This document discusses solidification processes and how they affect crystal structure and material properties. It covers topics like nucleation and grain growth during solidification, different crystal structures, the effects of imperfections and grain size, phase diagrams, and how heat treatments can modify material properties by changing the crystal structure.

1. Sp’ 05 W. Li
Solidification and Heat
Treatment
 Solidification
 Crystal structures
 Structure-property relationships
 Heat treatment

2. Pure Metal Solidification
• Temperature remains
constant while grains
grow.
• Some metals undergo
allotropic transformation
in solid state.
• For example on cooling
bcc -iron changes to
fcc -iron at 1400 C,
which again to bcc -
iron at 906 C.

3. Nucleation and Grain Growth
 Nucleation;
 Homogeneous nucleation: very pure metal, substantial
undercooling (0.2Tm)
 Heterogeneous nucleation: nucleation agents (5ºC
undercooling)
 Grain growth
 Planar: pure metal
 Dendritic: solid solution
 Grain size
 depends on number of nuclei and cooling rate.

4. Crystal Nucleation and Growth
“Manufacturing Processes for Engineering Materials,” by Serope Kalpakjian

5. Crystal Structure of Metals
 Atoms arrange themselves into various orderly
configuration, called crystals.
 The arrangement of the atoms in the crystal is
called crystalline structure.
 The smallest group of atoms showing the
characteristic lattice structure of a particular
metal is known as a unit cell.

6. Crystal Structure of Metals

7. Slip Systems
 Deformation (dislocation) occurs on preferential
crystallographic planes and directions, called slip
systems.
 The slip plane/direction is the plane/direction with the
most closely packed atoms.
6x2=12 4x3=12 1x3=3

8. Slip Systems
 BCC has 6 slip planes and 2 slip directions per plane (12
slip systems), but distance between slip planes is small,
therefore the required stress is high. Good Strength and
moderate ductility, e.g. Steel, Titanium, Molybdenum,
Tungsten.
 FCC has 4 slip planes and 3 slip directions per plane (12
Slip Systems), but distance between slip planes is larger
than BCC. Therefore, probability of slip is moderate,
shear stress to cause slip is low. Moderate Strength and
Good Ductility, e.g., Aluminum, Copper, Gold, Silver
 HCP has 1 slip plane and 3 slip directions on that plane
(3 systems). Low probability of slip. Generally brittle
materials, e.g., Beryllium, Magnesium, and Zinc

9. Plastic Deformation of Single
Crystals

10. Theoretical Shear Strength and
Tensile Strength
 Theoretical shear stress is the shear stress to
cause permanent deformation in a perfect
crystal.
 Theoretical or ideal tensile strength of material is
the tensile stress required to break the atomic
bonds between two neighboring atomic planes.
30
/
~
10
/
2
max G
G
between
a
b
G




10
/
max E



11. Solid Solutions
 Most metals are not pure but contain a number of
other metallic or non-metallic elements, either
alloying elements or contaminants. Alloying elements
are uniformly distributed in the base metal, forming a
solid solution.
 Substitutional solid solution
 Interstitial solid solution

12. Effect of Imperfections
 Pure metal: dislocation
 Solid solutions
 Solute atoms of slightly different size distort the lattice and makes
dislocation propagation more difficult, thus strength increases
without necessarily reducing ductility.
 Interstitial elements play a similar role in impeding dislocation
mobility although they can have an embrittling effect.
 Interfaces, inclusions, gases

13. Grain Size Effect
 Grain boundaries present
obstacles to dislocation
propagation. Therefore, it is
generally found that the yield
strength of a material
increase with decreasing
grain size according to the
Hall-Petch equation.
 However at low strain rate
and close to Tm, dislocation
is resolved by diffusion.
Material deforms by sliding
of grains or reshaping of
grains. Both processes are
easier if grain size is small.

14. Phase Diagrams
• A phase diagram, also called equilibrium diagram or a constitutional
diagram, graphically illustrates the relationships among temperature,
composition, and the phases present in a particular alloy system.

15. Lever Rule
 The composition of various phases in a phase
diagram can be determined by a procedure
called the lever rule.
 Example: Calculate the relative proportions of
the phases in a Cu-Ag alloy of eutectic
composition just below the eutectic temperature.
L
s
s
L
s
L
C
C
C
C
L
S
L
or
C
C
C
C
L
S
S








0
0
%
2
.
23
2
.
91
9
.
7
2
.
91
9
.
71







 





C
C
C
CE

16. The Structure of a Cu-Ag Solid
Solution with 20% Ag

17. Iron/Iron Carbide Phase Diagram

18. Nonequilibrium Solidification
Microsegregation or coring

19. Heat Treatment
 Most parts will require heat treatment either after or
during the processing for proper in-service properties
 Annealing
 Heat to elevated temp, hold, cool
 Softens the material and removes stress
 Precipitation Hardening
 Diffusion of alloys to produce two phase structure that
promote good strength and ductility
 (Aging – Aluminum for example)
 Heat Treatment of Steel

20. Heat Treatment of Steel (TTT
Diagram)

21. Summary
 Solidification process affects crystal structures
which in turn affect material properties.
 Single crystal materials behave very differently
than metal alloys.
 The effect of imperfections and grain size in
solid solutions.
 Heat treatment can modify material properties by
changing the crystal structure.