NPI + HMI 10/20/2015 1 SANS examination of precipitate microstructure in creep-exposed single- crystal Ni-base superalloy SC16 1 Hahn-Meitner-Institut,

NPI + HMI 04/21/23 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

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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 ...

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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

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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%

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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]

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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.)

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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

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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

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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

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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

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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 (%)

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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.