•Grade 92 steel exhibited compressive dwell sensitivity.•Weld joints showed a greater reduction in fatigue life compared to the base metal.•Weld joint specimens failed in the base metal region in all ...the tests.•Dynamic strain aging has enhanced with progression of cycling.•Shifting of strain localization from the HAZ to the base metal region led to failures in the latter.
Creep-fatigue interaction (CFI) behavior of 9Cr-1.8W-0.5Mo-VNb steel (Grade 92 steel) base metal and its weld joint has been investigated at 823 K using a constant strain rate of 3 × 10–3 s−1. Both the base metal and the weld joint exhibited continuous softening with an almost equal cyclic stress response. The fatigue life decreased with increasing hold time. The hold applied in the compressive peak strain proved more deleterious compared to the tensile hold, indicating a compressive dwell sensitivity. Besides, the weld joints showed a greater reduction in fatigue life compared to the base metal. Fracture surfaces of the tested specimens revealed the prevalence of secondary cracks and extensive oxidation in the material. Weld joint specimens failed in the base metal region in all the tests. Evidence of dynamic strain aging in the form of serrations in the stress-strain hysteresis loops was also noted.
Abstract The present study investigated the effect of shear load on the 110 asymmetric tilt grain boundaries of body-centered cubic iron with molecular dynamics simulations. Various mechanisms like ...grain boundary (GB) sliding, shear-coupled migration, GB migration with rotation, GB decomposition, and GB deformation with dislocations emission were observed. Low-angle tilt GBs migrated with the help of two dislocation mechanisms, gliding and climbing. Two types of twin nucleation mechanisms were noticed in high-angle tilt GBs with varying tilt angles ( η ). For 16° ≤ η ≤ 112° and η ≥ 130° cases, dynamic self-adjustment of the atoms at the GBs caused GBs decomposition with the formation of twin boundaries (TBs). Newly nucleated TBs migrated under further applied shear load. For 112° < η < 130° case, TBs nucleated from the GB on either side of the misfit dislocations due to their localized core structure on the GB plane.
HCF-creep interaction in type 316LN stainless steel is investigated at 923 K by conducting HCF tests well below the endurance limit with high mean stress (high R-ratio). High mean stress inflicts ...creep damage at 923 K, which when imposed on a small alternating stress, leads to significant HCF-creep interaction. Depending on the magnitude of mean stress (R-ratio), three distinct regimes are identified, viz. (a) HCF regime (b) HCF-creep interaction regime and (c) creep regime. Detailed fractographic investigation revealed distinct fracture characteristics pertaining to each of the three regimes. HCF-creep interaction regime is marked by significant intergranularity at low values of mean stress which changes to predominantly dimpled fracture with increasing mean stress. The above observations are corroborated through detailed EBSD investigations which bring out the underlying deformation mechanisms responsible for failure under HCF-creep interaction.
Size plays an important role on the deformation mechanism of nanopillars. With decreasing size, many FCC nanopillars exhibit dislocation starvation which is responsible for their high strength. ...However, many details about the dislocation starvation like how often it occurs, and how much is its contribution to the total plastic strain, are still elusive. Similarly, the size below which the dislocation starvation occurs is not clearly established. In this context, atomistic simulations have been performed on the compression of Cu nanopillars with size (d) ranging from 5 to 21.5 nm. Molecular dynamics (MD) simulation results indicate that the nanopillars deform by the slip of extended dislocations and exhibit dislocation starvation mainly at small sizes (<20 nm). The frequency of the occurrence of dislocation starvation is highest in small-sized nanowires and it decreases with increasing size. Above the size of 20 nm, no dislocation starvation has been observed. Furthermore, we define the dislocation starvation strain and based on this, it has been shown that the contribution of the dislocation starvation to the total plastic strain decreases from 70% in small-sized nanopillars to below 5% in large-sized pillars. The present results suggest that dislocation starvation is a dominant phenomenon in small-sized nanopillars.
The study presents an insight into the deformation mechanisms under high cycle fatigue-creep interaction (HCF-creep interaction) in a Ni-based superalloy (Alloy 617 M), using electron backscatter ...diffraction (EBSD) as an important tool. HCF-creep interaction tests are carried out at 700 °C by imposing periodic holds at mean stress (σm) during HCF cycling. Detailed EBSD investigation using kernel average misorientation (KAM) map indicates a non-uniform distribution of local misorientation especially at the grain boundaries, with increase in σm, which is attributed to the increased creep damage therein. Creep cavitation through grain boundary sliding is found to increase with an increase in the hold temperature to 750 °C owing to a higher dislocation mobility triggered by precipitate coarsening and dislocation climb. Change in twin boundary character from Σ3 to other higher angle boundaries caused by dislocation-twin interaction is noticed under HCF-creep interaction, leading to partial detwinning. A decrease in the fraction of Σ3 annealing twin boundaries through partial detwinning is observed, which is found to be more pronounced when the creep hold is maintained at 750 °C. This caused a loss of strength at the above temperature, through coalescence of secondary intergranular cracks, in turn leading to intergranular failure. In contrast, the hold imposed at 700 °C with similar loading parameters resulted in run out.
Glucocorticoid receptor (GR) exerts anti-inflammatory action in part by antagonizing proinflammatory transcription factors such as the nuclear factor kappa-b (NFKB). Here, we assess the crosstalk of ...activated GR and RELA (p65, major NFKB component) by global identification of their binding sites and target genes. We show that coactivation of GR and p65 alters the repertoire of regulated genes and results in their association with novel sites in a mutually dependent manner. These novel sites predominantly cluster with p65 target genes that are antagonized by activated GR and vice versa. Our data show that coactivation of GR and NFKB alters signaling pathways that are regulated by each factor separately and provide insight into the networks underlying the GR and NFKB crosstalk.
Present investigation is aimed at understanding the deformation behavior and the development of damage in a type 316 LN austenitic stainless steel (SS) weld joint (WJ) under isothermal low cycle ...fatigue (IF) and thermomechanical fatigue (TMF). In-phase (IP) and out-of-phase (OP) TMF tests were carried out by maintaining a phasing relation of 0° and 180° respectively, between the mechanical strain and temperature cycles. Dynamic strain ageing (DSA) was found to exert a strong influence on the cyclic stress response (CSR) of the joint under both IF and TMF cycling. The CSR was observed to be higher under TMF compared to IF cycling due to the activation of additional hardening mechanisms in the former tests. The difference in fatigue life under TMF and IF is rationalized based on the development of deformation and damage through optical and scanning electron microscopy (SEM) coupled with electron backscatter diffraction (EBSD) studies. A clear demarcation of the strain distribution in the base metal (BM) and weld metal (WM) region of the joint is achieved through detailed EBSD analysis of the tested specimens. Cyclic plastic deformation and the associated development of damage take place independently in the BM and the WM parts of the joint, competing to cause the failure. The build-up of damage under IF and TMF cycling is dictated by a combination of DSA and δ-ferrite transformation, depending on the applied strain amplitude and the strain-temperature phasing employed. The observed life variations have been rationalized in terms of the substructural evolution and fracture behavior under different testing conditions.
•The CSR and fatigue life depend on the type of fatigue cycle (IF and IP/OP TMF).•Thermal cycling and imposed strain influenced the dynamic strain ageing behavior.•Failure location across the joint changes with the applied strain and type of cycle.•Localized deformation in the joint depends on the δ-ferrite transformation and DSA.
The study presents an EBSD based investigation on the nature of deformation occurring under different modes of cyclic loading viz. low cycle fatigue (LCF), creep-fatigue interaction (CF) and ...LCF-creep-HCF. Compared to LCF, cyclic life was found to decrease for CF or LCF-creep-HCF loading conditions. This was attributed to the additional damage contributions from creep in CF cycling and a combination of creep and HCF in LCF-creep-HCF loading conditions, as substantiated through a highly intergranular fracture observed in those cases. Local misorientation map derived from EBSD showed that the misorientation spread is highest for the LCF condition compared to CF and LCF-creep-HCF. This was attributed to the enhanced thermal recovery taking place during CF and LCF-creep-HCF conditions in comparison to LCF. The dislocations generated during cycling rearrange themselves into a stabilized substructure in the form of cells in the above loading conditions leading to a lower value of local misorientation in those cases. This was also accompanied by a significant decrease in number of twins in CF and LCF-creep-HCF conditions compared to LCF. The dislocation-twin interaction responsible for the process of de-twinning was found to be expedited in the former cases owing to a higher thermal recovery.
•Residual stress distribution was investigated in curved samples of laser peened Ti-6Al-4V.•Elevated temperature (300 °C) fatigue tests were done (R = −1) on similar curved samples.•Induced residual ...stress distribution was favorably retained at elevated temperature.•Crack initiation site shifted to a depth of ∼ 200 μm with 3 times increase in fatigue life at 400 MPa for the peened samples.•Fatigue striation spacings decreased for the peened samples.
The influence of laser shock peening without coating (LPwC) on the induced compressive residual stress and the fatigue behaviour (R = −1 at 300 °C) was investigated on curved samples of Ti-6Al-4V. The stress relaxation after fatigue was also examined. In the peened samples crack initiation shifted to a depth of ∼ 200 μm, with 3 fold increase in fatigue life at 400 MPa. Decrease in fatigue striations in the laser peened (331.48 nm/cycle) samples compared to unpeened (410.54 nm/cycle) samples indicated retardation of crack propagation rate. Further, the effect of grain refinement and misorientation angles on fatigue behaviour is also considered.
The study presents an insight into the mechanism of crack initiation under high cycle fatigue (HCF) and estimation of the initiation life thereof, using electron backscatter diffraction (EBSD) as an ...important tool in a 10% Cr ferritic steel. The localized plastic deformation facilitating crack initiation under HCF is found to be associated with dislocation multiplication and localized cyclic hardening. This process is found to occur concomitantly with the substructural development through the localized deformation leading to formation of new subgrains which act as barriers to the dislocation motion. The subgrain strengthening that arises as a consequence, adds to the local internal stress through dislocation pile-ups, aiding the process of crack initiation. The local internal stress is computed using the average subgrain size, which in turn is used to estimate the critical length required for the crack to self-propagate under the applied HCF stress, applying the principle of fracture mechanics. This concept is also utilized to identify the crack propagation threshold demarcating the regimes of propagating and non-propagating cracks.
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