晶体取向对镍基单晶高温合金枝状组织的影响

本文研究了一种镍基单晶高温合金不同晶体取向形成的枝晶组织的变化规律，发现枝晶择优生长方向偏离热流方向时，一次枝晶形态发生明显的变化，二次枝晶分枝出现明显的不对称性。     

高温合金定向凝固过程中枝晶生长与溶质对流数值模拟

目的 针对高温合金叶片在定向凝固过程中容易出现雀斑缺陷,从而导致叶片报废的问题,对定向凝固枝晶生长与溶质对流进行模拟研究,以揭示雀斑缺陷的形成规律。方法 针对CM247LC合金定向凝固过程,采用相场模型模拟凝固过程枝晶生长,采用格子Boltzmann模型模拟溶质浓度差引起的自然对流。采用基于双重网格的GPU并行算法对相场-格子Boltzmann模型进行数值求解。研究在不同晶体取向角度与取向差条件下的枝晶形貌、对流速度及溶质羽流的演变规律。结果 当晶体取向角度不同时,在枝晶生长过程中,液相区域的平均对流速度均表现为周期性变化。当晶体取向角度较大时,随着晶体取向角度的变大,一次枝晶臂间距变大。当枝晶间存在晶体取向差时,溶质羽流倾向于在发散型晶界附近发起;随着晶体取向差的增大,溶质羽流发起时间提前。溶质羽流的形成阻碍了枝晶尖端及附近枝晶侧臂的生长。结论 晶体取向角度对溶质羽流形成的影响较小,较大的晶体取向差对溶质羽流的形成有促进作用。     

Simulation of dendrite growth of Fe–C alloy using phase field method

采用相场法模拟了Fe--0.5%C合金等温凝固过程中单个枝晶和多个枝晶的生长, 研究了过冷度、各向异性、界面厚度、晶体取向以及扰动对枝晶形貌的影响, 获得了具有二次分枝的枝晶形貌, 再现了枝晶生长过程及枝晶臂之间的竞争生长. 模拟结果表明: 凝固过程中存在溶质富集和枝晶偏析, 枝晶主干溶质浓度最低, 枝晶臂之间的液相浓度最高. 随着过冷度的增大, 枝晶生长加快且分枝发达; 界面厚度直接影响枝晶的生长速度; 各向异性影响枝晶的形态; 晶体取向与坐标轴方向一致时枝晶优先生长;扰动的加入导致枝晶分枝的形成.     

用相场方法模拟Fe-C合金枝晶生长

采用相场法模拟了Fe-0.5%C合金等温凝固过程中单个枝晶和多个枝晶的生长,研究了过冷度、各向异性、界面厚度、晶体取向以及扰动对枝晶形貌的影响,获得了具有二次分枝的枝晶形貌,再现了枝晶生长过程及枝晶臂之间的竞争生长.模拟结果表明:凝固过程中存在溶质富集和枝晶偏析,枝晶主干溶质浓度最低,枝晶臂之间的液相浓度最高.随着过冷度的增大,枝晶生长加快且分枝发达;界面厚度直接影响枝晶的生长速度;各向异性影响枝晶的形态;晶体取向与坐标轴方向一致时枝晶优先生长;扰动的加入导致枝晶分枝的形成.     

相场法模拟Zn-Al二元合金凝固过程中的枝晶生长

建立了与温度场和溶质场相耦合的 Zn-Al 二元合金枝晶生长的相场模型,用相场法模拟了枝晶形貌及生长过程。通过分析得到的图像,研究了各向异性强度系数、界面动力学系数以及界面能对枝晶生长形貌和枝晶尖端生长速度的影响。结果表明：随着各向异性强度系数的增大,枝晶生长速率增加,且容易出现二次支臂。此外,界面动力学系数和界面能的增大也会不同程度地加速枝晶生长速率并对二次支臂的产生有着较为明显的促进作用。     

单晶高温合金DD6的中温横向持久性能

在800,850,900℃的温度条件下,对第二代单晶高温合金DD6的横向持久性能进行了研究.结果表明:与相同条件下的纵向即[001]取向持久性能相比,合金横向持久寿命低于纵向持久寿命.在相对较高温度下,合金横向与纵向的持久寿命在数值上相差较小;在相对较低温度下,合金横向持久寿命明显低于纵向.随着实验温度的升高,扩散蠕变作用增强,韧窝断裂的倾向增加,同时枝晶间和枝晶干变形均匀,横向持久性能相对提高而接近于纵向持久性能.垂直于应力轴的一次枝晶界和枝晶间区域的存在,是横向持久性能低于[001]取向的主要原因.     

强磁场对Al-7%Si合金凝固过程中初生α-Al枝晶分布的影响

采用超导强磁场装置研究了磁场强度对有、无细化剂颗粒Al-7%Si(质量分数)合金凝固组织的作用效果。研究发现,施加强磁场使无细化剂颗粒合金中的初生α-Al枝晶转变为发达枝晶形貌,二次枝晶生长充分,三次枝晶分支明显,枝晶尖端清晰可见;晶粒细化,且枝晶主轴与磁场方向呈30°规则排列。施加强磁场后初生α-Al枝晶数量和Si在α-Al中的溶解度都有少量增加。强磁场抑制添加细化剂合金熔体中的对流,加剧了Ti的重力偏析,使初生α-Al相出现明显枝晶化趋势,方向性增强,枝晶臂粗化明显。     

镍基单晶高温合金小角度晶界的形成机制、影响因素与控制措施

镍基单晶高温合金被广泛用于制备先进航空发动机及工业燃气轮机的关键热端部件,随着铸件结构的复杂化和大型化以及合金中难熔元素的增多,凝固缺陷的形成倾向增大。其中,小角度晶界是定向凝固制备单晶高温合金铸件过程中经常出现的一类缺陷,它会破坏单晶的完整性,一旦超过容限就会对铸件的力学性能造成恶劣影响,随着单晶高温合金服役温度的不断提高,小角度晶界对性能的损害会更为严重。因此,小角度晶界日益成为镍基单晶高温合金发展和应用中需要解决的重要课题,受到国内外研究者的广泛关注。单晶中的小角度晶界与传统意义上的小角度晶界有所不同,是指相邻枝晶间的取向偏离。研究者们在不同晶界偏离角及不同温度条件下就小角度晶界对合金持久性能、蠕变性能及疲劳性能的影响进行了研究,结果发现:当偏离角较小时,小角度晶界对合金性能的影响并不明显,但是随着偏离角的增大及温度的升高,合金的性能均会降低。为了探寻有效的预防和控制措施,研究者们就小角度晶界的形成机制及影响因素进行了研究。关于小角度晶界的形成机制,被大家普遍认同的观点是:枝晶在分枝生长过程中发生了塑性变形,从而导致了枝晶的取向偏离,当枝晶再次汇聚时就会产生小角度晶界。但是,关于枝晶变形的原因则没有一致看法。另外,关于小角度晶界影响因素的研究还不是很系统,主要集中在合金成分及晶界强化元素、凝固参数及取向、铸件尺寸等方面。镍基单晶高温合金中的小角度晶界归根结底是由枝晶的取向偏离导致的,而取向偏离的影响因素复杂,因此小角度晶界的出现很难完全避免。目前,主要通过取向控制以减少小角度晶界的产生,并通过晶界强化以提高合金对小角度晶界的容限。本文阐明了单晶高温合金中的小角度晶界的概念,总结了小角度晶界对合金力学性能的影响,综述了小角度晶界形成机制的研究进展,分析了合金元素、微量元素、凝固条件和铸件结构等因素对小角度晶界形成的影响,在此基础上,提出了减少小角度晶界的措施和强化晶界的途径,最后就未来的研究方向进行了展望。     

一次枝晶间距对DD26单晶高温合金持久各向异性的影响

研究了取向对高速冷却法(DD26-HRS)和液态金属冷却法(DD26-LMC)制备的DD26单晶高温合金760℃/670 MPa条件下持久性能的影响。结果表明,DD26-HRS合金一次枝晶间距明显大于DD26-LMC合金,而且DD26-HRS合金持久各向异性更为严重。位错形貌显示靠近[001]-[012]边界试样变形以基体内多滑移为主。由于DD26-HRS合金γ′相尺寸较大,抑制了层错带剪切γ′相的变形机制,从而使得DD26-HRS合金持久寿命高于LMC合金。然而,靠近[001]-[112]边界试样变形以单方向层错剪切γ′相为主,基体内开动的位错较少,DD26-HRS合金枝晶间粗大的碳化物相周围易于位错的堆积,从而萌生裂纹并沿碳化物界面迅速开裂,造成DD26-HRS合金持久寿命低于DD26-LMC合金。     

强制对流作用下多枝晶生长的相场法研究

采用耦合溶质场、温度场和流场的相场模型,对Ni-Cu合金凝固过程中多枝晶生长进行模拟,研究了多枝晶生长形貌及温度场和溶质场分布.结果表明:熔体流动显著改变凝固前沿的传热和传质,从而影响枝晶生长.受过冷熔体冲刷,枝晶逆流侧前沿溶质浓度和温度低,枝晶臂尖端生长迅速;枝晶顺流侧前沿溶质浓度和温度高,枝晶臂尖端生长缓慢.在熔体...     

Influence of Crystal Orientation on the Morphology of a Single Crystal Superalloy during Directional Solidification

The solid-liquid(S/L) intedecial morphology of a single crystal superalloy DD8 has been in-vestigated. The evolutive behavior of cellular morphology with tilted preferred crystallographic orientation near cell-dendrite transition was dynamically observed, and the efFect of crystallo-graphic orientation on primary dendritic arm spacing has been examined. The experimental results show that for planar and cellular morphology no any S/L interfacial anisotropy exists,but near cell-dendrite transition, the S/L intedecial anisotropy appears and gives rise to the cellular crystal fingers tilted from thermal flow direction to preferred crystallographic orientation.The crystal fingers with their preferred orientation parallel to DS growth direction are more stable than that with tilted orientation. For the tilted fingers, the sudece on the side facing DS growth direction is less stable than that on the reverse side, the different stability on the two sides will lead to forming unsymmetrical dendritical microstructure. With the increase of tilted angle of preferred crystallographic orientation, the primary dendrite arm spacing decreases.     

Microstructure and orientation variation during cell/dendrite transition in directional solidification of a single crystal nickel-base superalloy

The crystallographic orientation and microstructure variation during cell/dendrite transition of a nickel-base single crystal superalloy have been analyzed. The crystal with the off-<0 1 1> orientation grew with a cellular interface at 6 μm s^?1 while with a dendrite interface at 100 μm s^?1. The crystal kept the orientation of bottom seed in the dendritic growth stage, and there was a short transitional zone. The deviation angle between the cellular orientation and the heat flow direction became small in the cellular growth stage. The cellular growth direction was in accordance with the heat flow direction and was independent of the crystallographic orientation. At the same crystallographic orientation, the dendrites had a large primary dendrite arm spacing during the cell/dendrite transition. The cellular crystal with a smaller orientation deviation from the cylindrical axis showed a larger primary arm spacing.     

History-dependent Selection of Primary Dendritic Spacing in Directionally Solidified Alloy

Directional solidification experiments were carried out for succinonitrile-1.0 wt pct acetone alloy with the orientation of dendritic arrays being not parallel to the direction of the temperature gradient. Experimental results show that there exists an allowable range of primary dendritic spacing under a given growth condition.The average primary spacing depends not only on the current growth conditions but also on the way by which the conditions were achieved. The upper limit of the allowable range becomes smaller in comparison with that with 〈001〉 direction of dendrite arrays parallel to the direction of the temperature gradient, which means that the history-dependence of dendritic growth is weaker under this condition. The lower limit obtained is compared with a self-consistent model, which shows a good agreement with experimental results.     

Microstructure development of different orientated nickel-base single crystal superalloy in directional solidification

Single crystal superalloy AM3 oriented along <001> and <011> lattice directions with a deviation were produced by the bottom seeding technique in a modified Bridgman directional solidification furnace. It was found that differently oriented seeds resulted in different dendritic structures. The dendrites of the crystal with <011> orientation arrayed in rows in transverse microstructure. The inclined primary dendrites grew along their initial crystallographic direction irrespective of heat flow direction. As the deviation angle was increased to 17°, the orientation of the crystal changed from <001> to <011>. In addition, crystal growth orientations could affect the primary dendrite arm spacings.     

Effect of Re Addition and Withdrawal Rate on the Solidification Behavior of Directionally Solidified Superalloy AM3

The influence of Re addition and withdrawal rate on the solidification behavior of the first generation single crystal superalloy AM3 was investigated by directional solidification and quenching experiments.The primary dendrite arm spacing and eutectic volume fraction were measured from directionally solidified superalloy AM3 with different Re contents.It is found that the primary dendrite arm spacing is determined by the withdrawal rate,and Re does not influence on the value.The eutectic fraction increases with increasing Re addition.Partition coeffcients of alloying elements were investigated with energy-dispersive X-ray spectrometry(EDS) analysis.The data was submitted to a statistical treatment to establish the solidification path,and the partition coeficients were measured by fitting the curve with a modified Scheil formula.It is shown that the addition of Re results in bigger microsegregation of alloying elements in directionally solidified AM3 superalloy.     

用相场方法模拟镍的枝晶生长

采用相场模型,计算了纯镍的过冷溶液在凝固过程中的枝晶生长。在相场方程中,采用均匀网格的一般显示有限差分方法求解,通过数值模拟显示了枝晶的形貌,包括一次臂、二次臂,以及在枝晶生长过程中出现的“缩颈”现象。     

静磁场对定向凝固镍基高温合金组织影响的研究进展

综述了国内外静磁场对镍基高温合金组织影响的研究现状,重点分析了施加不同方式、强度的静磁场对定向凝固镍基高温合金枝晶组织、元素偏析、凝固缺陷及高温力学性能的影响规律,并从变截面处杂晶的控制、晶体取向偏离的控制以及对凝固特性的影响机制等方面提出了静磁场在定向凝固镍基高温合金研究中潜在的发展方向。     

The 3-dimensional morphology of dendrite during equiaxed solidification of an Al-5 wt.% Cu alloy

In a sample quenched during equiaxed solidification of an Al-5 wt.% Cu alloy, the multi-scales 3-dimensional morphology of equiaxed dendrite was observed. The slim primary stem and secondary branches constitute the frame of dendrite, and rows of dense tertiary branches further divide the 3-dimensional space. In the divided space, the quartic branches grow further. The dendritic branches,which are perpendicular to each other, can change their growth directions and coalesce into a whole. In the tertiary branches and quartic branches, the formation of double branch structures is induced by competitive growth. The branch that wins in the competitive growth will produce a cabbage-like structure by wrapping the failed branches. In addition, the side branch can also wrap the original parent branch to produce cabbage-like structures. Depending on the historical growth direction, the dendritic arms can form vein-like and spicate structures, and the shapes of single dendritic arm may be the cylinder, plate and trapezoid platform. According to the compositions and etching morphology, the single dendritic arm in the final solidification structures should coalesce from a fine porous structure. The porous structures at different length-scales are principally induced by the preferential growth. Based on 3-dimensional morphology of equiaxed dendrite, a new research object for the investigation of microsegregation was suggested.