Current and candidate Space Shuttle Main Engine (SSME) turbopump blade materials are anisotropic, both in their elastic and plastic response. The major objective is to characterize the plastic deformation behavior of a typical single crystal nickel-base superalloy, PWA 1480, and to use this information to help guide the development of anisotropic constitutive models

Antolovich, Stephen D.

Milligan, Walter W.

English

NASA Technical Reports Server

N90-28641
Micromechanics of Cyclic Deformation in
SSME Turbopump Blade Materials
Walter W. Milligan and Stephen D. Antolovich
Georgia Tech
School of Materials Engineering
Atlanta, Ga 30332-0245
INTRODUCTION
Current and candidate SSME turbopump blade materials are
anisotropic, both in their elastic and plastic response.
The major objective of this study is to characterize the
plastic deformation behavior of a typical single crystal
nlckel-base superalloy, PWA 1480, and to use this
information to help guide the development of anisotropic
constitutive models.
In order to predict structural response in the blade, and
therefore in order to predict fatigue lives in the
structure, a constitutive model is necessary. For ordinary
isotroplc materials, vlscoplastic constitutive models are
fairly advanced. However, due to the plastic anisotropy of
the single crystal superalloys such as PWA 1480,
constitutive models for these materials are still in the
early stages of development. The two models which currently
have the most promise are both based on the crystal
plasticity approach. In this framework, the plastic shear
strains on each individual sllp system are computed from a
micromechanical model, and then the strains are added to
calculate the total strain in the material. Anisotropy is
taken into account from a fundamental metallurgical point of
view, and incorporated into the crystal plasticity
formulation. Unfortunately, the current understanding of
deformation in these alloys (from a metallurgical point of
vlew) is not very complete. The current study is an attempt
to extend this understanding.
To date, monotonic and cyclic deformation studies have
been conducted on Mar-M 246, and <001> oriented single
crystals of PWA 1480 over the temperature range of blade
operation. Tests on <123> crystals, which are necessary to
characterize the anisotropy, are being conducted presently.
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THE CONSTITUTIVE MODELS
For ordinary isotropic materials undergoing deformation
in the viscoplastic regime, the currently accepted models do
an adequate job of correllatlng and sometimes predicting
constitutive response under high temperature loading. These
models are unified, in the sense that they treat "creep" and
"plasticity" in the same analytical expression. This is
very desirable for single crystal superalloys, because both
"creep" and "plasticity" are the result of the same
mechanism: thermally activated dislocation motion. The
flow rule is of the following general form [1]:
_lJ = _ ( (slj - QAj ) / K } (I)
where sil is the plastic strain rate, sil is the deviatoric
stress,-_i_ is a state variable usually _eferred to as the
"back stre_s", which handles kinematic hardening, and K is
another state variable referred to as the "drag stress",
which handles isotropic hardening. The functional form of
the flow rule is model-dependent, but is usually either an
exponential function or a power-law function. Both state
variables evolve with deformation.
For anisotropic materials, this general framework must be
modified to account for the anlsotropy. In particular,
single crystal nickel base superalloys have an orientation-
dependent yield strength which cannot be explained by a
maximum shear stress (Schmid's Law) model. Many
investigators have postulated that this anisotropy is due to
the onset of cube plane slip within the strengthening T'
precipitates [2]. Since the anlsotropy is due to variations
in the type of active sllp plane, the crystal plasticity
approach was a natural choice. Several models of this type
are currently being developed [3,4]. Both models assign a
unified viscoplastic flow rule for slip on octahedral planes
(the usual mode in FCC crystals), and a unified viscoplastlc
flow rule for slip on cube planes. The total plastic strain
rate is calculated to be a sum of the two:
s = Soctahedral + ecube (2)
Of course, the functional forms of the _'s are orientation
dependent, and the state variable for cube and octahedral
sllp can evolve independent of one another. This approach
has been successful in correlating anlsotroplc behavior
under limited testing for several alloys [3,4].
488
DEFORMATIONBEHAVIOR
Since PWA 1480 is two-phase, and since the postulated
cube slip occurs only in the strengthening r' phase, the
first question which needs to be addressed is how mobile
dislocations interact with the strengthening precipitates.
In two-phase systems with very large particles and high
volume fractions (like PWA 1480), there are two possible
mechanisms of deformation. As shown in Figure l(a), the
first mechanism is a classical shearing operation. Gliding
dislocations under a stress high enough to penetrate the
precipitate can continue to glide, thus shearing the
particle. The second possible mechanism is illustrated in
Figure l(b). At high temperatures where diffusion is
relatively easy, a gliding dislocation can by-pass the
precipitate by a combination of climb and glide. In this
case, the dislocation never enters the particle. Obviously,
the behavior of the alloy under these two mechanisms will be
very different.
As documented in earlier reports from this research
[5,6], the deformation mechanism which is observed at yield
in <001> oriented single crystals is a strong function of
temperature and strain rate. At low temperatures (below
760_C), the T' is sheared by pairs of a/2<110> dislocations
on (111} planes. Typical deformation microstructures in
this temperature regime are shown in Figure 3(a), in which
planar sllp bands are seen to be shearing the precipitates.
At high temperatures (above 815 to 927°C depending on
strain rate), the T' is by-passed by the mechanism described
above. Typical structures in this regime are shown in
Figure 3(b), in which all dislocations are seen to be in the
matrix and interracial regions. This is indicative that no
shearing has taken place, and further analysis [5] confirmed
this conclusion. An analysis of current micromechanical
models of both the shearing and by-pass mechanisms is
contained in Reference [5].
In addition to the monotonic experiments described above,
cyclic deformation tests have been conducted on <001>
oriented single crystals over the same temperature range.
The tests were strain controlled, fully reversed low cycle
fatigue tests, and the plastic strain levels varied with
test condition between 0.05_ to 0.5_. In these tests, the
exact same deformation mechanisms were observed as a
function of tmeperature and strain rate as were observed in
the monotonic experiments. At low temperatures, T' shearing
was observed, while at high temperatures r' by-pass was
observed. This is illustrated by Figure 4.
489
The anisotropy is currently being studied. However, data
from another study [7] seems to indlcate that the trends
observed here will be general. As shown in Figure 5 (from
Reference [7]), the yield strength is very anisotropic at
temperatures up to and including 760°C, while the anisotropy
becomes much less severe
at higher temperatures. Since T' by-pass should be
essentially isotropic, this indicates that by-pass probably
is occurring regardless of orientation at higher
temperatures.
IMPLICATIONS
Although cube sllp has not been verified yet, the
occurrence of T' shearing at low temperatures indicates that
cube sllp is a possible source of anisotropy. Experiments
which are currently being conducted should help clarify this
shortly. If cube slip is observed in these tests, then the
physical foundations of the constitutive models will be
verified. In addition to identifying the type of
deformation mechanism, these tests should lead to advanced
micromechanical models which will be useful in the
constitutive model foundation.
At high temperatures, however, no shearing occurs. Since
by-pass appears to be isotroplc, the anlsotroplc models need
not be used. Again, future experiments will clarify these
observations.
ACKNOWLEDGEMENTS
This work was funded by NASA-Lewis Research Center under
Grant NAG3-503, Michael Verilll, grant monitor.
REFERENCES
i. R.L. Thompson, ed.: Nonlinear Constitutive Relations for
High Temperature Applications - 1984, NASA CP-2369.
2. D.M. Shah and D.N. Duhl: in Superalloys 1984, Proceedings
of the 5th International Conference on Superalloys, M. Gell
et.al., eds., AIME, Warrendale, Pa, 1984, p.105.
3. K.P. Walker and E.H. Jordan: in Reference 1, p. 65.
4. L.T. Dame: Ph.D. Thesis, University of Cincinnati, 1985.
490
5. W.W. Milligan and S.D. Antolovich: Met. Trans. A, 18A,
January 1987, p.85.
6. W.W. Milligan: NASA CR-175100, 1986.
7. G.A. Swanson et.a1: NASA CR-174952, 1986.
491
.L
Fig. 1 Possible deformation mechanisms in PWA 1480.
T' shearing. (b) cllmb-assisted r' by-pass.
(a)
492
OR!GINAL FAGE
;',_ ACK A_':D V:,_WlitE t,_:D[OGRAPH
Fig. 2. As-received microstructure snowing two-onase _/['
morpnology.
493
Fig. 3. Deformation structures at yield.
{al Typ_cai iow temperature T' shearing 705°C,
0.50%/min. (b) Typical high temperature r' by-
pass, 982°C, 0.5%/min.
494
BLACK A,,i_ ,,-:,i L. ,-, ;.:)iOGR/,,pFI
Fig. 4. Deformation structures after low cycle fatigue. (a)
shearing at low temperatures, 20°C, 50%/min, A_ =
0. 1% (b) By-pass at high temperatures, 915_C,
50%/m_n, ASp = 0.2%.
495
1400.0 -
7)
VJ
o.,=4
>.,
0
1200.0
1000.0
800.0
800.0
400.0
200.0
s = (001)
# = (011)
• = (Iii)
0.0 J l
0.0 200.0 400.0
I I I J
600.0 800.0 1000.0 120(
T,C
Fig. 5. Yield strength of PWA 1480 vs. Temperature for
several orientations. Note dimlnlshlng an2sotropy
at high temperatures. (Reference 7)
496


Micromechanics of cyclic deformation in SSME turbopump blade materials

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