The effect of electric field on the solidification microstructure of metal and alloys has been investigated extensively since the process of nucleation and evolution of microstructure modified by electric current, such as the evolution of dendrite morphology and grain refinement, were discovered in 1960s. However, the previous experiments mentioned above were done by researchers via metallograph and scanning electron microscope to analyze final solidification after complete solidification by quenching. It will lead to the missing of some crucial solidification dynamic process. Professor Tongmin Wang’s research team from School of Materials Science and Engineering, Dalian University of Technology has gained insight into equiaxed growth of Sn-Pb alloy under an applied electric field. Their research has been published on volume 89th, 2012 in Materials Letters.

The team has found that the equiaxed dendrite morphology and growth rate of Sn-Pb alloy have been modified obviously due to imposing direct current (DC). The sequence of images of the evolution of dendrite morphology has been provided without DC and with DC treatment. The results show that without DC treatment, the primary dendrite arms of a grain grow rapidly in one particular direction, while the growth is restricted in the other directions. The grains tend to grow in an irregular equiaxed morphology (denoted by white arrows in figure a). This phenomenon is referred to as “self-poisoning” by the researcher, Bogno, in France. With DC treatment, the induced electromagnetic flow washes the sides and underpart of the dendrites, decreasing the solute concentration nearby. The dendrites in every direction grow with the same rate and the dendrite “self-poisoning” is thus weakened. Finally, a regular equiaxed dendrite appears. This phenomenon is termed as “electricity-detoxicating”.

Sequence of images and quantitative analysis showing the evolution of equiaxed dendrite morphology on Sn-Pb under different conditions,determined at 6.7μm resolution using synchrotron radiation at BSRF: (a) without DC, the primary dendrite arms of a grain grow rapidly in one particular direction, while the growth is restricted in the other directions. This phenomenon of dendrite “self-poisoning” occurs and the grains tend to grow in an irregular equiaxed morphology; (b) with DC, the induced electromagnetic flow washes the sides and underpart of the dendrites, decreasing the solute concentration nearby. The dendrites in every direction grow with the same rate and the dendrite “self-poisoning” is relieved via electric current. Finally, a regular equiaxed dendrite appears. (c) variation in mean grain size versus time without DC and with DC. (d) time evolution of the mean growth rates without DC and with DC.

《Scientific Abstracts》, 16 Nov 2012, has reported the work of this research team in details: “Professor Tongmin Wang’s research team tracks the international scientific frontiers and raises the new research ideas. They firstly observe dendrite growth behavior of metal alloy with regulation of electromagnetic field via in situ visualization and reveal the internal mechanism of dendrite branches and grain refinement suppressed and promoted by electric field, respectively. They also find that when electric density increases, a special tip-split occurs to the primary arm leading to the “branching” of the primary dendrite and then give the reasonable explanation. This work is a highly significant advance that is certain to cause positive attention and repercussion of the same profession at home and abroad. ”

The research provides the direct proofs to understand dendrite growth behavior of alloys and the modification of solidification microstructure under an applied electromagnetic field. In present work, synchrotron sources have helped the team to unveil the essence of the modification of dendrite morphology and growth rate under an applied electric field. "At present, there are still the great difficulties to numerical modeling the dendrite motion and dendrite arm fracture in micro-scale. So, it is important to study the dendrite growth under convection condition using synchrotron X-ray imaging technology. We hope that the law and mechanism of the evolution of dendrite morphology, competition and selection, dendrite arm fracture and dissociation affected by convection during solidification can be made certain by combining with synchrotron radiation experiments and micro-scale numerical simulation." explains Tongmin Wang.

Article:

Jing Zhu, Tongmin Wang*, Fei Cao,Wanxia Huang,Hongwang Fu,Zongning Chen. Real time observation of equiaxed growth of Sn-Pb alloy under an applied direct current by synchrotron microradiography. Materials Letters 89(2012), 137-139.