第五章 二元相图和合金的凝固.docx

1、第五章 二元相图和合金的凝固第五章 二元相图和合金的凝固Chapter 5 Binary Phase Diagrams and The Solidification of Alloys本章基本问题：1. 组元、合金系、相、相变和相平衡等基本概念。2 相图意义及相图测定方法。3 相律、杠杆定律的意义及在相图中的应用。4匀晶相图、固溶体合金的平衡结晶和非平衡结晶。5共晶相图、包晶相图以及相图中各种合金的平衡和非平衡结晶。6用合金热力学分析绘制相图。7铁碳合金相图及各种钢铁材料的组织分析。8合金铸件的组织缺陷分析。Questions for chapter 51. What are componen

2、ts, alloy systems, phases and phase equilibrium?2. What is the meaning of phase diagrams and how to determine phase diagrams?3. What are the meanings of phase rule and lever rule? How to use them?4. What do isomorphous phase diagrams look like? How to analyze equilibrium and non-equilibrium solidifi

3、cations of solid solutions?5 . What do eutectic and peritectic phase diagrams look like? 6. How to study phase diagrams by using alloy thermodynamics? 7. How to describe Iron-carbon phase diagrams and microstructures of all kinds of iron-carbon alloys?8. How to analyze microstructure defects of iron

4、-carbon alloys castings? 5-1 相、相平衡及相图Sec. 5-1 Phase, Phase Equilibrium and Phase DiagramsComponent: A component can be loosely defined as a chemically distinct and essentially indivisible substance. The components are often the metallic elements that make up the system. Metallic elements are not the

5、 only types of components that can be used to form metallic systems, it is possible to have systems with components that are pure chemical compounds.Alloy: we define an alloy as a material that exhibits properties of a metallic material and is made from multiple elements.Alloy system: alloy system m

6、eans all the possible alloys that can be formed from a given set of components. An alloy of one composition is representative of a thermodynamic system, while an alloy system signifies all compositions considered together.Phase: A phase is defined as a macroscopically homogeneous body of matter.Phas

7、e diagrams: phase diagrams, also called equilibrium or constitution diagrams, are a very important tool in the study of alloys. The interrelationships between the phases, the temperature, and the composition in an ally system are shown by phase diagrams only under equilibrium conditions.Phase rule:

8、the phase rule describes the relationship between degrees of freedom, the number of components, and number of phases.Lever rule: a very important relationship, which applies to any two-phase region of a two-component, or binary , phase diagram, is the so-called lever rule.Phase diagram: a phase diag

9、ram is a graphical representation of the phase present and the ranges in composition, temperature, and pressure over which the phases are stable.5-2 匀晶相图及固溶体结晶Sec. 5.2 Isomorphous Phase Diagram and Solidification of Isomorphous Alloys 1. Introduction of isomorphous phase diagramsIn isomorphous syste

10、m only a single type of crystal structure is observed for all ratios of the components.Liquidus line, solidus line, phase regionsPhase regions: a. the area above the liquidus line is the region of liquid; b. the area between liquidus and solidus lines is the region of solid and liquid. c. the area b

11、elow the solidus line is the region of solid.At a given temperature: draw a horizontal line passing through the given temperature, make the intersecting points by a and b:We will get information as follow:a. the alloys whose compositions are lower that point a are in the solid state;b. the alloys wh

12、ose compositions are higher than point b are in the liquid state;c. the alloys whose compositions are between points a and b are in the liquid and solid states, these two phases are in equilibrium, and we can know the fractions of two phases.At a given composition: draw a vertical line passing throu

13、gh the given composition, make the intersecting points by 1 and 2:We will get information as follow: a. if the temperature is higher than point 1, it will be in liquid state; b. at point I, it starts to solidify; c. it the temperature is between points I and 2, it will be in liquid and solid states;

14、 as the temperature decreases, the compositions of liquid and solid will change along liquidus and solidus lines respectively.d. at point 2, it finishes solidification.e. if the temperature is lower than point 2, it will be in solid state.1 匀晶相同及其分析（1） 匀晶转变：由液相直接结晶出单相固溶体的转变。（2） 匀晶相图：具有匀晶转变特征的相图。（3）

15、相图分析（以Cu-Ni相图为例）两点：纯组元的熔点；两线：L, S相线；三区：L, , L+。2 固溶体合金的平衡结晶（1） 平衡结晶：每个时刻都能达到平衡的结晶过程。（2） 平衡结晶过程分析 冷却曲线：温度时间曲线； 相（组织）与相变（各温区相的类型、相变反应式，杠杆定律应用。）； 组织示意图； 成分均匀化：每时刻结晶出的固溶体的成分不同。（3） 与纯金属结晶的比较1 相同点：基本过程：形核长大； 热力学条件：T0； 能量条件：能量起伏； 结构条件：结构起伏。 不同点：合金在一个温度范围内结晶（可能性：相率分析，必要性：成分均匀化。） 合金结晶是选分结晶：需成分起伏。3 固溶体的不平衡结晶（

16、1） 原因：冷速快（假设液相成分均匀、固相成分不均匀）。（2） 结晶过程特点：固相成分按平均成分线变化（但每一时刻符合相图）； 结晶的温度范围增大； 组织多为树枝状。（3） 成分偏析：晶内偏析：一个晶粒内部化学成分不均匀现象。 枝晶偏析：树枝晶的枝干和枝间化学成分不均匀的现象。 （消除：扩散退火，在低于固相线温度长时间保温。）4 稳态凝固时的溶质分布 （1）稳态凝固：从液固界面输出溶质速度等于溶质从边界层扩散出去速度的凝固过程。 （2）平衡分配系数：在一定温度下，固、液两平衡相中溶质浓度的比值。k0=Cs/Cl （3）溶质分布：液、固相内溶质完全混合（平衡凝固）a; 固相不混合、液相完全混合b

17、; 固相不混合、液相完全不混合c； 固相不混合、液相部分混合d。（4） 区域熔炼（上述溶质分布规律的应用）5 成分过冷及其对晶体生长形态的影响（1） 成分过冷：由成分变化与实际温度分布共同决定的过冷。（2） 形成：界面溶质浓度从高到低液相线温度从低到高。 （图示：溶质分布曲线匀晶相图液相线温度分布曲线实际温度分布曲线成分过冷区。）（3）成分过冷形成的条件和影响因素 条件：G/RTE (eutectic temperature): the freezing process of all alloys is similar to that of isomorphous; but if the co

18、mpositions are lower than point E(eutectic point), the primary phase is , for those higher than point E, the primary phase is , we named them proeutectic or proeutectic . When T= TE, there will be a three phase reaction: L will transforms to two different solids and simultaniously at a constant T an

19、d constant composition. One phase is converted into two other phases or two solid form simultaniously from a single liquid phase. Three phases are in equilibrium at a constant composition and a constant temperature. Compositions lying to the left of eutectic point are designated as hypoeutectic, whi

20、le those to the right are called hypereutectic. Their microstructures consist of a mixture of the proeutectic phase and the eutectic structure. When TTE , there are two solvus lines MF and NG, MF represents the solubility of B in , NG represents the solubility of A in . When cooled below the solvus

21、lines, the compositions become supersaturated and precipitate particles.共晶转变：由一定成分的液相同时结晶出两个一定成分固相的转变。共晶相图：具有共晶转变特征的相图。（液态无限互溶、固态有限互溶或完全不溶，且发生共晶反应。共晶组织：共晶转变产物。（是两相混合物）1 相图分析（相图三要素）（1） 点：纯组元熔点；最大溶解度点；共晶点（是亚共晶、过共晶成分分界点）等。（2） 线：结晶开始、结束线；溶解度曲线；共晶线等。（3） 区：3个单相区；3个两相区；1个三相区。2 合金的平衡结晶及其组织（以Pb-Sn相图为例）（1） Ws

22、nTP (peritectic temperature) : the freezing process is similar to that of isomorphous, , when the alloys compositions are lower than point D.When T= TP : a liquid and a solid react to form a new and different solid phase. Cooling through the peritectic temperature combines the and liquid phases to f

23、orm the . Phase. When T TP: solubility of or and will decrease with the temperature decreasing, then if the temperature is below solvus lines, the secondary phase will precipitate. For those compositions is higher than point P, the freezing process is similar to the of isomorphous:. 包晶转变：由一个特定成分的固相和液相生成另一个特点成分固相的转变。 包晶相图：具有包晶转变特征的相图。1 相图分析点、线、区。2 平衡结晶过程及其组织（1） 包晶合金的结晶结晶过程：包晶线以下，L, 对过饱和界面生成三相间存在浓度梯度扩散长大全部转变为。室温组织：或。（2） 成分在C-D之间合金的结晶结晶过程：剩余；室温组织：。3 不平衡结晶及其组织异常相导致包晶偏析包晶转变要经扩散。包晶偏析：因包晶转变不能充分进行而导致的成分不均匀现象。异常相由不平衡包晶转变引起。成分在靠近固相、包晶线以外端点附件。4 包晶转变的应用