chapter 7 ME2733 - Dislocations and Strengthening Mechanisms

slip

The process by which plastic deformation is produced by "dislocation motion"

slip occurs:

along the most densely packed planes and the densest direction

dislocation motion - edge

stress direction parallel to dislocation motion

dislocation motions - screw

stress direction perpendicular to dislocation motion

dislocation density

total dislocation length per unit volume

slip systems =

slip planes x slip directions

how many possible slip system for FCC

12

for FCC and BCC _____ number of slip systems

large

for HCP _____ number of slip systems

few

Large number of slip systems:

ductile

few number of slip systems

brittle

strengthening mechanism 1. Strengthening by grain size reduction

grain boundaries act as barriers to dislocation motion

which has a higher strength:

-polycrystalline or single crystalline

-high angle grain boundary or small angle grain bound.

-fine grain bound. or coarse grain bound.

-polycrystalline > single crystalline

-high angle > small angle

-fine > coarse

grain sizes in grain size reduction hardening can be regulated by _____

the rate of solidification

strengthening mechanism 2. solid solution alloying

impurity atoms --> lattice strain --> restrict dislocation motion

in solid solution alloying what happens to tensile strength, yield strength, and ductility

tensile strength increases, yield strength increases, ductility decreases

strengthening mechanism 3. strain hardening or cold working

normally performed at T < recrystallization T or at room temp for metals

-plastic deformation --> increase number of dislocations, strain increases, strength increases

what happens to tensile strength, yield strength and ductility after strain hardening

-tensile strength increases, yield strength increases, but ductility decreases

Recovery

reduction of dislocation density

-tensile strength decreases, ductility increases

recrystallization

reduction of strain

-tensile strength decreases, ductility increases

grain growth

reduction of grain boundary

-tensile strength decreases, ductility increases