Ni-Al-Mo Single Crystal Rafting Studies

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Ni-Al-Mo Single Crystal Rafting Studies
Shuwei Ma, Tresa.M.Pollock The University of
Michigan, Ann Arbor, MI
MEANS Group Meeting, Columbus, OH April, 4, 2007
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Previous Models of Rafting
Elastic model
Rafting direction is determined by the sign of
lattice misfit, modulus mismatch and applied load
direction.
?
P type rafting
Plastic-elastic model
?
Take into account the contribution from plastic
deformation in ? channels (continuum). Rafting
direction is determined only by the sign of
lattice misfit and applied load direction.
N type rafting
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Accounting for Local Stress Fields with Phase
Field Model
Phase field modeling was applied to evaluate
elastic and plastic driving force in the Ni-Al-Mo
system.
Michael Millss plot
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Objective
Provide a complete experimental data set in model
Ni-Al-Mo single crystals for phase field
modeling.

• This information includes
• Misfit (sign and magnitude)
• Plastic strain processes in g matrix
• Elastic modulus
• Diffusion
• Rafting kinetics

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References
Many properties have already been measured in a
large set of Ni-Al-Mo single crystals

• M.Fahrmann, W.Hermann, E, Fahrmann, A.Boegli,
  T.M.Pollock. Materials Science and Engineering
  A260 (1999) pp.212-221.
• O.Paris, M.Fahrmann, E.Fahrmann, T.M.Pollock and
  P.Fratzl. Acta mater. Vol.45, No.3, 1997,
  pp.1085-1097.
• M.Fahrmann, E.Fahrmann, O.Paris, P.Fratzl, and
  T.M.Pollock. Superalloy 1996, pp.191-200.
• M.Fahrmann, P.Fratzl, O.Paris, E.Fahrmann and
  W.C.Johnson. Acta Metall.Mater, Vol.43, No.3,
  pp.1007-1022.
• M.Fahrmann, E.Fahrmann, T.M.Pollock, W.C.Johnson.
  Metallurgical and materials Transactions A.
  pp.1943-1945.

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Experimental Methods
1. Observation of rafting and coarsening of g
precipitate SAXS (small angle X-ray
scattering) Quantitatively image analysis on TEM
and SEM ( Fourier Analysis) 2. Misfit
measurement Hot stage X-ray diffraction on
over-aged sample, unconstrained misfit. (210)
diffraction. 3. Elastic Constant
measurement Free-free beam resonance technique.
4. Diffusion data
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Ternary Ni-Al-Mo Phase Diagram
Single Crystal Compositions
Vg?60
Vg?10
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Coherency Stress and Precipitate Coarsening in
Ni-Al-Mo Alloys
Vg?0.1
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Coherency Stress and Precipitate Coarsening in
Ni-Al-Mo Alloys

• The coarsening rate decreases with increasing Mo
  content of alloy
• A change in the rate controlling mechanism of
  coarsening from long range diffusion of Al to
  long range diffusion of Mo in this series of
  alloys.

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Misfit And Rafting in Experimental Alloys
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Misfit and Rafting in Experimental Ni-Al-Mo system
R1, Positive misfit d gt 0
R3, Negative misfit, d lt 0

• Elastic and plastic rafting models can predict
  these observation.
• Do rafting kinetics distinguish elastic and
  plastic effects?

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Youngs Modulus
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Elastic Stiffness of g and g phases in Ni-Al-Mo
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Elastic Modulus Mismatch
For Elastic model
For elastically anisotropic materials, the term
Eg-Eg has to be replaced by the differential C,
i.e, (C11-C12)g- (C11-C12)g
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Average Precipitate Aspect Ratio During Creep
Creep condition 980C/130MPa.
For Elastic model
R1 alloy with positive misfit has a lower rate of
rafting in spite of less Mo. CR3gtCR1. Effect
of elastic modulus mismatch?
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Role of Matrix Plasticity in the Rafting Kinetics
Ni-13.3Al-8.8Mo (at) Misfit, d-0.5, Vg0.60
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Microstructure Evolution During Stress Annealing
and Aging for Pre-strained interface

• The microstructure in samples pre-strained in
  tension or compression rafted during subsequent
  aging in a direction as if the former load was
  still present, whereas a sample with
  isotropically relaxed interfaces did not show
  directional coarsening
• The microstructure in a sample pre-strained in
  tension rafted under an applied compressive
  stress initially in a direction opposite to what
  is generally observed in compression for this
  alloy.

Kinetics and the driving force of rafting are
greatly affected by the state of the g/g
interfaces.
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Suggestions

• Channel dislocations and modulus mismatch could
  both affect the rafting. This may be reflected in
  differences in rafting kinetics.
• It is necessary to combine channel dislocations
  and modulus mismatch to evaluate their relative
  contributions via phase field modeling.
• Does phase field modeling have sufficient
  fidelity to evaluate elastic plastic driving
  force in the Ni-Al-Mo system?

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THANK YOU!
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Microstructure Evolution During Aging (980C/4hrs)
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Microstructure Evolution During Compression Creep
(980C/130MPa)
partially coherent, pre-strain in tension State b
Initially fully coherent (State a)
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Microstructure Evolution During Stress Annealing
and Aging for Pre-strained interface
Stress annealed in compression
Ordinary ageing