Wajira Mirihanage

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.

A combined enthalpy / front tracking method for modelling melting and solidification in laser welding

The authors present an integrated meso-scale 2D numerical model for the simulation of laser spot welding of a Fe-Cr-Ni steel. The melting of the parent materials due to the applied heating power is an important phenomenon, leading to the formation of the weld pool and the subsequent conditions from which solidification proceeds. This model deals with the dynamic formation of the weld pool whereby melting may be occurring at a given location while solidification has already commenced elsewhere throughout the weld pool. Considering both melting and possible simultaneous solidification in this manner ensures a more accurate simulation of temperature distribution. A source based enthalpy method is employed throughout the calculation domain in order to integrate the melting model with the UCD front tracking model for alloy solidification. Melting is tracked via interpolation of the liquidus isotherm, while solidification is treated via both the tracking of the advancing columnar dendritic front, and the nucleation and growth of equiaxed dendrites using a volume-averaging formulation. Heterogeneous nucleation is assumed to take place on TiN grain refiner particles at a grain refiner density of 1000 particles per mm2. A mechanical blocking criterion is used to define dendrite coherency, and the columnar-to-equiaxed transition within the weld pool is predicted.

In-situ observation of transient columnar dendrite growth in the presence of thermo-solutal convection

The time evolution of tip radii and velocities of growing columnar dendrites have been extracted from an image sequence measured in situ by synchrotron X-ray video microscopy during directional solidification of Al-15%Cu-9%wtSi alloy. At the scale of the primary dendrites, a highly transient growth was observed in conjunction with liquid flow caused by modest thermo-soutal convection. The nominal spatial and temporal resolutions employed in the experiment were 1.4 μm at approximately 100ms frame rate, respectively. A very prominent mutual inter-relation between the solute field and flow velocity field and the dendrite growth rate and morphology is evident in these observations. Image processing allowed extraction of the corresponding projection averaged solute field in each frame, and an estimate of the solute boundary velocity field by frame-to-frame tracking of its motion. These measurements may eventually support development of statistical dendrite kinetics models for transient states in alloy solidification processes that could be applied in microscopic scale solidification models such as phase field.

Ultra-fast in-situ X-ray studies of evolving columnar dendrites in solidifying steel weld pools

High-brilliance polychromatic synchrotron radiation has been used to conduct in-situ studies of the solidification microstructure evolution during simulated welding. The welding simulations were realized by rapidly fusing ~ 5 mm spot in Fe-Cr-Ni steel. During the solid- liquid-solid phase transformations, a section of the weld pool was placed in an incident 50-150 keV polychromatic synchrotron X-ray beam, in a near-horizontal position at a very low inclination angle. Multiple high-resolution 2D detectors with very high frame rates were utilized to capture time resolved X-ray diffraction data from suitably oriented solid dendrites evolving in the weld pool. Comprehensive analysis of the diffraction data revealed individual and overall dendritic growth characteristics and relevant melt and solid flow dynamics during weld pool solidification, which was completed within 1.5 s. Columnar dendrite tip velocities were estimated from the experimental data and during early stages of solidification were exceeded 4 mm/s. The most remarkable observation revealed through the time-resolved reciprocal space observations are correlated to significant tilting of columnar type dendrites at their root during solidification, presumably caused by convective currents in the weld pool. When the columnar dendrite tilting are transformed to respective metric linear tilting velocities at the dendrite tip; tilting velocities are found to be in the same order of magnitude as the columnar tip growth velocities, suggesting a highly transient nature of growth conditions.

Impacting Silicates Using a Fast Indentation Device

Material failure is determined by a suite of deformation mechanisms with differing kinetics, operating together to present an integrated response to an observer. To elucidate processes requires separating one from another in order to construct physically-based descriptions of behaviour. Observing a material, in which failure processes are controlled by a designed impulse and are at a suitable scale, offers the possibility of separating operating mechanisms. A highly synchronised, loading test frame has been developed by Diamond and Manchester. It has already been fielded at the ESRF and shown excellent results using ultra-high speed, single bunch imaging on simple test problems. Now that the device has been proven, we show studies on the compression and fracture of glass and quartz. The results indicate several modes of failure and emphasise the need for further fast radiography to elucidate failure mechanisms in solids. INTRODUCTION The process defining the end of elastic behaviour in silicates, typically glasses, is fracture. On exceeding the strength of the material, inelastic processes start when fracture is initiated at a flaw on the loaded surface. A crack then propagates with the tip travelling at a speed determined by the stress level. This then accelerates to the Rayleigh wave speed in the material in the limiting case, since the shear wave speed increases with pressure [1]. A regime exists ahead of a driven indenter where a material can support different states (unfailed and failed) for a time dependent upon the speed at which failure processes operate. The indenter morphology determines the deformation zone extent, and the degree of compaction of fragmented material within that absorbs the applied strain. Further, we can control indenter material and geometry to control the nature of the compact. This time is a few tens of microseconds in the case of glass and these processes were captured with radiography using single bunch imaging at the European Synchrotron Radiation Facility (ESRF) for the first time. Fast imaging at MHz allows us to observe the penetration of an indenter, and the compaction of the fragmented material, to capture these processes in this experiment. Glass shows the evolution of unsteady stress states through localisation, failure and compaction as the material accommodates strain and transits to a steady but often metastable state. In application, delayed failure is a feature of such materials, and this plays a critical role in the operation of armour and protection for key components and vital structures [2, 3]. Whilst it has been possible to measure stress states and wave speeds for these states using photography, we have never previously been able to track density change and identify fracture morphology in real time. Thus we have fielded X-ray and optical imaging to quantitatively define these states, observe fracture occurring in the glasses and then to extend to opaque brittle materials. In this work we present radiographic framing sequences with reconstructed streak of these radiographs down an impact axis. We have shown that capability exists to interrogate these phenomena, and we now present quantitative 4D results using X-ray imaging in silicates. This couples understanding of materials’ physics with characterisation of the geometry and kinetics of failure across regimes. We present an overview of the operating mechanisms and suggest analytical and numerical models that may be improved using this data for this class of brittle solids under load [4]. EXPERIMENTAL Fast indenter A precise and portable indentation device was designed to investigate failure in range of materials and material classes. The means of operation was a commercially available electrical solenoid (Ledex® Low Profile 6SFM) which delivers a force of approximately 900 N to a projecting indenter and allows it to reach velocities up to 10 m s. The solenoid shaft was assembled with the freely moving indenter tip inside the assembly shown in Figure 1. The sample stage of indenter device allows the positioning of samples of interest with rectangular cross sections with a range of thickness varies from 0.5 mm – 15 mm within a space that allows other initial conditions to be varied in a controlled manner. Two open sides allow the X-ray beam to pass uninterrupted through the target to collect transmission images with an adequate field of view. There is additional space to allow optical high speed photography of the target under load. FIGURE 1. Schematic set-up of the portable indenter. Single Bunch X-ray Imaging The experiments were carried out at ESRF ID19 beam line while the ESRF ring was operated in a dedicated 4 bunch machine operation. Under this operating mode four electron bunches were circulated in the storage ring with a current of ca. 10 mA per bunch. The measured bunch length was 140 ps (FWHM) [5, 6]. The revolution frequency of a single bunch is determined by the storage-ring circumference and is calculated to be ca. 2.8 μs for these experiments. For such single-bunch imaging experiments at ID19, the beam line was operating two U32 undulators in series with ca. 11 mm gap [7]. The usable polychromatic X-ray energy spectrum was largely confined to the energy range between 20 to 50 keV with a mean energy of approximately 30 keV. Imaging was adjusted so that phase contrast allowed cracks and their propagation to be captured and the imaging detector was therefore placed ca. 10 m distance from the experiment. The imaging detector configuration consisted of a 250 μm thick, LuAG scintillator coupled to either Shimadzu HPV-X2 cameras with 1000 x 1000 pixels or a pco.dimax camera. The Shimadzu HPV-X2 camera was used only for single bunch imaging while the pco.dimax was used to capture imaging at the slower speeds, capturing longer time windows. In the latter case, photons corresponding to more than one X-ray bunch were integrated to produce a single exposure. The Shimadzu HPV-X2 camera captured 128, 10 bit greyscale images, allowing fast and continuous image capture without any data transfer problems. The pco.dimax camera was used to capture images at 14.5 kHz frame rate with 400 x 250 pixels per image. The coupling optics and magnification allowed effective spatial resolutions of 11 μm and 8 μm for the pco.pimax and the Shimadzu cameras respectively. One of the critical aspects of in situ single bunch imaging configuration was to capture indentation and crack propagation to take place with the camera’s recording window. This required the camera aperture to be temporally synchronized with the X-ray flashes from the electron bunches after their travel through the two U32 undulators. Thus the initial triggering signal was passed to the indenter and a proximity sensor detected the position (accuracy ±20 μm) of the shaft sending a 5 V DC pulse to a digital delay generator to trigger image capture in synchronization with the X-ray flashes. In this mode, images were acquired every 1.404 μs and camera aperture was controlled to have a single frame for each X-ray bunch. Sample

Morphological Transition of α-Mg Dendrites During Near-Isothermal Solidification of a Mg–Nd–Gd–Zn–Zr Casting Alloy

Microstructure evolution in the Mg–Nd–Gd–Zn–Zr commercial casting alloy Elektron21 and in a Zn-free alloy variant, solidified under near-isothermal conditions at six constant cooling rates, has been studied via in situ X-ray radiography. In the Zn-free alloy, equiaxed α-Mg primary dendrites are always observed to develop with a steady growth rate. Conversely, in the Elektron21 alloy, primary dendrites undergo a morphological transition after nucleation and an initial transient growth for cooling rates \( \dot{T} \) ≤ 0.075 K/s. Such transition leads to a change in the growth morphology from volume spanning 3D to anisotropic sheet-like growth occurring mainly along \( \left\langle {11\bar{2}0} \right\rangle \) directions, with 4–5 times increase in the growth velocity. Experiments and simulations highlight the pivotal role of Zn, indicating that the morphological transition occurs due to the formation of ordered rare earth-zinc arrangements in the \( \{ 10\bar{1}1\} \) pyramidal and \( \{ 0001\} \) basal planes of the α-Mg lattice within a layer extending a few micrometres from the solid-liquid interface into α-Mg.

In‐Situ X‐Ray Radiographic Observations of Eutectic Transformations in Al‐Cu Alloys

In-situ X-ray radiography is currently being used to study solidification processes in a wide range of binary and ternary alloy systems. In this study, thin samples of hypoeutectic Al-Cu alloys were mounted in a Bridgman furnace and solidified near-isothermally from above the liquidus to below the eutectic. The eutectic transformation was easily identified from a contrast difference between the semi-solid mush and the fully solid phase in the field of view (FOV). By virtue of real time in-situ X-ray radiography, and image analysis techniques, it was possible to directly observe and measure eutectic nucleation as well as transformation rate across the FOV. Post-solidified samples were then subjected to further microstructural analysis, whereby the lamellar eutectic spacings were measured at random locations across the FOV region. The lamellar spacings were correlated to the eutectic transformation rate and the results compared to theoretical predictions. Reasonable agreement was observed and some interesting observations are noted, e.g. liquid feeding induced grain motion and the possible precursor to hot tearing.