【作者】 袁海；

【导师】 薛祥；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2010， 硕士

【摘要】 微观组织数值模拟对金属材料的发展和应用有着重要意义,也是计算机应用于材料科学领域的主要发展方向之一。微观组织数值模拟的方法主要有:确定性方法、随机方法及相场法。其中相场法是一种用于描述在非平衡状态中复杂相界面演变强有力的工具,不需要跟踪复杂固液界面,就可实现模拟金属凝固过程中枝晶生长的复杂形貌,是目前凝固组织模拟的国际前沿研究领域。本文采用相场法对合金凝固过程的枝晶生长进行了数值模拟,深入探讨了合金凝固过程的枝晶生长机制,为最终实现铸件机械性能的预测奠定了良好的基础。采用基于均匀网格的有限差分离散控制方程,在网格剖分时采用了双重网格法。数值计算时,为了避免时间步长的限制,温度控制方程则采用交替方向隐式法(ADI算法);并采用了窄带固液界面法和捕获溶质扩散层边界法两种方法共同优化相场模型的数值求解算法,大大减少了计算量,提高了计算效率。在等温凝固条件下,模拟了Ni-Cu二元合金凝固过程枝晶的等轴生长的演变过程;再现了等轴枝晶生长过程侧向分支的竞争生长、熟化过程和溶质偏析。采用Neumann温度边界条件进行了合金凝固枝晶生长的非等温模拟,研究了过冷度对枝晶生长模拟结果的影响,随着温度的降低,枝晶的生长速率越快,枝晶的二次晶臂越发达,枝晶中的溶质偏析越严重,但当温度降到临界温度1569K时,微观偏析迅速降低。多晶粒的模拟结果不同于单晶粒,枝晶相互接触后,枝晶生长收到抑制,发生弯曲,枝晶不再对称,连续形核模型下的多晶粒模拟结果与真实情况更为逼近。定向条件下出现初始平界面失稳,形成胞晶,胞晶间距逐渐调整,最后稳定生长的胞晶间距约为1.8μm。随着初始温度的降低,界面逐渐由柱状晶向胞状晶再到平界面演变。更多还原

【Abstract】 Phase-field method can be used to describe the complicated morphologies of dendritic growth without explicitly tracking the complex phase boundaries. It is expected as a powerful tool to describe complex phase transitions in non-equilibrium state. It is the frontier domain of the numerical simulation during solidification processes at present.The dendritic growth in the solidification of alloy is simulated by the phase-field method, the mechanism of the dendritic growth during solidification of the alloy is discussed, and a favorable base for prediction of mechanical property of casting is established. The governing equations are discretized on uniform grids using the Finite Difference method, and a double grid method is used for the mesh slice. The thermal governing equation is numerically solved using an alternating direct implicit (ADI) method, which is unconditionally stable, irrespective of the time step employed. The narrow solid/liquid interface method and the capturing solute diffusing boundary method are put forward to optimize the numerical computation of the phase-field model.The free growth of dendrite in the solidification of Ni-Cu binary alloy are simulated on the isothermal condition, the competitive growth of the secondary arms, ripen and solute microsegregation in the free dendritic growth are realized. The dendritic growth of alloy solidification is simulated using the non-isothermal model with Neumann boundary conditions, and the effect of undercooling on the dendritic growth is studied. With the decreasing of freezing storage temperature, the dendrite growth rate of the dendrite faster, secondary crystal arm more developed, and solute segregation moreserious, but when the temperature drops to the critical temperature 1569K, micro segregation lowers quickly.More grain results and single grain are not the same. When the dendrite interaction, dendrite growth is suppressed, bending, dendrite no longer symmetrical. More grain simulation of Continuous nucleation model and real situation is approaching. In the directional conditions, flat interface chip cell instability, cellular crystal spacing gradually adjusted, and finally the steady growth of cellular crystal spacing is about 1.8 um. As the initial temperature decrease, interface gradually from the columnar crystal to the cellular crystal to flat interface evolution.更多还原