ACTA

向上滑动阅览英文摘要

Tempering-induced carbon redistribution in martensitic steels from clustering to carbide precipitation is relevant to various steel processing operations and resulting property combinations. In low carbon steels with martensite start temperatures well above room temperature, carbon redistribution is already initiated during quenching. The extent of autotempering varies locally across the as-quenched microstructure, even within a single prior austenite grain, depending on the sequential transformation of martensite laths. To overcome this practical limitation in the study of carbon redistribution, we systematically track the tempering-induced microstructure evolution within a single martensite lath. This is achieved through interrupted tempering treatments and microstructure analyses by electron channeling contrast imaging and atom probe tomography, both correlated with crystallographic orientation information from electron backscatter diffraction. The results show plate-shaped carbon clusters parallel to {100} martensite lattice planes in the as-quenched state, where the maximum carbon compositions remain below ε- or η-carbide compositions. The driving force for initial cluster formation is discussed in relation to spinodal decomposition - analogous to room temperature aged Fe-Ni-C alloys after martensitic transformation during cryogenic quenching. Upon tempering, direct evidence for cluster dissolution and simultaneous cementite nucleation at carbon-rich dislocation cores is obtained. A crucial role is ascribed to the dislocation network which serves as a carbon diffusion path from clusters to cementite nucleation sites.

ACTA

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Heterophase interfaces play a crucial role in deformation microstructures and thus govern mechanical properties of multilayered composites. Here, we fabricated Ti/Nb multilayers by accumulative roll bonding (ARB) where shear bands became predominant with increasing rolling cycles. To explore correlation between micromechanical behavior and mechanical properties of the composites with various lamellar morphologies, in-situ high-energy X-ray diffraction tensile tests were performed. The results quantitatively reveal that the rapid strengthening of the composites with increasing ARB cycles mainly originates from the Nb layers strengthened by dislocations, grain boundaries and heterophase interfaces, and the {211} grains mostly contribute to the global strain hardening. The softer Ti grains also extend global strain hardening to a wide range and postpone necking. Furthermore, complete stress state analysis show that in the presence of extensive shear bands, significant load partitioning between the neighboring metals leads to triaxial stresses in each constituent and dislocations tend to slip along the shear direction. This promotes dislocation multiplication and motion, which is conducive to overall strength enhancement while maintaining a satisfactory ductility. These findings elucidate the effect of strong constraints of the interfaces on mechanical properties, which provides a fundamental understanding of load partitioning and strengthening mechanisms of the multilayers processed by multiple ARB cycles.

ACTA

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We have thoroughly clarified the mesoscopic nature of serration behavior in a high-Mn austenitic steel in connection with its characteristic localized deformation. A typical high-Mn steel, Fe-22Mn-0.6C (wt. %), with a face centered cubic (FCC) single-phase structure was used in the present study. After 4 cycles of repeated cold-rolling and annealing process, a specimen with a fully recrystallized microstructure having a mean grain size of 2.0 μm was obtained. The specimen was tensile tested at room temperature at an initial strain rate of 8.3 × 10⁻⁴ s⁻¹, during which the digital image correlation (DIC) technique was applied for analyzing local strain and strain-rate distributions in the specimen. Obtained results indicated that a unique strain localization behavior characterized by the formation, propagation and annihilation of deformation localized bands, so-called Portevin–Le Chatelier (PLC) bands, determined the global mechanical response appearing as serration on the stress-strain curve. In addition, the in-situ synchrotron XRD diffraction during the tensile test was utilized to understand what was happening in the material with respect to the PLC banding. Lattice strain of (200) plane nearly perpendicular to the tensile direction dropped when every PLC band passed through the beam position, which indicated a stress relaxation occurred inside the PLC band. At the same time, the dislocation density increased drastically when the PLC band passed the beam position, which described that the material was plastically deformed and work-hardened mostly within the PLC band. All the results obtained consistently explained the serration behavior in a mesoscopic scale. The serration behavior on the stress-strain curve totally corresponded to the formation, propagation and annihilation of the PLC band in the 22Mn-0.6C steel, and the localized deformation, i.e., the PLC banding, governed the characteristic strain hardening of the material.

ACTA

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High Entropy Alloys (HEAs) are a new class of multi-component alloys with excellent tensile strength-ductility combination. Their yield strength levels, however, are still low as compared to other high strength materials. Here, Equal Channel Angular Pressing (ECAP) was employed to improve the yield strength of the most well-known HEA, CoCrFeMnNi, through microstructural refinement. The cyclic response of both coarse and ultrafine grained CoCrFeMnNi was investigated during strain-controlled low cycle fatigue tests under fully reversed push-pull loading at room temperature. The microstructural evolution during cyclic loading was compared to the microstructure under quasi-static monotonic loading. Very high yield strength levels around 1 GPa were obtained after ECAP. In addition, ECAP samples demonstrated a superior fatigue life at relatively low strain amplitudes while coarse grained samples exhibited a better fatigue life at the highest strain amplitude considered. X-ray diffraction, electron backscattered diffraction and transmission electron microscopy were performed to reveal underlying mechanisms for the superior fatigue life and the overall transient behavior upon cycling. The fatigue life and hardening behavior were governed by the refined grain size, high density dislocation walls, and the annihilation of existing dislocations resulting in the formation of cell structures in the ECAP samples. The lower fatigue life of the ECAP samples at the highest strain amplitude is attributed to the higher stress amplitudes and cyclic softening due to accelerated dislocation annihilation. Finally, the formation of dislocation cell structures and high-density dislocation walls simultaneously is rationalized by the effect of applied stress on the partial dislocation separation.

ACTA

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An extension of the pioneer work by Mullins and Sekerka is proposed to analyze the stability conditions of a spherical particle growing in a matrix phase. The present model describes the onset of instabilities in an unsteady growth regime for multi-component alloys with cross-diffusion of chemical species. The developments are based on the decomposition of initial perturbations in spherical harmonics to determine their time evolution. Expressions for the threshold particle radii associated to the limits for absolute and relative stability criteria are derived. They depend on the degree of the spherical harmonic functions, the growth parameter for a sphere (proportional to the interface velocity times the square root of time) and the eigenvalues of the diffusion matrix. A large effect of the interface velocity on the stability domains is demonstrated considering the time-dependent solutions. Comparisons are provided regarding more recent solutions proposed in the literature. A similar evolution is only observed for slow regimes. An application for solidification of a ternary alloy is finally given.

ACTA

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Wear degradation is a key parameter to evaluate the performance of ceramic-metal composites applied in manufacturing and mining industries. We selected a typical composite (high Mn steel + 50 wt.% TiC) as the model system and studied its wear behavior. The integrity and damage of TiC ceramic particles were analyzed under different temperatures and atmospheres. A critical temperature of ~125 °C at which transitions of wear mechanism occurred was observed for the first time. Below this temperature, hardness dominated the wear behavior as the presence of oxygen induced surface decarburization of ceramic particles with an increased wear rate. The thermodynamics of oxygen-assisted decarburization were investigated using the CALPHAD (CALculation of PHAse Diagram) approach, and the resultant hardness reduction of TiC particles was clarified using density functional theory (DFT) calculations. At temperatures above 125 °C, toughness became the governing factor as the ceramic-metal deformation incompatibility induced fracture of ceramic particles. The formation of nano-crystalline oxide tribolayers improved the fracture toughness of ceramic particles and restrained the wear loss, which was elucidated using DFT calculations. Thus, the current work has delineated the competitive micro-mechanisms consisting of surface decarburization and toughening of ceramic particles, which could be used to interpret the wear behavior of ceramic-metal composites.

ACTA

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A novel Cr-alloyed medium-Mn steel was designed and manufactured for the hot forming application in automobile. Compared to the typical press hardening steel (PHS) grade of 22MnB5, it possesses more excellent mechanical combination and enormously reduced oxidization after press hardening. The synchrotron-based high-energy X-ray diffraction experiment indicated that the solute C atoms could be partitioned from martensite to austenite during the baking process and then segregated on geometry necessary dislocations that had been generated due to the austenite-to-martensite transformation during the previous cooling. This led to much hardened austenite grains, which could bear more load partitioned and accommodate the greater deformation until they transformed to martensite during deformation than the non-baked austenite grains, thus greatly improving both strength and ductility. Moreover, the thickness of oxidization layer on the invented steel is less than 3 μm after the hot forming at 750−810°C, much thinner than the 100 μm-thick oxidization layer on 22MnB5. Such a tremendously reduced oxidation results from both the lower soaking temperature employed for the hot forming and the formation of dense Cr/Al/Si oxide band at the bottom of the oxidation layer, the latter is because the designed steel contains much higher contents of Cr and Al than 22MnB5.