Govindarajan Muralidharan

In Situ Determination of the Nanoscale Chemistry and Behavior of Solid-Liquid Systems

Many fundamental questions in crystal-growth behavior remain unanswered because of the difficulties encountered in simultaneously observing phases and determining elemental concentrations and redistributions while crystals nucleate and grow at the nanoscale. We show that these obstacles can be overcome by performing energy-dispersive x-ray spectroscopy on partially molten Al-Si-Cu-Mg alloy particles during in situ heating in a transmission electron microscope. Using this technique, we were able to (i) determine that the aluminum and silicon concentrations change in a complementary and symmetric manner about the solid-liquid interface as a function of temperature; (ii) directly measure the solid- and liquid-phase compositions at equilibrium and in highly undercooled conditions for quantitative comparison with thermodynamic calculations of the liquidus and solidus phase boundaries; and (iii) provide direct evidence for homogeneous nucleation of the aluminum-rich solid.

Dynamics of self-driven microcantilevers

The small amplitude thermal vibrations of the microcantilever of an atomic force microscope can be enhanced via a delayed feedback system. This is verified experimentally for a triangular cantilever, and modeled theoretically as a boundary value problem resulting in a second order functional differential equation for the temporal behavior of the cantilever. The eigenvalues of the resulting delay differential equation describing the transverse vibrations of the cantilever are calculated and analyzed. These values are compared with the corresponding resonant frequencies predicted by a point mass model and with the experimentally observed values.

Manipulation of microcantilever oscillations.

Experimental observation of self-sustaining oscillations via a delayed feedback system is presented for a rectangular silicon microcantilever. The system is modeled as one and two-dimensional damped oscillator and the resulting delay differential equations are studied in frequency and time domain. The shortcomings of each model are outlined, and an improved formulation of the dynamics of the cantilever is presented.

Considerations of Alloy N for Fluoride Salt-Cooled High-Temperature Reactor Applications

Fluoride Salt-Cooled High-Temperature Reactors (FHRs) are a promising new class of thermal-spectrum nuclear reactors. The reactor structural materials must possess high-temperature strength and chemical compatibility with the liquid fluoride salt as well as with a power cycle fluid such as supercritical water while remaining resistant to residual air within the containment. Alloy N was developed for use with liquid fluoride salts and it possesses adequate strength and chemical compatibility up to about 700°C. A distinctive property of FHRs is that their maximum allowable coolant temperature is restricted by their structural alloy maximum service temperature. As the reactor thermal efficiency directly increases with the maximum coolant temperature, higher temperature resistant alloys are strongly desired. This paper reviews the current status of Alloy N and its relevance to FHRs including its design principles, development history, high temperature strength, environmental resistance, metallurgical stability, component manufacturability, ASME codification status, and reactor service requirements. The review will identify issues and provide guidance for improving the alloy properties or implementing engineering solutions.Copyright

Analysis of amplification of thermal vibrations of a microcantilever

We examine the conditions under which the small amplitude of thermal vibrations of cantilevers typically used for atomic force microscopy and sensor applications can be enhanced through a feedback mechanism. Using a simple mathematical model with two independent measurable physical parameters, a time delay τ and a gain factor G, we show that for certain values of these two parameters, such amplification is feasible. Experimental measurements of the two parameters when amplification succeeded show that these fall in the range predicted by the calculations.

Reliability of Sn-3.5Ag solder joints in high temperature packaging applications

There is a significant need for next-generation, highperformance power electronic packages and systems with wide band gap devices that operate at high temperatures in automotive and electric grid applications. Sn-3.5Ag solder is a candidate for use in such packages with potential operating temperatures up to 200°C. However, there is a need to understand thermal cycling reliability of Sn-3.5Ag solders. The results of a study on the damage evolution occurring in large area Sn-3.5Ag solder joints between silicon dies and Direct Bonded Copper (DBC) substrates subject to thermal cycling between 200°C and 5°C is presented in this paper. Damage accumulation was followed using high resolution X-ray radiography techniques, and nonlinear finite element models were developed based on the mechanical property data available in literature to understand the relationship between the stress state within the solder joint and the damage occurring under thermal cycling conditions. It was observed that regions of damage observed in the experiments do not correspond to the finite element predictions of the location of regions of maximum plastic work.

Mesoscale Modeling and Validation of Texture Evolution during Asymmetric Rolling and Static Recrystallization of Magnesium Alloy AZ31B

The focus of the present research is to develop an integrated deformation and recrystallization model for magnesium alloys at the microstructural length scale. It is known that in magnesium alloys nucleation of recrystallized grains occurs at various microstructural inhomogeneities such as twins and localized deformation bands. However, models need to be developed that can predict the evolution of the grain structure and texture developed during recrystallization and grain growth, especially when the deformation process follows a complicated deformation path such as in asymmetric rolling. The deformation model is based on a crystal plasticity approach implemented at the length scale of the microstructure that includes deformation mechanisms based on dislocation slip and twinning. The recrystallization simulation is based on a Monte Carlo technique that operates on the output of the deformation simulations. The nucleation criterion during recrystallization is based on the local stored energy, and the Monte Carlo technique is used to simulate the growth of the nuclei resulting from local stored energy differences and curvature. The model predictions are compared with experimental data obtained through electron backscatter analysis and neutron diffraction.

Microstructure and Mechanical Property Performance of Commercial Grade API Pipeline Steels in High Pressure Gaseous Hydrogen

The continued growth of the world’s developing countries has placed an ever increasing demand on traditional fossil fuels. This increased demand for fossil fuels has lead to increasing research and development of alternative energy sources. Hydrogen gas is one of the potential alternatives under development. It is anticipated that the least expensive method of transporting large quantities of hydrogen gas is through steel pipelines. It is well known that hydrogen embrittlement has the potential to degrade steel’s mechanical properties. Consequently, the current pipeline infrastructure used in hydrogen transport is typically operated in a conservative fashion, in particular lower operating pressures, lower strength steels, and heavier pipe wall thicknesses. This operational practice is not conducive to economical movement of significant volumes of hydrogen gas as an alternative to fossil fuels. The degradation of the mechanical properties of steels in hydrogen service depends on the microstructure of the steel. An understanding of the relationship of mechanical property degradation of a given microstructure on exposure to hydrogen gas under pressure can be used to evaluate the suitability of the existing pipeline infrastructure for hydrogen service and guide alloy and microstructure design for new hydrogen pipeline infrastructure. To this end, the microstructures of relevant steels and their mechanical properties in relevant gaseous hydrogen environments must be fully characterized to establish suitablity for transporting hydrogen. A project to evaluate four commercially available pipeline steels alloy/microstructure performance in the presences of gaseous hydrogen has been funded by the US Department of Energy along with the private sector. The microstructures of four pipeline steels were characterized and tensile testing was conducted in gaseous hydrogen and helium at pressures of 5.5 MPa (800 psi), 11 MPa (1600 psi) and 20.7 MPa (3000 psi). Based on reduction of area, two of the four steels that performed the best across the pressure range were selected for evaluation of fracture and fatigue performance in gaseous hydrogen at 5.5 MPa (800 psi) and 20.7 MPa (3000 psi). This paper describes the work performed on four commercially available pipeline steels in the presence of gaseous hydrogen at pressures relevant for transport of hydrogen in pipelines. Microstructures and mechanical property performances are compared. In addition, recommendations for future work related to gaining a better understanding of steel pipeline performance in hydrogen service are discussed.Copyright

The Role of Continuous Cooling Transformation Diagrams in Material Design for High Strength Oil and Gas Transmission Pipeline Steels

The economical, environmental, and safe movement of gas and oil to the marketplace requires transmission pipelines to be designed to operate at higher pressures and/or with improved toughness over a variety of temperature ranges. To meet the higher strength and toughness specification requirements of these transmission pipelines, appropriate materials and processes must be used in their design and construction. This includes selection of appropriate alloy composition, processing routes, microstructure control, and cost. A continuous cooling transformation (CCT) diagram is a tool that can be used to select alloy composition and processing route in order to obtain a specific, desirable microstructure for transmission linepipe steels in a cost-effective manner. In the past, CCT diagrams were developed experimentally under laboratory conditions, thus requiring extensive time and effort. However, with the vast data available and improved computational tools, reasonably accurate computer generated CCT diagrams can be produced quickly. These computer generated diagrams can give the materials design engineer a reasonable understanding of the effect of subjecting a given alloy to various processing routes and hence the resultant microstructures. Since final microstructure is a key variable in determining the linepipe steel material properties, the chosen alloy/processing route and its effect on the final microstructure needs to be understood. This paper will discuss the role of CCT diagrams in the design of steels (cost, alloy, processing, and microstructure) for oil and gas transmission pipelines. Examples of computer generated CCT digrams for various API alloy designs are included.Copyright

An in situ USAXS–SAXS–WAXS study of precipitate size distribution evolution in a model Ni-based alloy

Combined ultra-small-, small- and wide-angle X-ray scattering (USAXS–SAXS–WAXS) provides in situ evaluation of the precipitate size distribution (PSD) and phase structure temporal evolution during heat treatment. A method for extraction of an arbitrary PSD in the presence of interparticle interactions is described and illustrated for study of PSD evolution.