SANS examination of precipitate microstructure in creep-exposed single-crystal Ni-base superalloy SC16 - PowerPoint PPT Presentation

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SANS examination of precipitate microstructure in creep-exposed single-crystal Ni-base superalloy SC16

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SANS, Single-crystal Ni-base superalloys. SANS examination of precipitate microstructure in creep-exposed single-crystal Ni-base superalloy SC16 – PowerPoint PPT presentation

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Title: SANS examination of precipitate microstructure in creep-exposed single-crystal Ni-base superalloy SC16


1
SANS examination of precipitate microstructure in
creep-exposed single-crystal Ni-base superalloy
SC16
P. Strunz1,2, G. Schumacher1, W. Chen3, D.
Mukherji4, R. Gilles5 and A. Wiedenmann1
  • 1Hahn-Meitner-Institut, Glienickerstr. 100, 14109
    Berlin, Germany
  • 2Nuclear Physics Institute, 25068 Rež near
    Prague, Czech Republic
  • 3Bundesanstalt für Materialforschung und
    -prüfung, Unter den Eichen 87, 12205F Berlin,
    Germany
  • 4Technische Universität Braunschweig, 38106
    Braunschweig, Germany
  • 5Technische Universität Darmstadt, Petersenstr.
    23, 64287 Darmstadt, Germany

2
nickel base superalloys - rafting
  • High-temperature slow-strain-rate exposure an
    important regime of operation of turbine blades
    made of Ni-base superalloys (precipitation
    hardened alloys g precipitates in g matrix).
  • In this regime rafting (the g morphological
    change which significantly influences the
    lifetime of the blades)
  • Rafting the initial cuboidal g precipitates
    coarsen to a plate like or needle like morphology
    (the rafts)
  • Very complex phenomenon depending on the g/g
    lattice misfit, rate and temperature of
    deformation, initial microstructure, orientation
    ...

3
objectives
  • Rafting simultaneous particle agglomeration and
    particle growth but the mechanisms of
    raft formation not fully understood at present
  • Small-angle neutron scattering (SANS) measurement
    of initial stages of the morphological changes in
    the bulk material help to resolve some of
    the questions in the rafting phenomenon
  • The aim to study the initial stages of
    morphological changes during the formation of
    rafted g-precipitate structure in the SC16
    single crystal Ni-superalloy after high
    temperature creep

4
experimental
  • SC16 single crystal bars deformed at 950C to
    different strains (tensile stress of 150 MPa
    along 001 crystal direction, strain rates
    lt10-6 s-1)
  • SEM, strain 0.1
  • SEM, strain 0.5

5
experimental
  • V4 facility of BENSC in HMI Berlin
  • sample-to-detector distance 16 m
  • l 19.4 Å (low-Q range) and l 6.0 Å
    (large-Q range).
  • low-Q range low flux of source gt measured
    without the beam-stop normally protecting 2D PSD
    against overloading
  • samples of thickness 1.5-2 mm for SANS were cut
    out of these bars after unloading and cooling to
    the room temperature
  • The normal direction to the samples was parallel
    to 010

6
measured data (SC16, creep, low-Q)
  • Measured (gray scale) and fitted (solid lines)
    differential cross-sections dS/dW (in cm-1sr-1,
    logarithmic scale)
  • strains 0, 0.1, 0.5 and 1.4
  • low-Q region the effect dominated by the
    scattering from g' phase
  • w-scan fitted at once (3 meas.)

7
measured data (SC16, creep, large-Q)
  • SANS pattern measured in large-Q range for the
    most deformed sample (1.4 strain)
  • streaks in lt320gt directions gt presence of
    topologically close packed (TCP) phase
  • scattering from TCP is comparable with the
    scattering from g' in this Q-range

8
microstructural model and evaluation
  • Anisotropic SANS evaluation direct 3D "binary
    map" modeling followed by Transformed Model
    Fitting
  • The used model partially ordered cubiodal
    and/or plate-like particles
  • Realistic approximation of a partial ordering
    a Monte Carlo based simulation of positions and
    sizes of particles
  • A long-range size distribution included into one
    3D "binary map"
  • The model of the individual cuboidal particle
    according to the model introduced by Schneider et
    al. (J. Appl. Cryst. 33, 465-468 (2000))
  • In 3D space, the point belongs to the particle
    when the following is fulfilled
  • x0, y0, z0 ... coordinates of the center Rx,
    Ry, Rz ... "radii
  • b defines shape sphere or ellipsoid for b1
    it becomes more cuboidal, rod-like or plate-like
    when b decreases towards zero (exact cube or
    block with rectangular edges for b -gt 0)

9
models resulting from the fit
  • two models used to fit the SANS data the
    cuboidal one (RxRyRz) and the
    plate-like one (RxRygtRz) - rafts
  • Real-space models resulting from the SANS-data
    evaluation (corresponding to the presented fits)
  • For 0.5 strain, both models were necessary to
    apply simultaneously
  • The gray scale a slice of the 3D model having
    the thickness approximately equal to twice mean
    distance between precipitates was projected to 2D
    assuming a certain transparency of the modeled
    precipitates

0.1
0.0
0.5
1.4
10
results and discussion
  • For the deformations 0.0 and 0.1, cuboidal
    precipitates were sufficient to describe the
    observed SANS patterns
  • A combination of both cuboidal and plate-like
    precipitates was necessary to apply for the
    deformation 0.5
  • The data from 1.4 deformation could be
    successfully described by plate-like rafts alone
  • --------------------------------------------------
    ---------------------
  • Nearly no indication of rafting after deformation
    to 0.1. However, the change of the shape of
    precipitates during this initial deformation
    period occurred originally rather cubic
    precipitates transform to cuboids at 0.1 strain
  • Indications that diffusion flow during initial
    stages of creep can cause such rounding were
    published earlier

11
results and discussion
  • The evolution of the refined shape parameter b
    for cuboids
  • The evolution of the proportion "individual
    cuboids - rafts"

12
conclusions
  • Presented SANS bulk information on g'-phase
    morphology changes during creep deformation of
    SC16
  • Evolution of precipitate microstructure
    three stages
  • First stage no rafting occurs but the
    precipitates become significantly more rounded
  • Second stage the rafts develop as more and more
    cuboidal precipitates agglomerate with each other
  • Transition between 1st and 2nd stage between
    0.1 and 0.5
  • Above 1.4 strain, practically all precipitates
    in the bulk of the sample are rafted

ACKNOWLEDGEMENT Two of the authors (R. Gilles and
D. Mukherji) thank BENSC for support enabling to
carry out the SANS experiment.
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