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Deformation & Strengthening Mechanisms of Materials Chapter 8.

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Presentation on theme: "Deformation & Strengthening Mechanisms of Materials Chapter 8."— Presentation transcript:

1 Deformation & Strengthening Mechanisms of Materials Chapter 8

2 Deformation & Strengthening Mechanisms of Materials Deformation of materials… - is refer to plastic deformation which is occur due to motion of dislocations. -- consider metals only. -- show a significant plastic deformation before failure/rupture. Strengthening of materials… - restrict the dislocation motion & makes a material harder and stronger strategies/mechanisms to strengthen the materials. Annealing of materials… - heat treatment for cold-worked metals. -- change mech. properties & microstructure. Dislocation motion in selected materials… 1. Metals (Cu, Al): - dislocation motion easiest. -- non-directional bonding. -- close-packed directions for slip. electron cloud ion cores Covalent Ceramics (Si, diamond): - dislocation motion difficult. -- directional (angular) bonding. 3. Ionic Ceramics (NaCl): - dislocation motion difficult. -- need to avoid nearest neighbors of like sign (- and +) slip dislocation motion mechanisms stages grain size reduction solid solution strengthening precipitation strengthening strain cold working recovery recrystallization grain growth deformationstrengtheningannealing

3 Deformation & Strengthening Mechanisms of Materials Deformation mechanisms… Represent a slip plane & slip direction combination. Slip plane - plane on which easiest slippage occurs -- Highest planar densities (and large interplanar spacings). Slip directions - directions of movement -- Highest linear densities. - If dislocations can't move, plastic deformation doesn't occur! example: Dislocation cell structure in lightly deformed Aluminum Wall of high dislocation density Deformation of materials… - plastic deformation which is occur due to dislocation motion. -- consider metals only. -- show a significant plastic deformation before failure/rupture. (1) slip - atom slide over each other (slip) due to movement of dislocations. -- exist external shear stress. - Occurs on specific crystallographic planes & within these planes only in certain direction. Slip system… Screw dislocation Edge dislocation

4 Deformation & Strengthening Mechanisms of Materials Slip systems… Slip in crystals… - slip occurs in densely or close packed planes. - lower shear stress is required for slip to occur in densely packed planes. - if slip is restricted in close planes, then less dense planes become operative. - Less energy is required to move atoms along denser planes. Close packed plane Non-close-packed plane - Slip systems are combination of slip planes & slip direction. - Each crystal has a number of characteristic slip systems. - i.e: In FCC crystal, slip takes place in {111} octahedral planes and directions (111) type planes and 3 [110] type directions. 4 x 3 = 12 slip systems. Adapted from Fig. 8.6, Callister & Rethwisch 3e.

5 Deformation & Strengthening Mechanisms of Materials Strengthening mechanisms…Strengthening of materials (1) grain size reduction (2) solid solution strengthening (3) precipitation strengthening - grain boundaries are barriers to slip. - barrier "strength" increases with increasing angle of misorientation. - smaller grain size: more barriers to slip. - impurity atoms distort the lattice & generate stress. - stress can produce a barrier to dislocation motion. Smaller substitutional impurity Impurity generates local stress at A and B that opposes dislocation motion to the right. A B Larger substitutional impurity Impurity generates local stress at C and D that opposes dislocation motion to the right. C D - hard precipitates are difficult to shear. i.e: ceramics in metals (SiC in Aluminum). Side View precipitate Top View Slipped part of slip plane Unslipped part of slip plane S Large shear stress needed to move dislocation toward precipitate and shear it. Dislocation “advances” but precipitates act as “pinning” sites with S.spacing - aim: -- restrict the dislocation motion. -- makes a material harder & stronger.

6 Deformation & Strengthening Mechanisms of Materials Strengthening mechanisms…Strengthening of materials (4) strain hardening - also known as cold work (%CW). -- low temperature deformation. - common forming operations change the cross sectional area. i.e: forging, extrusion, rolling & drawing process. -Forging A o A d force die blank force -Drawing tensile force A o A d die -Extrusion ram billet container force die holder die A o A d extrusion -Rolling roll A o A d - grain structure at different regions of brass rolled into a wedge. -- grains elongate in rolling direction. -- dislocations get rearranged. Cold work (rolling process) - aim: -- restrict the dislocation motion. -- makes a material harder & stronger. cold-worked grains - As cold work (%CW) is increased -- Yield strength (  y ) increases. -- Tensile strength (  TS ) increases. -- Ductility (%EL or %AR) decreases. Impact of Cold Work

7 Deformation & Strengthening Mechanisms of Materials (4) strain hardening Strain Cold work analysis What is the yield strength, tensile strength & ductility of copper rod after cold working? D o =15.2mm Cold Work D d =12.2mm Copper Answer: Brass Copper 1040 Steel Yield strength (MPa) Yield strength (ksi) Percent cold work Example 1: Percent cold work Tensile strength (MPa) Tensile strength (ksi) Brass Copper 1040 Steel Brass Copper 1040 Steel Percent cold work Ductility (%EL) Yield strength = 310 MPa Tensile strength = 340 MPa Ductility = 7%

8 Deformation & Strengthening Mechanisms of Materials (4) strain hardening Strain Cold work analysis Answer: Brass Copper 1040 Steel Yield strength (MPa) Yield strength (ksi) Percent cold work Example 2: Percent cold work Tensile strength (MPa) Tensile strength (ksi) Brass Copper 1040 Steel Brass Copper 1040 Steel Percent cold work Ductility (%EL) A cylindrical specimen of cold-worked copper has a tensile strength of 300 MPa and the cold- worked radius is 7.0 mm. (a) calculate its radius before deformation. (b) estimate its yield strength & ductility (% EL). (a) %CW = 20 (b) Yield strength = 250 MPa Ductility (% EL) = 17 % r o = mm

9 Deformation & Strengthening Mechanisms of Materials Stages of annealing process…Annealing of materials (1) recovery (2) recrystallization (3) grain growth - relieve stored internal strain energy. - enhance atomic diffusion at the elevated temperature. - reduce the number of dislocations. - new grains are formed at T R that: -- have a small dislocation density. -- are small. -- replace cold-worked grains. - cold worked metals become brittle. - reheating, which increases ductility results in recovery, recrystallization & grain growth. - this is called annealing & changes material properties. -- reduce tensile strength. -- reduce hardness. -- increase ductility (% EL). - further recrystallization -- all cold-worked grains are replaced. After 4 seconds After 8 seconds 0.6 mm i.e: 33% cold-worked brass is reheat at 580 o C new crystals nucleate 0.6 mm After 3 seconds - at longer times, larger grains consume smaller ones. - why? Grain boundary area & energy is reduced. After 8 sAfter 15 min 0.6 mm º º If metal is held at recrystallization temperature, T R long enough, cold worked structure is completely replaced with recrystallized grains. TR = recrystallization temperature - point of highest rate of property change.

10 Dislocations are observed primarily in metals and alloys. Strength is increased by making dislocation motion difficult. Particular ways to increase strength are to: -- decrease grain size -- solid solution strengthening -- precipitate strengthening -- cold work Heating (annealing) can reduce dislocation density and increase grain size. This decreases the strength. Summary

11 End of Chapter 8


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