Recent progress in microstructural hydrogen mapping in steels: quantification,kinetic analysis,and multi-scale characterisation

ABSTRACT

This paper gives an overview of recent progress in microstructure-specific hydrogen mapping techniques. The challenging nature of mapping hydrogen with high spatial resolution, i.e. at the scale of finest microstructural features, led to the development of various methodologies: thermal desorption spectrometry, silver decoration, the hydrogen microprint technique, secondary ion mass spectroscopy, atom probe tomography, neutron radiography, and the scanning Kelvin probe. These techniques have different characteristics regarding spatial and temporal resolution associated with microstructure-sensitive hydrogen detection. Employing these techniques in a site-specific manner together with other microstructure probing methods enables multi-scale, quantitative, three-dimensional, high spatial, and kinetic resolution hydrogen mapping, depending on the specific multi-probe approaches used. Here, we present a brief overview of the specific characteristics of each method and the progress resulting from their combined application to the field of hydrogen embrittlement.

This paper is part of a thematic issue on Hydrogen in Metallic Alloys     

Fast Fluid Simulations with Sparse Volumes on the GPU

We introduce efficient, large scale fluid simulation on GPU hardware using the fluid‐implicit particle (FLIP) method over a sparse hierarchy of grids represented in NVIDIA^® GVDB Voxels. Our approach handles tens of millions of particles within a virtually unbounded simulation domain. We describe novel techniques for parallel sparse grid hierarchy construction and fast incremental updates on the GPU for moving particles. In addition, our FLIP technique introduces sparse, work efficient parallel data gathering from particle to voxel, and a matrix‐free GPU‐based conjugate gradient solver optimized for sparse grids. Our results show that our method can achieve up to an order of magnitude faster simulations on the GPU as compared to FLIP simulations running on the CPU.     

Nary coded structured light-based range scanners using color invariants

Three dimensional range data provides useful information for various computer vision and computer graphics applications. For these, extracting the range data reliably is of utmost importance. Therefore, various range scanners based on different working principles are proposed in the literature. Among these, coded structured light-based range scanners are popular and used in most industrial applications. Unfortunately, these range scanners cannot scan shiny objects reliably. Either highlights on the shiny object surface or the ambient light in the environment disturb the code word. As the code is changed, the range data extracted from it will also be disturbed. In this study, we focus on developing a system that can scan shiny and matte objects under ambient light. Therefore, we propose color invariant-based binary, ternary, and quaternary coded structured light-based range scanners. We hypothesize that, by using color invariants, we can eliminate the effect of highlights and ambient light in the scanning process. Thus, we can extract the range data of shiny and matte objects in a robust manner. We implemented these scanners using a TI DM6437 EVM board with a flexible system setup such that the user can select the scanning type. Furthermore, we implemented a TI MSP430 microcontroller-based rotating table system that accompanies our scanner. With the help of this system, we can obtain the range data of the target object from different viewpoints. We also implemented a range image registration method to obtain the complete object model from the range data extracted. We tested our scanner system on various objects and provided their range and model data.     

748 K (475 °C) Embrittlement of Duplex Stainless Steel: Effect on Microstructure and Fracture Behavior

22Cr-5Ni duplex stainless steel (DSS) was aged at 748 K (475 °C) and the microstructure development correlated to changes in mechanical properties and fracture behavior. Tensile testing of aged microstructures confirmed the occurrence of 748 K (475 °C) embrittlement, which was accompanied by an increase of strength and hardness and loss of toughness. Aging caused spinodal decomposition of the ferrite phase, consisting of Cr-enriched α″ and Fe-rich α′ and the formation of a large number of R-phase precipitates, with sizes between 50 and 400 nm. Fracture surface analyses revealed a gradual change of the fracture mode from ductile to brittle delamination fracture, associated with slip incompatibility between ferrite and austenite. Ferrite became highly brittle after 255 hours of aging, mainly due to the presence of precipitates, while austenite was ductile and accommodated most plastic strain. The fracture mechanism as a function of 748 K (475 °C) embrittlement is discussed in light of microstructure development.     

Mechanical performance of resol type phenolic resin/layered silicate nanocomposites

Clay addition has been shown to affect polymer resins positively in terms of several physical and chemical properties, including mechanical performance, high temperature endurance and durability. These increases are limited only to relatively low concentrations of reinforcement phase, but at these low concentrations polymer/layered silicate nanocomposites (P/LS NC) have shown to exhibit higher mechanical performance than fiber reinforced polymer composites. This is among the several reasons that make P/LS NC's one of the most widely studied class of materials today. In this study, the mechanical performance of resol type phenolic resin/layered silicate nanocomposite specimens was examined by carrying out 3‐point bending, Charpy impact and fracture toughness tests to couple the observations, microstructural analysis is done through X‐ray diffraction and scanning electron microscopy. The effects of especially three factors; cure method, clay amount, and clay modification were investigated. It was concluded that highest mechanical performance was obtained by the acid curing of the phenolic resin with very low amounts (e.g., 0.5%) of either very hydrophobic or very hydrophilic Na‐montmorillonite clay additions. Improvements as high as 7% in flexural strength, 11% in flexural strain at break, 16% in Charpy impact strength, and 66% in fracture toughness values were obtained. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Elimination of Hot Tears in Steel Castings by Means of Solidification Pattern Optimization

A methodology of how to exploit the Niyama criterion for the elimination of various defects such as centerline porosity, macrosegregation, and hot tearing in steel castings is presented. The tendency of forming centerline porosity is governed by the temperature distribution close to the end of the solidification interval, specifically by thermal gradients and cooling rates. The physics behind macrosegregation and hot tears indicate that these two defects also are dependent heavily on thermal gradients and pressure drop in the mushy zone. The objective of this work is to show that by optimizing the solidification pattern, i.e., establishing directional and progressive solidification with the help of the Niyama criterion, macrosegregation and hot tearing issues can be both minimized or eliminated entirely. An original casting layout was simulated using a transient three-dimensional (3-D) thermal fluid model incorporated in a commercial simulation software package to determine potential flaws and inadequacies. Based on the initial casting process assessment, multiobjective optimization of the solidification pattern of the considered steel part followed. That is, the multiobjective optimization problem of choosing the proper riser and chill designs has been investigated using genetic algorithms while simultaneously considering their impact on centerline porosity, the macrosegregation pattern, and primarily on hot tear formation.     

Sintering behaviour of silicon nitride powders produced by carbothermal reduction and nitridation

In this study, the sintering behaviour of silicon nitride (Si₃N₄) powders (having in situ form sintering aids/self-sintering additives) produced directly by the carbothermal reduction and nitridation (CRN) process is reported. The sintering of as-synthesised α-phase Si₃N₄ powders was studied, and the results were compared with a commercial powder. The α-Si₃N₄ powders, as-received contains magnesium, yttrium or lithium–yttrium-based oxides that were shaped with cold isostatic pressing and tape casting techniques. The compacts and tape casted samples are then pressureless-sintered at 1650–1750 °C for up to 2 h. After sintering, the density and the amount of β-phase formation were examined in relation to the sintering temperature and time. The highest density value of 3.20 g cm^?3 was obtained after only 30 min of pressureless sintering (at 1700 °C) of Si₃N₄ powders produced by CRN from silica initially containing 5 wt.% Y₂O₃. Silicon nitride powders produced by the CRN process performed similarly or even better than results from the pressureless sintering process compared with the commercial one.     

Comparison of the structural and magnetic properties of submicron barium hexaferrite powders prepared by molten salt and solid state calcination routes

The effect of crystallinity and particle morphology of the submicron barium hexaferrite (BaFe₁₂O₁₉) powders on the magnetic properties was investigated on powders synthesized by solid-state calcination (BHF-c) and molten salt synthesis (BHF-m) methods. Solid-state calcination route was found to yield agglomerated powders with poor crystallinity, whereas molten salt synthesis resulted in well crystallized powders with an anisometric morphology. The saturation magnetization of the BHF-m and BHF-c samples is 59 emu/g, and 56 emu/g at 300 K, and 90 emu/g, and 86 emu/g at 10 K. The temperature dependence of magnetization of the BHF-m is higher and the increase in magnetocrystal anisotropy with decreasing temperature is also steeper than that of the BHF-c due to the higher crystallinity. The magnetocrystalline anisotropy constant, K, calculated from the Stoner–Wohlfarth theory, of the BHF-m and BHF-c powders is 14.24 and 10.14 HA²/kg, respectively. The higher effective anisotropy, K_eff of the BHF-m is also confirmed through ferromagnetic resonance measurements. In conclusion, the higher crystallinity, slightly higher particle size and anisometric morphology of the BHF-m particles translated into higher magnetic properties and magnetocrystalline anisotropy.     

The effect of reactor temperature gradient on microstructures of sodium borosilicate glass powders produced by ultrasonic spray pyrolysis technique

Sodium borosilicate glass powders were produced by ultrasonic spray pyrolysis technique for three different reactor temperature gradients and the effect of the reactor temperature gradient on the microstructures of the powders produced was investigated. A three-zone ultrasonic spray pyrolysis system reactor in which the temperatures of each zone could be controlled separately was designed for this purpose. When the drying speed is high, early shell formation was observed in the aerosol droplets due to the difference of the drying speed between inner and outer parts and hollow powders were produced. In order to produce dense powder particles, shell formation on the aerosol droplet should be prevented, therefore drying speed should be decreased. The powders produced were glass-structured, spherical shaped and with smooth surfaces. The density of the hollow powder particle was 1.9 g/cm³ while the density of the dense particle was 2.5 g/cm³.     

Polymerization of epoxidized soybean oil with maleinized polybutadiene

Epoxidized soybean oil (ESO) triglycerides were reacted with maleinized polybutadiene (MMPBD) to give plant‐oil‐based thermoset polymers. MMPBD samples were of two different molecular weights [high‐molecular‐weight maleinized polybutadiene (MMPBD‐H), maleate content = 10%, number‐average molecular weight (M_n) = 9000, and low‐molecular‐weight maleinized polybutadiene (MMPBD‐L), maleate content = 15%, M_n = 5000]. To increase the crosslink density of the product, a free‐radical initiator, benzoyl peroxide, was added to this mixture to further crosslink MMPBD through its double bonds. The characterizations of the products were done by dynamic mechanical analysis, differential scanning calorimetry, thermogravimetric analysis, and IR spectroscopy. The ESO–MMPBD polymers were crosslinked rigid infusible polymers. ESO–MMPBD‐H–1 : 1 and ESO–MMPBD‐L–1 : 1 showed glass‐transition temperature values at −23, 78 and −17, 64°C, respectively, whereas the storage moduli of the two polymers at 25°C were 13 and 16 MPa, respectively. The storage moduli of the polymers remained the same or decreased with the addition of a free‐radical initiator. The storage moduli also decreased with increasing ESO concentration above a 1 : 1 epoxy‐to‐anhydride molar ratio. The surface hardness increased dramatically, and the equilibrium swelling ratio decreased with the addition of free‐radical initiator. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011