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Materials Science & Engineering Chapter 8. Deformation & Strengthening Mechanisms
Chapter 8. Deformation and StrengtheningMechanisms of Materials(재료의변형과강화메커니즘)
Deformation Mechanisms for Metals
Basic Concepts of Dislocations (전위의기본개념)
Edge dislocation (칼날전위)Screw dislocation (나선전위) 5장에서다룸Mixed dislocation (혼합전위)
소성변형(plastic deformation)은수많은전위(dislocation)의움직임(slip)에의한결과물
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Materials Science & Engineering Chapter 8. Deformation & Strengthening Mechanisms
Fig. 5.9 (a) 결정내의 screw dislocation, (b) 위에서본모양.
Fig. 5.8 Edge dislocation 주위에서의원자배치.
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Materials Science & Engineering Chapter 8. Deformation & Strengthening Mechanisms
Fig. 8.1 Edge dislocation (칼날전위)의움직임에따른원자재배열: A로표시된하나의잉여반평면은 shear stress를받으면전위가오른쪽으로이동하면서결국결정체에서원자거리만큼떨어진 step(계단)을형성함.
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Materials Science & Engineering Chapter 8. Deformation & Strengthening Mechanisms
Fig. 8.2 전위의이동에의한결정체표면에형성된계단(step): (a) edge dislocation (전위선이전단응력방향으로움직임), (b) screw dislocation(전위선이전단응력방향에수직으로움직임).
슬립면
전위선 전위선이동방향
전단응력방향
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Materials Science & Engineering Chapter 8. Deformation & Strengthening Mechanisms
Dislocation line (전위선): 원자잉여반평면의끝을따라존재하는국부적으로격자가뒤틀린선
Slip (슬립): 전위의움직임에따른소성변형과정
Slip plane (슬립면): 전위선이가로지르는결정학적면
Dislocation density (전위밀도): 단위체적당총전위길이 [단위: mm-2]or 임의의단위면적을교차하는전위수
금속결정의전위밀도: 103/mm2
소성변형이일어난금속의전위밀도: 109~1010/mm2
열처리과정을거친금속의전위밀도: 105~106/mm2
세라믹 재료의전위밀도: 102~104/mm2
IC (집적회로)에사용되는 Si 단결정의전위밀도: 0.1~1/mm2
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Characteristics of Dislocations (전위의특성)
소성변형 95% (열로분산)
5% (내부에잔류: 전위와관련된변형률에너지)
전위선주위의격자뒤틀림근처원자에압축, 인장,
전단성분의 lattice strain (격자변형률)을부과
전위가인접한변형장(strain field)
상호작용에의한힘이부과(Fig. 8.5)
Fig. 8.4 Edge dislocation 주위의압축및인장.
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Materials Science & Engineering Chapter 8. Deformation & Strengthening Mechanisms
Fig. 8.5 (a) 동일부호의두칼날전위 (척력작용), (b) 반대부호의두칼날전위 (전위가소멸되어완전한결정구조로됨).
(전위소멸)
C: compression(압축)
T: tension(인장)
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Slip Systems (슬립계)
전위의움직임은결정학적면이나결정학적방향에따라다름.전위가움직이는특정한면
slip plane (슬립면)
전위가움직이는특정한방향 slip direction (슬립방향)
결정구조와연관슬립면은가장조밀하게원자가충전된면슬립방향은가장조밀하게원자들이늘어선방향
FCC 결정구조는 {111} 면에서 <110> 방향으로슬립이일어남 FCC의슬립면과슬립방향을나타내는슬립계는
{111}<110>로표시함.
slip system (슬립계)
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Fig. 8.6 (a) FCC unit cell 내에표시한 {111}<110> 슬립계,(b) (111) 슬립면에서의세개의 <110> 슬립방향.
cf.) (111): 특정한결정면, {111}: (111) 결정면의등가 family[110]: 특정한결정방향, <110>: [110] 결정방향의등가 family
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FCC에는 4개의 {111} 면과각면에서 3개의 <110> 방향으로이루어진 12개의슬립계가존재
BCC는 {110}, {211}, {321} 슬립면이존재해슬립계의수가많음
HCP는슬립계가적음 HCP 금속은보통취성(brittle)임.
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Fig. 8.8 단결정의거시적슬립. Fig. 8.9 아연단결정의슬립.
Slip in Single Crystals (단결정의슬립)
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Plastic Deformation of PolycrystallineMetals (다결정금속의소성변형)
슬립이일어나더라도주변결정립에의해구속이생김슬립이나항복을일으킬때큰응력요구
다결정금속의강도 > 단결정금속
Fig. 8.10 소성변형시킨다결정구리시편의슬립선. (i) 각결정립의결정방향이다름결정립간의슬립선이다름,(ii) 결정립내에서슬립선이교차결정립내에복수의슬립계가생성)
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Fig. 8.11 다결정금속재료의 rolling (압연)에따른결정립구조변화(a) 소성변형전의 isotropic (등방성) 구조, (b) 변형후에늘어난anisotropic (비등방성) 구조.
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Mechanisms of Strengthening in Metals(금속의강화메커니즘)
~ 전위움직임과기계적거동사이의관계에기초고강도, 적절한연성및강인성을갖는합금이바람직함소성변형은전위움직임의용이성에의존전위움직임을방해할수록재료는더단단하고강해짐.
Strengthening by Grain Size Reduction(결정립크기감소에의한강화)
~ 결정립의크기는다결정금속의기계적성질에영향을끼침결정립계(grain boundary)가전위의이동을방해하는이유:
1. 전위가결정립을지나가기위해서는방향을바꾸어야함2. 입계부근의원자는무질서하므로슬립면이불연속이됨
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Fig. 8.14 결정립계를지날때의전위의움직임(입계를경계로슬립면이불연속이며슬립방향도바뀜).
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미세결정립재료가굵은결정립재료보다더강함(전위의이동을방해하는입계의면적이큼)
항복강도(σy)와결정립크기(d)와의관계 (Hall-Petch equation):
결정립크기는냉각속도및열처리로조절가능
Fig. 8.15 황동(70Cu-30Zn)의항복강도에대한결정립크기의영향.
0σσ +=d
kyy
재료상수
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Solid-Solution Strengthening (고용체강화)~ 침입형 or 치환형고용체합금을이용한금속강화
Fig. 8.16 Cu-Ni 합금의 Ni 함량에따른물성변화: (a) 인장강도, (b) 항복강도, (c) 신장백분율.
불순물함량 ↑ 강도 ↑
경도 ↑연성 ↓
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불순물원자는전위주위에모여격자변형률을상쇄전체변형률에너지감소슬립에대한저항성커짐 (소성변형시큰응력이요구됨)강도와경도증가
Fig. 8.17 주원자보다작은치환형불순물원자에의한전위격자변형률감쇄.
Fig. 8.18 주원자보다큰치환형불순물원자에의한전위격자변형률감쇄.
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Strain Hardening (변형경화)
~ 연성금속이소성변형을겪으면서더단단해지는현상
Cold working (냉간가공)
: 금속의재결정온도보다낮은온도에서변형을주는가공
cf.) Hot working (열간가공) ~ 변형경화현상없음
Percent cold work (냉간가공백분율, %CW):
A0, Ad : 변형전초기단면적및변형후단면적
100CW%0
0 ×
−=
AAA d
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Fig. 8.19 Steel (강), brass (황동),copper (구리)의 %CW에따른항복강도, 인장강도및연성의변화.
냉간가공 ↑ 강도 ↑
경도 ↑연성 ↓
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Fig. 8.20 저탄소강의냉간가공에따른응력-변형률거동.
변형경화메커니즘:
소성변형 ↑전위밀도 ↑전위사이의간격조밀한전위의움직임은다른전위에의해방해받음변형시많은응력을요구
냉간가공에의한변형경화현상을이용하여기계적성질향상시킴
변형경화효과는 annealing 에의해제거가능
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Recovery and Recrystallization (회복및재결정)
Recovery (회복) ~ 원자확산에따른전위움직임에의해저장된변형률에너지가완화되는과정전위수감소, 열/전기전도도는가공전상태로회복
Recrystallization (재결정) ~ 가공전상태의낮은전위밀도와변형이없는새로운등방형결정립을형성하는과정결정핵형성후확산통해결정립성장기계적성질원상태로회복 (softer, weaker & more ductile)
Recrystallization temperature (재결정온도) ~ 1시간내에재결정이완결되는온도일반적인재결정온도는 Tm (용융온도)의 1/3~1/2 사이냉간가공량클수록재결정속도빨라져재결정온도낮아짐
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Fig. 8.21 황동의재결정및결정립성장의단계별사진: (a) 냉간가공(33%CW) 후의결정립구조, (b) 재결정초기단계 (580 oC, 3 s 가열).
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(c) 부분적재결정(580 oC,4 s 가열)
(d) 재결정완료(580 oC,8 s 가열)
(e) 결정립성장(580 oC,15 min 가열)
(f) 결정립성장(700 oC,10 min 가열).
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Fig. 8.22 황동의인장강도와연성에대한 annealing 온도의영향 (annealing 시간: 1h).
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Grain Growth (결정립성장)
~ 재결정완료후고온에서변형률없는결정립이계속성장하는현상
결정립크기증가총입계면적감소총에너지감소
결정립성장: 입계의이동에의한성장작은결정립이소멸되어큰결정립에합침(평균결정립크기증가)
다결정재료의시간에따른결정립지름의변화:
d0: initial grain diameter at t=0K, n: time-independent constants (보통, n ≥ 2)
미세결정립을갖는금속의기계적성질은큰결정립보다우수함결정립이큰경우소성가공후열처리, 재결정을통해미세화시킴
tKdd nn =− 0
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Fig. 8.24 원자확산에의한결정립성장개요도.
Fig. 8.25 황동의온도에대한결정립성장그래프.
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Deformation Mechanisms for Ceramics
~ 상온에서소성변형전에파괴
Crystalline Ceramics
결정질세라믹이단단하고취성이높은이유:하전된이온이나공유결합에의해슬립계의수가적음동일전하이온의경우척력으로슬립양식이제한금속에비해전위구조가복잡
Noncrystalline Ceramics
비결정질세라믹의소성변형은점성유동(viscous flow)에의해변형변형속도는가한응력에비례, 유동의척도: 점도
점도(viscosity) ~ 비결정질재료의변형에대한저항의크기
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Mechanisms of Deformation & Strengthening of Polymers(고분자재료의변형및강화메커니즘)
Deformation of Semicrystalline Polymers반결정성고분자는결정질판상(lamella)이비정질사슬로연결된구조
Mechanism of elastic deformation (탄성변형메커니즘) (Fig. 8.27) ~ 가역적(reversible)라멜라와라멜라간의비정질사슬의부분적연신
결정질영역사슬의굽힘및연신에따른라멜라두께의증가
Mechanism of plastic deformation (소성변형메커니즘) (Fig. 8.28) ~ 비가역적(irreversible)라멜라내의 인접한사슬이미끄러짐라멜라의사슬접힘구조가인장축방향으로정렬결정질블록이라멜라로부터분리결정질블록과결합사슬이인장축방향으로정렬
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Fig. 8.27 반결정성고분자의탄성변형단계에서의사슬구조의변화.
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Fig. 8.28 반결정성고분자의소성변형단계에서의사슬구조의변화.
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Factors That Influence Mechanical Properties of Semicrystalline Polymers (반결정성고분자의기계적성질에영향을미치는요소)
온도 ↑, 변형속도 ↓탄성률 ↓, 인장강도 ↓, 연성 ↑
Molecular weight (분자량) 탄성률은분자량과직접관계는없음인장강도 TS :
TS∞ : 분자량무한대인재료의 TSA : 상수, Mn : 수평균분자량
Degree of crystallinity (결정화도)
결정화도 ↑ 탄성률 ↑(PE의경우결정화도 0.30.6 증가시탄성률 10배정도증가)
nMATSTS −= ∞
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Fig. 8.29 결정화도및분자량이폴리에틸렌의물리적특성에미치는영향.
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Drawing (연신)
필름과섬유제조시강도및탄성률을향상시키기위해활용연신방향 ~ 인장강도및탄성률최대수직방향 ~ 인장강도최소인장축과 45o 방향 ~ 탄성률최소
Heat treating (열처리) 연신하지않은재료의열처리:
열처리온도 ↑ 탄성률 ↑, 항복강도 ↑, 연성 ↓(금속재료의열처리와는반대의현상)
연신한섬유의열처리:열처리온도 ↑탄성률 ↓(사슬의배향이파괴되고결정화도가감소하기때문)
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Deformation of Elastomers (고무탄성체의변형)
고무탄성체가되기위한요구조건:1. 정지상태에서는무정형(amorphous) 구조2. 사슬결합의회전이자유로워야함3. 큰탄성변형을일으키기위해소성변형발생이지연4. 유리전이온도(glass transition temperature) 이상에존재
Fig. 8.30 가교결합된고분자사슬: (a) 정지상태,(b) 응력부과에의한탄성변형상태.
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Vulcanization (가황)
: 고무탄성체의가교결합과정주로황화합물을사용(1-5 % 정도첨가)
Unvulcanized rubber (미가황고무)~ soft, tacky, poor resistance to abrasion
Vulcanized rubber (가황고무)~ 탄성률, 인장강도, resistance to degradation by oxidation 향상
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Materials Science & Engineering Chapter 8. Deformation & Strengthening Mechanisms
Fig. 8.30 가황및미가황천연고무의응력-변형률그래프.