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/MS371/ Structure and Properties of Engineering Alloys Chapter 1 Iron-Carbon Alloys Ⅰ

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/MS371/ Structure and Properties of Engineering Alloys pure iron : to be obtained through zone refining adding a small amount of C, Mn, P, S 증가 pure iron 의 allotropic forms Allotropic forms Crystallographic form Unit cube edge [Å] Temperature range Alphabcc2.86up to 910°C Gammafcc3.65910~1403°C Deltabcc2.931403~1535°C Density: 7.868g/cm 3, m.p.: 1535°C, b.p.: 3000°C Iron

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/MS371/ Structure and Properties of Engineering Alloys α-ferrite (bcc) γ-austenite (fcc) δ-ferrite (bcc) cementite (Fe 3 C) Iron-carbon phase diagram

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/MS371/ Structure and Properties of Engineering Alloys Calculated by TCCR L + Fe 3 C γ L + γ α + Fe 3 C γ + Fe 3 C Liquid δ Iron-carbon phase diagram

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/MS371/ Structure and Properties of Engineering Alloys α-ferrite: solid solution of in α-iron γ-austenite: solid solution of carbon in δ-ferrite: solid solution of carbon in δ-iron cementite: Fe 3 C, non-equilibrium (metastable) phase ○ iron atom ● carbon atom atomic structure of cementite (a≠b≠c, α=β=γ=90°) 12Fe and 4C per unit cell 6.67wt% carbon, 93.3wt% Iron Iron-carbon

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/MS371/ Structure and Properties of Engineering Alloys ① ② ③ Invariant reaction in the Fe-Fe 3 C phase diagram

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/MS371/ Structure and Properties of Engineering Alloys hypoeutectoidhypereutectoid ① ② ① 724°C: austenizing or (homogeneous γ-austenite transformation) ② below 723°C: eutectoid (pearlite is a mixture of α + Fe 3 C) Slow cooling of plain-carbon steels

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/MS371/ Structure and Properties of Engineering Alloys a. γ-austenite b. grain boundary 에 -eutectoid ferrite 형성 c. pro-eutectoid ferrite 성장 d. γ-austenite 가 로 phase transition Hypo-eutectoid carbon steel

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/MS371/ Structure and Properties of Engineering Alloys a. γ-austenite b. grain boundary 에 pro-eutectoid 형성 c. pro-eutectoid Fe 3 C 성장 d. γ-austenite 가 pearlite 로 phase transition Hyper-eutectoid carbon steel

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/MS371/ Structure and Properties of Engineering Alloys 1.2 ①②③ Carbon steel (below 723C)

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/MS371/ Structure and Properties of Engineering Alloys Experimental arrangement for determining the microscopic changes that occur during the isothermal transformation of austenite in an eutectoid plain-carbon steel Isothermal transformation of eutectoid carbon steel

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/MS371/ Structure and Properties of Engineering Alloys Microstructural changes which occur during the isothermal transformation of an eutectoid plain carbon steel austenite5.8min19.2min24.2min66.7min22min Isothermal transformation of an eutectoid carbon steel

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/MS371/ Structure and Properties of Engineering Alloys Isothermal transformation diagram for an eutectoid plain-carbon steel showing its relationship to the Fe-Fe 3 C phase diagram Isothermal transformation of an eutectoid carbon steel

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/MS371/ Structure and Properties of Engineering Alloys Transformation of austenite to pearlite

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/MS371/ Structure and Properties of Engineering Alloys Isothermal reaction curve first stage second stage third stage first stage: transformation rate 가 느리다 적은 양의 pearlite nodule 이 nucleation & grow second stage: transformation rate 가 빠르다 새로운 많은 nuclei 가 nucleation 되고 grow 되며 nodule 은 계속 성장함 third stage: transformation rate 가 느리다 nucleation rate 가 감소하고, pearlite nodule 의 growth 도 impingement 에 의해 감소 Transformation of austenite to pearlite

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/MS371/ Structure and Properties of Engineering Alloys grain size 小大 grain size 가 작아지면 grain boundary area 가 증가하므로 transformation rate 가 증가하 므로 S 자형 곡선이 왼쪽으로 이동한다 반대로 grain size 가 커지면 S 자형 곡선이 오른쪽으로 이동한다 Transformation of austenite to pearlite

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/MS371/ Structure and Properties of Engineering Alloys temperature effecteffect of prior austenite grain size Transformation of austenite to pearlite inter-lamellar spacing dependent on only

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/MS371/ Structure and Properties of Engineering Alloys martensite: metastable structure supersaturated solid solution of C in α-ferrite isothermal transformation diagram for an eutectoid steel Transformation of austenite to martensite What is Ae 1 ? A 1, Ac 1, Ar 1 A 3, Ac 3, Ar 3 A cm, Ac cm, Ar cm

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/MS371/ Structure and Properties of Engineering Alloys Characteristics of martensitic transformation 1) martensite structures to depend on C content low Carbon → (dislocated) martensite high Carbon → (twinned) martensite effect of C content on martensite transformation start temperature ( ) Transformation of austenite to martensite

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/MS371/ Structure and Properties of Engineering Alloys (b) mixed type(a) lath type(c) plate type Transformation of austenite to martensite Characteristics of martensitic transformation 1) martensite structures to depend on C content

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/MS371/ Structure and Properties of Engineering Alloys Characteristics of martensitic transformation 2) transformation to occur, transformation 이 매우 빠르게 일어나 no time for intermix 3) transformation 후에도 Fe atom 에 대한 C atom 의 상대적 위치 같다. 즉, no change in Transformation of austenite to martensite

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/MS371/ Structure and Properties of Engineering Alloys Characteristics of martensitic transformation 4) γ-austenite (fcc) → martensite ( ) martensite 가 bct 구조를 가지는 이유는 γ-austenite 는 fcc 구조로 bcc 구조보다 solid solubility 가 큰데, bcc 로 transformation 하게 되면 기존의 C atoms 들을 다 수용 못하여 excess C atoms 들을 수용하기 위해 c 축을 따라 bcc 구조가 distortion 되어서 bct 구조가 됨 bctbccfcc Transformation of austenite to martensite

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/MS371/ Structure and Properties of Engineering Alloys schematic representation of the martensite transformation in high- carbon iron-carbon alloys Transformation of austenite to martensite http://www.doitpoms.ac.uk/tlplib/superelasticity/displacive.php

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/MS371/ Structure and Properties of Engineering Alloys Morphology of martensite in Fe-C alloys lath martensite: dislocation martensite (slip 이 발생 ) domain 내의 각각의 lath 들은 일정한 orientation 을 가짐 lath 들은 highly distortion 되어 있고 높은 밀도의 dislocation 들이 tangle 된 지역을 이루고 있음 almost no retained γ-austenite plate martensite: needle-like plate 들이 habit plane {225} 에서 {259} 까지 위에서 independent 하게 형성됨 dislocation density 가 낮음 plate 의 크기는 다양하고 {112} 위에 twin 들이 존재 lath and plate (mixed) martensite: 탄소 함량 0.6~1.0%, 온도 범위 200~320°C Transformation of austenite to martensite

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/MS371/ Structure and Properties of Engineering Alloys Strength of martensite in Fe-C alloys Hardness of fully hardened martensitic plain- carbon steel as a function of carbon content 1.refinement of the martensite cell size with increasing carbon content 2.segregation of carbon to cell walls 3.solid solution hardening 4.dispersion hardening due to precipitation of carbide Transformation of austenite to martensite

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/MS371/ Structure and Properties of Engineering Alloys Transformation of pure Fe (fcc) to martensite (bcc) Displacive transformation: quench fcc iron from 914 °C to room temperature at a rate of about 10 5 °Cs -1 to prevent the diffusive fcc bcc, fast cooling rate: °Cs -1 in reality, below 550°C the iron will transform to bcc by a transformation instead

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/MS371/ Structure and Properties of Engineering Alloys Nucleation, growth & morphology of martensite bcc lenses –nucleation at fcc GB –growing almost instantaneously –stop growing at next GB martensite –a phase in any material by displacive transformation martensitic transformation –displacive transformation The mechanism of displacive transformation (martensite) in iron: nucleation and growth from grain boundary to next grain boundary

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/MS371/ Structure and Properties of Engineering Alloys Martensite lattice martensite lattice – with parent lattice –growing as thin on preferred planes and in preferred direction least distortion of the lattice The crystallographic relationships between martensite and parent lattice for pure iron

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/MS371/ Structure and Properties of Engineering Alloys Bain strain by atomic movements fcc bcc “Bain strain” undistorted bcc cell This atomic “switching” involves the least shuffling of atoms. As it stands the new lattice is not coherent with the old one. But we can get coherency by rotating the bcc lattice planes as well (a) The unit cells of fcc and bcc iron (b) Two adjacent fcc cells make a distorted bcc cell

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