定向凝固羽毛状孪生枝晶形成条件与生长形态研究

Feathery grain formation in semi-continuous casting of aluminum alloys has been studied over many years. Although it is well known that the feathery grain may be the kind of the twinned dendrites and the twinned plane (111) has been clearly revealed during the solidification pocess, there are still many open questions on the formation mechanism and growth morphology of the twinned dendrites. These questions are described as follows: (1) whether the convection conditon is necessary for the formation of the twinned dendrites in aluminum alloys; (2) what is the twinned dendrite morphology, the doublon or the edge dendrite or the regular dendrite with a re-entrant angle at the dendrite tip? (3) what is the relationship between the twinned dendrite and regular dendrite and why the twinned dendrite can overgrowth the regular dendrite. With these open questions, we aim to study the formation mechansim and three-dimensional morphology of the twinned dendrites with the orientation selection during directional solidification of aluminum alloys. In this project, Al-X alloys (X includes Cu, Mg, Zn elements) are employed to study the solute effect on the formation of twinned dendrites since these elements have different crystal structures causing the stack faults available for the growth of the twinned dendrite. By aiding the modern analysis methods including XRD, SEM and TEM and the serial sectioning technique, the three-dimensional patterns of twinned dendrites will be investigated. Futher, to remarkably reduce the convective effect, we use the small diameter sample to conduct the study on the microstructure evolution of directionally solidified aluminum alloys in diffusive and convective regime. Based on the experimental results, the twinned dendrite pattern will be eluciated and compared with the dublon. The similar and difference of their morphologies will be explained to clearify the the growth mechanism of the twinned dendrite. The twinned dendrites formed by the convective-inducing or solute-inducing will be addressed. Also, the solidfication processing paramaters for the formation of the twinned dendrites will be obtained to prcisely control the solidification microstrucutres of the aluminum alloys. The relationship between the twinned dendrite tip and solidification vleocity with the corresponding undercooling will be developed to illustrate the competitive growth of the twinned dendrites and regular dendrites.These novel results can not only contribute to the modern solidifiction theroy, but also directly optimize to control the alloy solidification pattern and develop of the next-generation light aluminum alloys.

半连续铸造铝合金凝固组织中常会出现羽毛状晶，该晶为一类孪生枝晶，它的出现严重消弱了合金力学性能，对材料后续加工成形不利。铝合金中孪生枝晶虽有明确的孪生面，生长方向，但其生长形态比较复杂性，有一定的争议，且形态与晶向对应关系不明确。生长机制与凝固工艺参数范围也不清楚，也没有建立孪生枝晶与正常枝晶竞争生长的规律。为此本项目利用高温度梯度定向凝固技术，结合现代材料分析方法（SEM/EBSD/TEM/HRTEM等）和连续切片技术，开展有无对流条件下孪生枝晶三维生长形态、生长机制与晶向选择研究。通过重构的孪生枝晶三维生长形态，来澄清孪生枝晶生长形态是否为双子晶，并给出确切的实验证据。揭示孪生枝晶形成机制――对流诱发孪生枝晶还是溶质诱发的孪生枝晶，给出其生长的条件，建立起孪生枝晶与正常枝晶之间竞争生长的规律。上述研究，不仅能促进现有合金凝固理论的发展，而且对铝合金凝固组织预测和控制有很好的指导作用。

定向凝固羽毛状孪生枝晶形成条件和生长形态研究，主要开展定向凝固工艺参数对孪生枝晶生长形态的影响，探讨定向凝固下铝合金中形成孪生枝晶机制，讨论对流对孪生枝晶生长的作用，以及孪生枝晶生长动力学行为。实验研究发现，孪生枝晶可以在常规枝晶出现的范围存在，打破了孪生枝晶必须在很大的凝固速率（~1mm/s）下形成的观点，极大拓宽了孪生枝晶凝固速率范围，发现合金对流作用非常微弱。提出铝合金中所谓的“孪生胞晶”概念，孪生胞晶的稳态生长过程中发现了共格孪晶界两侧区域连续取向旋转现象，并在接近平胞转变的更低生长速度下，出现孪晶界的退共格化与偏离孪晶的取向调整。发现层错与两孪晶界直接相连的微观结构，为层错形成与铝合金中孪生枝晶生长的紧密关系提供了直接的关键实验证据。针对孪晶生长形态的选择过程，证明合金凝固领域还存在另外一类包括孪晶缺陷的平-胞-枝晶结构，给出了相应的凝固工艺参数范围。同时针对孪生枝晶与常规枝晶、以及不同孪生枝晶之间竞争生长进行了系统研究，发现低速下孪生枝晶与常规枝晶相比，并不具有动力学的生长优势，孪生枝晶与常规枝晶共存。只有高速凝固下，孪生枝晶相对于常规枝晶才有动力学优势，而这种动力学优势来源于孪生枝晶的侧向快速生长和增殖机制，为此我们提出了一种不需借助于堆垛层错形核的孪生枝晶侧向增殖新机制。上述结果不仅丰富了合金凝固理论界面生长形态学，有较好的学术意义，而且也发现孪生枝晶性能较好，这对后续开发高强韧铝合金也有指导意义。