Indrajit Charit

Friction stir processing: a novel technique for fabrication of surface composite

Abstract A novel surface modifying technique, friction stir processing (FSP), has been developed for fabrication of surface composite. Al–SiC surface composites with different volume fractions of particles were successfully fabricated. The thickness of the surface composite layer ranged from 50 to 200 μm. The SiC particles were uniformly distributed in the aluminum matrix. The surface composites have excellent bonding with the aluminum alloy substrate. The microhardness of the surface composite reinforced with 27vol.%SiC of 0.7 μm average particle size was ∼173 HV, almost double of the 5083Al alloy substrate (85 HV). The solid-state processing and very fine microstructure that results are also desirable for high performance surface composites.

Multi-sheet structures in 7475 aluminum by friction stir welding in concert with post-weld superplastic forming

Abstract Diffusion bonding/superplastic forming (DB/SPF) is an established fabrication technology for titanium aerospace components. However, DB cannot be applied easily to aluminum alloys because of tenacious surface oxides. Friction stir welding may eliminate this problem and allow fabrication of multi-sheet structures via friction stir welding/SPF.

Experimental evidence for diffusion creep in the superplastic 3 mol% yttria-stabilized tetragonal zirconia

Although there have been numerous studies on the high temperature deformation characteristics of the superplastic 3 mol% yttria stabilized tetragonal zirconia (3YTZ), the rate controlling deformation mechanism has not been identified unambiguously. In the present study, experiments were conducted on 3YTZ at high stresses and at coarser grain sizes than used conventionally for superplasticity. The experimental results reveal, for the first time, an intragranular dislocation motion controlled high stress regime that is independent of the grain size. With a decrease in stress, there is a transition to a Newtonian viscous deformation regime consistent with Coble grain boundary diffusion creep. At sufficiently low stresses, or in materials with finer grain sizes, there is a further transition to a grain size dependent interface controlled deformation regime. Analysis of the experimental data suggests strongly that superplastic flow in 3YTZ occurs by an interface controlled deformation mechanism.

Newtonian viscous creep in Ti–3Al–2.5V

Biaxial creep tests were performed on fine-grained Ti–3Al–2.5V tubing at 823 and 873 K in the stress range σ/E  = 1.7  × 10−4 to σ/E  = 5.9  × 10−4. Subsequently, the creep data were analysed to determine the stress exponent and activation energy. A stress exponent value of 1 and an activation energy equal to that for grain boundary diffusion were suggestive of a Coble creep-controlled deformation regime. However, discrepancy between the experimental creep rates and Coble creep model predictions along with subsequent observation of deformed microstructures decorated with slip bands implied the operation of a different viscous creep mechanism. A slip band model proposed by Spingarn and Nix was found to provide a better description of the experimental strain rates rather than the conventional viscous creep mechanisms. High-resolution transmission electron microscopy studies confirmed the nature of these bands.

Computational Thermodynamic Modeling of Hot Corrosion of Alloys Haynes 242 and HastelloyTM N for Molten Salt Service in Advanced High Temperature Reactors

Computational Thermodynamic Modeling of Hot Corrosion of Alloys Haynes 242 and HastelloyTM N for Molten Salt Service in Advanced High Temperature Reactors An evaluation of thermodynamic aspects of hot corrosion of the superalloys Haynes 242 and HastelloyTM N in the eutectic mixtures of KF and ZrF4 is carried out for development of Advanced High Temperature Reactor (AHTR). This work models the behavior of several superalloys, potential candidates for the AHTR, using computational thermodynamics tool (ThermoCalc), leading to the development of thermodynamic description of the molten salt eutectic mixtures, and on that basis, mechanistic prediction of hot corrosion.

In Situ Tensile Deformation and Residual Stress Measurement by Neutron Diffraction in Modified 9Cr-1Mo Steel

AbstractThe deformation behavior of monolithic modified 9Cr-1Mo (Grade 91) steel during uniaxial tensile loading was studied using the in situ neutron diffraction technique. The residual stress distribution across gas tungsten arc welds in the Grade 91 steel was measured by the time-of-flight neutron diffraction method using the SMARTS diffractometer at Lujan Neutron Scattering Center, Los Alamos National Laboratory. Grade 91 plates were welded using the gas tungsten arc welding (GTAW) technique. The load sharing by different grain orientations was observed during the tensile loading. The residual stresses along three orthogonal directions were determined at the mid-thickness, 4.35 and 2.35 mm below the surface of both the as-welded and post-weld heat-treated plates. The residual stresses of the as-welded plates were compared with those of the post-weld heat-treated plates. The post-weld heat treatment significantly reduced the residual stress level in the base metal, the heat-affected zone, and the weld zone. Vickers microhardness across the weld zone of the as-welded and post-weld heat-treated specimens was evaluated and correlated with the observed residual stress profile and microstructure.

Texture and Creep Anisotropy in Zirconium Alloys

Zirconium (Zr) alloys are best known for their use in nuclear reactor applications. A hexagonally close-packed structure with a low c/a ratio and very limited slip systems leads to strong textures in these alloys during fabrication processes. These alloys are used in cladding applications for encapsulating fuel pellets, and undergo various stress conditions in-service. Hence, it is necessary to understand the creep properties of Zr alloys to predict the life of reactor claddings. Due to the unique texture, the creep deformation of these alloys is anisotropic in nature. The texture of Zircaloys was determined by X-ray diffraction experiments, and expressed in terms of pole figures and crystalline orientation distribution functions. Biaxial creep testing of thin walled tubing was used to study the creep anisotropy. Creep loci evaluation based on the experimental data and model predictions are compared. It is found that the models can predict the creep loci for recrystallized alloy very well. However, they fail to explain the behavior of the cold worked alloys. When stress enhancements due to the grain boundary sliding are taken into account, the predicted creep loci correlated well with that constructed from the experimental data.

Understanding Microstructure and Mechanical Properties of Friction Stir Processed Aluminum-Bearing High-Chromium Ferritic Stainless Steel

Kanthal APMT® steel (Fe–22Cr–5Al–3Mo) was developed mainly for using as high temperature furnace elements. This kind of high-Cr ferritic steels is not considered to have good weldability because of a variety of metallurgical issues. Friction stir welding (FSW), a solid state welding process, was applied to a Kanthal APMT® plate in a bead-on-plate configuration using a PcBN tool with a tool rotation rate of 600 RPM and a traverse speed of 25.4 mm/min. Microstructure and mechanical properties were evaluated to determine the weld quality and examine the feasibility of applying FSW as a joining technique for this steel. Microstructural characteristics were mainly studied by optical microscopy and transmission electron microscopy. The stir zone contained equiaxed grain structure with an average grain size of 13.7 μm. Interestingly, Vickers microhardness profile across the processed zone has revealed no significant change in microhardness.

Nickel-Chromium Alloys: Engineered Microstructure via Spark Plasma Sintering

There is a need to enhance or develop high temperature capabilities of structural materials for advanced coal‐fired power plants. These materials require a combination of high temperature strength, creep resistance and corrosion resistance in the oxygen‐rich and hydrogen‐rich high pressure environments. In this study, atomized Ni‐20Cr (wt.%) powder was mechanically milled with Y2O3 nanopowder (30‐40 nm powder size) to produce an alloy with a chemical composition of Ni‐20Cr‐1.2Y2O3 (wt.%) alloy using high energy ball milling. To minimize agglomeration during milling, 1 wt.% stearic acid was added to the powder mixture prior to milling. Microstructural characteristics of the powder were primarily characterized by the X‐ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The crystallite size and lattice strain were measured by XRD whereas powder morphology (powder size, shape) was studied by SEM. A milling time of 2 h was found to be optimal for the purpose that yttria particles are not dissolved yet uniformly distributed. Subsequently, the milled powder was consolidated into bulk specimens (12.5 mm in diameter) via spark plasma sintering (SPS) at 1100 °C for 30 minutes. Following SPS, the density and hardness of the specimens were measured. Microstructural characterization of the SPSed specimens was performed using SEM and TEM. The microstructural characteristics were correlated with the measured mechanical properties.

Creep deformation of materials in light water reactors (LWRs)

Abstract: The time-dependent deformation of materials or creep governs the useful life of many engineering structures. It assumes even higher significance in the case of structures constituting a nuclear reactor, wherein materials bombarded with neutrons develop defects that assist faster diffusion leading to greater plastic deformation. As a result, an understanding of the creep deformation process and factors controlling it is necessary for gauging the usefulness of materials in a nuclear reactor as well as for predicting life-times of various structures. Thus in this work we discuss the various mechanisms of creep, the rate controlling factors, deformation mechanism maps and useful life prediction methodologies. We also identify a few cases where direct application of simple creep correlations might not be feasible. Finally, we discuss the various factors that control the creep behavior of materials in light water reactors.