Data and videos for ultrafast synchrotron X-ray imaging studies of metal solidification under ultrasound

The data presented in this article are related to the paper entitled ‘Ultrafast synchrotron X-ray imaging studies of microstructure fragmentation in solidification under ultrasound’ [Wang et al., Acta Mater. 144 (2018) 505-515]. This data article provides further supporting information and analytical methods, including the data from both experimental and numerical simulation, as well as the Matlab code for processing the X-ray images. Six videos constructed from the processed synchrotron X-ray images are also provided.


Subject area
Materials Science and Engineering More specific subject area

Solidification of Metallic Alloys
Type of data The data are available with this article

Value of the data
The videos presented clearly demonstrate the microstructural fragmentation induced by ultrasound treatment in a liquid metal.
The datasets can be used for comparison with other experimental and theoretical results. The data on acoustic pressure can be used to determine the ultrasound induced pressure at any depth from the sonotrode tip.
The Matlab code can be used for future synchrotron X-ray image processing.
1. Data Table 1 shows the measured amplitudes of the sonotrode tip from the ultrafast X-ray images when different ultrasound powers were applied; and the resulting ultrasonic intensities. Fig. 1 presents the distribution of acoustic pressure in the Bi-8%Zn liquid metal along the distance below the sonotrode tip. Fig. 2 gives the growth in percentage of solid needle-shaped Zn phase during the solidification process of the Bi-8% Zn alloy without ultrasound under a cooling rate of 0.2°C/s. Fig. 3 demonstrates the experimental data used to determine the material constants for the Zn alloy at 20, 50 and 100°C. The Matlab code used to obtain the percentage and speed of the solid particles from synchrotron X-ray images is listed in Appendix A. Synchrotron X-ray images in supporting the evidences and arguments presented in Figs. 4-6 and 8 in [1] were processed into videos. Video 1 shows the ultrasonic bubble implosion and shock wave; Video 2 presents the bubble pulsating on a liquid-solid (L-S) interface; Video 3 gives the fragmentation of a needle-shaped Zn particle by an oscillating bubble; Videos 4-6 demonstrate the break-up of the L-S interface by acoustic flow under different ultrasonic intensities.

Experimental design, materials and methods
Ultrafast Synchrotron X-ray imaging experiments were carried out to study the effects of ultrasound on the fragmentation of metallic phases during the solidification process as detailed in [1]. A Bi-8%Zn alloy was used because of its low melting temperature and sufficient X-ray contrast between the matrix and the primary Zn phases. A Hielscher UP100H ultrasound processor with a MS2 sonotrode was adopted to introduce ultrasound into the melt. As supplementary to the full research paper [1], this article provides further experimental and simulation data as listed below.

Ultrasonic intensity
To measure the actual vibration amplitudes of the sonotrode tip, the same ultrasound processor as in [1] was used, with the Ø2 mm sonotrode tip submerged in a distilled water tank (50 × 50 × 50 mm). Ultrasound was applied for 1 s, and a Phantom TM V7.3 high speed camera was used to capture the images at 11,000 frame per second (fps) with a spatial resolution of 4.43 µm/pixel. Then, the real vibration amplitudes of the sonotrode tip can be measured from a set of images by using the FIJI [2] software, and the resulting data are listed in Table 1. Therefore, the corresponding ultrasonic intensity, I, can be calculated as below [3]: where, ρ is the density of the melt; C is the sound speed in the liquid media; A 0 is the amplitude of the cyclic pressure wave; ω ¼ 2πf is the angular frequency with f being the frequency of the sound wave.

Acoustic pressure
The alternating acoustic pressure field generated from the ultrasonic waves in a liquid medium attenuates exponentially along the distance below the sonotrode tip [4]. The acoustic pressure, P a , at a certain depth follows the Helmoholtz equation [5]: Eq. (2) can be solved by using the finite element-based software COMSOL Multiphysics as detailed in [6]. Fig. 1 presents the distribution of acoustic pressure in the Bi-8%Zn liquid metal along the distance below the sonotrode tip under the ultrasonic power of 20 W, 60 W and 100 W.

Solidification of the Bi-8%Zn alloy
Solidification of the Bi-8%Zn alloy without ultrasound was monitored by using the synchrotron X-ray imaging at Beamline I-12, Diamond Light Source (DLS), UK. Fig. 2 shows the growth of the solid needle-shaped Zn phase during the solidification process, the insets are the real-time X-ray images captured at 30 fps.

Fatigue analysis of Zn phases
Experimental data for a commercial Zn-4% Al alloy as reported by Sawalha [7] were used to predict the fatigue behavior of Zn under 270°C. The fatigue strength, S, and fatigue life, N, of alloys can be calculated by using the Basquin's Law as detailed in [1]. For high-cycle fatigue (with N 4 10 5 cycles), three data points below 90 MPa were selected at each temperature from [7] to determine the material constants. Fig. 3 shows the linear relationship between (log N) and (log C) at 20, 50 and 100°C.

Synchrotron X-ray image processing
The percentage and speed of the detached solid particles were obtained by analyzing the X-ray images using a Matlab script [8]. The code is provided in Appendix A.