Secondary Cyclic Hardening Research Articles

Strengthening mechanism of abnormally enhanced fatigue ductility of gradient nanostructured 316L stainless steel

Structural materials with a good combination of high fatigue strength and high fatigue ductility are highly desirable in engineering applications. However, it remains a challenge to “break” the exclusive relationship between fatigue strength and fatigue ductility. The cylindrical 316L stainless steel specimens with a gradient nanostructured surface layer were prepared using surface mechanical rolling treatment (SMRT). Fully reversed strain-controlled tension–compression fatigue experiments indicated that the SMRT processed 316L specimens achieved simultaneous improvement of fatigue strength and fatigue ductility. The abnormal increase in fatigue ductility is attributed to the significant secondary cyclic hardening caused by the martensitic phase transformation and the transition in fatigue crack initiation mode. The martensitic phase transformation enhances the fatigue strength while reducing the proportion of plastic deformation in the controlled strain amplitude. The competition between the driving force and resistance of fatigue nucleation in different structural units results in the migration of fatigue crack initiation sites from the surface to subsurface, thereby prolonging the fatigue crack initiation life. This work reveals the synergistic strengthening mechanism of fatigue strength and fatigue ductility of a gradient nanostructured 316L stainless steel.

Short-range ordering plays a determining role in the low-cycle fatigue life improvement of fcc metals: A conclusive study on low solid-solution hardening Ni-Cr alloys

The effect of short-range ordering (SRO) on the low-cycle fatigue (LCF) behavior of low solid-solution hardening Ni-Cr alloys with high stacking fault energies (SFEs) was systematically studied under cycling at constant total strain amplitude (Δεt/2) in the range of 0.1%–0.7%. The results show that an inducement of SRO structures can notably improve the fatigue life of the alloy regardless of Δεt/2, and several unique fatigue characteristics have been detected, including the transition of fatigue cracking mode from intergranular cracking to slip band cracking, the non-negligible evolution from non-Masing behavior in pure Ni to Masing behavior in the Ni-40Cr alloy, and the secondary cyclic hardening behavior in the Ni-10Cr and Ni-20Cr alloys. All these experimental phenomena are tightly associated with the transformation in cyclic deformation mechanisms that is induced by SRO based on the “glide plane softening” effect. Furthermore, a comprehensive fatigue life prediction model based on total hysteresis energy has been reasonably proposed, focusing on the analyses of the macroscopic model parameters (namely the fatigue cracking resistance exponent β and the crack propagation resistance parameter W0) and microscopic damage mechanisms. In brief, on the premise that the effects of SFE and friction stress can be nearly ignored, as in the case of the present low solid-solution hardening Ni-Cr alloys with high SFEs, an enhancement of SRO in face-centered cubic metals has been convincingly confirmed to be an effective strategy to improve their LCF performance.

Cyclic deformation behavior and cracking mechanism of Inconel Alloy 690TT steam generator tube at elevated temperature

Cyclic deformation behavior and cracking mechanism of the actual Alloy 690TT steam generator (SG) tube was systematically studied using the designed boat-shaped specimen and supporting fixtures at 325 °C in air. Results demonstrated that Alloy 690TT exhibited serrated secondary cyclic hardening (SCH) under 325 °C elevated temperature air at the test strain rate and led to fatigue crack initiation, which was the fundamental reason why the fatigue life of Alloy 690TT SG tube under 325 °C elevated temperature air was lower than that under room temperature air(about 70%). Under elevated temperature fatigue at 325 °C, a large number of deformation twins burst at the grain boundary, the interaction between stacking faults and dislocations and dislocation entanglement caused the occurrence of serrated SCH, the density of persistent slip bands (PSBs) on the internal surface increased significantly, and the fatigue crack initiated at the PSBs on the internal surface, resulting in the fatigue life under elevated temperature air being lower than that under room temperature air.

Behavior of 316L Stainless Steel Under Static and Dynamic Loading Conditions

In the present study, 316L stainless steel was characterized for microstructure and mechanical behavior under static and dynamic loading conditions. It showed a high ratio of tensile strength to yield strength with good ductility. Experiments on strain controlled tests, performed at 0.25 Hz frequency, resulted in reduction of fatigue life with increasing total strain amplitude. At higher strain amplitudes, secondary cyclic hardening was noticed. In contrast, cyclic hardening was observed for initial few cycles followed by softening up to failure at lower strain amplitudes.

Thermally activated dependence of fatigue behaviour of CrMnFeCoNi high entropy alloy fabricated by laser powder-bed fusion

The CrMnFeCoNi high-entropy alloy demonstrates a promising potential for applications over a range of temperature. The alloy also shows excellent printability to be fabricated by additive manufacturing for complex structures. Nevertheless, there are limited studies on the thermo-mechanical behaviour of the alloy, in particular when fabricated by laser powder-bed fusion. This study provides an in-depth understanding of the relationship between as-built cellular microstructures and fatigue behaviour at a range of temperatures (22–600 °C) in particular concerning the stability of dislocation cells and thermo-mechanical dependence of the fatigue behaviour of the alloy. At all tested temperatures, the alloy exhibits a very short duration cyclic hardening with a low hardening rate followed by a cyclic softening. The high density of dislocations already existing in as-built condition were able to accommodate most of the prescribed strain. Hence, only a small number of mobile dislocations needs to be generated, causing a short cyclic hardening phase. Upon further loading, the back stress associated with the long-range stress field was dominant factor governing the cyclic softening behaviour. The similitude relationship provided insights into the stability of as-built cells, in particular it explains why the size of as-built cells did not change during cyclic loading at 22 °C. The significant reduction in dislocation density due to the increased annihilation rate and untanglement of dislocation substructures thanks mainly to thermal assistance at elevated temperatures led to a decrease in cyclic strength and related properties (yield stress, friction and back stress, hysteresis loop shape parameter and energy per cycle). The LPBF HEA shows an insignificant strain rate dependence of the primary cyclic hardening and softening in the range of 10 −3 s −1 and 10 −2 s −1 . However, the dynamic strain ageing results in a secondary cyclic hardening at 400 °C and the reversed strain sensitivity at temperatures from 200° to 400 °C. The fracture mode was transgranular at 22–400 °C but changed to more intergranular-like at 600 °C due to the decohesion of grain boundaries, resulting in a reduction in fatigue life.

Grain-size-dependent microstructure effects on cyclic deformation mechanisms in CoCrFeMnNi high-entropy-alloys

The effect of grain size on strain-controlled low-cycle fatigue (LCF) properties in the CoCrFeMnNi high-entropy alloys (HEAs) was investigated towards the distinct microstructural developments during cyclic loading at a strain amplitude of ± 1.0%. A much more prominent secondary cyclic hardening (SCH) behavior at the final deformation stage was observed in the fine-grained (FG, 18 µm) than in the coarse-grained (CG, 184 µm) CoCrFeMnNi. In-situ neutron-diffraction and microscopic examination, strongly corroborated by molecular dynamic (MD) simulations, indicated that dislocation activities from planar slip to wavy slip-driven subgrain structures within the grains acted as the primary cyclic-deformation behaviors in the FG CoCrFeMnNi. Differently observed in the cyclic behavior of the CG CoCrFeMnNi was due to a transition from the planar dislocation slip to twinning. Our findings suggested that the fatigue-resistant HEAs can be designed via tuning the microstructure with an optimal range of grain size at a specific strain amplitude.

Effect of solution annealing on low cycle fatigue of 304L stainless steel

The present study investigates low cycle fatigue behaviour of the metastable austenitic 304L steel in two variants: i) as-received and ii) after additional solution annealing at 1050 °C/30 min. The applied heat treatment resulted in a decrease of δ-ferrite volume fraction and slight grain coarsening. After the solution annealing, significantly prolonged fatigue life within the whole range of studied total strain amplitudes was observed. Such improvement is attributed to the larger grain size possessing better ductility and enhanced formation of deformation-induced martensite. The phase transformation led to a significant secondary cyclic hardening stage at high strain amplitude loading. The low cycle fatigue behaviour and principal fatigue life stages were characterized by the analysis of cyclic response. A vital supplementary data were obtained by the analysis of hysteresis loop shape carried out, for the first time on a phase-transforming material, by the generalized statistical theory. The analysis was able to follow the evolution of cyclic stress sources during the fatigue life, especially the increasing contribution of deformation-induced martensite. The identified features of cyclic plasticity were supported by electron channeling contrast imaging observations. Furthermore, electron backscatter diffraction revealed a distinct preferential localization of α'-martensite into macrobands parallel to rolling direction. Energy dispersive spectroscopy mapping identified pronounced chemical segregation of Cr and Ni which correlated well with the observed macrobands. These compositional fluctuations led to local austenite metastability which is enhanced further by the applied solution annealing.

Correlation between microstructure evolution and mechanical response in a moderately low stacking-fault-energy austenitic Fe–Mn–Si–Al alloy during low-cycle fatigue deformation

Through in-depth microstructural and mechanical characterization, a mechanism for low-cycle fatigue (LCF) deformation of an Fe-30.7Mn-4.3Si-1.8Al (in wt%) austenitic transformation-induced-plasticity (TRIP) steel at a total strain range of 2% was investigated. The LCF deformation of the steel was performed through two main deformation modes which were planar slip of Shockley partials, and ε-martensitic transformation and its reverse transformation. The varying significance of each deformation mode as well as the relevance between the two modes determined the microstructures evolved during fatigue deformation and resulted in a four-stage cyclic hardening behavior. With increasing fatigue cycles, the LCF deformation was dominated sequentially by the multiplication of partial dislocations, development of slip bands, reversible ε-martensitic transformation, and formation of more thick and crossed ε-martensite along with enhancement of planar dislocation slip, thus generating the characteristic deformation stages of primary cyclic hardening, cyclic softening, cyclic saturation, and secondary cyclic hardening correspondingly. In general, the superior LCF resistance of the steel was attributed to appreciably high plasticity reversibility during the entire fatigue deformation which was caused by reversible planar dislocation slip and ε-martensitic transformation.

Low cycle fatigue of additively manufactured thin-walled stainless steel 316L

To ensure the robust design freedom of metallic additive manufacturing, the fatigue properties and the dimensional limitation of as-built components by laser powder bed fusion (PBF-LB) are investigated. Fully reversed and strain-controlled fatigue tests were carried out on tubular specimens with different wall thicknesses, 1 mm and 2 mm, for the purpose of studying the thin-wall effect without having risk of buckling problem during compression. Two wrought conditions are also enclosed as a comparison, which are the cold worked (CW) and solution annealed condition (SA). In the as-built PBF-LB tubular specimens, deformed microstructure and deformation twins are discovered close to the surface region, together with a higher roughness of the inner surface due to the heat accumulation. The surface roughness is evaluated as micro-notches, and a higher fatigue notch factor, Kf, at lower applied strain range is revealed. The factors influencing Kf include, the non-conductive inclusions serving as crack initiation sites at the surface region, and the deformation twins formed by the local stress concentration. The strain-life of PBF-LB samples is comparable with the wrought samples. However, the fatigue strength of the responding mid-life stress shows greater difference and is in the following order, CW wrought > PBF-LB > SA wrought. Secondary cyclic hardening owing to deformation induced martensitic transformation is found in both of the wrought samples. Yet, only cyclic softening exhibits in the PBF-LB samples, which is the result of the suppressed martensitic transformation and the dislocation unpinning from the cell boundaries.

A cyclic plasticity model for secondary hardening due to strain-induced martensitic transformation

A model for secondary cyclic hardening due to strain-induced martensitic transformation in metastable austenitic stainless steels is proposed. The model is built on an Armstrong–Frederick type kinematic hardening framework. Secondary hardening is described by relating the size of the limiting surface of each backstress to an evolution rule for the martensite content. A memory surface is used to account for the influence of the plastic strain amplitude on martensitic transformation. The model properly estimated the stress response of three austenitic stainless steels (types 304/304L, 321, and 348) subjected to strain-controlled axial, torsional, and proportional loading. The characteristics of the model, the determination of material constants, and suggestions for future research are discussed.

Low-cycle fatigue behavior and life prediction of fine-grained 316LN austenitic stainless steel

Abstract

Fatigue curve of microscale single-crystal copper: An in situ SEM tension-compression study

A series of quantitative in situ tension-compression fatigue experiments in SEM are performed to investigate the fatigue curve of microscale single-crystal copper specimens with a test part of 1 × 1 × 2 μm under various strain amplitudes. The variation of resolved shear stress on the primary slip plane (τB4) and the corresponding evolution of crystallographic slips in each cycle are analyzed and in situ observed, respectively. The variation of τB4 during fatigue shows that, (i) the variation of τB4 clearly includes the early stage, the saturation stage, and the final fracture of the specimen; (ii) apparent cyclic hardening and softening appear in the early stage of all specimens; (iii) τB4 shows a strain-dependent variation in the saturation stage. Continuous cyclic softening and secondary cyclic hardening occur in specimens with relatively higher strain, while only slight cyclic hardening emerges in the specimen with the minimum strain. Based on in situ SEM observations, the variation trend of τB4 is interpreted from the evolution of geometrical characteristics of slip traces. Finally, we obtain the fatigue curve of microscale single-crystal coppers. The fatigue life is apparently dependent on the strain amplitude, while insensitive to the frequency of cyclic loading. Moreover, the fatigue lives of microscale single-crystal Cu are much shorter than those of bulks, indicating a significant size effect. The difference in fatigue lives of microscale and bulk ones reduces from about 360 to only 7 times with the decrease of applied strain, revealing the strain-dependence of size effect of fatigue life for microscale single-crystal coppers.

The distinct influences of pre-strain on low cycle fatigue behavior of CP-Ti along rolling direction at different strain amplitudes

In this paper, the distinct influences of pre-strain on low cycle fatigue (LCF) behavior of CP-Ti at different strain amplitudes are clarified from macroscopic and microscopic aspects. At low strain amplitude, the sample of as-received material (AR) shows pronounced secondary cyclic hardening, however, this phenomenon gradually disappears in pre-strained (PS) sample with the increase of pre-strain. As cyclic stress amplitude and mean stress of PS sample increase with the decrease of strain amplitude, the detrimental effect of pre-strain on LCF life becomes more significant at lower strain amplitude, indicating the pre-strain dependent LCF behavior. At high strain amplitude, due to the more significant cyclic softening and mean stress reduction behavior, cyclic stress amplitude and mean stress rapidly decay to the value of AR sample in the initial few cycles, indicating the pre-strain independent LCF behavior. As the reduction of LCF life is related to the increase of mean stress and stress amplitude, the asymmetry coefficient related model can be applied to predict LCF life of CP-Ti. Further TEM observation shows that at low strain amplitude, dislocation structures of AR sample are mainly composed of planar slip structures and the secondary cyclic hardening is caused by corduroy structure. After pre-strain, both planar slip and wavy slip structures are observed in PS sample, indicating that the irreversibility of the dislocation structures inherited from pre-strain causes the decrease of LCF life. At high strain amplitude, the dislocation structures introduced by pre-strain can be erased completely during subsequent cyclic deformation, consequently, dislocation configurations of AR and PS samples are composed of the same wavy slip structures.

Influence of mean stress and light water reactor environment on fatigue life and dislocation microstructures of 316L austenitic steel

Influence of mean stress on fatigue life of the austenitic stainless steel 316 L in air and light water environments (boiling water reactor/hydrogen water chemistry) at 288 °C was determined with a series of tests carried out in load-control mode. Fatigue life was found to increase with application of compressive and tensile mean stress in air and light water reactor environments. Secondary hardening was regarded as the main reason for this behavior. A modified Smith-Watson-Topper (SWT) model was considered to account for mean stress and was shown to predict fatigue life accurately in air and water environments. The reduction of fatigue life in water environment, determined with the SWT curves, was about 2.5. Observations of the end-of-life dislocation arrangements by transmission electron microscopy showed that the dislocation microstructure depends essentially on plastic strain amplitude, which in turn is strongly correlated to stress amplitude and mean stress. The microstructures were found consistent with those usually observed after strain-controlled experiments. At rather low plastic strain amplitudes, corduroy structure consisting of small dislocation loops was observed. Acting as significant obstacle to dislocation motion, corduroy structure affects overall dislocation mobility therefore contributing to notable secondary cyclic hardening.

Low Cycle Fatigue Behavior of 316LN Stainless Steel Alloyed with Varying Nitrogen Content. Part I: Cyclic Deformation Behavior

In this study, the influence of cyclic strain amplitude on the evolution of cyclic stress–strain response and the associated cyclic deformation mechanisms in 316LN stainless steel with varying nitrogen content (0.07 to 0.22 wt pct) is reported in the temperature range 773 K to 873 K (500 °C to 600 °C). Two mechanisms, namely dynamic strain aging and secondary cyclic hardening, are found to strongly influence the cyclic stress response. Deformation substructures associated with both the mechanisms showed planar mode of deformation. These mechanisms are observed to be operative over certain combinations of temperature and strain amplitude. For strain amplitudes >0.6 pct, wavy or mixed mode of deformation is noticed to suppress both the mechanisms. Cyclic stress–strain curves revealed both single and dual-slope behavior depending on the test temperature. Increase in nitrogen content is found to increase the tendency toward planar mode of deformation, while increase in strain amplitude leads to transition from planar slip bands to dislocation cell/wall structure formation, irrespective of the nitrogen content in 316LN stainless steel.

Revealing the cyclic hardening mechanism of an austenitic stainless steel by real-time in situ neutron diffraction

Real-time in situ neutron diffraction was performed on an austenitic stainless steel under cyclic loading at room temperature. The evolutions of individual phase stresses during martensitic transformation were derived from the lattice parameters and volume fraction based on Rietveld refinements and Hooke’s law, while the phase-specific dislocation densities were elucidated by the single peak broadenings. Both results reveal that the increasing content of martensite phase, instead of individual phase strengthening, should be accounted for the remarkable secondary cyclic hardening.

Influence of Secondary Cyclic Hardening on the Low Cycle Fatigue Behavior of Nitrogen Alloyed 316LN Stainless Steel

In this article, the occurrence of secondary cyclic hardening (SCH) and its effect on high-temperature cyclic deformation and fatigue life of 316LN Stainless steel are presented. SCH is found to result from planar slip mode of deformation and enhance the degree of hardening over and above that resulted from dynamic strain aging. The occurrence of SCH is strongly governed by the applied strain amplitude, test temperature, and the nitrogen content in the 316LN SS. Under certain test conditions, SCH is noticed to decrease the low cycle fatigue life with the increasing nitrogen content.

The Effect of Nitrogen Alloying on the Low Cycle Fatigue and Creep-Fatigue Interaction Behavior of 316LN Stainless Steel

Low cycle fatigue (LCF) and Creep-fatigue interaction (CFI) behavior of 316LN austenitic stainless steel alloyed with 0.07, 0.11, 0.14, .22 wt.% nitrogen is briefly discussed in this paper. The strain-life fatigue behavior of these steels is found to be dictated by not only cyclic plasticity but also by dynamic strain aging (DSA) and secondary cyclic hardening (SCH). The influence of the above phenomenon on cyclic stress response and fatigue life is evaluated in the present study. The above mentioned steels exhibited both single-and dual-slope strain-life fatigue behavior depending on the test temperatures. Concomitant dislocation substructural evolution has revealed transition in substructures from planar to cell structures justifying the change in slope. The beneficial effect of nitrogen on LCF life is observed to be maximum for 316LN with nitrogen in the range 0.11 - 0.14 wt.%, for the tests conducted over a range of temperatures (773-873 K) and at ±0.4 and 0.6 % strain amplitudes at a strain rate of 3*10-3 s-1. A decrease in the applied strain rate from 3*10-3 s-1 to 3*10-5 s-1 or increase in the test temperature from 773 to 873 K led to a peak in the LCF life at a nitrogen content of 0.07 wt.%. Similar results are obtained in CFI tests conducted with tensile hold periods of 13 and 30 minutes. Fractography studies of low strain rate and hold time tested specimens revealed extensive intergranular cracking.

Low cycle fatigue behavior of Zircaloy-2 at room temperature

Fuel cladding and pressure tubes of Zircaloy-2 in pressurized light and heavy water nuclear reactors experience plastic strain cycles due to power fluctuations in the reactor, such strain cycles cause low cycle fatigue (LCF) and could be life limiting factor for them. Factors like strain rate, strain amplitude and temperature are known to have marked influence on LCF behavior. The effect of strain rate from 10−2 to 10−4s−1 on LCF behavior of Zircaloy-2 was studied, at different strain amplitudes between ±0.50% and ±1.25% at room temperature. Fatigue life was decreased with lowering of strain rate from 10−2 to 10−4s−1 at all the strain amplitudes studied. While there was cyclic softening at lower strain amplitudes (Δεt/2⩽±0.60%) cyclic hardening was exhibited at higher strain amplitudes (Δεt/2⩾±1.00%) at all the strain rates. Further, there was secondary cyclic hardening during the later stage of cycling at all the strain amplitudes and the strain rates. Cyclic stress–strain hysteresis loops at the lowest strain rate of 10−4s−1 were found to be heavily serrated, resulting from dynamic strain aging (DSA). There was significant effect of strain rate on dislocation substructure. The results are discussed in terms of high concentration of point defects generated during cyclic straining and their role in enhancing interaction between solutes and dislocations.

Role of microstructural condition on fatigue damage development of AISI 316L at 20 and 300 °C

At 300°C, when dynamic strain ageing takes place, the fatigue life of AISI 316L for lower strain amplitudes is lower than under equivalent conditions at 20°C. Exhaustive examination of the changes in: (1) apparent elastic modulus, (2) microstructural condition, and (3) fractographic features has been performed to reveal the reason for the life reduction. The analysis of apparent elastic modulus variations and the results of fractographic observations show that the propagation rates for fatigue cracks at 20°C are faster than for 300°C. Crack initiation however occurs earlier at 300°C, in particular for lower strain amplitude tests, due to the activity of localised deformation bands as a consequence of cyclic loading. In addition to persistent slip bands, a form of ladder-free deformation bands is also present at 300°C, in particular at low strain amplitudes. When the fatigue life is rather short, the influence of the ladder-free deformation bands on cyclic endurance is negligible. The ladder-free type of localised bands have a strong influence on crack initiation once the material endurance increases with lowering strain amplitude, leading to the relative life reduction at the elevated temperature. In addition, the incidence of secondary cyclic hardening for lower strain amplitude tests at 300°C partly contributes to the more evident life reduction. The influence of dislocation walls on the propagation of microstructurally short fatigue cracks is also examined.