Fe-C System and Mechanical Properties of Steel - Prof. Miqin Zhang, Study notes of Materials science

Download Fe-C System and Mechanical Properties of Steel - Prof. Miqin Zhang and more Study notes Materials science in PDF only on Docsity! 1 Mechanical prop: Fe-C System (1) • More wt% C: TS and YS increase, %EL decreases. Effect of wt% C Co < 0.76 wt% C Hypoeutectoid Pearlite (med) ferrite (soft) Co > 0.76 wt% C Hypereutectoid Pearlite (med) Cementite (hard) 300 500 700 900 1100YS(MPa) TS(MPa) wt% C0 0.5 1 hardness 0. 76 Hypo Hyper wt% C0 0.5 1 0 50 100 %EL Im pa ct e ne rg y (Iz od , f t-l b) 0 40 80 0. 76 Hypo Hyper Mechanical prop: Fe-C system (2) Fine vs coarse pearlite vs spheroidite • Hardness: • %RA: fine > coarse > spheroidite fine < coarse < spheroidite 80 160 240 320 wt%C0 0.5 1 Br in el lh ar dn es s fine pearlite coarse pearlite spheroidite Hypo Hyper 0 30 60 90 wt%C D uc til ity (% A R ) fine pearlite coarse pearlite spheroidite Hypo Hyper 0 0.5 1 2 3 Mechanical Prop: Fe-C system (3) Fine Pearlite vs Martensite: • Hardness: fine pearlite << martensite. 0 200 wt% C0 0.5 1 400 600 Br in el lh ar dn es s martensite fine pearlite Hypo Hyper Tempering martensite reduces brittleness of martensite, reduces internal stress caused by quenching decreases TS, YS but increases %RA produces extremely small Fe3C particles surrounded by α. 9 μm YS(MPa) TS(MPa) 800 1000 1200 1400 1600 1800 30 40 50 60 200 400 600 Tempering T (°C) %RA TS YS %RA 5 Chapter 12 Structures and properties of ceramics Bonding in ceramics Imperfection in ceramics Electric properties of ceramics Ceramic phase diagrams Brittle fracture of ceramics Stress-strain behavior Mechanisms of plastic deformation Ceramic bonding Bonding: --Mostly ionic, some covalent. --% ionic character increases with difference in electronegativity. He - Ne - Ar - Kr - Xe - Rn - Cl 3.0 Br 2.8 I 2.5 At 2.2 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.7 Fr 0.7 H 2.1 Be 1.5 Mg 1.2 Sr 1.0 Ba 0.9 Ra 0.9 Ti 1.5 Cr 1.6 Fe 1.8 Ni 1.8 Zn 1.8 As 2.0 C 2.5 Si 1.8 F 4.0 Ca 1.0 Table of Electronegativities CaF2: large SiC: small 6 Imperfections in ceramics Schottky defects: --a paired set of cation and anion vacancies. Frenkel defects: an atom from a lattice site to an interstitial position Shottky Defect: Frenkel Defect Point defects in ionic crystals Impurities must also satisfy charge balance • Ex: NaCl Na+ Cl- Substitutional cation impurity Substitutional anion impurity initial geometry Ca2+ impurity resulting geometry Ca2+ Na+ Na+ Ca2+ cation vacancy initial geometry O2- impurity O2- Cl- anion vacancy Cl- resulting geometry 7 Point defects in ionic crystals Defect examples for other ionic crystal systems In simple ionic crystals, both Schottky and Frenkel defects occur, but the concentration of one type generally exceeds that of the other • Schottky defects dominate in alkali halides • Cation Frenkel defects dominate in AgCl and AgBr • Anion Frenkel defects dominate in CaF2 and fluorites Electric properties Electrical conductivity: the mobility of charged point defects Since the cation vacancy is more mobile than anion vacancy n-- defect concentration e-- charge μ-- mobility )exp( )( KT Eaen en −≈≈ += + −+ μ μμσ 10 Room T behavior is usually elastic, with brittle failure. 3-Point Bend Testing often used. Tensile tests are difficult for brittle materials. F L/2 L/2 δ= midpoint deflect ion cross section R b d rect. circ. Determine elastic modulus according to: F x linear-elastic behavior δ F δ slope = E = F δ L3 4bd3 = F δ L3 12πR4 rect. cross section circ. cross section Measuring elastic modulus 3-point bend test to measure room T strength. F L/2 L/2 cross section R b d rect. circ. location of max tension x F Fmax δmax δ 10 Flexural strength: rect. σ fs = σm fail = 1.5Fmax L bd 2 = Fmax L πR3 • Typ. values: Material σfs (MPa) E(GPa) Si nitride Si carbide Al oxide glass (soda) 700-1000 550-860 275-550 69 300 430 390 69 Data from Table 12.5, Callister 6e. Measuring strength 11 Elastic behavior Typical stress- strain behavior to fracture for aluminum oxide and glass Mechanisms of plastic deformation Crystalline ceramics are brittle • Covalent bonds are relatively strong • There are limited numbers of slip systems • Dislocation structures are complex Noncrystalline ceramics • Plastic deformation does not occur by dislocation motion for noncrystalline ceramics • Viscosity is a measure of of non-crystalline material’s resistance to deformation 12 Influence of porosity on mechanical behavior E=E0(1-1.9P+0.9P2) σfs= σ0 exp (-nP) Hardness