MT 348 Outline No MECHANICAL PROPERTIES

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1 MT 348 Outline No MECHANICAL PROPERTIES I. Introduction A. Stresses and Strains, Normal and Shear Loading B. Elastic Behavior II. Stresses and Metal Failure A. ʺPrincipal Stressʺ Concept B. Plastic Yield Criteria C. Elastic Fracture Criteria D. Examples: Uniaxial Tension, Torsion III. Tensile Properties A. Engineering Stress and Strain B. Strength Characteristics 1. Youngʹs modulus 2. proportional limit (upper yield point or yield point) 3. yield strength 4. ultimate tensile strength C. Ductility 1. elongation at fracture 2. reduction of area at fracture D. Example Calculations IV. Additional Mechanical Properties A. Creep 1. creep strength 2. rupture life B. Fatigue (S/N Curves Only) 1. endurance limit 2. fatigue strength C. Fracture 1. impact strength 2. fracture toughness

2 I. Introduction A. τtheoretical vs. τobserved B. Observation of Dislocations 1. electron microscopy 2. etch pitting 3. slip lines MT 348 Outline No DISLOCATIONS AND SLIP PHENOMENA II. Dislocation Configurations A. Edge Dislocations (b t) 1. definitions: burgerʹs vector, burgerʹs circuit, and tangent vector 2. slip plane B. Screw Dislocations (b // t) 1. ease of changing slip planes C. Mixed Dislocations 1. glide loop D. Dislocation Sources III. Slip Phenomena A. Introduction 1. slip system as a close packed direction in a close packed (or closest packed) plane 2. changing slip planes a. cross slip (screw dislocation) b. climb (edge dislocation) B. Slip Systems in Metallic Crystals C. Resolved Shear Stress and Critical Resolved Shear Stress 1. resolved shear stress 2. τcrss 3. factors that affect τcrss 4. calculate resolved shear stress by vector algebra D. Deformation in Single Crystals

3 I. Introduction [6.1] A. Low Angle Boundaries [6.2] B. High Angle Boundaries [6.5, 6.8] MT 348 Outline No ELEMENTS OF GRAIN BOUNDARIES II. Boundary Energy A. Grain Boundaries [6.5] B. Interphase Boundaries [6.9] 1. force balance 2. 2 nd phase morphology (example: hot shortness) III. Grain Size [6.10] A. Measurement 1. linear intercept method 2. ASTM grain size number B. Mechanical Property Effects [6.11] 1. strengthening (Hall Petch Equation) 2. fabricability

4 MT 348 Outline No VACANCIES AND DIFFUSION I. Introduction A. Temperature Sensitive Processes B. Free Energy Decrease as ʺDriving Forceʺ 1. internal energy and enthalpy 2. configurational entropy II. Vacancy Formation A. Setting up ΔG B. Calculating Δ S = kln P B PA C. Minimizing Δ G d G 0 Δ = dn v D. The Result: Q n r v RT = e n III. Vacancy Motion A. Setting Up Calculation B. The Result: Q m RT Γ = Ae IV. Solid State Diffusion A. Vacancy Mechanism (Qf + Qm) B. Interstitial Mechanism (Qm only) v

5 MT 348 Outline No RECRYSTALLIZATION AND GRAIN GROWTH I. Introduction A. Cold Work / Stored Energy B. Release of Cold Work (Stages of Annealing) II. Recovery A. Subtle Microstructural Changes 1. dislocation annihilation 2. polygonization 3. sub boundary coalescence III. Recrystallization A. Time and Temperature Dependence 1. observations 2. apparent activation energy, Qr 3. ʺrecrystallization temperatureʺ B. Effect of Prior Strain on Recrystallization Rate C. Nucleation and Growth in Recrystallization 1. ε, T effects on nucleation 2. ε, T effects on growth D. Recrystallized Grain Size 1. effect of prior strain 2. effect of metal purity 3. effect of initial grain size E. Dynamic Recrystallization (Hot Working) IV. Controlling Strength and Ductility by Working and Annealing Processes A. Forming Parameters B. Example Problems V. Grain Growth A. Driving Force 2 2 B. Ideal Grain Growth Law ( D = Do = kt ) C. Factors Affecting Grain Growth 1. temperature 2. impurities or solute additions 3. second phase particles

6 I. Introduction A. Types of Solutions 1. substitutional 2. interstitial B. Intermediate Phases 1. solid solution 2. intermetallic compound II. Solubility Criteria A. Interstitial 1. size considerations 2. transition metal effect 3. examples B. Substitutional 1. size 2. chemical similarity 3. additional criteria 4. examples MT 348 Outline No SOLID SOLUTIONS III. Solid Solution Strengthening Solute/Dislocation Interactions A. Dislocation Stress Fields 1. screw dislocation (shear stresses only) 2. edge dislocation (shear and normal stresses) B. Solute Atom Stress Fields 1. substitutional solute 2. interstitial solute C. Stress Field Interactions 1. edge dislocations/all solute atoms 2. screw dislocations/interstitial solute only 3. solute atmospheres IV. Solute Atmospheres and Mechanical Effects A. Yield Points in Low Carbon Steel B. Luders Bands and Stretcher Strains

7 MT 348 Outline No SOLIDIFICATION OF METALS I. Introduction A. The Liquid Phase B. Nucleation C. Metallic Glass II. Crystal Growth from the Liquid Phase A. Heat of Fusion B. Heat of Vaporization C. Liquid Solid Interface III. Liquid Solid Interface Instability A. Continuous Growth B. Lateral Growth C. Cellular Structure D. Dendritic Structure IV. Freezing in Alloys A. Scheil Equation B. Segregation C. Homogenization

8 MT 348 Outline No PRECIPITATION HARDENED IN ALLOYS I. Introduction A. Importance of Strengthening Technique B. Relative Effectiveness C. Basis of Technique II. Nucleation and Growth Theory A. Homogeneous Nucleation 1. critical radius (r*) 2. activation energy for nucleation (ΔG*) 3. nucleation rate ( N ) B. Heterogeneous Nucleation C. Growth Rate ( G ) D. Transformation Rate 1. microstructural result of high N / G vs. low N / G III. The Strengthening Mechanism of Precipitates A. Observations B. The Orowan Bowing Equation C. Coherency Effects D. Overaging 1. loss of coherency 2. particle coarsening (ʺOstwald ripeningʺ) E. Additional Factors 1. transition phases 2. equilibrium phases and annealing

9 MT 348 Outline No METALLURGY OF STEELS I. The Iron Carbon Phase Diagram (pp ) A. Equilibrium Phases 1. austenite 2. ferrite 3. cementite (metastable) B. Eutectoid Decomposition (pp ) 1. the eutectoid reaction and pearlite 2. pearlite growth 3. hypoeutectoid compositions (< 0.8% C) 4. hypereutectoid compositions (> 0.8% C) C. Ferritic Pearlitic Steels 1. characteristics 2. effect, carbon content 3. effect, cooling rate 4. effect, alloying 5. AISI and SAE designations D. Isothermal Transformations Diagrams (pp , ) 1. experimental determination 2. examples (eutectoid, hypoeutectoid, hypereutectoid) E. Martensite Transformation (pp ) 1. mechanism 2. microstructure and properties 3. appearance on IT diagram F. Tempered Martensite (a misnomer) (pp ) 1. microstructure and properties G. Bainite Transformation (pp ) 1. mechanism and characteristics 2. microstructure and properties a. high magnification (similarity to tempered martensite) b. low magnification 3. austempering (isothermal transformation to bainite) H. Review and Practice 1. steel transformations as nucleation and growth 2. relation of IT diagram to phase diagram 3. IT diagram, hypoeutectoid steel (Figure 18.37) 4. hardenability 5. IT diagram, hypoeutectoid low alloy steel (Figure 19.10)

10 MT 348 Outline No THE HARDENING OF STEEL I. Continuous Cooling Diagrams A. CCT vs. IT Diagrams B. Characteristic Features of CCT Diagrams 1) transformation suppressed in temperature and time, 2) absence of bainite in transformation of plain carbon steels, 3) critical cooling rate, 4) bainite curves in low alloy steels II. Hardenability A. Introduction of Concept B. Measurement and Use of Hardenability 1. critical diameter, D, and ideal critical diameter, DI. 2. severity of quench, H 3. Jominy end quench test C. Hardenability Variables 1. prior γ grain size 2. carbon content effect 3. alloying effect D. Hardenability and Applications 1. applications for high hardenability 2. applications for low hardenability III. The Martensite Transformation A. Mechanism / Displacive Transformation B. Additional Factors 1. tetragonality and mechanical properties, 2. irreversibility of M reaction, 3. carbon effect 4. significance of retained γ, 5. alloying effect, 6. martensite hardness, 7. lath and plate M C. Dimensional Changes and Quench Cracking 1. positive transformation strain and stress, 2. differential thermal contraction, 3) quenching stress vs ΔT, 3. preventing quench cracking (small ΔT, martempering, reducing hardenability) IV. Tempering of Steel A. Basic Phenomena (ʺStagesʺ) B. Microstructure and Properties C. Time and Temperature in Tempering D. Secondary Hardening