Fatigue
Fatigue strength
§2 Iron-carbon phase diagram
铁碳合金相图
Phase and Phase diagrams （相与相图）
•Phase：a region that differs in its state, microstructure， and /or composition from another region.
两组元在液态下无限溶解，固态下有限 溶解的二元合金相图。
The characteristics of eutectic system （共晶相图特点）：
eutectic reaction(共晶反应)
Eutectic reaction equation
共晶反应式
Eutectic system
（共晶相图）
Toughness What’s the relationship between the
value of ak and toughness? ak ↑，toughness↑
Questions for you
4. When a metal is subjected to repetitive stresses ， even when the stress is much lower than that which the metal can withstand under a static stress, the metal will fail to work. What do we call this phenomenon?
（共析相图）
The eutectoid reaction takes place completely in the solid state.
Eutectoid system
（共析相图）
Explanation of some concepts(概念): • Eutectoid reactions （共析反应） • eutectoid steels(共析钢) • hypoeutectoid steels（亚共析钢 ） • hypereutectoid steels(过共析钢)
（相图类型）
• Only binary alloy phase diagrams (二 元合金相图) are discussed.
• Types: isomorphous system(匀晶相图) eutectic system （共晶相图） eutectoid system（共析相图）
Isomorphous system
A—纯Pb熔点； B—纯Sn熔点；
C—共晶点，
WSn=61.9%（共
晶成分）
温度/℃
400
A 327.5
300
200 α
100
D
Pb 10
L
L+α E 183
19.2
L+β
C
61.9
232 B
F
97.5 β
α+β G
20 30 40 50 60 70 80 90 Sn
WSn/ %
Eutectoid system
• From cooling curves of metals and alloys, equilibrium phase diagrams of alloys are constructed.
• Ways to obtain a phase diagram for an alloy:
Calculation Experiment---
相图：表征不同成分的合金在不同温度、 压力下相的存在范围的图形。说明了合金 的状态与温度、成分等之间的关系。
Application of phase diagrams（相图的用途）
Most phase diagrams are constructed by using equilibrium conditions and are used to understand and predict many aspects of the behavior of materials.
（匀晶相图）
The two components of the alloy are completely soluble in each other in both the liquid and solid states, only a single type of crystal structure exists for all compositions of the components.（Cu-Ni）
Iron-carbon alloys containing
over 6.69% carbon are too brittle to use in industry, so we only discuss part of the iron-carbon phase diagram-----the iron-iron carbide
---differential thermal analysis (差热分析)
• Take Cu-Ni phase diagram for example. It is constructed by the thermal analysis(热分析法).
Fig.2.2 Construction of Cu-Ni phase
相：具有相同化学成分、晶体结构、 聚集状态的均匀组成部分 。
Phase and Phase diagrams （相与相图）
•Phase diagrams： graphical representations(代表图形) of what
phases are present in an alloy system at various temperatures and composition.
δorψ↑， the ductility ↑
Questions for you
2. What are the two hardness tests we mentioned last class?
Brinell hardness (布氏硬度) Rockwell hardness test (洛氏硬度)
相图多是通过平衡条件下获得的，可用来 估计材料行为的很多方面。
equilibrium conditions （平衡条件）
• Cooling or heating very slow. • In most cases, equilibrium is
approached but never fully attained.
1. Crystallization and cooling curve
• Crystallization（结晶）：the process of a metal transforms from a melted liquid into a crystalline solid.
结晶：金属由液态转化为晶体的过程。 • This process can be expressed by
diagram （Cu-Ni合金相图的建立）
temp/℃
100%
70%
50%
1452
WNi=30%
c
b
a 0%
a’ 108b
a L+α
b’ a’
α
1452 c c’
time
a) 冷却曲线
Cu
30 50 70
Ni
WNi%→
b) 相图
2. Types of phase diagram
3. Iron-carbon phase diagram(铁碳合金相图)
<1>Introduction
Plain carbon steels and cast irons contain not only carbon and iron elements, but also minor amounts of other elements such as Si, Mn, S and P, etc. However, in this course, they are treated as iron-carbon binary alloys.
• Alloys: formed by two or more metals or metals & nonmetals and have metallic characteristics.
• Similar to that of pure metals, but normally there are no flats in the curves.（冷却曲线上无平台）
(Fe-Fe3C) phase diagram. Fe3C, is also called cementite,
containing 6.69% carbon.
The iron-iron carbide phase diagram is shown in Fig.2.4.
All the room temperature structures obtained under different conditions are filled in the diagram.
cooling curve(过冷曲线).
cooling curve(冷却曲线)
cooling curve of a pure metal
degree of undercooling
(过冷度)
• We must note that the above discuss is based on cooling under equilibrium co存nd在it过io冷ns度（是平金衡属条或件合下金结冷晶却的）必, 要it条’s 件a ！ theoretic cooling process.