Heat Treatment (Annealing) of Cold-Worked Metals Annealing: heat treatment whereby ________________________________ ______________________________________________________________ i. Cold working: increase y, TS; decrease ductility (see next slide) ii. Annealing: decrease y, TS; increase ductility 3 Annealing Stages tensile strength (MPa) ductility (%EL) tensile strength ductility Recovery Recrystallization Grain Growth 600 300 400 500 60 50 40 30 20 annealing temperature (ºC) 200 100 700 Three Stages of Anealling with Increasing Temp.: - (Note:Annealling stages may also occur by holding T above Trecrys)

Cold working (strain-hardening, work hardening) Plastic deformation  Recall…. Cold working (strain-hardening, work hardening) Plastic deformation  Increase # of or dislocation density (now, very high dislocation density)  ___________________________ (many atoms out of place)  Dislocation motion _______________________________: _____________ ___________________________________  __________________________________________

As temp. increases, ______ _______________ increases 3 Stages of Annealing: 1. Recovery: As temp. increases, ______ _______________ increases Some dislocations move to relieve strain Slight _________________ in # of dislocations Minor changes in mechanical properties Brass alloy

________________________ 2. Recrystallization: _______________________ ________________________ New set of ___________ grains: Have ______ dislocation density Gradually consume cold-worked grains Occurs when T increased above Trecrys or held at T > Trecrys Trecrys = min temp required for complete recrystallization in 1 h Trecrys = ½ to 1/3 of Tmelt T in Kelvin [see photomicrographs; magnification 75X] Brass alloy crystals

Initial recrystallization Cold-worked grains 3 s @ 580 C Initial recrystallization Brass Trecrys = 450 C 4 s @ 580 C 8 s @ 580 C Complete recrystallization (Annealling stages occur by holding T above Trecrys)

Next Stage : grain growth 15 min @ 580 C 10 min @ 700 C New set of strain-free grains w/ low dislocation density

If left at elevated temperature following recrystallization: 3. Grain Growth: If left at elevated temperature following recrystallization: _________________________ (coarsening) some grow while others shrink [see photomicrographs; magnification 75X] Brass alloy Cold-worked Grains: _______ dislocation density _______ strain  stronger After Annealing, Grains:  more ductile

______ dislocation density: 109 – 1010 / mm2 (highly deformed) Cold-worked Metals: Grains: ______ dislocation density: 109 – 1010 / mm2 (highly deformed) ______ strain state  stronger After Annealing, Metals: 105 – 106 / mm2 ______strain state  more ductile Brass alloy

Hot-Working “metal forming operation performed at T > Trecryst” Metal remains ductile or (if previously cold-worked) becomes more ductile -Forging A o d force die blank -Rolling roll A o d pp 383-384 -Drawing tensile force A o d die -Extrusion ram billet container force die holder die A o d extrusion Steel 1020 y (MPa) TS (MPa) %EL Cold-drawn 350 420 15 Hot-rolled 210 380 25

Mechanical Properties of Ceramics (12.8-12.11, 13.11)

Elastic Modulus Most Ceramics: E = ___________ GPa Most Metals: E = _____________ GPa Most polymers: E < ____ GPa

Density

Most ceramics fail/fracture __________________________________ (see below) Very __________ (not tough, not ductile) Exhibits ___________________________________ (which parallels?) ______________________ (ionic) prevents dislocation motion (i.e. “slip”) Recall: Ionic bonds: 600-1500 kJ/mol Metallic bonds: <850 kJ/mol Covalent bonds: 346 kJ/mol (for C-C)

Al2O3 and Soda-lime glass

Why Flexural Strength (not Tensile Strength) for Ceramics? Difficult to prepare and test specimens with the required geometry Difficult to grip brittle materials into tensile tester clamps w/o fracture Ceramics fail after ~ 0.1% strain Material s fs (MPa) E(GPa) Si nitride Si carbide Al2O3 Zirconia (ZrO2) glass (soda) 250-1000 100-820 275-700 800-1500 69 304 345 380 205 69

Measuring Flexural Strength (fs) • _____________________________ test to measure strength. F L/2 d = midpoint deflection cross section R b d rect. circ. location of max tension Adapted from Fig. 12.32, Callister 7e. For rectangular specimen: Specimen: - rod with circular or rectangular cross-section Apply force: - “top” face [ in compression] - “bottom” face [in tension] 3. Fracture occurs on “tension face” Why? For ceramics: TS = 1/10 of compressive strength So, flexural test good substitute for tensile test fs = (3Ff L)/(2bd2) Ff = load at fracture L = distance between support pts For rectangular specimen: fs = (Ff L)/(R2) R = radius of specimen

Influence of Pores in Ceramics Figure 13.16 powder With pressure Figure 13.14 pore With sintering Pores introduced during “powder processing”

fs = o exp (-nP) E = Eo(1-1.9P + 0.9P2) Fig. 12.35 Fig. 12.36 E versus P fs versus P fs = o exp (-nP) E = Eo(1-1.9P + 0.9P2) 0, n: experimental constants Increase P  _______________ fs 10% porosity  decrease fs 50% (vs non-porous) Eo = elastic modulus of non-porous material P = volume fraction porosity Increase P  __________________ E