MAT_101_Winter08_Lecture_11

MAT_101_Winter08_Lecture_11 - Effects of Cold Work...

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Effects of Cold Work Stress-strain curves for a material with progressively increasing cold work What happens to the energy to failure (toughness) with increasing cold work?
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•Resu l ts fo r polycrystalline iron: σ y and TS decrease with increasing test temperature. •%EL increaseswith increasing test temperature. • Why? Vacancies help dislocations past obstacles. 1. disl. trapped by obstacle 2. vacancies replace atoms on the disl. half plane 3. disl. glides past obstacle obstacle Adapted from Fig. 6.14, Callister 6e. σ - ε Behavior vs Temperature 0 0 0.1 0.2 0.3 0.4 0.5 200 400 600 800
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• 1 hour treatment at T anneal ... decreases TS and increases %EL. • Effects of cold work are reversed! •3 Annealing stages to discuss. .. Adapted from Fig. 7.20, Callister 6e. (Fig. 7.20 is adapted from G. Sachs and K.R. van Horn, Practical Metallurgy, Applied Metallurgy, and the Industrial Processing of Ferrous and Nonferrous Metals and Alloys , American Society for Metals, 1940, p. 139.) Effect of Heating After %CW tensile strength (MPa) ductility (%EL) Annealing Temperature (°C) 300 400 500 600 60 50 40 30 20 R e c o v r y s t a l iz io n G in w h ductility tensile strength 300 700 500 100
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Annihilation reduces dislocation density. • Scenario 1 • Scenario 2 atoms diffuse to regions of tension extra half-plane of atoms extra half-plane of atoms Disl. annhilate and form a perfect atomic plane. 1. dislocation blocked; can’t move to the right obstacle dislocation 2. grey atoms leave by vacancy diffusion allowing disl. to “climb” 4. opposite dislocations meet and annihilate 3. “Climbed” disl. can now move on new slip plane τ R Recovery
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• New crystals are formed that: --have a small disl. density --are small --consume cold-worked crystals. 33% cold worked brass New crystals nucleate after 3 sec. at 580C. Adapted from Fig. 7.19 (a),(b), Callister 6e. (Fig. 7.19 (a),(b) are courtesy of J.E. Burke, General Electric Company.) 0.6 mm 0.6 mm Recrystallization
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• All cold-worked crystals are consumed. After 4 seconds After 8 seconds Adapted from Fig. 7.19 (c),(d), Callister 6e. (Fig. 7.19 (c),(d) are courtesy of J.E. Burke, General Electric Company.) 0.6 mm 0.6 mm Further Recrystallization
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Recrystallization Temperature Larger deformation Higher stored elastic energy Larger deformation Higher dislocation density Larger deformation Lower recrystallization temperature
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• At longer times, larger grains consume smaller ones. • Why?
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This document was uploaded on 04/02/2010.

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MAT_101_Winter08_Lecture_11 - Effects of Cold Work...

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