Computational Micromechanics Modeling of Polycrystalline Superalloys. . .
137
of cycles shown in Fig. 6c. This mechanism is known as cyclic softening and is
attributed to the successive shearing by dislocations of the coherent and ordered γ
precipitates, during the cyclic deformation. The shearing leads into a progressive
reduction of the precipitate size up to a point in which they can no longer offer
any resistance to the movement of dislocations [31, 76]. The deformation is thus
localized in planar slip bands free of precipitates, and the “mechanical scrambling”
of the precipitates is responsible for the cyclic softening. A detailed analyses of the
influence of temperature and cyclic plastic strain amplitude on the cyclic softening
can be found in various papers [3, 21, 31]. Cyclic hardening was found to take
place during a few cycles at ambient temperature and was followed by a continuous
cyclic softening until failure. The short period of cyclic hardening disappeared at
high temperature and cyclic softening was dominant throughout the test at high
temperature.
The effect of the grain size is finally analyzed form the stabilized cyclic stressstrain curve in Fig. 6d. There are two remarkable differences from the monotonic
uniaxial tensile stress-strain curves: (1) the size effect found under monotonic
loading is strongly reduced being the stabilized curve very similar for the two
grain sizes considered and (2) the flow stress under cyclic deformation is below
its monotonic counterpart. Note that this second effect is consequence of the cyclic
softening previously described.
Low Cycle Fatigue Response
The low cycle fatigue (LCF) performance of the two microstructures studied is
represented in Fig. 7, where the cyclic plastic strain range is plotted as function of
the number of cycles to have a reduction of 5% in the maximum load during cyclic
loading. As suggested by [5, 75], this reduction in the strength can be associated
with the nucleation of a fatigue crack, and it has been observed that this number of
cycles was very near in all the cases to the final rupture so they can be considered as
an estimation of the fatigue life. The results show that this alloy follows a bilinear
Coffin-Manson law and the transition between both slopes takes place for N ≈ 2000.
This bilinear Coffin-Manson behavior presented in Inconel 718 has been reported by
many researchers [50–52, 61] and has been attributed to many different mechanisms.
Recently, using micromechanical models [17] showed that this effect is associated
with the transition from highly localized plasticity at low strain ranges to more
homogeneous deformation at high cyclic strain ranges.
The influence of the grain size in the fatigue crack nucleation for the two
microstructures considered can be observed in Fig. 7a. When the plastic strain range
is large, both microstructures present similar fatigue lives. However for small values
of the plastic strain range, when the cyclic stress-strain curves are nearly elastic, the
fine-grained microstructure exhibits much larger fatigue life. Note that the transition
to the grain size-controlled regime coincides with the transition point of the dual
slope Coffin-Manson behavior and thus can be associated with the different strain
localization observed in the two regimes.
137
of cycles shown in Fig. 6c. This mechanism is known as cyclic softening and is
attributed to the successive shearing by dislocations of the coherent and ordered γ
precipitates, during the cyclic deformation. The shearing leads into a progressive
reduction of the precipitate size up to a point in which they can no longer offer
any resistance to the movement of dislocations [31, 76]. The deformation is thus
localized in planar slip bands free of precipitates, and the “mechanical scrambling”
of the precipitates is responsible for the cyclic softening. A detailed analyses of the
influence of temperature and cyclic plastic strain amplitude on the cyclic softening
can be found in various papers [3, 21, 31]. Cyclic hardening was found to take
place during a few cycles at ambient temperature and was followed by a continuous
cyclic softening until failure. The short period of cyclic hardening disappeared at
high temperature and cyclic softening was dominant throughout the test at high
temperature.
The effect of the grain size is finally analyzed form the stabilized cyclic stressstrain curve in Fig. 6d. There are two remarkable differences from the monotonic
uniaxial tensile stress-strain curves: (1) the size effect found under monotonic
loading is strongly reduced being the stabilized curve very similar for the two
grain sizes considered and (2) the flow stress under cyclic deformation is below
its monotonic counterpart. Note that this second effect is consequence of the cyclic
softening previously described.
Low Cycle Fatigue Response
The low cycle fatigue (LCF) performance of the two microstructures studied is
represented in Fig. 7, where the cyclic plastic strain range is plotted as function of
the number of cycles to have a reduction of 5% in the maximum load during cyclic
loading. As suggested by [5, 75], this reduction in the strength can be associated
with the nucleation of a fatigue crack, and it has been observed that this number of
cycles was very near in all the cases to the final rupture so they can be considered as
an estimation of the fatigue life. The results show that this alloy follows a bilinear
Coffin-Manson law and the transition between both slopes takes place for N ≈ 2000.
This bilinear Coffin-Manson behavior presented in Inconel 718 has been reported by
many researchers [50–52, 61] and has been attributed to many different mechanisms.
Recently, using micromechanical models [17] showed that this effect is associated
with the transition from highly localized plasticity at low strain ranges to more
homogeneous deformation at high cyclic strain ranges.
The influence of the grain size in the fatigue crack nucleation for the two
microstructures considered can be observed in Fig. 7a. When the plastic strain range
is large, both microstructures present similar fatigue lives. However for small values
of the plastic strain range, when the cyclic stress-strain curves are nearly elastic, the
fine-grained microstructure exhibits much larger fatigue life. Note that the transition
to the grain size-controlled regime coincides with the transition point of the dual
slope Coffin-Manson behavior and thus can be associated with the different strain
localization observed in the two regimes.