https://doi.org/10.1016/j.actamat.2020.09.048
AbstractHydrogen (H) induced damage in metals has been a longstanding woe for many industrial applications. One form of such damage is linked to H clustering, for which the atomic origin remains contended, particularly for non-hydride forming metals. In this work, we systematically studied H clustering behavior in bcc metals represented by W, Fe, Mo, and Cr, combining first-principles calculations, atomistic and Monte Carlo simulations. H clustering has been shown to be energetically favorable, and can be strongly facilitated by anisotropic stress field, dominated by the tensile component along one of the 〈001〉 crystalline directions. We showed that the stress effect can be well predicted by the continuum model based on H formation volume tensor, and that H clustering is thermodynamically possible at edge dislocations, evidenced by nanohydride formation at rather low levels of H concentration. Moreover, anisotropy in the stress effect is well reflected in nanohydride morphology around dislocations, with nanohydride growth occurring in the form of thin platelet structures that maximize one 〈001〉 tension. In particular, the 〈001〉 type edge dislocation, with the 〈001〉 tensile component maximized, has been shown to be highly effective in facilitating H aggregation, thus expected to play an important role in H clustering in bcc metals, in close agreement with recent experimental observations. This work explicitly and quantitatively clarifies the anisotropic nature of stress effect on H energetics and H clustering behaviors, offering mechanistic insights critical towards understanding H-induced damages in metals.
https://doi.org/10.1016/j.actamat.2020.08.028
AbstractNew-generation multi-phase martensitic steels derive their high strength from the body-centered cubic (BCC) phase and high toughness from transformation of the metastable face-centered cubic (FCC) austenite that transforms into martensite upon loading. In spite of its critical importance, the in-situ transformation strain (or 「shape deformation」 tensor), which controls ductility and toughness, has never been measured in any alloy where the BCC lath martensite forms and has never been connected to underlying material properties. Here, we measure the in-situ transformation strain in a classic Fe-Ni-Mn alloy using high-resolution digital image correlation (HR-DIC). The experimentally obtained results can only be interpreted using a recent theory of lath martensite crystallography. The predicted in-situ transformation strain agrees with the measurements, simultaneously demonstrating the method and validating the theory. Theory then predicts that increasing the FCC to BCC lattice parameter ratio substantially increases the in-situ transformation strain magnitude. This new correlation is demonstrated using data on existing steels. These results thus establish a new additional basic design principle for ductile and tough alloys: control of the lattice parameter ratio by alloying. This provides a new path for development of even tougher advanced high-strength steels.https://doi.org/10.1016/j.actamat.2020.08.057
AbstractAn unusual tensile deformation behaviour in the form of a propagating band along the sample gauge was observed in two neutron-irradiated 316 stainless steel samples during room-temperature tests, leading to a combination of high strength and high ductility. These bands were not observed in an unirradiated counterpart. With the help of in situ high-energy synchrotron x-ray diffraction, the phase-specific crystal information was tracked at different deformation levels in each sample. Post-irradiation and post-deformation samples were examined using electron microscopy to characterize various microstructural features. All samples displayed a deformation-induced martensitic phase transformation, which was identified as a second strain-hardening mechanism accompanying the dislocation hardening. The deformation-induced martensitic transformation was rationalized by the effect of applied stress on the effective martensite start temperature. The results showed that the irradiation did not alter the dislocation hardening and the martensitic transformation mechanisms, but the increased yield strength in irradiated materials facilitated the localized phase transformation at the onset of plastic deformation, in contrast to the unirradiated material which required pre-straining. The hardening effect of the martensitic transformation reduced the tendency towards necking and mitigated the loss of ductility in the irradiated material by carrying the deformation in the form of a propagating band. Despite the beneficial effect from the martensitic transformation, this study indicates that this mechanism cannot not be activated at typical operating temperatures of nuclear reactors.https://doi.org/10.1016/j.actamat.2020.09.007
AbstractThe combination of different phase constituents to realize a mechanical composite effect for superior strength-ductility synergy has become an important strategy in microstructure design in advanced high-strength steels. Introducing multiple phases in the microstructure essentially produces a large number of phase boundaries. Such hetero-interfaces affect the materials in various aspects such as dislocation activity and damage formation. However, it remains a question whether the characteristics of phase boundaries, such as their chemical decoration states, would also have an impact on the mechanical behavior in multiphase steels. Here we reveal a phase boundary segregation-induced strengthening effect in ultrafine-grained duplex medium-Mn steels. We found that the carbon segregation at ferrite-austenite phase boundaries can be manipulated by adjusting the cooling conditions after intercritical annealing. Such phase boundary segregation in the investigated steels resulted in a yield strength enhancement by 100–120 MPa and simultaneously promoted discontinuous yielding. The sharp carbon segregation at the phase boundaries impeded interfacial dislocation emission, thus increasing the stress required to activate such dislocation nucleation process and initiate plastic deformation. This observation suggests that the enrichment of carbon at the phase boundaries can enhance the energy barrier for dislocation emission, which provides a favorable condition for plastic flow avalanches and thus discontinuous yielding. These findings extend the current understanding of the yielding behavior in medium-Mn steels, and more importantly, shed light on utilizing and manipulating phase boundary segregation to improve the mechanical performance of multiphase metallic materials.https://doi.org/10.1016/j.actamat.2020.09.005
AbstractWear and friction of nanocrystalline (NC) aluminum were investigated via molecular dynamics simulations and the effects of dopants were considered. Zr-doped NC Al was found to have a better wear resistance and a smaller friction force, which is consistent with a higher hardness and a higher strength of the doped sample. The underlying mechanisms are suppressed emission of dislocations from grain boundaries (GBs), suppressed GB migration, and suppressed GB sliding. After multiple sliding cycles, the trend in mechanical response was reversed, with the pure NC Al showing a better wear resistance and a lower friction force than the doped sample. One reason is that the higher dislocation density introduced during wear into the pure sample leads to more strain hardening. Another reason is that the pure NC Al has undergone more significant grain growth than the doped sample. Since the grain size of our samples is in the inverse Hall-Petch regime, here grain growth leads to strengthening of the pure sample. Mechanisms of grain growth in the pure NC Al and its suppression in the doped NC Al are analyzed and discussed.
https://doi.org/10.1016/j.actamat.2020.08.080
AbstractDuctile alloys fail in corrosive environments by intergranular stress corrosion cracking, through interactions between mechanical and chemical processes that are not yet understood. We investigate formation and mechanical effects of metal defects produced by grain boundary corrosion of low-alloy pipeline steel, at conditions of high susceptibility to stress corrosion cracking in the absence of hydrogen evolution. Nanoindentation measurements show local softening near corroded grain boundaries, indicated by significantly reduced critical loads for dislocation nucleation. Molecular dynamics simulations of nanoindentation of bulk iron showed that metal vacancies and not interstitial hydrogen atoms explain the observed critical load reduction. Both the dislocation activation volume and dislocation activation energy for vacancy-charged samples are found to be nearly one-half of that for a hydrogen charged samples. Quantitative agreement with experimentally measured indentation response was found for vacancy concentrations equivalent to the bulk silicon concentration in the steel, suggesting that vacancies originate from oxidation of reactive silicon solute atoms at grain boundaries. The results help explain the chemical mechanism of formation of vacancy defects that may participate in grain boundary degradation in the absence of hydrogen embrittlement environment.https://doi.org/10.1016/j.actamat.2020.09.045
AbstractCarbon trapping at dislocations and carbide precipitation in martensite could significantly reduce the amount of carbon partitioning into austenite, e.g. incomplete carbon partitioning phenomenon, which would alter austenite decomposition behavior and austenite stability during the Quenching and Partitioning (Q&P) process. In this study, an integrated model is developed to clarify the mechanism of incomplete carbon partitioning and quantify its effects on austenite stabilization in the low-alloy medium-carbon Fe–C–Mn–Si steels. The fraction of carbon consumed by Cottrell atmospheres around dislocations is described using a semi-empiric equation. Then, the kinetic competition among carbide precipitation, carbon partitioning and austenite decomposition during the partitioning step is simulated by coupling the Deschamps–Bréchet model and quenching and partitioning-local equilibrium (QP-LE) model. It is found that transition carbide precipitation in martensite and carbon partitioning into austenite are kinetically coupled at the very early stage of the partitioning step and subsequently promotes austenite decomposition. Taking the synergy effects of incomplete carbon partitioning and austenite decomposition into account, our model is capable of predicting the evolution of volume fraction of austenite and its carbon content during partitioning.https://doi.org/10.1016/j.actamat.2020.09.020
AbstractNano-oxide precipitates in a modern nanostructured ferritic alloy were investigated after extreme thermomechanical processing into a thin-walled tube geometry. It was found that the morphology of the precipitates changed from spherical to rod-shaped, with some increasing to aspect ratios of up to 9, despite the precipitate volume fraction (0.3%) and number density (> 1023 m−3) of precipitates remaining unchanged. High-resolution electron microscopy showed that the precipitates likely remained coherent with the Fe-matrix, while atom probe tomography confirmed that the precipitate compositions remained unaffected by the transformation. The morphological change was attributed to the shearable nature of the (Y,Ti,O)-rich precipitates, indicating they should be considered as 「soft」 obstacles to dislocation motion. The elongation was most pronounced in larger (>5 nm) precipitates, which may be caused by preferential dissolution of the smallest (1–3 nm) precipitates followed by the competition between re-precipitation and solute diffusion to larger precipitates during recovery heat treatments.https://doi.org/10.1016/j.actamat.2020.08.072
AbstractUnderstanding the local strain enhancement and lattice distortion resulting from different microstructure features in metal alloys is crucial in many engineering processes. The development of heterogeneous strain not only plays an important role in the work hardening of the material but also in other processes such as recrystallization and damage inheritance and fracture. Isolating the contribution of precipitates to the development of heterogeneous strain can be challenging due to the presence of grain boundaries or other microstructure features that might cause ambiguous interpretation. In this work a statistical analysis of local strains measured by electron back scatter diffraction and crystal plasticity based simulations are combined to determine the effect of M23C6 carbides on the deformation of an annealed AISI 420 steel. Results suggest that carbides provide a more effective hardening at low plastic strain by a predominant long-range interaction mechanism than that of a pure ferritic microstructure. Carbides not only influence local strain directly by elastic incompatibilities with the ferritic matrix, but also the spatial interactions between ferrite grains. Carbides placed at the grain boundaries enhanced the development of strain near ferrite grain boundaries. However the positive effect of carbides and grain boundaries to develop high local strains is mitigated at regions with high density of carbides and ferrite grain boundaries.