Al-4％Cu合金定向凝固枝晶／胞晶转变速率的研究

通过实验和理论对比研究Al-4%(质量分数)Cu合金定向凝固胞晶/枝晶转变过程,得到胞晶/枝晶转变发生在尖端半径变化的拐点处。采用KGT模型与非平衡效应研究与胞晶/枝晶转变过程相对应的高速枝晶/胞晶转变特征。结果表明:尖端半径和界面温度均随抽拉速率的增加而减小,到达临界值后又急速增大。枝晶/胞晶转变发生在尖端半径和界面温度的拐点处,即在尖端半径和界面温度最小时发生转变;溶质截留在枝晶/胞晶转变过程中作用明显,大大减小了微观偏析。     

单相凝固亚结构的特征尺度与形态转变

从定向凝固界面形的非稳态分析出发，对单相凝固亚结构的几个特征尺度包括平界面失稳初始扰动波长、胞晶间距、枝晶一次间距、枝晶尖端半径及二次间距与两有态转变即胞枝转变和高梯度绝对稳定性进行了实验研究和理论分析。     

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。     

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...     

凝固速度对奥氏体不锈钢定向凝固组织及其固液界面稳定性的影响

本文研究了凝固速率对1Cr18Ni9Ti不锈钢定向凝固组织及其固液界面稳定性转变规律的影响.结果表明,在某特一定的温度梯度下,随着凝固速度的增加,定向凝固的固液界面由平面转变为胞状晶,再转变为树枝晶.研究发现,随着凝固速率的增大,定向凝固组织枝晶形貌逐渐细化,枝晶间距减小.     

相场方法模拟氙枝晶生长研究

基于相场方法对纯物质的凝固过程进行了二维数值模拟.以氙为例,展示了界面形貌的演化过程,研究了相场参数对枝晶界面形貌的影响.结果表明,随着过冷度值的增加,晶核由一圆核逐渐发展为发达的枝晶,相应的枝晶稳态尖端速度明显加快;随着耦合系数的增大,晶核由一圆核发展为细长的枝晶,枝晶的稳态尖端速度呈单调递增趋势,界面的稳定性逐渐变差.     

单晶凝固组织的样品尺寸效应

通过对不同直径的单晶样品的凝固实验，研究了样品尺寸对单晶凝固组织的影响规律，研究发现，随样品尺寸减小，单晶凝固组织被细化，尤其一次枝晶间距λ1和枝晶间γ′相尺寸对样品尺寸的依赖性更为明显；样品尺寸影响的要质在于此表面积的变化，进而对凝固过程温度梯度产生影响，最终作用于凝固组织和力学性能，一次枝晶间距λ1和枝晶间γ′尺寸与样品的比表面积近拟呈线性关系，样品尺寸变化，明显改变了单晶生长过程的温度梯度和合金的高温持久寿命。     

溶质对流对SCN-5%ETH模拟合金垂直向上生长的影响

采用实时观测装置和定向凝固系统研究了SCN-5%ETH(succinonitrile-wt%ethanol,wt%表示质量分数)模拟合金的垂直向上生长过程。实验结果与水平定向生长的SCN-5%ETH模拟合金的胞晶组织,枝晶组织以及枝晶间距比较发现,胞晶垂直向上生长引起的溶质对流减小了胞晶的尖端半径,使胞晶组织呈现尖端尖细的形态;枝晶垂直向上生长时,二次枝晶臂生长速度减小,枝晶一次间距和二次臂长度也减小,并且减小幅度随着界面推移速度的增大而减小,枝晶二次间距和水平定向生长时相比有较小幅度的增大。     

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。     

激光快速凝固Al-Mn合金的组织选择规律研究

利用 5k W CO2 激光器对 Al-1 .1 ,3 .2和 5.6wt% Mn合金进行了一系列表面重熔实验 ,并对微观组织形成规律进行了研究。实验结果表明 :重熔区组织较基体组织大大细化 ;Al-3 .2 wt% Mn在所有速度范围内没有明显的共晶生长出现 ,而以α(Al)胞 /枝晶形式生长 ;随着生长速度的增大 ,Al-5.6wt% Mn合金的组织由 Al6Mn枝晶向 α(Al) + Al6Mn共晶、α(Al)胞 /枝晶及完全无偏析固溶体转变 ,三种合金达到绝对稳定的临界速度分别为 4 4 .1 ,1 3 4.6,2 3 0 .1 mm/s,与 M-S理论预言的达到绝对稳定的临界速度相吻合。     

Modeling of a transition to diffusionless dendritic growth in rapid solidification of a binary alloy

Diffusionless growth of dendritic crystals results in microsegregation-free microstructures with an initial (nominal) chemical composition of solidifying systems. Normally, a transition from chemically partitioned growth to diffusionless solidification is accompanied by the morphological transition in crystal shape with the appearance of nonlinearity in the kinetic behavior of growing crystals. This phenomenon is discussed using a model of local non-equilibrium rapid solidification. Considering the transition from the solute diffusion-limited growth to purely thermally controlled growth of dendritic crystals, the model predicts the abrupt change of growth kinetics with the break points in the “dendrite tip velocity-undercooling” and “dendrite tip radius-undercooling” relationships. It is shown that the abrupt change of growth kinetics occurs with the ending of the transition to purely thermally controlled growth and the onset of diffusionless solidification. To predict the dendrite growth kinetics in a whole region of undercooling, numeric analysis shows that the model has to take into account both anisotropies of solid–liquid interfacial properties. These are anisotropy of surface energy and anisotropy of atomic kinetics of solidification.     

Al-Cu 合金高梯度定向凝固过程中的形态转变

本文利用 ZMLMC 定向凝固装置,研究了 Al-Cu 合金系在不同温度梯度下定向凝固时凝固界面和组织形态的变化。发现存在着两种树枝状生长和胞状生长之间的转变,即在低速范围内胞状向树枝状转变,在高速范围内树枝状向胞状转变;当温度梯度足够高时,可以在整个生长速率范围内不出现树枝状生长,获得高度细化的胞状组织;合金的结晶温度间隔越宽,完全消除树枝状生长所需的温度梯度越高。     

Modes of dynamic shear failure in solids

The technique of loading edge cracks by edge impact (LECEI) for generating high rates of crack tip shear (mode-II) loading is presented. The LECEI-technique in combination with a gas gun for accelerating the impactor is used to study the high rate shear failure behaviour of three types of materials. Epoxy resin (Araldite B) shows failure by tensile cracks up to the highest experimentally achievable loading rate; steel (high strength maraging steel X2 NiCoMo 18 9 5) shows a failure mode transition: at low rates failure occurs by tensile cracks, at higher rates, above a certain limit velocity, failure by adiabatic shear bands is observed; aluminum alloy (Al 7075) shows failure due to shear band processes in the high rate regime, but this failure mode is observed over the entire range of lower loading rates, even down to quasi-static conditions. Characteristics of the failure modes are presented. When transitions are observed in the failure process from tensile cracks to shear bands the limit velocity for failure mode transition depends on the bluntness of the starter crack the failure is initiated from: The larger the bluntness of the starter crack the higher the critical limit velocity for failure mode transition. The data indicate that adiabatic shear bands require and absorb more energy for propagation than tensile cracks. Aspects of the energy balance controlling mode-II instability processes in general are considered. Effects very different than for the mode-I instability process are observed: When failure by a tensile crack occurs under mode-II initiation conditions, a notch is formed between the initiated kinked crack and the original starter crack, and, at this notch a compressive stress concentration builds up. The energy for building up this stress concentration field is not available for propagation of the initiated kinked crack. The energy density of a mode-II crack tip stress field, however, when compared to an equivalent mode-I crack tip field, is considerably larger, and, consequently, the remaining driving energy for any mode-II initiated failure process, nevertheless, is higher than for the case of equivalent mode-I initiation conditions. Furthermore, mode-II crack tip plastic zones are considerably larger than equivalent mode-I crack tip plastic zones. Consequently, validity conditions for linear-elastic or small scale yielding failure behaviour are harder to fulfill and possibilities for the activation of nonlinear high energy ductile type failure processes are enhanced. Speculations on how these effects might favour failure by high energy processes in general and by shear bands processes in particular for conditions of high rate shear mode-II loading are presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

In-situ observation of solid-liquid interface transition during directional solidification of Al-Zn alloy via X-ray imaging

The morphological instability of solid/liquid(S/L) interface during solidification will result in different patterns of microstructure. In this study, two dimension(2 D) and three dimension(3 D) in-situ observation of solid/liquid interfacial morphology transition in Al-Zn alloy during directional solidification were performed via X-ray imaging. Under a condition of increasing temperature gradient(G), the interface transition from dendritic pattern to cellular pattern, and then to planar growth with perturbation was captured. The effect of solidification parameter(the ratio of temperature gradient and growth velocity(v), G/v) on morphological instabilities was investigated and the experimental results were compared to classical "constitutional supercooling" theory. The results indicate that 2 D and 3 D evolution process of S/L interface morphology under the same thermal condition are different. It seems that the S/L interface in 2 D observation is easier to achieve planar growth than that in 3 D, implying higher S/L interface stability in 2 D thin plate samples. This can be explained as the restricted liquid flow under 2 D solidification which is beneficial to S/L interface stability. The in-situ observation in present study can provide coherent dataset for microstructural formation investigation and related model validation during solidification.     

Study on crack propagation behavior in a quenched glass plate

Crack pattern transition and crack propagation behavior in a quenched glass plate are investigated. Theoretical analysis indicates that the distance between the crack tip and the cold front is closely related to the crack pattern transition. This theoretical result is examined experimentally using instantaneous phase-stepping photoelasticity. As expected theoretically, when the crack tip remains close enough to the cold front, crack propagation remains straight. When this distance reaches a given value, the crack oscillates. These experimental results are in good agreement with the theory of crack pattern transition. Therefore, present theoretical analysis is valid in predicting the instability of crack propagation. The crack tip stress field is also examined by the present experimental method. In particular, in the oscillating regime, the mode-I stress intensity factor frequently becomes larger than the fracture toughness, and the mode-II stress intensity factor has a nonzero value during propagation. For the former result, some reasons are discussed, but the cause of this problem is still unknown. However, the latter result can be explained by the theoretical analysis of an infinitesimal kinked edge crack just after crack initiation.     

Macrosegregation and thermosolutal convection-induced freckle formation in dendritic mushy zone of directionally solidified Sn-Ni peritectic alloy

Compared with the growing applications of peritectic alloy,none research on the freckle formation during peritectic solidification has been reported before.Observation on the dendritic mushy zone of Sn-36 at.％Ni peritectic alloy during directional solidification at different growth velocities shows that the freckles are formed in two different regions:region Ⅰ before peritectic reaction and region Ⅱ after peritectic reaction.In addition,more freckles can be observed at lower growth velocities.Examination on the experimental results demonstrates that both the temperature gradient zone melting(TGZM)and Gibbs-Thomson(G-T)effects have obvious influences on the morphology of dendritic network during directional solidification.The current theories onKI Rayleigh number Ra characterizing the thermoso-lutal convection of dendritic mushy zone to predict freckle formation through the maximum of Ra can only explain the existence of region Ⅰ while the appearance of region Ⅱ after peritectic reaction cannot be predicted.Thus,a new Rayleigh number RaP is proposed in consideration of evolution of dendritic mushy zone by both effects and peritectic reaction.Theoretical prediction of RaP also shows a maximum after peritectic reaction in addition to that before peritectic reaction,thus,agreeing well with the freckle formation in region Ⅱ.In addition,more severe thermosolutal convection can be predicted by the new Rayleigh number RaP at lower growth velocities,which further demonstrates the reliability of RaP in describing the dependence of freckle formation on growth velocity.     

非稳态过程与凝固界面形态选择

以透明模型合金丁二腈－乙醇系为实验系统，直接观察其单相凝固过程中界面形态的非稳态演化过程，从三个方面：①平胞转变②胞枝转变③枝晶一次间距，系统地考察了凝固界面形态选择的时间相关性和历史相关性。结果表明：①导出了凝固初始过渡区溶质再分配的一个新公式；②提出了确定平界面稳定性临界条件的可测判据和实验方法；③从实验上和理论上解释了胞枝转变行为；④揭示了枝晶一次间距的历史相关性；⑤发现凝固形态选择的历史相     

硅灰石晶体在三元氧化物熔体中的生长行为研究

利用耦合FACTSage Toxide 热力学数据库的定量相场模型, 本研究模拟了硅灰石(CaSiO₃)在CaO-Al₂O₃-SiO₂体系中的恒温晶体生长过程, 并研究了熔体组分和温度对CaSiO₃结晶过程的影响。结果表明, 硅灰石的形貌主要由其表面能的各向异性所决定, 而几乎不受界面动力学的各向异性所影响。此外, 随着温度的降低, 析出的硅灰石的生长方式由平面生长向枝晶生长方式转变, 于此同时, 更加精细的枝晶结构也逐渐呈现出来。模拟所得的枝晶生长速度和尖端半径与Ivanstov理论所得结果一致, 和实验测得的数据也处于同一数量级。     

Self-consistent modeling of morphology evolution during unidirectional solidification

A self-consistent model is developed to describe the morphology evolution during unidirectional solidification, which shows that, for a given temperature gradient, the interface morphology will go planar → shallow cell → deep cell → dendrite → cell → planar with increasing growth velocity. By examining the interaction of adjacent cells/dendrites, a wide allowable range of primary spacing for given growth conditions is determined, which shows a good agreement with experimental results. Numerical results show that cellular/dendritic and dendritic/cellular transitions appear not at a unique velocity but over a range of velocities, the critical velocity for the transition being dependent on the primary spacing before the transition.     

Self-consistent modeling of morphology evolution during unidirectional solidification

A self-consistent model is developed to describe the morphology evolution during unidirectional solidification, which shows that, for a given temperature gradient, the interface morphology will go planar→shallow cell→deep cell→dendrite→cell→planar with increasing growth velocity. By examining the interaction of adjacent cells/dendrites, a wide allowable range of primary spacing for given growth conditions is determined, which shows a good agreement with experimental results. Numerical results show that cellular/dendritic and dendritic/cellular transitions appear not at a unique velocity but over a range of velocities, the critical velocity for the transition being dependent on the primary spacing before the transition.