TOPICAL COLLECTION: SUPERALLOYS AND THEIR APPLICATIONS
Segregation and Phase Transformations Along
Superlattice Intrinsic Stacking Faults in Ni-Based
T.M. SMITH, B.D. ESSER, B. GOOD, M.S. HOOSHMAND, G.B. VISWANATHAN,
C.M.F. RAE, M. GHAZISAEIDI, D.W. MCCOMB, and M.J. MILLS
In this study, local chemical and structural changes along superlattice intrinsic stacking faults
combine to represent an atomic-scale phase transformation. In order to elicit stacking fault
shear, creep tests of two diﬀerent single crystal Ni-based superalloys, ME501 and CMSX-4,
were performed near 750 °C using stresses of 552 and 750 MPa, respectively. Through
high-resolution scanning transmission electron microscopy (STEM) and state-of-the-art energy
dispersive X-ray spectroscopy, ordered compositional changes were measured along SISFs in
both alloys. For both instances, the elemental segregation and local crystal structure present
along the SISFs are consistent with a nanoscale c¢ to D0
phase transformation. Other
notable observations are prominent c-rich Cottrell atmospheres and new evidence of more
complex reordering processes responsible for the formation of these faults. These ﬁndings are
further supported using density functional theory calculations and high-angle annular dark-ﬁeld
(HAADF)-STEM image simulations.
Ó The Minerals, Metals & Materials Society and ASM International 2018
superalloys are frequently used in the
hot section of jet turbine engines due to their high
strength and excellent high-temperature properties.
Currently, new research is exploring on how to improve
these properties as demand for more eﬃcient turbine
engines requires ever-increasing operating temperatures.
As these temperatures are increased, athermal deforma-
tion modes, such as anti-phase boundary (APB) shear-
ing, begin to transition to mechanisms involving
For example, in the temperature
regime between 600 °C and 800 °C, reorder-mediated c¢
precipitate shearing modes become prevalent during
These modes include superlattice stacking
faults (SSF) and deformation twinning. One of these
important mechanisms which requires improved under-
standing is the formation of superlattice intrinsic stack-
ing faults (SISFs).
The ﬁrst models for the formation of SISFs inside a c¢
precipitate were presented by Kear et al.
In the Kear
models, to explain the formation of SISFs without
nearest neighbor violations which would form by the
h112i Shockley partials and
dislocations into the c¢ precipitate, climb by sessile
h111i Frank partials or the introduction of a dipole
dislocation near the dislocation core was introduced.
Both theories were never experimentally conﬁrmed and
have not been further expanded upon. However, Kear
did note that the local crystal structure along SISFs is a
phase with an ABAB stacking sequence.
proposed an atomic reordering process
which could explain the formation of superlattice
extrinsic stacking faults (SESFs) and microtwins. In
this process, a high-energy two-layer complex stacking
fault (CSF), caused by nearest neighbor Al-Al bonds,
can atomically reorder to form a low-energy, two-layer
SESF or microtwin. This reordering process was
expanded upon by Kovarik et al.
to include the
formation of SISFs. It was theorized that a high-energy
two-layer CSF, followed by an APB, could reorder to
create a low-energy SISF. Indeed, experimental evidence
of this process was ﬁrst presented by Vorontsov et al.
via the presence of a two-layer fault leading a one-layer
SISF terminated inside a c¢ precipitate. This was further
conﬁrmed by experimental evidence recently presented
by Rao et al.
Still, competing theories continue to be
presented proposing that SISFs are created by the shear
T.M. SMITH and B. GOOD are with the NASA Glenn Research
Center, 21000 Brookpark Road, Cleveland OH 44135. Contact e-mail:
firstname.lastname@example.org B.D. ESSER, M.S. HOOSHMAND, G.B.
VISWANATHAN, M. GHAZISAEIDI, D.W. MCCOMB, M.J.
MILLS are with the Center for Electron Microscopy and Analysis,
The Ohio State University, Columbus OH 43212. C.M.F. RAE is with
the Department of Materials Science and Metallurgy, University of
Cambridge, Cambridge CB2 3QZ, UK.
Manuscript submitted March 12, 2018.
Article published online June 1, 2018
4186—VOLUME 49A, SEPTEMBER 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A