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.