Chapter 7: Dislocation and Strengthening Mechanism. Why Study ?
extra half-plane in the bottom portion
Symbol in Figure
Adapted from Fig. 7.2, Callister 7e.
Plane having greatest planar density Slip Plane
Directions having highest linear density Slip Direction
A combination of a slip plane and a slip direction is known as Slip System.
=> total of 12 slip systems in FCC
Adapted from Fig. 7.6, Callister 7e.
4 unique slip planes x 3 independent slip directions = 12 slip systems
• Crystals slip due to a resolved shear stress, tR.
• Applied tension can produce such a stress.
• Condition for dislocation motion:
• Crystal orientation can make
it easy or hard to move disl.
Adapted from Fig. 7.9, Callister 7e.
Adapted from Fig. 7.8, Callister 7e.
a) Will the single crystal yield?
b) If not, what stress is needed?
crss = 3000 psi
So the applied stress of 6500 psi will not cause the crystal to yield.
Adapted from Fig. 7.7, Callister 7e.
= 6500 psi
So for deformation to occur the applied stress must be greater than or equal to the yield stressEx: Deformation of single crystal
What stress is necessary (i.e., what is the yield stress, sy)?
requires a higher applied stress level.
d: average grain diameter
s0 and ky are material constants
for very large and extremely
small grain materials
dislocation movement is restricted
Figure 7.17b and 7.18b
greater applied stress is needed to initiate and continue plastic deformation
micro-structural and property changes
enhanced atomic diffusion
some stored strain energy relieved
Recovery process Involves
(similar to Fig 4.8)
Metal becomes softer, weaker, yet ductile
The degree (or fraction ) of recrystallization increases with time (Figure 7.21a-d)
For alloys, it may run as high as 0.7(melting temperature)
Hot working : plastic deformation operations at temperatures above the recrystallization temperature
dn – don = Kt
do : initial grain diameter at t=0
K, n: time-dependent constants
n is equal to greater than 2
subject to recrystallization heat treatment
refine grain size