Prakash Srirangam

Synchrotron quantification of ultrasound cavitation and bubble dynamics in Al–10Cu melts

Knowledge of the kinetics of gas bubble formation and evolution under cavitation conditions in molten alloys is important for the control casting defects such as porosity and dissolved hydrogen. Using in situ synchrotron X-ray radiography, we studied the dynamic behaviour of ultrasonic cavitation gas bubbles in a molten Al-10 wt%Cu alloy. The size distribution, average radius and growth rate of cavitation gas bubbles were quantified under an acoustic intensity of 800 W/cm(2) and a maximum acoustic pressure of 4.5 MPa (45 atm). Bubbles exhibited a log-normal size distribution with an average radius of 15.3 ± 0.5 μm. Under applied sonication conditions the growth rate of bubble radius, R(t), followed a power law with a form of R(t)=αt(β), and α=0.0021 &β=0.89. The observed tendencies were discussed in relation to bubble growth mechanisms of Al alloy melts.

In Situ Synchrotron Radiography of Ultrasound Cavitation in a Molten Al-10Cu Alloy

In-situ synchrotron radiography was used to investigate bubble dynamics in a molten Al-10 wt% Cu alloy during ultrasound pulsing. Radiographs with an exposure time of 25 ms were collected continuously during sonication at a temperature of approximately 640oC. The formation, collapse, and movement of bubbles, including parameters such as size distribution were quantified using image analysis. Results show that the average bubble radius is 16.0±0.5 μm, and that the average bubble radius increases linearly with sonication time.

Partial pair correlation functions and viscosity of liquid Al–Si hypoeutectic alloys via high-energy X-ray diffraction experiments

The liquid structure of Al–Si hypoeutectic binary alloys was characterized by diffraction experiments using a high-energy X-ray (synchrotron) beam source. The diffraction experiments were carried out for liquid pure Al, Al–3 wt% Si, Al–7 wt% Si, Al–10 wt% Si and Al–12.5 wt% Si alloys at several temperatures. The salient structure information such as structure factor (SF), pair distribution function (PDF), radial distribution function (RDF), coordination number (CN) and atomic packing densities (PD) were quantified as a function of Si concentration and melt temperatures. Reverse Monte Carlo (RMC) analysis was carried out using the diffraction experimental data to quantify the partial pair correlation functions, such as partial structure factor, partial pair distribution function (PPDF) and partial radial distribution function. Furthermore, the partial pair distribution function and the liquid atomic structure information were used in a semi-empirical model to evaluate the viscosity of these liquid alloys at various melt temperatures. The results show that the viscosity determined by semi-empirical methods using the atomic structure information is in good agreement with the experimentally determined viscosity values.

Ultra-small-angle X-ray scattering study of second-phase particles in heat-treated Zircaloy-4

The ultra-small-angle X-ray scattering (USAXS) technique has been used to investigate and to quantify the morphology and size distribution of second-phase particles in Zircaloy-4 under various heat-treatment conditions. The alloy samples were solutionized in the β phase field at 1293 K for 15 min and then cooled at different rates, including water quenching, air cooling and furnace cooling. The water-quenched samples were subsequently subjected to a thermal aging treatment at 873 K for different aging times (30, 60, 120 and 300 min). The USAXS results show that water quenching and air cooling from the β phase field produces a narrow size distribution of fine-size precipitates with an average diameter of 300–800 A, while furnace cooling resulted in coarsening of the particles, with a broad size distribution having an average precipitate size of 600–1200 A. Further, the furnace-cooled sample shows a higher volume fraction of particles than the water-quenched or air-cooled sample. The USAXS results on the quenched then aged samples show that aging at 873 K for 10 min resulted in very fine size precipitates with an average diameter of 200–350 A. A rapid precipitation with the highest number density of second-phase particles amongst all the heat-treated samples (4.3 × 1020 m−3) was observed in the sample aged for 10 min at 873 K. Particles of larger size and with a broad size distribution were observed in the sample aged at 873 K for 300 min. A bimodal type of particle size distribution was observed in all the heat-treated samples. Important parameters in the characterization of second-phase particles, such as the average size, size distribution, volume fraction and number density, were evaluated and quantified. These parameters are discussed for both β heat-treated and aged specimens. Transmission and scanning transmission electron microscopy characterization were carried out on all heat-treated samples, to assist in interpretation and to substantiate the results from the USAXS measurements.

Modification of oxide inclusions in calcium-treated Al-killed high sulphur steels

ABSTRACT A study has been carried out to understand the modification of alumina inclusions in Al-killed high sulphur steel with calcium treatment. For calcium treatment to be effective, a general practice is to desulphurise the steel to prevent the formation of solid CaS inclusions that are harmful to steel quality and final properties. To avoid this additional desulphurising step, the authors developed a new approach of calcium treatment of steel at an industrial scale. This approach involves treating the liquid steel with calcium treatment at low aluminium levels which enables formation of liquid calcium aluminate inclusions (C12A7) in the melt and then adding remaining amount of required aluminium. Based on this principle, Al-S diagram has been developed and calcium treatment has been modified accordingly. The inclusion transformation and morphology were studied using scanning electron microscope /energy dispersive spectroscopy analysis and. activity of CaS was calculated.

2D SEM and 3D XCT investigation of in-situ Al-Cu-TiB2 semi-solid forged composites

The present work compares the 2D and 3D distribution of TiB2 particles in a semisolid processed Al-4.5%Cu-5wt.% TiB2 in-situ composite prepared by flux assisted synthesis. The composite was synthesized by the reaction of K2TiF6 and KBF4 salts in molten Al-4.5Cu alloy held at 800 oC for an hour. The extent of distribution of TiB2 particles was investigated using Field Emission Scanning Electron Microscopy (FESEM) and X-ray computed tomography (XCT) to obtain 2Dand 3D images respectively. The studies indicated improved distribution of TiB2 particles after semi-solid forging of composites (at 0.1 volume fraction of liquid and 50% reduction) as compared to as cast composites. The hardness of the semisolid forged composites showed a significant increase and is uniform in all directions. The increase in hardness could be attributed to particle fragmentation and its redistribution in the matrix. Further investigation will be needed to understand the mechanism of redistribution and investigate the mechanical properties of such composites in detail. Copyright