T. Karthikeyan

Weldability and microstructural variations in weldments of Ti-5Ta-1.8Nb alloy

The successful replacement of the present generation of corrosion-resistant materials (nitric acid-grade stainless steel and Ti) by Ti-5Ta-1.8Nb, which has better corrosion resistance, depends on its weldability characteristics. This article presents the results of a study on the fabrication, qualification, and microstructural characterization of the welds. Welding was carried out using the direct current electrode negative (DCEN) polarity tungsten inert gas (TIG) (manual) welding method with high-purity Ar shielding. Testing was carried out as per the ASME standard (section IX, welding and brazing). Qualification tests found that the weldment met the required properties. The weldment showed heterogeneous microstructures, which are rationalized based on differences in phase transformation mechanisms that are dictated by the thermal cycles experienced by various microscopic regions. The results, described in this article, confirm that the weldability of the developmental Ti-Ta-Nb alloy is excellent. A preliminary evaluation of the corrosion behavior of the welds showed rates comparable to that of the base metal, establishing that this alloy could be considered as an alternative material for use in highly corrosive environments.

Solidification structure in Ti–5Ta–1·8Nb weld

Abstract Solidification structure of a weld in a manual gas tungsten arc welding of 3 mm thick Ti–5Ta–1·8Nb plates has been examined using optical/scanning electron microscopy, EDX spectroscopy and electron backscattered diffraction techniques. The weld exhibited columnar grains of prior β, with a substructure of martensitic α′. The backscattered electron image revealed the solidification mode to be cellular–dendritic, and the EDX analysis confirmed the partitioning of Ta atoms between the dendrites. The crystal orientation of the product α′ obtained by electron backscattered diffraction and the Burgers orientation relationship (for β→α′ transformation) were used to predict the orientation of parent β grains in the weld zone, and the long dendritic arms were found to be aligned parallel to β<100>. Analytical solution based on Rosenthal equation was used to calculate the temperature cycle of the welding process. The solidification mode predicted from calculated G/R ratio (where G is solidification gradient and R is rate of solidification) and Kurz–Fischer map matched with the observed cellular–dendritic solidification structure.

Microstructure, microchemistry, and prediction of long-term diffusion behavior of chloride in concrete

Microstructural and microchemical features of two types of concrete are investigated employing electron and ion optical techniques. The first type is the concrete cured in seawater or normal water. The second type is concrete cured in normal water and exposed subsequently to seawater. Major constituent phases of concrete and differences in their distribution due to different curing media are identified. Chloride profiles in different concretes are evaluated using the proton induced x-ray emission technique. Diffusion coefficientD was calculated by modeling the diffusion process and comparing with measured profiles.D, thus estimated, is found to be ∼1.8×10−9 m2/s, which is higher than the reported values of ∼10−11 to 10−13 m2/s. The faster diffusion of chloride in seawater-cured concrete can be attributed to the availability of water medium in wet concrete, in the initial stages of the hydration of cement. The prediction of the concentration profile of chloride in a layer of 100 mm of 28% fly ash containing concrete over concrete exposed to seawater is carried out. For the worst scenario, analytical estimates of the concentration of chloride as a function of time at a distance of 100 mm in the fly ash containing concrete were made. The concentration profiles of chloride expected after 40 years in the fly ash-containing concrete were also estimated using diffusion coefficient values available in the literature.

Evaluation of misindexing of EBSD patterns in a ferritic steel.

The systematic misindexing caused by pseudo‐symmetry Kikuchi diffraction patterns in automated Electron Backscatter Diffraction analysis has been studied in a 9Cr‐1Mo ferritic steel. Grains with its [1 1 1] directed towards detector centre were found to be prone to misindexing, and the solutions exhibit a relative orientation of ±30° and 60° about the common [1 1 1] axis (as compared to the true orientation). Fictitious boundaries were detected within such grains, which satisfy the Σ3 or Σ13b type coincidence site lattice boundary criteria. Misindexing rate was reduced with more than six detected bands, but 30° rotated solution was comparatively more persistent, as the additional bands of (3 1 0)‐type exhibited a nearly good pattern match. Increase in detector collection angle to 0.96 sr or number of detected bands to nine were found to be beneficial in preventing the misindexing problem.

Five-Parameter Grain Boundary Determination in Annealed Ferrite Structure Using Electron Backscatter Diffraction and Serial Sectioning Technique

The SEM/electron back scattered diffraction and serial sectioning procedure have been used to study the five parameter description of grain boundaries in a polygonal ferrite microstructure of 9Cr-1Mo steel. A routine has been evolved to correlate the successive images of a selected region and determine the grain boundary plane morphology. The relative misorientation of the crystallites, in terms of misorientation angle-axis (ω,ȓ), the grain boundary inclination angles [azimuth (γ) and polar (β) angles], and the crystallographic description of the two meeting planes have been studied and compared with random distribution. The low-angle boundaries are found to be persistently present in higher amounts after the grain growth induced by extended annealing.

Orientational Equidistance Method for Solving Orientation Relationship From Product Variants: Application to Steel

A three-step procedure has been evolved for finding the orientation relationship (OR) between parent austenite and product ferrite phase, from the room temperature microtexture data of ferritic/martensitic steel. In the first step, the frequently occurring misorientations between product ferrite variants are sought from the electron backscatter diffraction (EBSD) dataset. This is carried out by calculating the ‘misorientation angle-rotation axis’ values across all neighboring pair of ferrite pixels, and plotting them in a parametric space to detect the prominent misorientation peaks. Identification of ‘concurrent relative orientation’ of product variants, i.e., the second step, is resolved by comparing the observed misorientation peak positions with the inter-variant relation set of Kurdjumov–Sachs (KS) OR. The final crucial step of solving the OR based on the ‘Orientational Equidistance’ principle of parent phase from all its product phase variants is implemented in the Rodrigues–Frank parametric space. The procedure is illustrated using an large area EBSD dataset of normalized and tempered 9Cr–1Mo–0.1C steel, and the optimum orientation relation is determined to be ~44.0°[0.151 0.137 0.979], which is 2.4° deviated from KS OR.

Area-preserving colour coding of inverse pole figure domain

The inverse pole figure (IPF) map is a routinely displayed output in microtexture studies, interpreted using the attached colour legend/diagram. An area‐preserving relation between the IPF domain and RGB colour domain has been developed here, and the resultant IPF colour diagrams of different crystal point group symmetries are presented.

Restitution of Prior-Austenite Grain Orientation by Microtexture Analysis of Tempered Martensite Structure in 9Cr-1Mo Ferritic Steel

The 9Cr-1Mo-0.1C steel was solutionzed (1323 K for 1hr) and step cooled (at 40 K/hr between Ms and Mf) to effect martensitic transformation at slower rates, and then subsequently tempered (1023 K for 2 hr). The microtexture of the tempered martensite structure was evaluated using SEM-Electron Back Scattered Diffraction (EBSD) technique, and the crystal orientation data of ferrite crystallites formed within a single prior-γ grain were analyzed in detail. The Kurdjumov-Sachs (K S) orientation relationship (111)γ // (011)α´ [-101]γ // [-1-11]α´ is commonly obeyed in low carbon steels, and a restitution methodology was used to back calculate the orientation of parent γ grain assuming the K-S relationship. The austenite was identified as the common solution of the data set, and its mean orientation has been calculated. The mean deviation from the K-S was calculated to be 3.75º.

Common Region between Euler Subspace and Rodrigues Fundamental Zone for a Cubic Crystal and Orthotropy Sample Symmetry

Due to crystal and sample symmetry, texture data can be presented uniquely in a smaller domain of Euler box or Rodrigues space. As these sub-domains are obtained by different criteria, a one-to-one relation does not always exist for conversion between Euler and Rodrigues coordinates. This work presents a methodology for finding the common region between the Euler sub-space and Rodrigues fundamental zone for the case of cubic crystal and orthotropy sample symmetry, and its size is found to be 0.0864068.