Deformation & Strengthening Mechanisms of Materials

1. Chapter 8 Deformation & Strengthening Mechanisms of Materials

2. Deformation & Strengthening Mechanisms of Materials + + + + + + + + + + + + + + + + + + + + + + + + ion cores electron cloud - - - + + + + - - - - + + + - - - + + + + Dislocation motion in selected 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. 1. Metals (Cu, Al): - dislocation motion easiest. -- non-directional bonding. -- close-packed directions for slip. • Strengthening of materials… • restrict the dislocation motion & makes a • material harder and stronger. • -- 4 strategies/mechanisms to strengthen the • materials. • Annealing of materials… • heat treatment for cold-worked metals. • -- change mech. properties & microstructure. 2. Covalent Ceramics (Si, diamond): - dislocation motion difficult. -- directional (angular) bonding. deformation strengthening annealing dislocation motion grain size reduction slip solid solution strengthening mechanisms precipitation strengthening 3. Ionic Ceramics (NaCl): - dislocation motion difficult. -- need to avoid nearest neighbors of like sign (- and +). strain hardening @ cold working recovery recrystallization stages grain growth

3. Deformation & Strengthening Mechanisms of Materials • Deformation of materials… • plastic deformation which is occur due to • dislocation motion. • -- consider metals only. • -- show a significant plastic deformation • before failure/rupture. Deformation mechanisms… (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. Wall of high dislocation density example: Dislocation cell structure in lightly deformed Aluminum Edge dislocation Slip system… • 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. Screw dislocation • If dislocations can't move, plastic • deformation doesn't occur!

4. Deformation & Strengthening Mechanisms of Materials • 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. Slip systems… • 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 <110> directions. -- 4 (111) type planes and 3 [110] type directions. 4 x 3 = 12 slip systems. Close packed plane Non-close-packed plane Adapted from Fig. 8.6, Callister & Rethwisch 3e.

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

6. Deformation & Strengthening Mechanisms of Materials Impact of Cold Work - As cold work (%CW) is increased -- Yield strength (y) increases. -- Tensile strength (TS) increases. -- Ductility (%EL or %AR) decreases. -Rolling roll A d force -Forging A o roll die A A blank o d -Extrusion A o container die holder force ram A billet extrusion force d die container -Drawing die A d tensile A o force die Strengthening of materials Strengthening mechanisms… - aim: -- restrict the dislocation motion. -- makes a material harder & stronger. (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. Cold work (rolling process) cold-worked grains - grain structure at different regions of brass rolled into a wedge. -- grains elongate in rolling direction. -- dislocations get rearranged.

7. Deformation & Strengthening Mechanisms of Materials Copper Cold Work D =15.2mm D =12.2mm o d 70 140 900 120 60 800 1040 Steel 120 1040 Steel 800 700 50 100 Ductility (%EL) 700 100 600 40 Tensile strength (MPa) 80 Yield strength (ksi) Tensile strength (ksi) Yield strength (MPa) Brass 600 500 Brass 30 80 60 Brass 500 400 20 Copper 60 300 400 40 1040 Steel Copper 10 200 300 Copper 40 20 0 100 10 30 20 0 40 50 60 70 200 10 30 20 0 40 50 60 70 10 30 20 0 40 50 60 70 Percent cold work Percent cold work Percent cold work (4) strain hardening Strain hardening @ Cold work analysis Example 1: What is the yield strength, tensile strength & ductility of copper rod after cold working? Answer: Yield strength = 310 MPa Tensile strength = 340 MPa Ductility = 7%

8. Deformation & Strengthening Mechanisms of Materials 70 140 900 120 60 800 1040 Steel 120 1040 Steel 800 700 50 100 Ductility (%EL) 700 100 600 40 Tensile strength (MPa) 80 Yield strength (ksi) Tensile strength (ksi) Yield strength (MPa) Brass 600 500 Brass 30 80 60 Brass 500 400 20 Copper 60 300 400 40 1040 Steel Copper 10 200 300 Copper 40 20 0 100 10 30 20 0 40 50 60 70 200 10 30 20 0 40 50 60 70 10 30 20 0 40 50 60 70 Percent cold work Percent cold work Percent cold work (4) strain hardening Strain hardening @ Cold work analysis Example 2: 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). Answer: (a) %CW = 20 (b) Yield strength = 250 MPa Ductility (% EL) = 17 % ro = mm

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

10. Summary • 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.

11. End of Chapter 8