【作者】 王狂飞；

【导师】 郭景杰； 李邦盛；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2007， 博士

【摘要】 本文采用数值模拟方法研究了Ti-Al合金定向凝固组织演变过程,论文工作有助于深入理解Ti-Al合金定向凝固组织形成规律,并为今后实验研究及控制组织形貌提供参考。对于受溶质扩散控制的单相凝固过程,提出了溶质扩散控制模型结合元胞自动机方法模拟合金组织演化的模型。在模型中,采用扩散流方法来处理固/液界面两侧不连续的物理性质。随后利用这种模型模拟了Al-30wt.%Cu合金组织演变过程,并与实验结果和理论分析结果对比,以验证模型的准确性。结果表明,模拟结果与经典凝固理论相符合,说明该模型的准确性。模拟结果同时与实验结果吻合良好,证明这种方法的有效性。此外,模拟还再现了定向凝固实验经常观察到的尖端分裂现象,模拟获得的枝晶形态与实验结果一致。借助上述模型,对Ti-Al合金在发生液固相变L→β或者L→α单相定向凝固过程中组织的动态演化过程进行了模拟。通过模拟,取得了与实验相吻合的一些重要结果,如枝晶臂的粗化和根部颈缩,以及枝晶臂间的碰撞、融合和竞争生长等。在此基础上,模拟了定向凝固参数影响下组织演化过程,结果表明,随着抽拉速度增大,凝固形态经历了平面→胞晶→胞状树枝晶→树枝晶转变过程。在平界面生长阶段,整个固液界面是以近平界面方式生长,而在胞晶/枝晶混合生长阶段,胞/枝晶间距不均匀,形核数量的增加有助于改善间距的不均匀度。在一定的抽拉速度下,随着温度梯度增大,枝晶间距减小;而温度梯度较低时,在胞晶/枝晶混合生长阶段,胞/枝晶间距出现最大值。对凝固相为β和α作为单相生长时组织演化过程进行了对比模拟,发现由于溶质分配系数、液相线斜率存在很大差别,在凝固条件相同的情况下,β相凝固界面形态更多表现为胞晶或枝晶,而α相多为平面或胞晶,模拟结果与理论推导相一致。在溶质扩散控制模型基础上,建立了柱状晶/等轴晶转变(CET)的溶质影响模型,利用该模型模拟了定向凝固参数(温度梯度、抽拉速度、晶核间距、形核过冷度)对CET的影响。结果表明,温度梯度、抽拉速度对CET影响定性地符合Hunt解析解。横向晶核间距影响比纵向大;形核过冷度增加,CET推迟;柱状晶间距不仅与温度梯度、抽拉速度有关,而且可通过激活预置晶核使其生长为柱状晶而得以调整,特别是温度梯度在30K/mm~50K/mm之间,形核有较强的调节柱状晶间距的作用。根据成分过冷准则以及相稳定生长的最高界面温度判据,对Ti-Al合金初生β相作为单相领先生长时,包晶相直接凝固过程进行了模拟,获得了凝固组织的动态演化过程,以及与实验观察相吻合的现象。研究表明α相依附于β相生长,在β相胞/枝晶生长间距较小或者溶质成分为49at.%Al时,α相仅沿β相侧面生长较快,侧向增厚是α相的主要推进方式,而在β相胞/枝晶生长间距较大、合金成分接近47at.%Al时,两相倾向于以胞/枝晶形态生长。更多还原

【Abstract】 In this paper, the evolutions of interface morphology and structure are simulated for Ti-Al alloy during its liquid-solid phase transition in directional solidification, which helps us to further understand the directional solidification mechanism of Ti-Al alloy, and it also provides a theorical reference for controlling the microstructures in experiment.During directional solidification of single phase, the model which combines solute diffusion controlled model with Cellular Automation( CA) is presented to simulate microstructure evolution by using, and a diffusion flow method is employed to deal with the discontinuous thermophysical properties in both sides of the solid/liquid interface. And then a microstructure evolution of Al-30wt.%Cu alloy is simulated so as to verify the model, and at the same time, simulated results are compared with experimental and theoretical analysis results. The quantitative analysis of solute distributions along dendritic tips at different growth conditions shows that the simulation results are identified with the classical theory of solute diffusion controlled solidification, which testifies the accuracy of the model. The simulation also reproduces the phenomenon of tip splitting observed often in directionally solidified experiments, and the simulated dendritic morphologies agree well with experimental observation.Using above the solute diffusion controlled model and choosing Ti-(40-50) Al (at.%) alloy , the microstructure evolution of single phase alloy is simulated during its liquid-solid transition of L→βor L→α. The dynamic evolutions of microstructures in directional solidification are derived, and they also match well with experimental observations, such as the coarsening and necking of dendritic arms, the impingments, coalescences and competitional growths etc. Furthermore, a parametric study is performed to investigate the effects of the applied temperature gradient and pulling velocity, the results show that, with increasing pulling velocity, the morphology of the interface varies from plane to cell to dendrite. While in planar growth, whole interface is near planar, and in cell/dendrite transition, the uniform degree of dendrite spacing tends to decrease, and with increasing the numbers of the seed, the uniform degree of dendrite spacing tends to increase.At a fixed pulling velocity, increasing thermal gradient decreses the dendrite arm spacing, and at low thermal gradient, and at cell/dendrite transition zone, the cellular spacing increases to a maximum.Comparision of mcrostructure evolution for directional solidificatedβphase withαphase is done by numerical simulation, the results show that due to different liquidus slope and solute partition coefficient, even at same solidification conditions, cell/dendrite structure is often formed forβphase, and plane/cell structure is often formed forαphase, which is identical with the theory of solidification.Solute diffusion controlled model is presented to simulate the columnar-to-equiaxed transition (CET) by using pre-setted seeds. A parametric study is performed to investigate the effects of the applied temperature gradient and pulling speed, the seed spacing and nucleation undercooling for the equiaxed grains on the CET.The results illustvelocity that the CET depends quantatively on the temperature gradient and pulling velocity, And the present simulations agree with the analytical model of Hunt. And the seed spacings of a cross section has a stronger effect on CET than that of a longitudinal section; for large nucleation undercooling, columnar grain growth is favored; the columnar branch spacing depends not only on the temperature gradient and the pulling velocity, but also on the pre-setted seeds, this hints us that a spacing adjustment can occur through initiation of seeds that develop into new columnar grains.According to criterions of compositional undercooling and the highest interface temperature of steady growth of one phase,the direct growth ofαphase from liquid phase is simulated. The dynamic evolutions of solidification microstructures are derived, and also some phenomena are identical with experimental observations, such as, the peritecticαphase surrounding the primaryβcells. The present simulations also reveal a number of other interesting phenomena related to the growth ofαphase, for example, for large cell/dendrite spacings of a leading primaryβphase or Ti-49at.%Al alloy,αphase tends to grow atβphase interface, and for samll cell/dendrite spacings of a leading primaryβphase or near Ti-47at.%Al alloy, and two phases tend to grow independently.更多还原