Affect of Variables on Recrystallization 1.Minimum amount of deformation is required 2.The smaller the deformation, the higher the temperature required for recrystallization 3.Increasing annealing time decreases required recrystallization temperature. Temperature is more important than time. Doubling annealing time is approximately equivalent to increasing annealing temperature 10 o C 4.Final grain size depends most on the degree of deformation and to lesser extent on the annealing temperature. The greater the deformation & the lower the annealing temp., the smaller the recrystallized grain size. 5.The larger the original grain size, the greater the amount of cold work required to produce same recrystallization temp. Source: G. Dieter, Mechanical Metallurgy, 3rd Edition , McGraw-Hill, 1986.
Affect of Variables on Recrystallization The recrystallization temperature decreases with increasing purity of the metal. Solid solution alloying additions ALWAYS raise the recrystallization temperature. The amount of deformation required to produce equivalent recrystallization behavior increases with increased working temperature For a given reduction in cross-section – different metal working processes produce different effective deformations. Therefore, identical recrystallization behavior may not be obtained. Source: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986.
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Affect of Variables on Recrystallization
1. Minimum amount of deformation is required2. The smaller the deformation, the higher the temperature
required for recrystallization3. Increasing annealing time decreases required recrystallization
temperature. Temperature is more important than time. Doubling annealing time is approximately equivalent to increasing annealing temperature 10oC
4. Final grain size depends most on the degree of deformation and to lesser extent on the annealing temperature. The greater the deformation & the lower the annealing temp., the smaller the recrystallized grain size.
5. The larger the original grain size, the greater the amount of cold work required to produce same recrystallization temp.
Source: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986.
Affect of Variables on Recrystallization
6. The recrystallization temperature decreases with increasing purity of the metal. Solid solution alloying additions ALWAYS raise the recrystallization temperature.
7. The amount of deformation required to produce equivalent recrystallization behavior increases with increased working temperature
8. For a given reduction in cross-section – different metal working processes produce different effective deformations. Therefore, identical recrystallization behavior may not be obtained.
Source: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986.
• Larger grains consume smaller ones. • Grain boundaries have curvature• Migration of atoms across grain boundaries always moves
toward the center of curvature• Small grains that are not hexagonal and have corners at angles
less than 120o (a perfect hexagon has 120o) tend to have center of curvature towards center of grain – they shrink
• Big grains, or grains with more than 6 sides grow
After 8 s,580ºC
After 15 min,580ºC
0.6 mm 0.6 mm
Grain Growth – How does it occurGrain size is the mean diameter of an aggregate of grainsAs grains grow the number of grains decreases but the mean diameter continues to grow
Mathematical RelationshipsEmpirical Relationship:
Ktdd no
n elapsed time
coefficient dependenton material and T.
grain diam.at time t.
exponent typ. ~ 2
• Most reported experimental work does not conform to grain growth equation
• Many of the data sets correspond to empirical equation of the form
D = ktn Where
n is less than value ½ n is not usually constant for given
Definitions• Component: Pure metal or compound from which an alloy is composed
– Components are Zn and Cu in Brass Diagram– We have also used the terms solvent and solute when we were discussing
solid solutions
• Phase: A homogeneous portion of a system that has uniform physical and chemical characteristics
– Every pure metal is a phase– Every liquid, solid, or gaseous solution is a phase– When two or more phases are present there is a boundary between the two
• Phase diagram: is a graphical representation of phase stability– Phase stability is dependent on temperature, pressure, and composition– Phase diagrams are constructed to show the interplay of these parameters
Definition of Equilibrium• Definition of Gibb’s Free Energy, G:
G = H – TSG = Go – RTlnQ
• Gibb’s Free Energy is used to determine if a reaction will occur – must be negative
• At equilibrium – G = 0, reaction rates forward and backward are equal
• Phase equilibrium is stability in the chemical and physical makeup of phases present with time
Solution thermodynamics can be used to derive phase diagrams – not gonna happen here.
One Component Phase Diagram
Beyond “critical point” physico-chemical properties of water and steam converge to the point where they are identical. Beyond the critical point: "supercritical fluid".
Water phase diagram can be used to explain ice skating…
Gibbs Phase Rule:
P + F = C + 2
P: Number of PhasesF: Degrees of Freedom(What variables may be independently changed without altering state of system)C: Number of Components
Invariant point – no degrees of freedom
Curves represent chemical reaction that describes a phase transformation
Definition of Solubility Limit
• Solubility Limit: Max concentration for which only a single phase solution occurs.
Question: What is the solubility limit at 20°C?
Answer: 65 wt% sugar. If Co < 65 wt% sugar: syrup
If Co > 65 wt% sugar: syrup + sugar.
65
Sucrose/Water Phase Diagram
Pure
Su
gar
Tem
pera
ture
(°C
)
0 20 40 60 80 100Co =Composition (wt% sugar)
L (liquid solution
i.e., syrup)
Solubility Limit L
(liquid) + S
(solid sugar)20
40
60
80
100
Pure
W
ater
Effect of T & Composition (Co)• Changing T can change # of phases:
Adapted from Fig. 9.1, Callister 7e.
D (100°C,90)2 phases
B (100°C,70)1 phase
path A to B.• Changing Co can change # of phases: path B to D.
A (20°C,70)2 phases
70 80 1006040200
Tem
pera
ture
(°C
)
Co =Composition (wt% sugar)
L (liquid solution
i.e., syrup)
20
100
40
60
80
0
L (liquid)
+ S
(solid sugar)
water-sugarsystem
Binary Phase Diagram• Hold pressure constant (typically 1 atm)• Allow temperature and composition to vary• Binary phase diagram has 2 components• Ternary phase diagram has 3 components (not going to
cover in this class)• Maps of equilibrium phase structures
Fully Miscible Solution
CrystalStructure
electroneg r (nm)
Ni FCC 1.9 0.1246Cu FCC 1.8 0.1278
• Both have the same crystal structure (FCC) and have similar electronegativities and atomic radii (W. Hume – Rothery rules) suggesting high mutual solubility.
Simple solution system (e.g., Ni-Cu solution)
• Ni and Cu are totally miscible at all mixture compositions – isomorphous
Copper-Nickel Binary Equilibrium Phase Diagram
• Solid solutions are typically designated by lower case Greek letters: etc.
• Liquidus line separates liquid from two phase field
• Solidus line separates two phase field from a solid solution