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Nucleation

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Designing High Strength Aluminium Alloys for Aerospace Applications H.Aourag Aluminium Alloys in Aerospace Design Requirements Components must be Lightweight Damage ... – PowerPoint PPT presentation

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Title: Nucleation


1
Nucleation
  • Nucleation rate (number of new particles
    formed/s) depends on
  • Thermodynamic driving force for formation of new
    phase
  • Diffusion rate (temperature)
  • Interfacial energy between nucleus and matrix

Driving force increasing but diffusion
rate decreasing
Temperature
Nucleation rate
2
Growth
  • Growth rate for each particle depends on
  • Concentration gradient ahead of particle
  • Equilibrium compositions from phase diagram
  • Particle size
  • Diffusion rate

Concentration profiles
Zr in particle
Small particle
Large particle
Zr concentration
Zr in matrix at interface (depends on particles
size)
distance
3
Coarsening
Coarsening does not need to be modelled
separately but arises naturally from growth model
in later stages of precipitation
Early stages
Late stages
shrinking
growing
c
c
Concentration Zr
Concentration Zr
All particles growing
Large particles growing, small particles shrinking
4
Testing the Model
  • First test model against experiment for a single
    initial Zr concentration

Comparison of model prediction and experiment at
500oC
Number
Size
Evolution of size distribution with time
5
Effect of Zirconium Segregation
  • In practice, Zr concentration varies across a
    grain due to segregation during casting
  • Leads to non-uniform dispersoid precipitation
    during homogenization

EDGE
CENTRE
Observed dispersoid distribution after
homogenization
Zr concentration after casting
6
Including Effect of Segregation
  • To model Al3Zr distribution across a grain
  • Divide the distance from grain edge to centre
    into large number of elements
  • Model dispersoid evolution in each element
  • Allow zirconium redistribution by diffusion
    between elements

Zr diffusing out of element
Zr diffusing into element
Zr removed into Al3Zr dispersoids
Zr concentration
Centre
Edge
7
Predicting Across a Grain
Can the model reproduce the observed behaviour?
Edge
Centre
Mean radius
Zr in solution
Volume Fraction
8
Effect of Dispersoid Distribution
  • Inhomogeneously distributed dispersoids are not
    best for control of grain structure
  • In regions where there are few dispersoids, new
    grains can form (recrystallization) - this is
    undesirable

Structure after processing New grains have formed
and partially consumed original grains - this
structure does not give best properties
9
Optimizing Dispersoid Distribution
  • Use model to determine optimum homogenization
    conditions to promote dispersoid precipitation in
    low Zr regions
  • Aim is to reduce the formation of new
    (recrystallized) grains during processing
  • For best recrystallization resistance, want a
    large number of small dispersoid particles, as
    uniformly distributed as possible

10
Model Predictions
Use model to investigate kinetics in detail
Growth
Nucleation
Temperature /oC
Temperature /oC
Time /h
To promote dispersoid nucleation in low Zr
regions need to hold at 425oC
11
Optimizing Homogenization
  • BUT Homogenization temperature for 7050 is
    restricted

Need to dissolve these phases during
homogenization
Must avoid onset of melting
  • Model suggests that best temperature for
    precipitating dispersoids in low Zr regions lies
    below this range

12
Two Step Practice
  • Two step homogenization practice may be of
    benefit
  • Step 1 Hold at a temperature to precipitate
    optimum dispersoid distribution
  • Step 2 Hold at final homogenization temperature
  • Model used to determine best conditions for step
    1
  • 5h Hold time at 430oC
  • Test 2 step homogenization practice

13
Effect on Dispersoids
Standard Homogenization
14
Comparison of Recrystallization
Standard Practice Recrystallized Fraction 30.4
Hold Homogenize Practice Recrystallized
Fraction 14.0
Two step homogenization practice, developed
entirely by computer modelling, is effective in
significantly reducing the fraction of
recrystallization
15
Summary
Aerospace aluminium alloys are complex materials,
developed over a long period of time by empirical
experiment to meet industrial needs
In recent years, the understanding of the
metallurgical processes governing the
microstructure and properties of these alloys has
greatly increased
This has led to the development of models that
have practical application in the design of new
alloys and processes
16
Acknowledgements
  • For provision of data and examples of FE and
    thermodynamic modelling
  • Dr Qiang Li, Birmingham University
  • Dr Andy Norman, Manchester Materials Science
    Centre
  • Luxfer and Alcoa for funding some of this research
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