Morphological Evolution During Directional Solidification Of AL-CU Alloys

How to control the solidification process of castings as well as their microstructure is an important interest in materials science. For large castings with complicated shapes, the mechanical properties can be greatly enhanced by reasonable designs of structure and casting technique, which can also make full use the potential properties of materials. Many factors are concerned with structure design, including solidification conditions, morphological evolution, material properties and the relationships among them, of which the study of morphological evolution is one of the key contents to be considered.One difficulty in the imaging study of the morphological evolution in solidification is that metals, most with high melting points, are opaque to visible light. The other is that the grow front is hard to be continuously tracked and recorded. Therefore, transparent organic alloys and rapid quenching technique were commonly used to study the grow front. However, in many cases, there is a great difference between the model and the reality. Real-time observations of the solidification process during the solidification of metals are still necessary. Fortunately, at present, the high-brilliance synchrotron X-radiation imaging technique based on phase-contrast has shown promise for in situ observations of evolving solidification microstructuresIn the present work, binary Al-Cu alloys with different compositions were selected as the model materials. With the facilitation of the beam-line BL13W1 in SSRF, the phenomena in crystal growth during the solidification process of the alloys were systematically observed and studied. Several crucial problems, such as the reason for the detachment of dendrite arms, columnar-to-equiaxed transition (CET), the selection of dendrite morphology and morphological evolution of Al2Cu crystals, were studied in this thesis.The dendrite fragments can serve as the cores for the nucleation of equiaxed dendrites at the growth front, promoting the formation of equiaxed dendrites and interrupting the continuous growth of columnar dendrites. Therefore, they play important roles in the evolution of dendrite morphology. Selecting hypoeutectic Al-Cu alloys as the model materials, the reasons for fragmentation are discussed from the viewpoints of the mechanical and remelting mechanisms, respectively. The relationship between the detachment of dendrite arms and constitutional supercooling as well as the melt flow at the growth front is unveiled. It is found that the mechanical and remelting effects coexist and promote each other in the detachment process. The remelting effect plays a leading role in the initial stage of the detachment. Then the effect of stress concentration becomes obvious as the root radius of the dendrite arms decreases. The breakdown occurs once the root radius decreases to a critical value corresponding to the yield stress of the dendrite arms. It is also found that there is a sensitive region for dendrite detachment, which is about 800-1000μm from the growth front. The detachment of dendrite arms can be consecutive.It is found that at the stage before CET, the tips of the columnar dendrites with small spacing of primary dendrite arms grow in a regular manner. There are steady temperature and composition gradients along the longitudinal direction of the sample. A strong melt flow is hard to be caused and the columnar growth is in a dynamic steady state. However, the orientation mismatch of these primary arms will gradually lead to the formation of larger melt zones at the growth front, which is the liquid pocket. High-order dendrite arms around the liquid pocket are more developed and get more exposed to the stronger melt flow formed here, and the high-order dendrite arms at the detachment-sensitive area will be detached from the dendrite network in this condition. Then they are transported upwards by buoyancy, move out of the pockets due to the density difference and grow dendritically in the melt. Because the density difference disappears, their movement will stop at the growth front, which blocks the continuous growth of the columnar dendrites. For alloys with high purity, the fragments by consecutive fragmentation can serve as the cores for the nucleation of equiaxed dendrites.A new dendrite morphology, which is named anaxial columnar dendrite, was found to appear in directional solidification of the Al-15wt.% Cu alloy. The dendrite is composed of a pair of stems, which are divided by a narrow liquid zone located in its center. It is confirmed that it grows along <110> directions. According to the basic morphology of the dendrite, a tip model was constructed to explain the growth preference of the secondary arms.The crystallization process of Al2Cu was also studied in details. These crystals are long and slender and grow in a tuft, with their tips needlelike. The neighboring tips gradually adhere with each other at the root. The cross-section of the tuft-like crystals is irregularly square, with its center surrounded by small outlying crystals. According to the analysis of crystal lattice of Al2Cu and the heat transfer in the solidification process, the formation reason for the tuft-like morphology of Al2Cu crystals was analyzed.In summary, the laws for some key morphological evolutions in the growth process of crystals are unveiled by real-time in situ studies of the solidification process of Al-Cu alloys with different compositions. The conclusions are useful for structure controlling for hypo/hyper-eutectic alloys and are helpful to improve the understanding of morphological evolution of crystals and provide important references for casting technology and solidification theory.