Ajay Kumar Revelly

Additive Manufacturing (AM) is a layer-by-layer material addition process. Direct Metal Deposition (DMD) is one of the metal powders feeding AM methods. It can produce Functionally Graded Material (FGM) used in extreme service conditions. Two gradient path FGM blocks were deposited by a continuous co-axial nozzle of a commercial DMD machine. Each deposited block was cut into ASTM E8 samples by a wire cut Electric Discharge Machine (EDM). The powders used for deposition were Stainless Steel 316L (SS316L) and Inconel 625 (IN625). The research paper is primarily focused on tensile properties of the samples. The first sample has higher Ultimate Tensile Strength (UTS) of 454 MPa as compared to the second sample and also has higher micro-hardness of 263 HV on its top surface. These properties are due to presence of cellular grains micro-structures in the sample. Fractography revealed that the samples are ductile. However, rounded indications and Face-Centered-Cubic (F.C.C) structure of the specimens have confirmed by radiography test and X-Ray Diffraction (XRD).

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’.

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 γ_GB (grain boundary energy). Controlled focused ion beam damage, with Ga⁺ (galium) ions, provided a clear scaling between γ_GB 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 γ_GB. 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 γ_GB dependence of experimental ion damage.

The anodic polarization behavior of Sanicro 28 austenitic stainless steels has been established in the cold worked (10 to 80% reduction in thickness) samples in 1N HCl, 1M H₂SO₄+ 1.5N HCl and 3N HCl solutions at room temperature. The current oscillations during the potentiodynamic scans appeared in 3N HCl implying formation of meta stable pits and this solution more severe than 1N HCl and 1M H₂SO₄+ 1.5N HCl. Two anodic peaks were observed in as-received and the cold worked samples in 3N HCl. In as-received state, the first anodic peak appeared at-3.95 mV_SCEand the second anodic peak appeared at 116 mV_SCE. It was established that selective dissolution started from 18mV_SCE. The grains and grain boundaries were not attacked at the beginning of the first peak-168 mV_SCEand started revealing at 18 and 216 mV_SCEin the potentiodynamic polarization test. The fore-scatter detector (FSD) attached to FEI Quanta EBSD revealed the pitting morphology of the specimens. It is shown that pit dimensions, types, distribution, the shape of pits in all deformation conditions is hemispherical and number of pits did not depend on the microstructural features, irrespective of the plastic deformation and it may not be related to severity of plastic deformation.