【作者】 郭锐；

【导师】 任忠鸣；

【作者基本信息】 上海大学 ， 钢铁冶金， 2022， 博士

【摘要】 凝固过程作为一种常见的物理现象,广泛存在于自然界和工业生产中。凝固过程很大程度上决定了工业产品的微观组织和成分分布,从而影响产品的性能。作为凝固过程的初始阶段,形核过程对凝固组织的相组成、晶粒大小及分布均能够产生不可忽略的影响。近年来,静磁场已经成为了一种非常重要的凝固过程调控手段,静磁场的多种效应能够很大程度上影响金属熔体的凝固过程,其中静磁场对金属熔体形核过程的影响非常值得关注。然而,目前对静磁场下金属熔体形核过程的研究十分有限,其中大多数研究以报道实验现象为主,对实验现象仅进行定性解释,缺乏进一步的实验验证,说服力较弱;同时,大多数研究主要对温度等宏观物理量进行观测,没有针对处于微观尺度的异质形核过程及静磁场的作用机理进行研究。本文以纯Al等纯金属及Al-Cu合金等一系列的非铁磁性金属为研究对象,采用不同实验方法对触发形核的异质核心进行选择性调控,通过热分析技术等实验手段研究了静磁场对金属熔体异质形核过程的影响,对异质形核界面进行了微观观察及分析,主要内容如下:首先,采用差热分析法研究了磁场下纯Al、纯Sn和纯Zn三种纯金属的熔化和形核过程以及Al-4.5wt%Cu、Al-26wt%Cu及Al-45wt%Cu三种Al-Cu合金的形核过程。实验结果表明,纯金属的熔化温度在磁场下未发生明显变化;同时,通过热力学分析发现,磁场的施加基本不改变非铁磁性金属液固相变过程的热力学参数,对液固相变温度基本无影响。而对于本实验中不同冷却速率、具有不同磁性以及不同晶体结构的形核相,磁场的施加均会引起其形核温度降低,过冷度增大。这说明磁场对金属熔体的形核过程会产生抑制作用,且该抑制作用与冷却速率、形核相的磁性及晶体结构等因素均未表现出明显的相关性。通过形核动力学分析发现,磁场的施加仅改变了形核过程的动力学参数,包括液相与晶核之间的界面能和润湿角,而并未引起金属熔体形核模式的改变。其次,采用差示扫描量热法对磁场下添加Al5TiB形核剂的2024Al合金熔体中初生α-Al相的形核及生长过程进行了研究。实验结果表明,磁场下初生α-Al相的形核温度降低,过冷度增加;凝固组织中初生α-Al晶粒数量显著减少、尺寸显著增加,出现了形核剂效果衰退现象,直接证实了磁场对形核过程的抑制作用。根据自由生长模型分析表明,对于以TiAl3或TiB2颗粒作为异质核心的形核过程,熔体过冷度增加及形核剂效果衰退现象均可归因于磁场引起的液相/晶核界面能的增加。此外,还观察到磁场下初生α-Al晶粒发生了胞状晶-树枝晶转变,该转变主要源于液固界面前沿的成分过冷程度增加。根据晶体生长理论,磁场对形核过程的抑制作用促使初生α-Al晶粒生长更加充分,晶粒尺寸更大,液相中溶质富集程度更高,成分过冷趋势更强烈,因此初生α-Al晶粒更容易生长为树枝晶。根据界面稳定性理论分析,磁场对熔体对流及扩散的抑制作用同样会改变界面处的溶质分布,导致成分过冷趋势增大,使胞状晶-树枝晶转变趋势更加显著,从而影响初生α-Al晶粒的形态。同时,磁场对扩散的抑制作用还会引起胞状晶向树枝晶转变的临界生长速率降低,使初生α-Al晶粒更容易发生形态转变。除此之外,磁场引起的初生α-Al晶粒界面能增大及生长维度增加同样可能导致界面处溶质的不均匀分布,增加成分过冷趋势。最后,在磁场环境下对由A12O3异质基底触发的纯Al及Al-4.5wt%Cu合金样品的形核过程进行了显微分析研究。实验结果发现,当磁场施加后,纯Al及初生α-Al相的形核温度均降低,过冷度均增加。通过晶体位相分析发现,无磁场时,纯Al的(110)Al晶面和初生α-Al相的(311)Al晶面与Al2O3基底的外露晶面(1010)均呈现良好的匹配结构,晶格错配度较低;而施加磁场后,纯Al和初生α-Al相的原匹配晶面均发生8～10°的偏转,其与Al2O3基底外露晶面的匹配晶面均向高指数晶面转变,晶格错配度增加。基于熔体/异质基底界面处有序原子层的研究,对于本实验结果,可以运用磁偶极子相互作用理论进行解释:对于熔体/异质基底界面处形成的有序原子层,磁场的施加能够改变有序层内原子间结合力,引起原子之间的排斥作用,促使原子间距增加,从而影响了有序层内的原子排布,表现为形核相与异质基底的匹配晶面改变,错配度增加;同时,有序层内原子排布的改变导致有序层与Al2O3基底和金属熔体之间的界面能均增加,从而抑制了以有序层作为晶胚的形核过程,增加了纯Al和初生α-Al相的形核能垒,需要更大的过冷度方能开始形核。更多还原

【Abstract】 As a common physical phenomenon,the solidification process widely exists in the nature and industrial production.The solidification process largely determines the microstructure and composition distribution of industrial products,thereby affecting product performance.As the initial stage of the solidification process,the nucleation process has a non-negligible effect on the phase composition,grain size and distribution of the solidification structure.In recent years,the steady magnetic field has become a very important means of controlling the solidification process owing to its various effects influencing on the solidification structure.Among these effects,the effect of the steady magnetic field on the nucleation is very worthy of attention.However,the current researches on the nucleation of metal melts in the steady magnetic field are very limited.Most of the studies focus on the experimental phenomena only providing qualitative explanations,which are lack of experimental verifications and not convincing.Meanwhile,the most physical quantities observed in these reseaches are in macro-scale,such as temperature,which are difficult to reveal the micro-scale heterogeneous nucleation and the mechanism of the effect of the steady magnetic field.The present work aims to investigate the influence of the steady magnetic field on the heterogeneous nucleation and solidification process of metal melts.Nonferromagnetic metals such as pure A1 and Al-Cu alloys are selected and different experiment methods are employed to control the nucleant during the solidification.The main results are as follows:Firstly,the undercooling behaviors of pure metals,i.e.,pure Al,pure Sn and pure Zn,and three Al-Cu alloys,i.e.,Al-4.5wt%Cu,Al-26wt%Cu and Al-45wt%Cu,in a steady magnetic field were investigated by differential thermal analysis.There was no obvious change in melting temperatures of pure metals in the steady magnetic field and through the thermodynamic estimation,the transformation temperature of liquid-solid phase transformation of the non-ferromagnetic metal was found to be almost unvaried under the action of a steady magnetic field.The nucleation temperatures of three pure metals and three Al-Cu alloys were found to decrease and correspondingly,their undercoolings increased in the steady magnetic field,implying that the nucleations of all pure metals and alloys were suppressed in the steady magnetic field,regardless of cooling rate,magnetism and crystal structure of the nucleated phases.From the nucleation kinetics analysis,the parameters in the nucleation,for instance,the interfacial energy between the liquid phase and nucleus and/or contact angle,can be influenced in the steady magnetic field,while the mechanism of nucleation is unaffected.Secondly,the nucleation and growth of primary α-Al phase in 2024Al alloy melt inoculated with Al5TiB master alloy in a steady magnetic field were studied by differential scanning calorimeter.It was found that the nucleation temperature of primary α-Al phase decreased in the steady magnetic field,i.e.,the undercooling was enhanced.The micro structure of primary α-Al phase in the steady magnetic field showed an enhanced degradation in grain refinement and a cellular-dendritic transition of equiaxed primary a-Al grains.According to the free growth model,the enhanced degradation in grain refinement and increase in undercooling in the steady magnetic field can be ascribed to the modified liquid/nucleus interfacial energy and the delay of formation of critical nucleus due to the retarded migration rate of atoms in the liquid phase.The cellular-dendritic transition of primary α-Al grains is attributed to the modified constitutional undercooling at the solid/liquid interface,which results from the suppressed nucleation of primary α-Al grains and the change in solute distribution by the damped convection and retarded diffusivity in the SMF.The critical growth velocity of cellular-dendritic transition is also suppressed due to the retarded diffusivity in the SMF,resulting in the dendritic morphology of primary α-Al grains.Additionally,the increase in growth dimension and the modified solid/liquid interfacial free energy under the SMF are also responsible for the enhanced constitutional undercooling and the cellular-dendritic transition.Finally,the nucleations of pure Al and Al-4.5wt%Cu alloy triggered by the singlecrystal Al2O3 substrate with a certain orientation in the steady magnetic field were investigated.The nucleation temperatures of pure Al and Al-4.5wt%Cu alloy melts were decreased and their undercoolings were increased when the steady magnetic field was applied.The good lattice matchings of(110)Al/(1010)Al2O3 and(311)Al/(1010)Al2O3 were shown at Al/Al2O3 and α-Al/Al2O3 interfaces without the steady magnetic field,respectively.However,(110)and(311)Al planes in pure Al and primary α-Al phase were deviated about 10°from the interface in the steady magnetic field and thereby,the lattice misfits increased.The increase in undercooling and the change in lattice matching can be attributed to the repulsive effect of Al atoms in the ordered layer induced by the magnetic dipole-dipole interaction in the steady magnetic field.The repulsive effect results in the increase in atom distance and the change in atom arrangement in the ordered layer,which directly influences the lattice matching between the nucleus and substrate.Besides,the increase in interfacial energies of ordered layer/substrate and ordered layer/melt can be induced by the varied atom arrangement in the ordered layer.Therefore,the nucleation where the ordered layer acts as the nucleus is suppressed due to the increased nucleation barrier and a larger undercooling is needed to initiate the nucleation.更多还原