Ch 8: Deformation & Strengthening Mechanisms

plastic deformation occurs by motions of

dislocation

direction of edge dislocation is

in line with applied shear stress

direction of screw dislocation is

perpendicular to applied shear stress

is it easy for metal to dislocate?

yes → bonds are loose (electron cloud)

is it easy for covalent ceramics to dislocate?

relatively difficulte

is it easy for ionic ceramics to dislocate?

relatively difficult → motion of nearby ions of like charge (+ and -) restricted by electrostatic repulsive forces

what is a slip system?

combo of slip plane and direction

what is a slip plane

crystollagraphic place where the most slip occurs easily

• high planar density

what is slip direction

crystallographic direction along which slip occurs most easily

• high linear density

what is slip system for FCC

{111} (planes) <110> (directions)

how many independent slip systems does an FCC have?

12

what is resolved shear stress

force applied tangent to slip plane and parallel to slip direction

what is critical resolved shear stress

minimum shear stress on a slip plane and in s lip direction needed to cause a dislocation

what slip system is most favored

one where resolved shear stress is the highest

grains change during plastic deformaiton due to

slip

what properties do grains exhibit before rolling?

isotropic → uniform properties in all directions

what properties do grains exhibit after rolling

anisotropic → different properties depending on direction

what happens when metal hardened

dislocation mobility reduces

what are 3 ways to strengthen metals

• grain size reduction

• solid solution strengthening

• strain hardening (cold working)

what happens when grain size is reduced?

• grain boundary area increases

• more barriers to dislocation motion

• increases yield strength, tensile strength and hardness

what happens in solid-solution strengthening

the lattice strains around dislocations

• high shear stress is needed to dislocate

small substitutional impurities introduce

tensile strains

large substitutional impurities introduce

compressive strains

what happens in strain hardening?

plastically deforming metals at room temp makes them harder and stronger

• reduces cross sectional area

as % cold work increases…

• yield strength increases

• tensile strength increases

• ductility decreases

by forming new dislocations, cold work

increases dislocation density

what does heat treating cold worked metals do

causes annealing → effects of cold work are nullified

what are the 3 annealing stages?

• recovery

• reccrystallization

• grain growth

what happens during recovery?

dislocation density is reduced → dislocations removed

what happens during recrystallization?

new grains form that

• have low dislocation densities

• small in size

• consume and replace parent cold-worked grains

what is recrystallization temp (TR)

temp where recrystallization reaches completion in 1 hr

• depends on %CW (TR decreases with increasing %CW)

• depends on purity of metal

hot working vs cold working

• hot working → deformation above TR

• cold working → deformation below TR

metals with small grain are

strong and tough at low temps

metals with large grains

have good creep resistance at high temps

what is drawing

stretches the polymer prior to use

• aligns chains in the stretching direction

what does drawing do

• increase elastic modulus and tensile strength

• decreases ductility

annealing after drawing…

• decreases chain alignment

• reverses effects of drawing

what is creep

progressive deformation over time at high temp and constant stress

what is primary creep

strain increases at a decreasing rate

what is secondary creep

strain increases at a constant rate

what is tertiary creep

strain increases at an increasing rate