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Austenite reversion treatment (ART) is usually used to obtain retained austenite (RA) in medium Mn steel (MMS). In this contribution, we proposed a flash-annealing strategy to produce MMS, which allows us to efficiently obtain a large amount of RA in MMS via the “explosive” nucleation of austenite and the fast C partitioning. Furthermore, flash-annealing can also render a heterogeneous ferritic matrix consisting of recrystallized/non-recrystallized ferrite grains, leading to heterogeneous deformation-induced hardening. As compared to the conventional ART processed MMS, the unique heterogeneous microstructure in the flash-annealed MMS leads to a higher yield and ultimate tensile strength but a marginal ductility loss. Flash-annealing provides an alternative route to tailor microstructure and optimize the mechanical performance of steels.

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Porous W with high porosity of 75.7 % was achieved by selective dissolution using Fe as active elements. The results showed that W-Fe pseudo alloy precursor was sintered at 800°C, the nano C powder helped in-situ formation of the W₆Fe₆C phase and the interface of W-W and W-Fe were both combined by W₆Fe₆C. It enabled the precursor to inherit the morphology of the green compacts without deformation. W-W connection remained and formed W framework after the selective dissolution of Fe. With the increasing of heat treatment temperature, the porosity decreased and the compressive strength increased, the change of the pore size is very little due to the effect of the W₆Fe₆C in the W framework.

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Abnormal nodular ferrite (NF) with vanadium carbide (VC) interphase precipitation was observed in a vanadium-microalloyed low carbon steel. The NF is characterized as a structure with a high density of low-angle boundaries, having an irrational ferrite/austenite orientation relationship (OR) with angular deviation of 15-30 ° from the exact Kurdjumov-Sachs OR with its neighboring austenite grain into which it grows. The growth of NF is substantially faster than that of normal grain boundary ferrite in the same alloy. Even in one single NF, the formation of multiple variants of VC interphase precipitation was confirmed, each of which was bordered by low-angle boundaries.

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The TiC dispersed strengthened Mo alloys were synthesized by novel freeze-drying technology and subsequent low-temperature sintering. With the introduction of TiC nanoparticles using freeze-drying technology, the sintered Mo-TiC alloys possess ultrafine grain size (2.38 μm), high density (99.3%) and high hardness (402±29 HV_0.2). The agglomeration and growth of second phase particles in Mo-TiC alloys were effectively depressed by freeze-drying technology. Moreover, a theory about purification and strengthening is proposed. C-deficient TiC (Ti₈C₅) will adsorb nearby oxygen impurities and even form TiO₂ to purify Mo matrix. Furthermore, TiC (002), Ti₈C₅ (0-24) and TiO₂ (111) can form coherent interfaces with Mo (110) to strengthen their phase boundary and improve material strength. The stable phase interface can pin and limit the growth of these second phase particles, thus maintaining their small size (<50 nm).

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In this study, a new single-phase bcc Ti35Zr35Hf15Nb5Ta5Sn5 high-entropy alloy (HEA) was developed and investigated by means of cyclic tensile tests, transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD) and in situ synchrotron X-ray diffraction (SXRD). The alloy reveals a superelastic behavior due to the reversible stress-induced martensitic transformation that occurs between the β phase and the α'' phase. A high strain recovery of 3.8% is obtained by tensile test, which is attributed to the <110>β{447}β recrystallization texture observed.

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Strain rate sensitivity and activation volume that dictate the kinetics of plastic deformation along with strain hardening response determine the plastic deformation behaviour of novel high entropy alloys. Although, the deformation behaviour of FCC single-phase Cantor alloy exhibits some similarity to that of conventional FCC solid solutions, the deformation behaviour over a wide range of temperature, strain rate, strain and grain size is unexplored. More importantly, theoretical understanding of solid solution strengthening, stacking fault energy and dislocation activation in 100 percent solute high entropy alloys (HEAs) and their flow kinetics is not yet established. This necessitates development of combined experimental and computational approaches to realise the mechanistic design of new single-phase and multi-phase transformative FCC HEAs to achieve optimum combination of strength and ductility in the novel complex concentrated alloys.

Vol. 198, 1 June. 2021, 113827

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