B. Vishwanadh

Bio: B. Vishwanadh is an academic researcher from Bhabha Atomic Research Centre. The author has contributed to research in topics: Alloy & Microstructure. The author has an hindex of 5, co-authored 10 publications receiving 53 citations.

Textural and microstructural evolutions during deformation and annealing of Nb–1% Zr–0.1% C (wt%) alloy

Abstract: Evolution of texture and microstructure in the Nb–1% Zr–0.1% C (wt%) alloy has been studied as a function of deformation and annealing treatments. The Nb alloy was deformed by rolling at room temperature up to 40%, 60% and 80% thickness reduction. Samples reduced to 60% thickness were annealed at 1300 °C for different soaking periods (0, 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 6 and 7 h). All the samples were characterised using X-ray diffraction (XRD), electron back scattered diffraction (EBSD) and analytical transmission electron microscopy (ATEM). Texture results have shown that deformed and annealed samples exhibited the development of typical bcc texture i.e., α and γ-fibre texture. With increasing percentage of deformation (>40%), γ-fibre becomes the dominant texture. Samples deformed up to 80% have predominantly {111}〈110〉 texture. Rolling textures were simulated using Taylor models. The pancake relaxed constraint Taylor model, consideration of {110}〈111〉 type of slip systems exhibited good agreement with experimentally observed rolling texture. In the case of annealed samples, with increasing soaking time, volume fraction of α-fibre remained constant and the γ-fibre volume fraction decreased up to 0.5 h soaking time, followed by it being constant. Samples annealed at 1300 °C for 7 h showed {100}〈110〉 orientation texture. ATEM results showed that with increasing annealing time at 1300 °C, Nb/Zr ratio in the carbide precipitates decreases.

Effect of Specific Energy Input on Microstructure and Mechanical Properties of Nickel-Base Intermetallic Alloy Deposited by Laser Cladding

Abstract: This article describes the microstructural features and mechanical properties of nickel-base intermetallic alloy laser-clad layers on stainless steel-316 L substrate, with specific attention on the effect of laser-specific energy input (defined as the energy required per unit of the clad mass, kJ/g) on the microstructure and properties of the clad layer, keeping the other laser-cladding parameters same. Defect-free clad layers were observed, in which various solidified zones could be distinguished: planar crystallization near the substrate/clad interface, followed by cellular and dendritic morphology towards the surface of the clad layer. The clad layers were characterized by the presence of a hard molybdenum-rich hexagonal close-packed (hcp) intermetallic Laves phase dispersed in a relatively softer face-centered cubic (fcc) gamma solid solution or a fine lamellar eutectic phase mixture of an intermetallic Laves phase and gamma solid solution. The microstructure and properties of the clad layers showed a strong correlation with the laser-specific energy input. As the specific energy input increased, the dilution of the clad layer increased and the microstructure changed from a hypereutectic structure (with a compact dispersion of characteristic primary hard intermetallic Laves phase in eutectic phase mixture) to near eutectic or hypoeutectic structure (with reduced fraction of primary hard intermetallic Laves phase) with a corresponding decrease in the clad layer hardness.

A Study on Synthesis, Crystallization, and Magnetic Behavior of Nanocrystalline Fe-Based Metallic Glasses

Abstract: In the present work, structure of the as-cast melt-spun ribbons, nonisothermal crystallization kinetics, and the effect of heat treatment on the magnetic properties have been studied. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses have revealed the presence of amorphous and partly crystalline structures in the as-cast Fe67Co18Si1B14 and Fe57Co26Cr3B14C0.2 metallic-glass ribbons, respectively. The crystalline phase present in the as-cast Fe57Co26Cr3B14C0.2 metallic-glass was identified as α-Fe. Direct transformation from liquid to α-Fe has been analyzed from a thermodynamic and kinetics point of view. The differential scanning calorimetry (DSC) studies have shown two-stage crystallization behavior. The primary and secondary crystallization phases were identified as bcc-Fe(Co) and bct-(Fe,Co)3(Si,B), respectively. Kissinger and Gao et al. methods were employed for nonisothermal crystallization kinetic studies. The activation-energy values obtained by the two models were in good agreement. The nucleation and growth morphologies of crystalline phases have been explained on the basis of the Avrami exponent, which were found to be consistent with the observed microstructures. The magnetic properties of as-cast amorphous ribbons showed low coercivity, and this has been attributed to averaging of magnetocrystalline anisotropy over grains coupled within an exchange length, i.e., based on a random anisotropy model. The influence of microstructure on magnetic properties was studied by crystallizing the amorphous phase at 400 °C for 3 hours. The saturation magnetization and coercivity had increased after crystallization for both alloys.

The Mechanism of Atmospheric Rusting and the Effect of Cu and P on the Rust Formation of Low Alloy Steels

Abstract: The oxidation processes of Fe(II) hydroxo-complexes to α-, β-, γ-, and δ-FeOOH and Fe3O4, which are important atmospheric products of steels, and the effect of Cu2+, PO43− ion on the oxidation of the Fe(II) hydroxo-complexes in aqueous solutions have been investigated. The mechanism of atmospheric rusting deduced from the results obtained in the present investigation has been used to explain the difference in behaviour between ordinary mild steels and low alloy steels during atmospheric exposure. It is concluded that Fe(II) complexes are transformed to amorphous δ-FeOOH by the catalytic effect of Cu and P present in steels. The amorphous δ-FeOOH forms a compact rust layer that enhances corrosion resistance of the steel.

A dislocation density based crystal plasticity finite element model: Application to a two-phase polycrystalline HCP/BCC composites

Abstract: We present a multiscale model for anisotropic, elasto-plastic, rate- and temperature-sensitive deformation of polycrystalline aggregates to large plastic strains. The model accounts for a dislocation-based hardening law for multiple slip modes and links a single-crystal to a polycrystalline response using a crystal plasticity finite element based homogenization. It is capable of predicting local stress and strain fields based on evolving microstructure including the explicit evolution of dislocation density and crystallographic grain reorientation. We apply the model to simulate monotonic mechanical response of a hexagonal close-packed metal, zirconium (Zr), and a body-centered cubic metal, niobium (Nb), and study the texture evolution and deformation mechanisms in a two-phase Zr/Nb layered composite under severe plastic deformation. The model predicts well the texture in both co-deforming phases to very large plastic strains. In addition, it offers insights into the active slip systems underlying texture evolution, indicating that the observed textures develop by a combination of prismatic, pyramidal, and anomalous basal slip in Zr and primarily {110}〈111〉 slip and secondly {112}〈111〉 slip in Nb.

Silicide Coating Fabricated by HAPC/SAPS Combination to Protect Niobium Alloy from Oxidation

Abstract: A combined silicide coating, including inner NbSi2 layer and outer MoSi2 layer, was fabricated through a two-step method. The NbSi2 was deposited on niobium alloy by halide activated pack cementation (HAPC) in the first step. Then, supersonic atmospheric plasma spray (SAPS) was applied to obtain the outer MoSi2 layer, forming a combined silicide coating. Results show that the combined coating possessed a compact structure. The phase constitution of the combined coating prepared by HAPC and SAPS was NbSi2 and MoSi2, respectively. The adhesion strength of the combined coating increased nearly two times than that for single sprayed coating, attributing to the rougher surface of the HAPC-bond layer whose roughness increased about three times than that of the grit-blast substrate. After exposure at 1200 °C in air, the mass increasing rate for single HAPC-silicide coating was 3.5 mg/cm2 because of the pest oxidation of niobium alloy, whereas the combined coating displayed better oxidation resistance with a mass...