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Martensite is a supersaturated solid solution of carbon in body-centered iron wherein interstitial carbon atoms preferentially occupy a single octahedral sublattice. Despite a century of research, the mechanism of this long-range ordering is still a subject of debate. Recently, Zener’s theory of ordering was challenged both experimentally and theoretically. In an attempt to settle the controversy, we investigated by density functional theory the ground states of Fe-C configurations having various degrees of order. We conclude that the fully Zener-ordered configurations are always the most stable energetically, thus confirming Zener’s theory. Comparison with mean-field elasticity and Ising-type modelling supports the elastic origin of Zener ordering.

SCRIPTA

High ductility after quenching has significant benefits for ultra-high strength steels, especially from the perspective of formability. Intercritical annealing was applied prior to rapid quenching (flash) to obtain heterogeneous phase (HP) units, consisting of nano-twinned martensite and retained austenite. The uniform elongation was significantly enhanced without compromising ultimate tensile strength because of nanoscale twinned martensite and retained austenite. The formation of nanoscale twinned martensite and retained austenite occurred because of micro-segregation of alloying elements (Mn and Al) by combining annealing and flash process. Thermodynamic calculations enabled us to design the alloy and heat treatment.

SCRIPTA

This work quantified nonuniform creep deformation across the heterogeneous heat-affected zone (HAZ) of Grade 91 steel with sophisticated experiments, including an electric-thermal finite element model–assisted Gleeble thermomechanical simulation and a high-temperature creep testing with in situ digital image correlation (DIC). High temperature creep properties of HAZ sub-zones were quantitatively measured by the DIC. By utilizing peak temperature, hardness, local creep strain, and underlying microstructures, creep deformation mechanisms in HAZ were further understood. DIC measurements reveal a creep-vulnerable zone (CVZ) exposed to a peak temperature of 932°C (close to AC3) in the intercritical HAZ experienced the fastest creep strength degradation instead of the soft zone with the lowest hardness prior to creep. The significantly reduced precipitation strengthening from misplacement of undissolved and coarsened M23C6 carbides led to a faster recrystallization of tempered martensite in the CVZ. Weak untransformed tempered martensite (ferrite grains) stabilized by local Cr enrichment from dissolved M23C6 also harmed the CVZ's creep resistance.

SCRIPTA

Various multi-principle element alloys exhibit superior mechanical properties across a wide range of temperatures including cryogenic temperatures. In most systems, the remarkable properties are attributed to deformation twinning (twinning induced plasticity). The recent VFeCoCrNi multi-principle element alloy displays a combined twinning induced plasticity effect, at room-temperature, and phase transformation at low deformation temperatures (transformation induced plasticity). This work considers single crystalline specimens of VFeCoCrNi loaded along the [111] direction. At room-temperature, slip dominates at low strains, below 22% plastic strain, and a delayed activation of twinning is observed. At 77 K, early activation of transformation, face-center cubic to body-center cubic having {111}_FCC//{110}_BCC and ⟨110⟩_FCC//⟨111⟩_BCC parallel relations, was revealed.

SCRIPTA

Tensile creep tests were performed on a CrMnFeCoNi high-entropy alloy at temperatures from 1023 K to 1173 K. A uniform stress exponent 3.7 ± 0.1 was found across all temperatures. The apparent activation energies of creep under various applied stresses were determined to be around 230 kJ/mol and decrease with increasing stress, indicating a stress-assisted, thermally activated behavior. Steady-state creep microstructures feature no subgrain formation and high dislocation density within grains. Based on our results, the creep rate of CrMnFeCoNi is believed to be controlled by both dislocation-dislocation interactions and dislocation-lattice interactions.

SCRIPTA

In the present study, the effect of nitrogen alloying on hydrogen embrittlement in FeMnNiCoCr high-entropy alloys was investigated. In tension, hydrogen-free nitrogen-alloyed FeMnNiCoCrN alloy (0.37 wt.% N) demonstrated higher strength, strain hardening, and elongation-to-failure than the interstitial-free FeMnNiCoCr Cantor alloy. Despite the different tensile properties, both alloys fractured via a ductile dimple micromechanism. After hydrogen charging, the nitrogen-alloyed material demonstrated lower strain hardening and higher sensitivity to hydrogen-assisted embrittlement than the interstitial-free alloy. Both alloys featured a stable austenitic structure and similar grain size, yet, the nitrogen-alloyed FeMnNiCoCr alloy was more susceptible to hydrogen embrittlement. Although, the overall degradation effects appear similar, there are pronounced differences in mechanical behavior and hydrogen transport upon hydrogen charging when the high-entropy alloys are compared to conventional austenitic stainless steels, and the experiments reveales that nitrogen alloying enhances hydrogen diffusivity in the Cantor alloy.

SCRIPTA

In martensitic/bainitic bearing steels, non-observable critical cracks can initiate at inner inclusions well below the endurance limit (at 10⁷ cycles), leading to very high cycle fatigue failure. The stability of these cracks is not ensured even if they have a stress intensity factor lower than the threshold for long crack propagation, strongly compromising the safety of the mechanical component. Propagation of such cracks is reported to be preceded by the formation of a fine granular area (FGA) structure. Due to the high demanding characterization and testing experimental circumstances, the crystallography of FGA is far from understood. Recent advances have come hand by hand from an experimental procedure strategy which triggers the FGA formation at predicted crack initiation sites. The novel EBSD-dictionary indexing has proven to be a powerful tool from which FGA formation acquires new value and opens the door for a systematic microstructural characterization at different crack length values.

SCRIPTA

Faceted grain boundaries exhibit unusual segregation and migration tendencies. To gain a deeper understanding of how solute atoms interact with faceted interfacial structures during migration, this study probes the migration behavior of a faceted Σ11 boundary in Cu doped with Ag atoms. The solutes are found to segregate to the facet with more free volume and strongly reduce boundary velocity in one migration direction, but not the other, due to the presence of a directionally-dependent motion mechanism that can escape solute pinning and therefore speed up migration. Hence, a new mechanism of chemically-induced anisotropy in grain boundary mobility is uncovered by these simulations.

SCRIPTA

Element redistributions after initial oxidation of a Ni₃₈Cr₂₂Fe₂₀Mn₁₀Co₁₀ (at.%) multi-principle element alloy at 120 °C and 300 °C is captured via in situ atom probe tomography. All cations contribute to the oxide in both conditions, consisting of a Cr-rich inner oxide and a Fe, Mn, Co and Ni-rich outer oxide. At lower temperature, Ni tends to be trapped at the outer/inner oxide interface, while Ni enriches at the oxide/metal interface at higher temperature in tandem with Cr metal depletion. These observations confirm oxidation of complex alloys involves the formation of metastable, multi-layered oxide films, with a distinct tendency for solute trapping.

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We demonstrate the formation of partially recrystallized bimodal microstructures through single-step heat treatment after cold-rolling in Fe₆₀Co₁₅Ni₁₅Cr₁₀ (at%) ferrous medium-entropy alloy that leads to the enhanced yield-strength of ~1.1 GPa and ultimate-tensile-strength of 1.7 GPa at persistent ductility of 61%, evading strength-ductility trade-off. The retention of deformation substructures allows a yield-strength increase by ~79% compared to that in a fully-recrystallized state, together with the modification of mechanical stability without compositional variation, resulting in multi-stage strain hardening capability during tensile deformation at 77 K.

Vol. 194, 15 Mar. 2021, 113663

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