Materials are typically ductile at higher temperatures and become brittle at lower temperatures. In contrast to the typical ductile-to-brittle transition behavior of body-centered cubic (bcc) steels, we observed an inverse temperature dependence of toughness in an ultrahigh-strength bcc steel with an ultrafine elongated ferrite grain structure that was processed by a thermomechanical treatment without the addition of a large amount of an alloying element. The enhanced toughness is attributed to a delamination that was a result of crack branching on the aligned {100} cleavage planes in the bundles of the ultrafine elongated ferrite grains strengthened by nanometer-sized carbides. In the temperature range from 60 degrees to -60 degrees C, the yield strength was greater, leading to the enhancement of the toughness.

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3D Mxene based gas sensors demonstrated a highly sensitive detection for VOCs in an ultra-wide sensing range at room temperature.

The effects of grain boundary (GB) character and impurity contamination on the recombination activity of grain boundaries (GBs) in multicrystalline silicon (mc-Si) were systematically studied through an electron-beam-induced current (EBIC) technique. First, clean GBs of various characters were checked at 300 and 100K. The EBIC contrasts of these GBs were in the same range of 0%–2% at 300K and 2%–4% at 100K, suggesting that the recombination activity of clean GBs is weak and the GB character has no significant effect on it. Second, the effect of impurities was studied by comparing the EBIC contrasts of the same type of the GBs in mc-Si with different Fe contamination levels. The recombination activity of GBs became stronger as the contamination level rose. The variation in the recombination activity related to the GB character was also observed in these specimens. The random or high-Σ GBs showed a stronger EBIC contrast than the low-Σ GBs. Moreover, we found that the EBIC contrast was not uniform along one Σ3 GB. In a clean mc-Si, the variation in the EBIC contrast of different parts of the Σ3 GB was 1% at 300K, whereas it was more than 10% in the contaminated mc-Si. This indicates that not only the GB character but also the GB plane would affect the recombination activity of GBs. These results suggest that the recombination activity of GBs is principally determined by the gettering ability of GBs. Specifically, that random GBs may have the strongest gettering ability, whereas Σ3 {111} has the weakest.

Bulk ultrafine-grained (UFG) low-carbon steel bars were produced by caliber rolling, and the impact and tensile properties were investigated. Initial samples with two different microstructures, ferrite-pearlite and martensite (or bainite), were prepared and then caliber rolling was conducted at 500°C. The microstructures in the rolled bars consisted of an elongated UFG structure with a strong a-fiber texture. The rolled bar consisting of spheroidal cementite particles that distributed uniformly in the elongated ferrite matrix of transverse grain sizes 0.8 to 1.0 lm exhibited the best strength-ductility balance and impact properties. Although the yield strength in the rolled bar increased 2.4 times by grain refinement, the upper-shelf energy did not change, and its value was maintained from 100°C to À40°C. In the rolled bars, cracks during an impact test branched parallel to the longitudinal direction of the test samples as temperatures decreased. Delamination caused by such crack branching appeared, remarkably, near the ductile-to-brittle transition temperature (DBTT). The effect of delamination on the impact properties was associated with crack propagation on the basis of the microstructural features in the rolled bars. In conclusion, the strength-toughness balance is improved by refining crystal grains and controlling their shape and orientation; in addition, delamination effectively enhances the low-temperature toughness.

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A highly flexible and stretchable network-structured MXene/polyurethane mat demonstrated ultra-high sensitivity and an ultra-wide sensing range for strain sensing.

To analyze the good toughness of ultrafine ferrite/cementite steels, the concept of effective grain size (d EFF ) is applied to ductile-to-brittle transition temperature, DBTT, for ultrafine ferrite/cementite (Uf-F/C), ferrite/pearlite (F/P), quenched (Q), and quench-and-tempered (QT) microstructures in a low carbon steel. The d EFF is determined to be 8, 20, 100, and 25 mm for Uf-F/C, F/P, Q, and QT, respectively. In F/P and Q, it is in accordance with the ferrite grain size and the prior austenite grain size, respectively. In QT, the d EFF fits the martensite packet size. In Uf-F/C, the ferrite grain size has a bimodal distribution and the larger grain size corresponds to the d EFF , which is the smallest among the four microstructures. In terms of the relationship between d EFF and DBTT, the Uf-F/C, Q, and QT microstructures can be placed into the same group and the F/P to a different one. Furthermore, the Uf-F/C has the highest estimated fracture stress among the four microstructures. These might be the result of the difference in the surface energy of fracture, namely the former is estimated to have a surface energy of 34.6 J/m 2 and latter a surface energy of 7.7 J/m 2 . Thus, the excellent toughness of the ultrafine ferrite/cementite steel can be attributed to the small d EFF and the high surface energy of fracture.KEY WORDS: effective grain size; ductile-to-brittle transition temperature; toughness; fracture strength; low carbon steel; ultrafine grain structure. Fig. 1. Relationship between the change in DBTT and the change of 9.8 MPa in yield strength for different microstructural factors, i.e. dislocation strengthening, dispersion strengthening, and grain-size strengthening.