Fig. 16. Cyclic hardening/softening curves in TMF cycling with and
without dwell for total strain amplitude 6×10-3.
4.2. Internal structure
Investigation of the internal structure of the specimens subjected to
TMF cycling with and without dwells revealed the important effect of the
dwells on the dislocation substructure. Substructure in Fig. 4a and Fig.
4b is very similar to the structure observed by Heczko et al.10 after constant strain rate high temperature cycling
of Sanicro 25. High dislocation density and dislocations arranged in
bands parallel to the primary slip plane are typical for IP-TMF cycling
(Fig. 4a). It is apparent that the dominant effect in the formation of
the dislocation structure has the part of the cycle when the specimen is
deformed at high temperature. In specimens cycled in OP-TMF cycle, the
density of bands is lower and the areas between the bands are filled
with a homogeneous distribution of dislocations.
Dislocation structure in IPD-TMF and OPD-TMF cycled specimens differ
substantially from the previous ones. 10 min dwell at 700 °C in a cycle
has an important effect on the dislocation distribution. Dislocations
produced in the cycle have enough time during 10 min relaxation at high
temperature to be rearranged and even to disappear due to climb
processes. Dislocations are not any more arranged in bands but
individual segments are randomly arranged. High densities of
precipitates which often pin dislocations are the characteristic feature
of the internal structure. The role of small precipitates is important
as obstacles for dislocation motion. The dislocation density in
specimens subjected to OPD-TMF testing is even lower than in specimens
subjected to IPD-TMF testing. This finding encourages the hypothesis
that high temperature fatigue resistance of Sanicro 25 can be ascribed
preferably to the dense distribution of precipitates produced during
cyclic loading than to the high dislocation density.
A considerable number of different precipitates were found in specimens
subjected to IPD-TMF and OPD-TMF cycling. This corresponds to the
finding 29 that the density and in some cases also the
size of the particles got larger with increasing time and temperature.
Laves phases were also found in the microstructural study of the
specimens Sanicro 25 exposed to 700 °C for 3000 hours reported by Chai
et al.2. Warner et al. 27investigated thermomechanical behaviour of virgin and pre-aged specimens
subjected to IP-TMF loading in the temperature range from 100 °C to
800 °C. Microstructural investigation revealed the formation of Nb(C,N),
Z-phase, Cr2N, M23C6 and
µ-phase in virgin specimens while strained pre-aged specimens resulted
in the extra formation of Laves phase (Fe,Cr)2W.
Precipitates having larger dimensions and lower density like tungsten
carbides or Z-phase do not affect substantially the material strength.
The small precipitates containing copper or niobium which precipitate
during cycling 11 and very effectively also during
dwell periods are supposed to be mostly responsible for the high cyclic
stress-strain response of the material.
M23C6 particles which nucleate at high
angle grain boundaries do not affect the stress-strain response but have
an important effect on the depletion of grain boundaries by chromium and
their preferential oxidation (see 4.3).
Recently Li et al. 30 measured strain rate dependence
of the cyclic plastic stress-strain response in Sanicro 25 steel and
explained it in terms of the different distributions of
M23C6 precipitates. Present results
point to the appearance of the M23C6precipitates preferentially at the grain boundaries and to the
homogeneous distribution of fine Cu- and Nb-rich nanoprecipitates which
determine the high cyclic stress of the material.
4.3. Damage mechanism and fatigue life.
Mechanisms of fatigue crack initiation in Sanicro 25 at ambient
temperature and at elevated temperature differ substantially. While
straining at room temperature, the principal role is played by the
localization of the cyclic plastic strain and formation of extrusions
and intrusions within the grains 13, 31. In high
temperature cycling the oxidation, namely the oxidation of the grain
boundaries is a dominant damage mechanism 8, 14. It is
similar to high temperature fatigue with dwells in high-strength
superalloys 32. Formation of Cr rich carbides
M23C6 at grain boundaries leads to the
depletion of the neighbour matrix from Cr and easy oxidation in high
temperature environment. The same is true for thermomechanical cycling
since high temperature in a cycle leads to grain boundary oxidation20. Different mechanisms of crack initiation were
found in IP-TMF and OP-TMF cycling. The introduction of the dwell
periods at maximum temperature in IP-TMF cycle substantially increases
the oxidation and moreover it can launch the internal damage in the form
of internal cracks.
Li et al. 24 studied damage mechanisms in austenitic
steel X2CrNiMo18.12 subjected to in-phase thermomechanical fatigue in
the lower temperature range, namely 250 °C to 450 °C. They found
localization of the cyclic plastic strain in PSBs and formation of PSMs
on the surface of the material. Both grain boundaries and PSMs were more
vulnerable to oxidation under thermo-mechanical fatigue loadings and
thus were preferentially oxidized and fatigue cracks started from these
locations. In the case of thermomechanical fatigue in the temperature
range 250 °C to 700 °C studied here PSBs and surface PSMs do not play a
significant role in the evolution of the damage and either grain
boundaries in IP-TMF cycling or parallel cracks in a thick oxide layer
in OP-TMF cycling are responsible for fatigue crack initiation.
Dwell periods in TMF cycle do not change the damage mechanisms
radically. In IPD-TMF cycling the grain boundaries play the most
significant role. Grain boundaries become oxidized more intensively than
the rest of the surface and oxide extrusions and oxide intrusions are
intensively produced. Preferentially in grain boundaries perpendicular
to the loading axis, the oxide is cracked in tensile half-cycle. Oxygen
is admitted to the tip of the crack and local oxidation of the crack tip
and repeated cracking of the produced oxide during tensile loading leads
to the early crack growth. Dwell time periods at maximum temperature
enhance significantly the local oxidation of the crack tip neighbourhood
since the crack in IPD-TMF cycle is open. The crack growth rate
therefore increases and fatigue life of specimens subjected to IPD-TMF
loading decreases.
In OPD-TMF cycling the preferential oxidation of the grain boundaries is
suppressed since at high temperature the specimen is in compression.
Thick oxide layer develops on the whole surface of the specimen. As soon
as the thickness of the oxide exceeds a certain quantity in the surface
layer several cracks perpendicular to the stress axis arise. Many
surface cracks remain open even in maximum compression at high
temperature (see Fig. 13) and this allows admitting of the oxidising
atmosphere to the metal. Local oxidation (see Fig. 14) and subsequent
cracking of the oxide lead to the crack growth, this time
transgranularly, perpendicular to the loading axis. The growth rate of
these cracks is substantially smaller than in IPD-TMF cycling and thus
the fatigue life is longer. Nevertheless, the introduction of the dwell
period in maximum compression at high temperature enhances oxidation and
in comparison with OP-TMF cycling, the fatigue life decreases.
Different damage mechanisms and enhancement of the damage due to longer
oxidation during the dwells is reflected in the fatigue life of tested
specimens. Fig. 17 shows the Manson-Coffin law for specimens subjected
to TMF cycling with dwells and without dwells. It is apparent that in
the case of IPD-TMF cycling the fatigue life slightly decreases possibly
due to the increased plastic strain amplitude as a result of the dwell.
Long oxidation time at high temperature and participation of the
internal damage are the main factors contributing to the decrease of the
fatigue life. OPD-TMF cycling with the total strain amplitudes higher
than in OP-TMF cycling led to the substantial decrease of the fatigue
life (see Fig. 17 where all points are shifted to lower fatigue lives).
However, plastic strain amplitude due to the dwell increased
substantially and all data of OPD-TMF cycling in Fig. 17 are also
shifted to higher plastic strain amplitudes. As a result, both sets of
data (OP-TMF and OPD-TMF) can be also approximated by a single
Manson-Coffin law.