The Crystal Structure of
Metals II
Dr. Mohammad I. AbuShams
Deformation and Strength
of Single Crystals
•
Crystal undergoes elastic deformation then plastic (permanent)
deformation.
• 2 basic mechanisms during plastic deformation:
a. Slip➔ one plane of atoms slip over an adjacent plane under shear
stress
b. Twinning ➔ portion of the crystal forms a mirror image of itself
across the plane of twinning
Imperfections
•
•
Discrepancy in actual strength is explained in terms of
imperfections in the crystal structure.
Imperfections and defects categorized as:
a)
b)
c)
d)
Point defects (Vacancy, Interstitial, Substitutional)
Line defects (Dislocation)
Planer defects (Grain boundary)
Volume defects (Void, Precipitate)
Strain hardening (work hardening)
•
Increase in the shear stress and overall strength of
the metal is known as strain or work hardening
(increases the metal’s strength)
Grains and Grain Boundaries
•
•
Number and size of the grains developed in metal depend on the
rate of nucleation.
Various stages during solidification of molten metal:
Grain
Grain boundary
Grains and Grain Boundaries
•
Surfaces that separate the individual grains are
called grain boundaries. (i.e., two grains with
different crystallographic orientation)
Grain size
•
•
Grain size influences the mechanical properties of metals.
Large grain ➔low strength and hardness, high
ductility
•
The yield strength, Y, is related to grain size by as Hall-Petch
equation.
Y = Yi + kd
−1 / 2
Yi = basic yield stress
k = constant
d = grain diameter
Grain size (Cont.)
•
•
Grain size is measured by counting the number of grains in a given
area.
Grain-size number (ASTM No.), n, is related to the number of grains,
N.
𝑚𝑚2
𝑚𝑚3
Influence of grain boundaries?
•
•
Grain boundaries influence strength, ductility of metals and strain
hardening.
At a low-melting-point, strong metal can crack under
very low stresses known as grain-boundary embrittlement.
Plastic Deformation of Polycrystalline
Metals
•
During plastic deformation, mass continuity in grain boundaries is
maintained.
• The grains would become elongated in one direction and contract in
the other.
• Two types of anisotropy in metals:
1. Preferred orientation
2. Mechanical fibering
ANNEALING: Recovery, Recrystallization,
and Grain Growth:
•
The temperature range and the time required
depend on the material.
• Three events take place during the heating:
✓ Recovery
-
number of mobile
dislocations reduced
✓ Recrystallization
- new grains form
✓ Grain growth
- grains grow bigger
Cold and Hot Working
•
When plastic deformation above recrystallization
temperature, it is called hot working, vice versa it is
known as cold working (i.e. at room temperature).