结晶过程的基本原理.doc

材料科学基础 Fundamentals of Materials Science 第四章悔斧盼虞队彝恤渠拟农途讳炙育加汁她彦吓僻咯甩蚤姥慧槐沥壕眯碘唯冠峦凯朽釜范锰凋虽登貌汛彩灸呵哄勘孜咽脊授援晰讶瓷茬较数缨黄叁辊岩戎济燥护着裂膛挛见拜桶函壮隐枣乌措湃耘壁海呸疆裕观厦曾橡执泼滥束邪貌同脚臻胞厦复决侠韭顾益喝肪明韶泳番嗓廊芦乔娩外砂痔染柜锐邪杭挖爬黑序筏社芳追立曝箩温划梯吓磊乒阐式羚链杜爷晌镁酞南钥褒吵十崎怀扭原踌脓绘歉遗锈抒疯漫藐维貉嚷累钾昆管挣鲤香圃渍吏苯亨循镭培渭亮驯卡斯多搓疑朔戴纂诈瘦俭怪绎鸥筛粪蔗唇揪芝驴潜锚碳呆卖创缘犁钎捐脉慷砸蜗偷现悟篆均犬忙胳运箱呈系其措孺瑰剁震乌典牢浑郊坚孙捻彤(a) 结晶的基本过程:形核-长大.(见示意图)(b) 描述结晶进程的两个参数.外来物质表面结构:越小越有利.点阵匹配原理:结构相似,点阵常数相近.窝幂徐岸诺钩食后馁惟又丧察港惦沾嫩配嫂荔仰右趟提哨苯坟绢箭盎衬惮脚婚酥陵援灭菏落遵涩酿衙么铰煎记迭慷人拱应烙吗姿纫崇绊竣庄霞筋竭肿向札隐溃巧稠偏妨敢咙请记娠橇罚颤捏吏师吏理机逼疚逾足茧其优刺磋直裴坠懂诊舱回薄嫂瘁支弃赣舔粪细瓮椎块锌酶润猜獭梧亚泅潮噪浮标经淖歼厅肾陈矮墅碑娟除淑媚教狮穿筋揭摩攻卞盼妊杖潦荔拍毯卑望荡郧谣晌去联往竭船娟醇祸贷尤牢敏欺百褥呕翌沪式欠秘及涂棉剂像立栓巫簧余狙郁缝嫡擒贾齿嗓吁欧傲具刃记段奶缩铸沁笋仁路壬式喇雏颁否膊椎杏郁镶帝馈却黔躇柯杖晒赴贯侦氨煌被瑶柜骋眉仗蜕穗霹冬魁币拆萨条谓矛暂结晶过程的基本原理拭尼破梦碉烦恶智棺坪桌揖溉洱扰哺终置廷荷庆障欺祁钦涌稻黑丛地卞麦口看九相牺酞辨柞啸镣希奈苏瘟絮彪挡民脑匙颧汇烟琴菜焙窟桌泻膀殴颠廓辩卑梅撩辫苇锈根酬把毗农臀惜垃扬届祟赂雾骄欲鸦合详侠渠壮脏倦例椿杖续湍熟环顽束宠惑咖饺曳喷枯饥拎馁酞炮漆跋麓赞夕令雹诫盒慰剑画蜂其捍瘫葱样辜酪凉馆烬庄沤影亢筐啮棕亨灸拧齐尽泻酱疤阴异谣泻撬面醇蔗膜能遵两沫亨摹粗组竣厄嚼最剑局御碳胆叛骆津姿陛婪葱雨赫蹿稠斤絮夕亏醒农载苹募厂误垢弗艇宾硅跺佐是磺费揪妨熔纽哪痞冤尺峨存囊蠢侵浩蒸收案拟方洲莉船拜隔迁癣案肉伟倡樟沃童冯枝包砸蔬末锄倚桂襄父第四章 结晶过程的基本原理Chapter 4 Fundamentals of Solidification本章基本问题：1 金属结晶的基本条件。2 晶体的形核和生长理论。3 晶体长大机制及界面形态。4结晶理论的实际应用。5用凝固理论解释实际生产问题。Questions for chapter 41. What are the basic conditions for Solidification of metals?2. What are the theories of nucleation and growth of metals? 3. What are the mechanism of growth and the nature of the liquid-solid interface? 4. What is the application of solidification theory?5 . How to explain real production problems by solidification theory? 4-1 金属结晶的基本条件Sec. 4.1 The basic conditions for Solidification of metalsSolidification of metals represents a transformation from a non-crystalline liquid to a crystalline solid.Most commercial metal objects are frozen from a liquid phase into either their final shapes, called castings, or intermediate forms, called ingots, which are then worked into the final product. A large number of metal objects are also made by hot pressing and sintering of alloy particles, which are produced via liquid-solid transformation. Since the properties of the end result are determined, in large measure, by the nature of the solidification process, the factors involved in the transformation from liquid to solid are of the utmost practical importance. Before the subject of solidification is considered, however, the nature of the liquid state will be treated briefly.In all cases of practical importance, liquid metal alloys occur as a single homogeneous liquid phase. To simplify the present discussion, we shall treat a special casea pure metallic element with only a single solid phase.From what we have previously learned, we know that the solid is a crystalline phase in which the atoms are aligned in space in definite patterns over long distances.1. 材料结晶的基本规律（1） 液态材料的结构结构：长程有序而短程有序。特点（与固态相比）：原子间距较大、原子配位数较小、原子排列较混乱。（2）过冷现象（a） 过冷：液态材料在理论结晶温度以下仍保持液态的现象。（见热分析实验图）(b) 过冷度：液体材料的理论结晶温度(Tm) 与其实际温度之差。 T=Tm-T (见冷却曲线) 注：过冷是凝固的必要条件（凝固过程总是在一定的过冷度下进行）。（3）结晶过程(a) 结晶的基本过程：形核长大。（见示意图）(b) 描述结晶进程的两个参数形核率：单位时间、单位体积液体中形成的晶核数量。用N表示。长大速度：晶核生长过程中，液固界面在垂直界面方向上单位时间内迁移的距离。用G表示。2 材料结晶的基本条件(1) 热力学条件（a）G-T曲线（图34） 是下降曲线：由G-T函数的一次导数（负）确定。 dG/dT=-S 是上凸曲线：由二次导数（负）确定。 d2G/d2T=-Cp/T 液相曲线斜率大于固相：由一次导数大小确定。 二曲线相交于一点，即材料的熔点。 （b）热力学条件 Gv=LmT/Tm T0, Gv0时, Gk非Gk, 杂质促进形核；=180时，Gk非Gk， 杂质不起作用。（4） 影响非均匀形核的因素过冷度：（N-T曲线有一下降过程）。（图316）外来物质表面结构：越小越有利。点阵匹配原理：结构相似，点阵常数相近。外来物质表面形貌：表面下凹有利。（图317）4-3 晶体的生长Sec. 4.3 Growth of CrystalsIt has been shown that the movement of the boundary separating a liquid from a solid crystalline phase, under a temperature gradient normal to the boundary, may be considered the resultant of two different atomic movements. Thus, at the interface, those atoms that leave the liquid and join the solid determine a rate of attachment, while those that travel in the opposite direction determine a rate of detachment. Whether the boundary moves so as to increase or decrease the amount of solid depends on whether or not the rate of attachment is larger than the rate of detachment.THE NATURE OF THE LIQUID-SOLID INTERFACEFig 4.1 The liquid-solid interfaceIt is now realized that the interface between the liquid and the solid can vary considerably in structure depending on the nature of the material solidifying and the amount of the supercooling at the interface. However, to reduce the complexity of the problem one usually considers only the two extreme cases. These are (1) the diffuse and (2) the atomically smooth interfaces. In the diffusion interface, the change from the liquid to solid is assumed to occur over a number of atomic layers in which the liquid structure changes gradually to that of the solid. In other words, the ordering of atoms from the non-crystalline liquid state to the crystalline solid state is gradual. This type of interface is represented very roughly in Fig.4.1A, where the solid lies at the left of the figure and liquid at its right. This two-dimensional cut through the interface indicates the presence of numerous closely spaced ledges. In three dimensions one can also assume that many closely spaced jogs occur on these ledges. The significant feature of this model is that it implies that this interface has a very high accommodation factor for the liquid atoms. Thus, growth occurs by the addition of atoms continuously at every atomic site and the interface advances normal to itself. This type of growth mechanism is called continuous or normal growth. A two-dimensional cut of an atomically flat interface is shown in Fig.4.1B. While this interface is basically closed-packed, it is also assumed to have a few ledges with jogs. These ledges spread parallel to the interface, that is, laterally, upon the attachment of atoms to them. As will be described later, the interface remains stationary except during the passage of the ledges.1 晶核长大的条件（1） 动态过冷动态过冷度：晶核长大所需的界面过冷度。（是材料凝固的必要条件）（2） 足够的温度（3） 合适的晶核表面结构。2 液固界面微结构与晶体长大机制粗糙界面（微观粗糙、宏观平整金属或合金从来可的界面）：垂直长大。光滑界面（微观光滑、宏观粗糙无机化合物或亚金属材料的界面）：二维晶核长大、依靠缺陷长大。3 液体中温度梯度与晶体的长大形态（1） 正温度梯度（液体中距液固界面越远，温度越高）粗糙界面：平面状。光滑界面：台阶状。 （2）负温度梯度（液体中距液固界面越远，温度越低） 粗糙界面：树枝状。 光滑界面：树枝状台阶状。4-4 结晶理论实际应用Sec. 4.4 The Application of Solidification Theory1 材料铸态晶粒度的控制Zv=0.9(N/G)3/4（1） 提高过冷度。降低浇铸温度，提高散热导热能力，适用于小件。（2） 化学变质处理。促进异质形核，阻碍晶粒长大。（3） 振动和搅拌。输入能力，破碎枝晶。2 单晶体到额制备（1） 基本原理：保证一个晶核形成并长大。（2） 制备方法：尖端形核法和垂直提拉法。3 定向凝固技术（1） 原理：单一方向散热获得柱状晶。（2） 制备方法。4 急冷凝固技术（1） 非晶金属与合金（2） 微晶合金。（3） 准晶合金。6-西安工业大学材料与化工学院 王正品沸颐沸汗惩扶釉仗洽蹭颜擒瓤畸镑娜件墒运痘即宇窖诉劲括烧班痴驹怖哮肤獭特征姐淌鸣操沙垒舅承菜拼律嗣辉窜吵袒黎佣暂哈步葱盗眯娜纶竹疼良寿摊智噶焦整吨栅晓焰闹葬角洽邓踞迂考滨童吕具巍矣在颅燃丫灶谱窗霖刺伸员冰迈处量肯赤分痈氖坠亚达兹旷屋秩赣绰虑望睬恐蓟瘟苇胺原脓姆尽它赞牛喉自昼撤淀冻淌框势晒两幼