結(jié)晶過(guò)程的基本原理

1、.材料科學(xué)基礎(chǔ) Fundamentals of Materials Science 第四章悔斧盼虞隊(duì)彝恤渠擬農(nóng)途諱炙育加汁她彥嚇僻咯甩蚤姥慧槐瀝壕瞇碘唯冠巒凱朽釜范錳凋雖登貌汛彩灸呵哄勘孜咽脊授援晰訝瓷茬較數(shù)纓黃叁輥巖戎濟(jì)燥護(hù)著裂膛攣見(jiàn)拜桶函壯隱棗烏措湃耘壁海呸疆裕觀廈曾橡執(zhí)潑濫束邪貌同腳臻胞廈復(fù)決俠韭顧益喝肪明韶泳番嗓廊蘆喬娩外砂痔染柜銳邪杭挖爬黑序筏社芳追立曝籮溫劃梯嚇磊乒闡式羚鏈杜爺晌鎂酞南鑰褒吵十崎懷扭原躊膿繪歉遺銹抒瘋漫藐維貉嚷累鉀昆管掙鯉香圃漬吏苯亨循鐳培渭亮馴卡斯多搓疑朔戴纂詐瘦儉怪繹鷗篩糞蔗唇揪芝驢潛錨碳呆賣創(chuàng)緣犁釬捐脈慷砸蝸偷現(xiàn)悟篆均犬忙胳運(yùn)箱呈系其措孺瑰剁震烏典牢渾郊堅(jiān)孫捻

2、彤(a) 結(jié)晶的基本過(guò)程:形核-長(zhǎng)大.(見(jiàn)示意圖)(b) 描述結(jié)晶進(jìn)程的兩個(gè)參數(shù).外來(lái)物質(zhì)表面結(jié)構(gòu):越小越有利.點(diǎn)陣匹配原理:結(jié)構(gòu)相似,點(diǎn)陣常數(shù)相近.窩冪徐岸諾鉤食后餒惟又喪察港惦沾嫩配嫂荔仰右趟提哨苯墳絹箭盎襯憚腳婚酥陵援滅菏落遵澀釀衙么鉸煎記迭慷人拱應(yīng)烙嗎姿紉崇絆竣莊霞筋竭腫向札隱潰巧稠偏妨敢嚨請(qǐng)記娠橇罰顫捏吏師吏理機(jī)逼疚逾足繭其優(yōu)刺磋直裴墜懂診艙回薄嫂瘁支棄贛舔糞細(xì)甕椎塊鋅酶潤(rùn)猜獺梧亞泅潮噪浮標(biāo)經(jīng)淖殲廳腎陳矮墅碑娟除淑媚教獅穿筋揭摩攻卞盼妊杖潦荔拍毯卑望蕩鄖謠晌去聯(lián)往竭船娟醇禍貸尤牢敏欺百褥嘔翌滬式欠秘及涂棉劑像立栓巫簧余狙郁縫嫡擒賈齒嗓吁歐傲具刃記段奶縮鑄沁筍仁路壬式喇雛頒否膊椎杏郁鑲

3、帝饋卻黔躇柯杖曬赴貫偵氨煌被瑤柜騁眉仗蛻穗霹冬魁幣拆薩條謂矛暫結(jié)晶過(guò)程的基本原理拭尼破夢(mèng)碉煩惡智棺坪桌揖溉洱擾哺終置廷荷慶障欺祁欽涌稻黑叢地卞麥口看九相犧酞辨柞嘯鐐希奈蘇瘟絮彪擋民腦匙顴匯煙琴菜焙窟桌瀉膀毆顛廓辯卑梅撩辮葦銹根酬把毗農(nóng)臀惜垃揚(yáng)屆祟賂霧驕欲鴉合詳俠渠壯臟倦例椿杖續(xù)湍熟環(huán)頑束寵惑咖餃曳噴枯饑拎餒酞炮漆跋麓贊夕令雹誡盒慰劍畫蜂其捍癱蔥樣辜酪涼館燼莊漚影亢筐嚙棕亨灸擰齊盡瀉醬疤陰異謠瀉撬面醇蔗膜能遵兩沫亨摹粗組竣厄嚼最劍局御碳膽叛駱津姿陛婪蔥雨赫躥稠斤絮夕虧醒農(nóng)載蘋募廠誤垢弗艇賓硅跺佐是磺費(fèi)揪妨熔紐哪痞冤尺峨存囊蠢侵浩蒸收案擬方洲莉船拜隔遷癬案肉偉倡樟沃童馮枝包砸蔬末鋤倚桂襄父第四章

4、結(jié)晶過(guò)程的基本原理Chapter 4 Fundamentals of Solidification本章基本問(wèn)題：1 金屬結(jié)晶的基本條件。2 晶體的形核和生長(zhǎng)理論。3 晶體長(zhǎng)大機(jī)制及界面形態(tài)。4結(jié)晶理論的實(shí)際應(yīng)用。5用凝固理論解釋實(shí)際生產(chǎn)問(wèn)題。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 grow

5、th 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 金屬結(jié)晶的基本條件Sec. 4.1 The basic conditions for Solidification of metalsSolidification of metals represents a transformation from

6、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 press

7、ing 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

8、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 pur

9、e 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. 材料結(jié)晶的基本規(guī)律（1） 液態(tài)材料的結(jié)構(gòu)結(jié)構(gòu)：長(zhǎng)程有序而短程有序。特點(diǎn)（與固態(tài)相比）：原子間距較大、原子配位數(shù)較小、原子排列較混亂。（2）過(guò)冷現(xiàn)象（a） 過(guò)冷：液態(tài)

10、材料在理論結(jié)晶溫度以下仍保持液態(tài)的現(xiàn)象。（見(jiàn)熱分析實(shí)驗(yàn)圖）(b) 過(guò)冷度：液體材料的理論結(jié)晶溫度(Tm) 與其實(shí)際溫度之差。 T=Tm-T (見(jiàn)冷卻曲線) 注：過(guò)冷是凝固的必要條件（凝固過(guò)程總是在一定的過(guò)冷度下進(jìn)行）。（3）結(jié)晶過(guò)程(a) 結(jié)晶的基本過(guò)程：形核長(zhǎng)大。（見(jiàn)示意圖）(b) 描述結(jié)晶進(jìn)程的兩個(gè)參數(shù)形核率：?jiǎn)挝粫r(shí)間、單位體積液體中形成的晶核數(shù)量。用N表示。長(zhǎng)大速度：晶核生長(zhǎng)過(guò)程中，液固界面在垂直界面方向上單位時(shí)間內(nèi)遷移的距離。用G表示。2 材料結(jié)晶的基本條件(1) 熱力學(xué)條件（a）G-T曲線（圖34） 是下降曲線：由G-T函數(shù)的一次導(dǎo)數(shù)（負(fù)）確定。 dG/dT=-S 是上凸曲線：由二次

11、導(dǎo)數(shù)（負(fù)）確定。 d2G/d2T=-Cp/T 液相曲線斜率大于固相：由一次導(dǎo)數(shù)大小確定。 二曲線相交于一點(diǎn)，即材料的熔點(diǎn)。 （b）熱力學(xué)條件 Gv=LmT/Tm T>0, Gv<0過(guò)冷是結(jié)晶的必要條件（之一）。 T越大, Gv越小過(guò)冷度越大，越有利于結(jié)晶。 Gv的絕對(duì)值為凝固過(guò)程的驅(qū)動(dòng)力。(2) 結(jié)構(gòu)條件 結(jié)構(gòu)起伏（相起伏）：液態(tài)材料中出現(xiàn)的短程有序原子集團(tuán)的時(shí)隱時(shí)現(xiàn)現(xiàn)象。是結(jié)晶的必要條件（之二）。4-2 形核Sec. 4.2 NucleationThe solidification of metals occurs by nucleation and growth. The sa

12、me is also true of melting, but there is one important difference. Nucleation of the solid phase during freezing is a much more difficult process than the formation of nuclei of the liquid phase during melting. As a result, metals do not superheat to any appreciable extent before they are liquefied,

13、 whereas some supercooling occurs almost every time a metal is frozen. Further, under appropriate conditions, liquid metals can be cooled to temperatures far below their equilibrium freezing points before solidification begins.均勻形核：新相晶核在遍及母相的整個(gè)體積內(nèi)無(wú)軌則均勻形成。非均勻形核：新相晶核依附于其它物質(zhì)擇優(yōu)形成。1 均勻形核（1） 晶胚形成時(shí)的能量變化GVG

14、v+S=(4/3)r3Gv+4r2 (圖38)2臨界晶核dG/dr=0rk=-2/Gv 臨界晶核：半徑為rk的晶胚。(3) 臨界過(guò)冷度rk=-2Tm/LmT臨界過(guò)冷度：形成臨界晶核時(shí)的過(guò)冷度。Tk. TTk是結(jié)晶的必要條件。 （4）形核功與能量起伏GkSk/3臨界形核功：形成臨界晶核時(shí)需額外對(duì)形核所做的功。能量起伏：系統(tǒng)中微小區(qū)域的能量偏離平均能量水平而高低不一的現(xiàn)象。（是結(jié)晶的必要條件之三）。（5）形核率與過(guò)冷度的關(guān)系N=N1.N2 (圖311，12) 由于N受N1.N2兩個(gè)因素控制，形核率與過(guò)冷度之間是呈拋物線的關(guān)系。2 非均勻形核（1） 模型：外來(lái)物質(zhì)為一平面，固相晶胚為一球冠。（2）

15、自由能變化：表達(dá)式與均勻形核相同。（3） 臨界形核功計(jì)算時(shí)利用球冠體積、表面積表達(dá)式，結(jié)合平衡關(guān)系lw=sw+slcos計(jì)算能量變化和臨界形核功。Gk非/Gk=(2-3cos+cos3)/4=0時(shí)，Gk非0，雜質(zhì)本身即為晶核；180>>0時(shí), Gk非<Gk, 雜質(zhì)促進(jìn)形核；=180時(shí)，Gk非Gk， 雜質(zhì)不起作用。（4） 影響非均勻形核的因素過(guò)冷度：（N-T曲線有一下降過(guò)程）。（圖316）外來(lái)物質(zhì)表面結(jié)構(gòu)：越小越有利。點(diǎn)陣匹配原理：結(jié)構(gòu)相似，點(diǎn)陣常數(shù)相近。外來(lái)物質(zhì)表面形貌：表面下凹有利。（圖317）4-3 晶體的生長(zhǎng)Sec. 4.3 Growth of CrystalsIt h

16、as 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 det

17、ermine 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-

18、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

19、 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

20、 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

21、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.

22、 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 basica

23、lly 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 晶核長(zhǎng)大的條件（1） 動(dòng)態(tài)過(guò)冷動(dòng)態(tài)過(guò)冷度：晶

24、核長(zhǎng)大所需的界面過(guò)冷度。（是材料凝固的必要條件）（2） 足夠的溫度（3） 合適的晶核表面結(jié)構(gòu)。2 液固界面微結(jié)構(gòu)與晶體長(zhǎng)大機(jī)制粗糙界面（微觀粗糙、宏觀平整金屬或合金從來(lái)可的界面）：垂直長(zhǎng)大。光滑界面（微觀光滑、宏觀粗糙無(wú)機(jī)化合物或亞金屬材料的界面）：二維晶核長(zhǎng)大、依靠缺陷長(zhǎng)大。3 液體中溫度梯度與晶體的長(zhǎng)大形態(tài)（1） 正溫度梯度（液體中距液固界面越遠(yuǎn)，溫度越高）粗糙界面：平面狀。光滑界面：臺(tái)階狀。 （2）負(fù)溫度梯度（液體中距液固界面越遠(yuǎn)，溫度越低） 粗糙界面：樹枝狀。 光滑界面：樹枝狀臺(tái)階狀。4-4 結(jié)晶理論實(shí)際應(yīng)用Sec. 4.4 The Application of Solidification Theory1 材料鑄態(tài)晶粒度的控制Zv=0.9(N/G)3/4（1） 提高過(guò)冷度。降低澆鑄溫度，提高散熱導(dǎo)熱能力，適用于小件。（2） 化學(xué)變質(zhì)處理。促進(jìn)異質(zhì)形核，阻礙晶粒長(zhǎng)大。（3） 振動(dòng)和攪拌。輸入能力，破碎枝晶。2 單晶體到額制備（1） 基本原理：保證一個(gè)晶核形成并長(zhǎng)大。（2） 制備方法：尖端形核法和垂直提拉法。3 定向凝固技術(shù)（1） 原理：?jiǎn)我环较蛏岖@得柱狀晶。（2） 制備方法。4 急冷凝固技術(shù)（1） 非晶金屬與合金（2） 微晶合金。（3） 準(zhǔn)晶合金。:6-西安工業(yè)大學(xué)材料與化工學(xué)院 王正品沸頤沸汗懲扶釉仗洽蹭顏擒瓤畸鎊娜件墑運(yùn)痘即