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NPI + HMI 04/20/23 1
SANS examination of precipitate microstructure in creep-exposed single-
crystal Ni-base superalloy SC16
1Hahn-Meitner-Institut, Glienickerstr. 100, 14109 Berlin, Germany
2Nuclear Physics Institute, 25068 Řež 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
P. Strunz1,2, G. Schumacher1, W. Chen3, D. Mukherji4, R. Gilles5 and A. Wiedenmann1
NPI + HMI 04/20/23 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: ’ precipitates in matrix).
In this regime: rafting (the ’ morphological change which significantly influences the lifetime of the blades)
Rafting: the initial cuboidal ’ precipitates coarsen to a plate like or needle like morphology (the rafts)
Very complex phenomenon depending on the /’ lattice misfit, rate and temperature of deformation, initial microstructure, orientation ...
NPI + HMI 04/20/23 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 ’-precipitate structure in the SC16 single crystal Ni-superalloy after high temperature creep
NPI + HMI 04/20/23 4
experimental
SC16 single crystal bars: deformed at 950°C to different strains (tensile stress of 150 MPa along [001] crystal direction, strain rates <10-6 s-1)
SEM, strain 0.1%
SEM, strain 0.5%
NPI + HMI 04/20/23 5
experimental V4 facility of BENSC in HMI Berlin
sample-to-detector distance 16 m
= 19.4 Å (“low-Q range”) and = 6.0 Å (“large-Q range”).
“low-Q range”: low flux of source => 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]
NPI + HMI 04/20/23 6
measured data (SC16, creep, low-Q) Measured (gray
scale) and fitted (solid lines) differential cross-sections d/d (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 ' phase
-scan: fitted at once (3 meas.)
NPI + HMI 04/20/23 7
measured data (SC16, creep, large-Q) SANS pattern measured in large-Q range for the most
deformed sample (1.4% strain)
streaks in <320> directions => presence of topologically close packed (TCP) phase
scattering from TCP is comparable with the scattering from ' in this Q-range
NPI + HMI 04/20/23 8
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”
defines shape: sphere or ellipsoid for =1; it becomes more cuboidal, rod-like or plate-like when decreases towards zero (exact cube or block with rectangular edges for 0)
microstructural model and evaluation
1
1112
z
0
2
y
0
2
x
0
R
zz
R
yy
R
xx
NPI + HMI 04/20/23 9
models resulting from the fit
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 %
two models used to fit the SANS data: the cuboidal one (Rx=Ry=Rz) and the plate-like one (Rx=Ry>Rz) - rafts
NPI + HMI 04/20/23 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
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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
NPI + HMI 04/20/23 11
results and discussion
The evolution of the proportion "individual cuboids - rafts"
The evolution of the refined shape parameter for cuboids
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0.0
0.2
0.4
0.6
0.8
1.0
Rel
ativ
e vo
lum
e fr
actio
n of
cub
oida
l an
d ra
fted
prec
ipita
tes
(dim
ensi
onle
ss)
Volume fraction: cuboidal precipitates rafted precipitates
strain (%)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.0
0.2
0.4
0.6
0.8
1.0
shape of the cuboidal precipitates
(
dim
ensi
onle
ss)
strain (%)
NPI + HMI 04/20/23 12
conclusions Presented SANS: bulk information on '-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.