facilitated further β-crystalline structure formation of the PP phase. In the presence of m-MWNTs, a higher β-fraction was observed in the PP phase as compared to the blend with pristine MWNTs. Addition of PP-g-MA has suppressed the β-phase formation in the PP phase in the blend. X-ray bulk texture analysis revealed that incorporation of PA6 as well as pristine/modified MWNTs has influenced the extent of orientation of the PP chains towards specific crystalline planes in various blend compositions of PP and PA6.
Texture development and plastic deformation in a pilgered Zircaloy-4 tube
(Springer, 2015-02) Kumar, Gulshan
The development of microstructure and crystallographic texture with effective strain at three through-thickness locations (near rolls, center, and near mandrel) in a partly pilgered Zircaloy-4 tube is described. Pilgering is found to eliminate through-thickness variation in grain size in the starting hot-extruded material and to generate location-dependent asymmetries in crystallographic texture. Deformation texture development during pilgering is modeled with polycrystal plasticity by idealizing the metal flow pattern as axisymmetric flow through a convergent channel. Good qualitative comparison of the predicted and experimental post-pilgering textures is obtained, provided location-dependent transverse shear component is superposed on the gross flow field, and localized deformation at grain boundaries is allowed. Frictional forces between tube and die are deduced from these observations.
Defining the stages of annealing in a moderately deformed commercial Zirconium alloy
(Elsevier, 2015-11) Kumar, Gulshan
Fully recrystallized Zircaloy-4 was cold rolled to 20% reduction in thickness. The deformed microstructure had fragmented and non-fragmented grains. Fragmentation represented deformation-induced refinement in grain size. Typically, the fragmented grains had more misorientation and were finer than the as-received grains. The deformed samples were subjected to 650°C annealing for different time periods, followed by water quenching. Based on experimental observations, three distinct stages of annealing were noted. Stage I caused changes in the misorientations of the non-fragmented grains, while the fragmented regions remained unaffected. This was also the most effective stage for residual stress relief. In stage II, discontinuous recrystallization and grain coarsening consumed the fragmented regions. This stage provided the highest softening. Finally, stage III created recovery-induced grain refinement of the larger non-fragmented grains. A combination of indirect and direct observations thus provided a complete picture of the annealing related microstructural changes in a moderately deformed commercial Zirconium alloy.
Effect of pre-annealing strains on annealing texture developments in commercially pure (CP) titanium
(Taylor & Francis, 2015-03) Kumar, Gulshan
Hexagonal commercially pure titanium (cp-titanium) plates were subjected to unidirectional-rolling (rolling), accumulative roll-bonding (ARB) and cross-rolling in a laboratory rolling mill. Rolling and cross-rolling were carried out to impart 90% reduction in thickness and ARB processing was performed for six passes. The deformed plates were then subjected to annealing at 600 °C for a large range of soaking time starting from 0.17 min (10 s) to 30 min. It was observed that the samples were fully recrystallized after 5 min of annealing, irrespective of the rolling processes employed in this study. Also, the samples were seen to develop almost similar texture when annealing was carried out beyond 5 min of annealing time. However, before annealing, the texture development was seen to be different in the respective samples subjected to different rolling processes. The initial texture present in the deformed structure got strengthened during annealing of the samples under investigation. It was also observed that the texture development was insignificant in ARB-processed samples after annealing.
Zirconium microstructures: uncharted possibilities
(Bhabha Atomic Research Centre, 2015-07) Kumar, Gulshan
The 'conventional' Zirconium microstructures can be significantly extended with information on: (i) microtexture, (ii) residual stresses and (iii) local mechanical properties. Though these involve different tools, but a consolidated microstructure can be crated. This is the theme of this presentation. Examples of this consolidated picture will be made from deformation twinning, recovery-recrystallization, burst ductility and orientation versus solid solution hardening.
Characterization of heavy ion induced defects in Zr-2.5 wt. % Nb by grazing incidence X-ray diffraction
(Bhabha Atomic Research Centre, 2015-07) Kumar, Gulshan
Zr-2.5 wt. % Nb alloy is used as a pressure tube material in pressurized heavy water reactor (PHWR). It is one of the most critical component which decides the life of the reactor. The in-reactor degrading phenomenon of prime concern is dimensional changes caused by irradiation induced creep and growth processes. The present study aims to understand the mechanism of irradiation damage by irradiating the alloy with heavy ion. Heavily charged Ar9+ ions have been used, as it leads to dense cascade similar to those observed in neutron beam damage. Such type of irradiation study would facilitate larger damage of material in a shorter time. Zr-2.5 %Nb alloy samples were irradiated using 315 keV Ar9+ ion for different durations. The irradiation doses were varied from 3.1x1015 to 4.17x1016 Ar9+/cm2. Grazing incidence X-ray diffraction (GI-XRD) is known as a direct, non-destructive, surface-sensitive technique for characterization of materials. Using GI-XRD domain size, micros train by Williamson hall method, dislocation density by line profile analysis (LPA), residual stress, and depth profiles of irradiated phase distributions were calculated. The analysis revealed that there is a significant decrease in domain size with corresponding increase in microstrain and dislocation density with irradiation dose.
Deformation twinning in zirconium: direct experimental observations and polycrystal plasticity predictions
(Springer, 2015-08) Kumar, Gulshan
Deformation twinning was directly observed in three commercial zirconium alloy samples during split channel die plane-strain compression. One pair of samples had similar starting texture but different grain size distributions, while another pair had similar grain size distribution but different starting textures. Extension twinning was found to be more sensitive to the starting texture than to the grain size distribution. Also, regions of intense deformation near grain boundaries were observed. A hierarchical binary tree-based polycrystal plasticity model, implementing the Chin-Hosford-Mendorf twinning criterion, captured the experimentally observed twinning grains’ lattice orientation distribution, and the twin volume fraction evolution, provided the critical resolved shear stress for extension twinning, was assumed much larger than any of the values reported in the literature, based on the viscoplastic self-consistent model. A comparison of the models suggests that obtained using the present model and the viscoplastic self-consistent models physically correspond to the critical stress required for twin nucleation, and twin growth, respectively.
Burst ductility of zirconium clads: the defining role of residual stress
(Springer, 2016-05) Kumar, Gulshan
Closed end burst tests, using room temperature water as pressurizing medium, were performed on a number of industrially produced zirconium (Zr) clads. A total of 31 samples were selected based on observed differences in burst ductility. The latter was represented as total circumferential elongation or TCE. The selected samples, with a range of TCE values (5 to 35 pct), did not show any correlation with mechanical properties along axial direction, microstructural parameters, crystallographic textures, and outer tube-surface normal (σ 11) and shear (τ 13) components of the residual stress matrix. TCEs, however, had a clear correlation with hydrostatic residual stress (P h), as estimated from tri-axial stress analysis on the outer tube surface. Estimated P h also scaled with measured normal stress (σ 33) at the tube cross section. An elastic–plastic finite element model with ductile damage failure criterion was developed to understand the burst mechanism of zirconium clads. Experimentally measured P h gradients were imposed on a solid element continuum finite element (FE) simulation to mimic the residual stresses present prior to pressurization. Trends in experimental TCEs were also brought out with computationally efficient shell element-based FE simulations imposing the outer tube-surface P h values. Suitable components of the residual stress matrix thus determined the burst performance of the Zr clads.

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