Formation mechanism of β’’-Mg5Si6 and its PFZ in an Al-Mg-Si-Mn alloy: experiment and first-principles calculations

Formation mechanism of β’’-Mg5Si6 and its PFZ in an Al-Mg-Si-Mn alloy was studied by the means of experiment and first-principles calculations. Results show that at 270°C during the 100 °C/h heating period, β’’-Mg5Si6 precipitated inside the dendrites, whereas precipitation free zones (PFZs) of β’’-Mg5Si6 presented near the dendrite arm regions. The formation of the β’’-Mg5Si6 and its PFZ were related to the concentration of vacancy. Low-concertation zones of vacancy formed near the eutectics during the solidification due to the constitutional supercooling, no β’’-Mg5Si6 precipitated in the low-concertation zones of vacancy due to the vacancy-dependence of β’’-Mg5Si6, the Si vacancy-containing β’’-Mg5Si6 was extremely unstable and Si vacancies prefer to stay away from distribution.

Formation mechanism of Cu/Cu3Sn–Cu/Cu interconnections based on solder-filled microporous copper as interlayer via a current-assisted thermal compression bonding

In this study, a Cu/Cu3Sn–Cu/Cu interconnection can be achieved based on solder-filled microporous copper (MPC) as interlayer via a current-assisted thermal compression bonding. The high-temperature soldering connection materials can be used in the third-generation semiconductor packaging, so as to meet the promising application in high-temperature power device packaging. The influence of auxiliary current on the microstructure evolution of Cu–Sn IMCs and its formation mechanism were studied. Experiments show that the action of Joule heat coupled with the electron wind significantly enhanced the interfacial reaction at the Cu/Sn metallization interface. The microstructure of Cu6Sn5 changed from scallop-like into columnar due to constitutional supercooling, and the dispersion distribution of Cu6Sn5 also changed. The growth constitutive equations of Cu3Sn in bondlines were established under current-assisted thermal compression bonding process.

Extended Stefan problem for the solidification of binary alloys in a sphere

We study the extended Stefan problem which includes constitutional supercooling for the solidification of a binary alloy in a finite spherical domain. We perform an asymptotic analysis in the limits of large Lewis number and small Stefan number which allows us to identify a number of spatio-temporal regimes signifying distinct behaviours in the solidification process, resulting in an intricate boundary layer structure. Our results generalise those present in the literature by considering all time regimes for the Stefan problem while also accounting for impurities and constitutional supercooling. These results also generalise recent work on the extended Stefan problem for finite planar domains to spherical domains, and we shall highlight key differences in the asymptotic solutions and the underlying boundary layer structure which result from this change in geometry. We compare our asymptotic solutions with both numerical simulations and real experimental data arising from the casting of molten metallurgical grade silicon through the water granulation process, with our analysis highlighting the role played by supercooling in the solidification of binary alloys appearing in such applications.

Segregation and Morphological Evolution of Si Phase during Electromagnetic Directional Solidification of Hypereutectic Al-Si Alloys

Understanding the Si segregation behavior in hypereutectic Al-Si alloys is important for controlling the micro- and macrostructures of ingots. The macrosegregation mechanism and morphological evolution of the primary Si phase were investigated during electromagnetic directional solidification (EMDS). Both numerical simulations and experimental results strongly suggested that the severe macrosegregation of the primary Si phase was caused by fluid flow and temperature distribution. Microscopic analysis showed that the morphological evolution of the Si crystal occurred as follows: planar → cellular → columnar → dendritic stages during EMDS. Based on constitutional supercooling theory, a predominance area diagram of Si morphology was established, indicating that the morphology could be precisely controlled by adjusting the values of temperature gradient (G), crystal growth rate (R), and solute concentration (C0). The results provide novel insight into controlling the morphologies of primary Si phases in hypereutectic Al-Si alloys and, simultaneously, strengthen our understanding of the macrosegregation mechanism in metallic alloys.

Microstructure of Directionally Solidified Mg-Zn Alloy with Different Growth Rates

The influence of withdrawal rate on the microstructure of directionally solidified Mg-x%Zn (x=2, 4, 6) alloys was investigated in this paper. It was found that with the withdrawal rates increased from 20 μm/s to 60 μm/s, the morphology of the solid-liquid interface changed from planer to cellular dendrite. When the growth rate was further increased to 120 μm/s, the solidification microstructure appeared to be the typical dendrite structure with the developed secondary dendrite arms. Meanwhile, the dendrite arm spacing decreased with the increase of growth rate. Under the same solidification conditions, the microstructure went through cell branch transformation with the increase of Zn content within a lower withdrawal rate range; while the Zn content did not affect the morphology at a higher withdrawal rate. As well, the microstructure was refined gradually with the increase of Zn content. The effects of withdrawal rate and alloying content on morphology were analyzed by constitutional supercooling and the MS theory.