In situ synchrotron x-ray study of ultrasound cavitation and its effect on solidification microstructures

Considerable progress has been made in studying the mechanism and effectiveness of using ultrasound waves to manipulate the solidification microstructures of metallic alloys. However, uncertainties remain in both the underlying physics of how microstructures evolve under ultrasonic waves, and the best technological approach to control the final microstructures and properties. We used the ultrafast synchrotron X-ray phase contrast imaging facility housed at the Advanced Photon Source, Argonne National Laboratory, US to study in situ the highly transient and dynamic interactions between the liquid metal and ultrasonic waves/bubbles. The dynamics of ultrasonic bubbles in liquid metal and their interactions with the solidifying phases in a transparent alloy were captured in situ. The experiments were complemented by the simulations of the acoustic pressure field, the pulsing of the bubbles, and the associated forces acting onto the solidifying dendrites. The study provides more quantitative understanding on how ultrasonic waves/bubbles influence the growth of dendritic grains and promote the grain multiplication effect for grain refinement

High speed synchrotron X-ray imaging studies of the ultrasound shockwave and enhanced flow during metal solidification processes

The highly dynamic behaviour of ultrasonic bubble implosion in liquid metal, the multiphase liquid metal flow containing bubbles and particles, and the interaction between ultrasonic waves and semisolid phases during solidification of metal were studied in situ using the complementary ultrafast and high speed synchrotron X-ray imaging facilities housed respectively at the Advanced Photon Source, Argonne National Laboratory, US, and Diamond Light Source, UK. Real-time ultrafast X-ray imaging of 135,780 frames per second (fps) revealed that ultrasonic bubble implosion in a liquid Bi-8 wt. %Zn alloy can occur in a single wave period (30 kHz), and the effective region affected by the shockwave at implosion was 3.5 times the original bubble diameter. Furthermore, ultrasound bubbles in liquid metal move faster than the primary particles, and the velocity of bubbles is 70 ~ 100% higher than that of the primary particles present in the same locations close to the sonotrode. Ultrasound waves can very effectively create a strong swirling flow in a semisolid melt in less than one second. The energetic flow can detach solid particles from the liquid-solid interface and redistribute them back into the bulk liquid very effectively

A study of the mechanical properties of PMMA and PS at strain rates of 10−4^{-4} to 103^3 over the temperature range 293–363 K

This paper describes work in which stress-strain data is obtained for samples of PMMA and PS during deformation at strain rates in the range 10$^{-4}$ to 10$^3$ s$^{-1}$ at temperatures in the range 293 K to 363 K. The effect of strain rate on the $\beta$-transition of PMMA and PS is also studied over a range of temperatures but a limited strain rate range. The relationship between strain rate and activation energy and volume is obtained by applying Eyring theory to the PMMA stress-strain curves at their yield point

Crystallisation effects during high rate deformation of polymers

Previous work with PEK, PEEK and nylon has shown a close correlation between the rapid increase in yield stress as a function of strain rate of these semi-crystalline polymers at strain rates > 103 s-1 and the crystallinity content of samples recovered after a test. It has been speculated that the crystallinity increase may be partially responsible for the flow stress increase. This paper describes work carried out to extend the previous work to PET and has confirmed that the correlation also holds for PET. An extensive investigation into the morphology of tested strain limited PET samples using X-rays and scanning calorimetry has shown that the crystallinity does not develop until the latter part of a test and that crystallinity changes cannot therefore contribute to yield and flow stress increases

A systematic investigation of the compressive behaviour of near equi-atomic NiTi over a range of strain rates and temperatures

An investigation of the compressive stress-strain behaviour of a 50.4% Ni by weight NiTi shape memory alloy when subjected to both quasi-static and high strain rate deformation across a range of temperatures is reported. Testing was carried out using a Hounsfield universal testing machine and a split Hopkinson pressure bar system. Transformation stress dependence on both increasing strain rate and temperature is presented, together with observed values of strain rate sensitivity and derived entropy of transformation per unit volume

Flow stress peaks at high strain rates in PMMA

The results of an investigation into the stress-strain behaviour of PMMA in the strain rate region of 102 to 104 s−1 over a range of temperatures from room temperature up to 90°C are reported. The investigation was carried out in an effort to detect the presence, or absence, of peaks in the flow stress – strain rate response. All tests were carried out in compression on a standard Hopkinson Bar system. Although not as clearly defined as the sharp peak in flow stress reported by previous workers in PEEK, there is good evidence of a peak in the flow stress in the strain rate range 102 to 103 which both narrows and moves to a higher strain rate as the temperature approaches the glass transition. Previously reported peaks in flow stress had only been observed in semi-crystalline polymers. The current observations in PMMA suggest that this may be a universal phenomenon. Preliminary work at similar strain rates and temperature ranges on PCTFE, show less conclusive indications of peaks in the flow stress at strain rates around 3×103 s−1

TEMPERATURE RISES DURING HIGH RATE DEFORMATION OF POLYMERS

Des expériences pour mesurer la réponse contrainte-déformation et l'élévation de température à la fois en volume et localement ont été effectuées sur une large gamme de polymères à des vitesses de 102 à 103 s-1. Les paramètres de décomposition et les chaleurs spécifiques ont été déterminées par calorimétrie différentielle. Les résultats obtenus suggèrent qu'une décomposition thermique significative peut se produire en avant des fissures dans certains matériaux et dans tous les cas, l'échauffement conduit à des déviations de la courbe isotherme contrainte-déformation vraies.Experiments to measure stress-strain response and both bulk and localised temperature rises have been carried out on a wide range of polymers at strain rates of 102 - 103 s-1. Decomposition parameters and specific heats have been determined by Differential Scanning Calorimetry. Results obtained suggest significant thermal decomposition may occur ahead of running cracks in some materials and in al1 cases heating leads to deviations from a true isothermal stress-strain curve

Crystallinity Increases in Semi Crystalline Polymers During High Rate Testing

During low rate deformation in tension semi-crystalline polymers can undergo a strain induced crystallisation during deformation which leads to increases in flow stress. This work extends studies of crystallinity in the polymers PELK PEEK and PET to compression at high strain rates at a variety of temperatures. All three polymers show large increases in crystallinity when tested at high rates with PEK and PEEK showing increases only at rates of 103/s. It is speculated that the rapid increases in flow stress reported in polymers at high strain rates may be due to the rapidity of strain induced crystallisation at these strain rates.Pendant une déformation à basse vitesse en traction, les polymères semi-cristallins peuvent subir une cristallisation induite par déformation qui conduit à une augmentation de la contrainte d'écoulement. Ce travail concerne l'étude de la cristallinité des polymères PEK, PEEK et PET provoquée par la déformation en compression à vitesses de déformation élevées et à différentes températures. Les trois polymères montrent une forte augmentation de cristallinité lorsqu'ils sont testés à grande vitesse ; le PEK et le PEEK ne montrant qu'une augmentation qu'à partir d'une vitesse de 103/s. On suppose que l'augmentation rapide de la contrainte d'écoulement révélée par les polymères à vitesse élevée pourrait être due à la rapidité de la cristallisation induite à ces vitesses de déformation correspondantes

THE RESPONSE OF VARIOUS POLYMERS TO UNIAXIAL COMPRESSIVE LOADING AT VERY HIGH RATES OF STRAIN

Une barre de Kolsky à choc direct a été utilisée pour obtenir des courbes contrainte-déformation à vitesse de déformation de 2 x 104 s-1 dans un essai de compression sur polymères. De plus, la déformation de disques de polymères imposée par la chute d'un poids a été étudiée par photographie ultrarapide ; l'intervalle entre chaque image était de 7 µs et la vitesse de déformation de 5 x 103 s-1. Enfin, une nouvelle technique basée sur l'utilisation de film sensible à la chaleur a été développée afin d'évaluer l'accroissement de température associé à une déformation rapide.A direct impact Kolsky bar has been used to obtain stress-strain curves for polymers at compressive strain rates of 2 x 104 s-1. High speed photography with an interframe time of 7 µs has been used to study the deformation of solid polymer discs in a drop-weight machine at strain rates of ca. 5 x 103 s-1. A novel technique using heat sensitive film has been developed to estimate the temperature rises associated with rapid deformation