Transcript Solidification and Grain Size Strengthening

Solidification and Grain Size Strengthening
Solidification and Grain Size Strengthening
• Stages of Solidification
– Nucleation: occurs when a small piece of solid forms in the liquid
and must attain a minimum critical size before it is stable
– Growth: occurs as atoms from the liquid are attached to the tiny
solid until no liquid remains
– Both conditions are met when the free energy of the particular
phase is lower
Solidification Terminology
• To get the solidification process started, the
liquid phase must be undercooled, cooled to
a temperature below the freezing point.
• Once a nucleus forms, it can proceed to grow
as fast as the latent heat of solidification and
specific heat can be carried away. Controlled
by:
– thermal conductivities
– relative masses
– shapes of the melt, the solid, and mold
Growth Interfaces
• Growth of Interfaces depends on
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Concentration Gradient
Temperature Gradient
undercooling
Growth Rate
• Types of Interfaces
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Planar
Cellular
Dendritic
Equiaxed
Schematic Illustration of
Solidification Morphologies
Solidification Time
• Chvorinov’s Rule
V 
t s  B 
 A
2
ts: Solidification time
B: Mold Constant
V: Volume of the Casting
A: Surface Area of the casting in contact with the mold
Cooling Curves
Casting or Ingot Structure
Solidification Defects
• Shrinkage - the degree, size, and location of
shrinkage defects is dependent on the type of
solidification of the alloy. For castings, can
be corrected with risers.
• Microshrinkage - commonly occurs in
dendritic and equiaxed growth types.
• Gas Porosity - metal dissolve considerable
gas, higher concentration in liquid than solid.
Thus, on solidification, gas bubbles form and
are trapped by surrounding solid metal.
Control of Solidification
• Inoculation or Grain Refining Practice introduction of nuclei to encourage rapid
solidification (reduce and minimize undercooling)
• Directional Solidification - control the growth rate
and temperature gradient to manipulate growth
interface.
– Use of chills in castings produce finer grain structure
– Specialized solidification techniques to create single crystals
castings
Atom Movement in Materials
• Diffusion is the movement of atoms to
produce a homogenous, uniform composition
• Applications
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Heat Treatment of Metals
Manufacturing of Ceramics
Solidification of Materials
Curing of Cement and Refractories
Manufacture of Electronic Components
Stability of Atoms
• Imperfections and normal atoms are not
stable or at rest
• Atoms possess thermal energy that allow
from atom movement
 Q 
Rate  co exp 

 RT 
Co is a material constant
Q is the activation energy
R is the gas constant
T is temperature
Diffusion Mechanism
Diffusion Mechanisms
Activation Energy
• A diffusing atom must squeeze past the
surrounding to reach its new location
Rate of Diffusion
Fick’s First Law
C
J  D
x
• Diffusion Coefficient or
Diffusivity is dependent
on temperature from
Arrhenius rate equation
Composition Profile
Fick’s Second Law
C C
C C
s
x
s
o
 erf (
x
)
2 Dt
• Applications
– Heat Treatment of Steels
– Plating and Metallic Coating
Diffusion and Material Processing
Grain Growth
Diffusion and Material Processing
Diffusion Bonding
Diffusion and Material Processing
Sintering
Solidification and Solid
Solution Strengthening
• Mechanical properties of materials can be
controlled by the addition of point defects,
particularly substitutional and interstitial atoms.
• In addition, the introduction of point defects
changes the composition of the material and
influences the solidification behavior.
• Definition of a Phase
– has the same structure or atomic arrangement throughout
– has roughly the same composition and properties throughout
– is a definite interface between the phase and any surrounding
or adjoining phases
Illustration of Phases and Solubility
Unary Phase Diagram
Unlimited Solubility
• Regardless of the ratio of A and B, only one
phase is produced by mixing them together.
• Example: Cu-Ni alloy system
Limited Solubility
• A is soluble in B, only one phase is found.
However, if excess A is added to B and either
A precipitates or A-B compound forms, then A
has limited solubility in B.
• Example: Cu-Zn alloy system
Conditions for Unlimited Solid
Solubility in Metals and Some Ceramics
• Atoms of the metals must be of similar size, with
no more than a 15% difference in atomic radius.
• Metals must have the same crystal structure.
• Atoms of the metals must have the same valence.
• Atoms of the metals must have about the same
electronegativity.
Solid Solution Strengthening
Effect of Solid Solution Strengthening
Isomorphous Phase Diagrams
Phase Compositions
Lever Law
Relationship Between Strength
and the Phase Diagram
Equilibrium Solidification of a
Solid Solution Alloy
Cooling Curve
Solidification Simulation
“Growth of Solutal Dendrites: A Cellular Automation Model and Its Quantitative Capabilities”, L. Beltran-Sanchez
and D.M. Stefanescu, Metallurgical and Materials Transactions A, Volume 34A, Feb. 2003, p. 367-382
Nonequilibrium Solidification of
Solid Solution Alloys
Effect of Nonequilibrium
Solidification
• Microsegregation - centers of dendrites (typical
growth condition) are rich in solute. Lower
mechanical properties result. Effect reduced by
homogenization heat treatment.
• Macrosegregation - similar to microsegregation but
on a large scale. Effect reduced by hot working.
• Zone Refining
Effect of Freezing Range on Shrinkage