© GNU Su-Jin Kim Deformation & Strengthening Material Science Deformation & Strengthening 변형 경화

© GNU Su-Jin Kim Deformation & Strengthening Material Science 2 Dislocation Motion ( 전위 ) Metals - plastic deformation occurs by dislocation movement If dislocations can't move, plastic deformation doesn't occur! Single Crystal Slip ( 단결정미끄럼 )

© GNU Su-Jin Kim Deformation & Strengthening Material Science Slip Motion in Polycrystals ( 다결정미끄럼 ) Polycrystals stronger than single crystals grain boundaries are barriers to dislocation motion.

© GNU Su-Jin Kim Deformation & Strengthening Material Science 4 Anisotropy can be induced by rolling a polycrystalline metal. - before rolling 235  m - after rolling - anisotropic( 이방성 ) since rolling affects grain orientation and shape. rolling direction Anisotropy in  y - isotropic( 등방성 ) since grains are equiaxed & randomly oriented.

© GNU Su-Jin Kim Deformation & Strengthening Material Science 5 Strengthening( 강화 ): 1: Reduce Grain Size( 결정립 미세화 ) Smaller grain size increase grain boundaries, barriers to make slip difficult.

© GNU Su-Jin Kim Deformation & Strengthening Material Science 6 Strengthening by 2. Solid Solution Alloying ( 합금 ) Reduce mobility of dislocations and increase strength

© GNU Su-Jin Kim Deformation & Strengthening Material Science 7 Ex: Solid Solution Strengthening in Copper Tensile strength & yield strength increase with wt% Ni. Alloying increases  y and TS. Adapted from Fig. 7.16(a) and (b), Callister & Rethwisch 8e. Tensile strength (MPa) wt.% Ni, (Concentration C) Yield strength (MPa) wt.%Ni, (Concentration C)

© GNU Su-Jin Kim Deformation & Strengthening Material Science 8 Strengthening: 3: Cold Work (Strain Hardening, 변형경화 ) Deformation at room temperature (for most metals). Cod work increase strength and decrease ductility. -Forging A o A d force die blank force -Rolling roll A o A d

© GNU Su-Jin Kim Deformation & Strengthening Material Science 9 Impact of Cold Work ( 냉간가공 효과 ) Yield strength (  y ) & Tensile strength (TS) increases. Ductility (%EL or %AR) decreases. As cold work is increased low carbon steel

© GNU Su-Jin Kim Deformation & Strengthening Material Science 10 1 hour treatment at T anneal... ( 풀림 ) decreases TS and increases %EL. Effects of cold work are nullified! Annealing( 풀림 ) After Cold Working tensile strength (MPa) ductility (%EL) tensile strength ductility Recovery Recrystallization Grain Growth annealing temperature (ºC) Three Annealing stages: 1.Recovery 2.Recrystallization 3.Grain Growth

© GNU Su-Jin Kim Deformation & Strengthening Material Science 11 33% cold worked brass New crystals nucleate after 3 sec. at 580  C. 0.6 mm Three Stages During Heat Treatment: 2. Recrystallization( 재결정 ) New grains are formed T R = recrystallization temperature = 0.3T m < T R < 0.6T m After 4 seconds After 8 seconds 0.6 mm All cold-worked grains are eventually consumed/replaced.

© GNU Su-Jin Kim Deformation & Strengthening Material Science 12 Three Stages During Heat Treatment: 3. Grain Growth( 결정립성정 ) At longer times, average grain size increases. After 8 s, 580ºC After 15 min, 580ºC 0.6 mm - Small grains shrink and large grains continue to grow

© GNU Su-Jin Kim Deformation & Strengthening Material Science 13 TRTR T R = recrystallization temperature º