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The face-centered cubic medium-entropy alloy NiCoCr has received considerable attention for its good mechanical properties, uncertain stacking fault energy, etc, some of which have been attributed to chemical short-range order (SRO). Here, we examine the yield strength and misfit volumes of NiCoCr to determine whether SRO has measurably influenced mechanical properties. Polycrystalline strengths show no systematic trend with different processing conditions. Measured misfit volumes in NiCoCr are consistent with those in random binaries. Yield strength prediction of a random NiCoCr alloy matches well with experiments. Finally, we show that standard spin-polarized density functional theory (DFT) calculations of misfit volumes are not accurate for NiCoCr. This implies that DFT may be inaccurate for other subtle structural quantities such as atom-atom bond distance so that caution is required in drawing conclusions about NiCoCr based on DFT. These findings all lead to the conclusion that, under typical processing conditions, SRO in NiCoCr is either negligible or has no systematic measurable effect on strength. Chemical short-range order (SRO) NiCoCr has been proposed to account for its positive stacking fault energy and good mechanical properties. Here, a combination of theory and experiment shows that SRO is of negligible importance in NiCoCr processed by standard methods.

In polycrystalline materials, grain boundaries are known to be a critical microstructural component controlling material’s mechanical properties, and their characters such as misorientation and crystallographic boundary planes would also influence the dislocation dynamics. Nevertheless, many of generally used mechanistic models for deformation twin nucleation in fcc metal do not take considerable care of the role of grain boundary characters. Here, we experimentally reveal that deformation twin nucleation occurs at an annealing twin (Σ3{111}) boundary in a high-Mn austenitic steel when dislocation pile-up at Σ3{111} boundary produced a local stress exceeding the twining stress, while no obvious local stress concentration was required at relatively high-energy grain boundaries such as Σ21 or Σ31. A periodic contrast reversal associated with a sequential stacking faults emission from Σ3{111} boundary was observed by in-situ transmission electron microscopy (TEM) deformation experiments, proving the successive layer-by-layer stacking fault emission was the deformation twin nucleation mechanism, different from the previously reported observations in the high-Mn steels. Since this is also true for the observed high Σ-value boundaries in this study, our observation demonstrates the practical importance of taking grain boundary characters into account to understand the deformation twin nucleation mechanism besides well-known factors such as stacking fault energy and grain size.

Interstitial oxygen embrittles titanium, particularly at cryogenic temperatures, which necessitates a stringent control of oxygen content in fabricating titanium and its alloys. Here, we propose a structural strategy, via grain refinement, to alleviate this problem. Compared to a coarse-grained counterpart that is extremely brittle at 77 K, the uniform elongation of an ultrafine-grained (UFG) microstructure (grain size ~ 2.0 µm) in Ti-0.3wt.%O is successfully increased by an order of magnitude, maintaining an ultrahigh yield strength inherent to the UFG microstructure. This unique strength-ductility synergy in UFG Ti-0.3wt.%O is achieved via the combined effects of diluted grain boundary segregation of oxygen that helps to improve the grain boundary cohesive energy and enhanced dislocation activities that contribute to the excellent strain hardening ability. The present strategy will not only boost the potential applications of high strength Ti-O alloys at low temperatures, but can also be applied to other alloy systems, where interstitial solution hardening results into an undesirable loss of ductility. Oxygen has long been considered as a detrimental impurity in pure titanium since it can severely deteriorate the ductility. Here, the authors propose a simple, yet effective strategy via grain refinement to solve this long-standing issue, while preserving its potential hardening effect.

Tensile mechanical properties of fully recrystallized TWIP steel specimens having various grain sizes ( d ) ranging from 0.79 μm to 85.6 μm were investigated. It was confirmed that the UFG specimens having the mean grain sizes of 1.5 μm or smaller abnormally showed discontinuous yielding characterized by a clear yield-drop while the specimens having grain sizes larger than 2.4 μm showed normal continuous yielding. In-situ synchrotron radiation XRD showed dislocation density around yield-drop in the UFG specimen quickly increased. ECCI observations revealed the nucleation of deformation twins and stacking faults from grain boundaries in the UFG specimen around yielding. Although it had been conventionally reported that the grain refinement suppresses deformation twinning in FCC metals and alloys, the number density of deformation twins in the 0.79 μm grain-sized specimen was much higher than that in the specimens with grain sizes of 4.5 μm and 15.4 μm. The unusual change of yielding behavior from continuous to discontinuous manner by grain refinement could be understood on the basis of limited number of free dislocations in each ultrafine grain. The results indicated that the scarcity of free dislocations in the recrystallized UFG specimens changed the deformation and twinning mechanisms in the TWIP steel.

We observe that a nanostructured metal can be hardened by annealing and softened when subsequently deformed, which is in contrast to the typical behavior of a metal. Microstructural investigation points to an effect of the structural scale on fundamental mechanisms of dislocation-dislocation and dislocation-interface reactions, such that heat treatment reduces the generation and interaction of dislocations, leading to an increase in strength and a reduction in ductility. A subsequent deformation step may restore the dislocation structure and facilitate the yielding process when the metal is stressed. As a consequence, the strength decreases and the ductility increases. These observations suggest that for materials such as the nanostructured aluminum studied here, deformation should be used as an optimizing procedure instead of annealing.

Some of ultrafine-grained (UFG) metals including UFG twinning induced plasticity (TWIP) steels have been found to overcome the paradox of strength and ductility in metals benefiting from their unique deformation modes. Here, this study provides insights into the atomistic process of deformation twin nucleation at Σ3{111} twin boundaries, the dominant type of grain boundary in this UFG high manganese TWIP steel. In response to the applied tensile stresses, grain boundary sliding takes place which changes the structure of coherent Σ3{111} twin boundary from atomistically smooth to partly defective. High resolution transmission electron microscopy demonstrates that the formation of disconnection on Σ3{111} twin boundaries is associated with the motion of Shockley partial dislocations on the boundaries. The twin boundary disconnections act as preferential nucleation sites for deformation twin that is a characteristic difference from the coarse-grained counterpart, and is likely correlated with the lethargy of grain interior dislocation activities, frequently seen in UFG metals. The deformation twin nucleation behavior will be discussed based on in-situ TEM deformation experiments and nanoscale strain distribution analyses results.

Hydrogen-related fracture behaviors in low-carbon (Fe-0.1wtpctC) and medium-carbon (Fe-0.4wtpctC) martensitic steels were characterized through crystallographic orientation analysis using electron backscattering diffraction. The martensitic steels with lower strength (Fe-0.1C specimen or Fe-0.4C specimen tempered at higher temperature) exhibited transgranular fracture, where fractured surfaces consisted of dimples and quasi-cleavage patterns. Crystallographic orientation analysis revealed that several of the micro-cracks that formed around the prior austenite grain boundaries propagated along {011} planes. In contrast, fracture surface morphologies of the martensitic steels with higher strength (Fe-0.4C specimen tempered at lower temperature) appeared to be intergranular-like. Crystallographic orientation analysis demonstrated that, on a microscopic level, the fracture surfaces comprised the facets parallel to {011} planes. These results suggest that the hydrogen-related fractures in martensitic steels with higher strength are not exactly intergranular at the prior austenite grain boundaries, but they are transgranular fractures propagated along {011} planes close to the prior austenite grain boundaries. A description of the mechanism of hydrogen-related fracture is proposed based on the results.

Scanning electron microscope/electron back-scattering diffraction was used to investigate local misorientation development within an individual α plate of a Ti-6Al-4V alloy with an α lamellar microstructure during hot deformation at 1223 K (950 °C) and a strain rate of 0.1 s −1 . The correlation between the local misorientation development and the globularization behavior of α plates during subsequent annealing at 1223 K (950 °C) was discussed. The misorientation profile along an individual α plate showed that not only a continuous and smooth change in orientation but also a discontinuous change in orientation was developed by the hot deformation. We assume that the points where discontinuous change in orientation occurs, P d , became α / α boundaries and resulted in splitting α plates the annealing. The mean length between adjacent discontinuous points, L I , was determined and compared with the actual mean length of the α plates after hot deformation and subsequent annealing, L a , as measured by optical microscopy. The two kinds of length parameters coincided at lower strains, but significant differences were observed at higher strains, i.e ., L I was larger than L a . Further analysis showed that rotation axes (R.A.s) changed even within regions where orientation changes were continuous. By taking into account the points where the R.A.s changed, P r and P d , the mean length between adjacent points, L II , appeared to coincide with L a at higher strains. A higher lattice distortion is expected near points P r at higher strains, which results in the formation of new α / α boundaries in subsequent annealing. Consequently, points P d already developed by hot deformation were considered to become α / α boundaries and led to splitting α plates in annealing. New α / α boundaries formed at points P r in subsequent annealing after a higher strain deformation, which led to a splitting of α plates as well.

Hydrogen embrittlement behaviors of a 22Mn–0.6C (mass%) twinning induced plasticity (TWIP) steel with the grain sizes of 21 μm (coarse grain) and 0.58 μm (ultrafine grain) were investigated by means of hydrogen precharging and subsequent slow strain rate tensile tests. The total elongation and fracture stress for both of the coarse-grained and ultrafine-grained specimens decreased by hydrogen charging. The area fraction of the brittle fracture surfaces in the ultrafine-grained specimen was much smaller than that in the coarse-grained specimen. Three-point bending test also showed that the reduction of the fracture toughness by the introduction of hydrogen was much smaller in the ultrafine-grained specimen than that in the coarse-grained specimen. It was concluded that the suppressed hydrogen embrittlement by grain refinement in the 22Mn–0.6C TWIP steel was probably due to the smaller hydrogen contents per unit grain boundary area in the finer grain-sized material.

Medium-entropy alloys (MEAs) have recently emerged as potential replacements for traditional alloys in high-performance applications. Among the various MEAs developed, equi-atomic CoCrNi MEA stands out for its excellent mechanical performance. Nevertheless, the yield strength of this MEA having a single-phase FCC structure is not exceptionally high. This study suggests a novel approach to strengthen the MEA by adding a small quantity of Zirconium (0.2 at.%). A (CoCrNi) 99.8 Zr 0.2 MEA was cast and homogenized, alongside a CoCrNi MEA for comparison purposes. The alloys underwent high-pressure torsion followed by annealing in a temperature range of 700–1100 °C. In the Zr-doped MEA, annealing resulted in the recrystallization within the FCC matrix, accompanied by the precipitation of finely dispersed particles (Ni 7 Zr 2 ). The concurrent precipitation hindered the grain growth, maintaining refined grain sizes (0.17 – 4 µm) over a wide range of temperatures. In contrast, the grain size in the Zr-free counterpart increased rapidly above 10 µm at elevated temperatures. The Zr doping substantially increased the yield strength with a minor negative impact on the ductility. The strength enhancement was due to grain refinement, precipitation, and dislocation locking caused by the Zr segregation at dislocation sites. These phenomena and their impacts on the Hall–Petch relationship were investigated and discussed. This study proves micro-alloying as a promising strategy for strengthening MEAs, making these alloys superior to their traditional counterparts. Graphical abstract