查看更多>>摘要：? 2022Structural stability of high entropy alloys (HEAs) at intermediate temperatures during which the configurational entropy loses its dominance is crucial. In this work, we have scrutinized the effects of cooling rates on the phase stability of the HfTaTiZr(Ta1) and HfNbTiZr (Nb1) refractory HEAs (RHEAs). Ta1 exhibited strong microstructure and tensile property dependence on the cooling route, but not Nb1. Ta1 retained the BCC lattice under water quenching but experienced complex phase decomposition upon air and furnace cooling. The different dependence on cooling rates in these RHEAs arises from three main reasons: (i) the wider metastable temperature range in Ta1, (ii) the larger mobility difference between Ta and other constituents, and (iii) the larger electronegativity difference of Nb1. The current work not only sheds new insights into the understanding of the thermal and mechanical stability of HEAs but also provides a new paradigm for the optimization of their overall properties.
查看更多>>摘要：? 2022 Acta Materialia Inc.The intrinsic shear strength (1.25 GPa) of In4Se3 remarkably lower than those of classic thermoelectric (TE) materials such as CoSb3 (7.17 GPa), PbTe (3.46 GPa), TiNiSn (10.52 GPa), which limits the commercial applications of In4Se3 based TE materials. To improve the shear strength of single-crystalline In4Se3, we used density functional theory to study the influence of point defects on the mechanical behavior of In4Se3 under the pure shear loading. We found that doping with Ca, Ag, Yb, Pb, Zn, I, and Br can improve the shear strength of In4Se3. In particular, Ca-doped In4Se3 obtained the highest shear strength (1.43 GPa), an increase of 14.4%. These point defects can improve the van der Waals interaction between In/Se layers significantly, hence enhancing the shear strength, while the slippage between the In/Se layers is still predominating its deformation and failure. Our work offers a possibility in strengthening layered materials with robust mechanical properties.
查看更多>>摘要：? 2022Flash sintering is a novel electric field and current assisted sintering technique which densify ceramics at moderate furnace temperature and in short time (few seconds) as compared to the conventional sintering techniques. The onset of flash encompasses three unique characteristics, (i) non-linear rise in conductivity, (ii) luminescence and (iii) rapid densification. the mechanism behind the flash phenomena is not fully understood yet. The present work experimentally demonstrated that the densification in flash sintering is primarily dominated by Joule heating. Herein, we distinguished the onset of flash and the degree of densification of cubic zirconia in sintering atmospheres with different oxygen partial pressures. In this study, we demonstrate that the flash onset temperature and densification are independently influenced by the sintering atmosphere during flash sintering. Furthermore, the extent of densification is not directly influenced by the current density rather, by the power dissipated in the sample.
查看更多>>摘要：? 2022 Acta Materialia Inc.This paper presents a method for refining the coarse lamellar colonies in α-solidified TiAl alloys using α-segregation. The prerequisite of this method is control of the solidification rate. Isothermal holding at upper and/or middle of α phase region and control of cooling rate from α to (α+γ) phase region is also necessary for better refinement. In addition, the formation of α-segregation and characteristics of the refined lamellar colonies are studied. The results show that the formation of α-segregation is mainly related to the change of the phase transformation path caused by Al-depletion. Moreover, α-segregation is composed of two kinds of β phases, which can be distinguished by orientation relationship with matrix colony. Although the coarse colonies are refined, the refined colonies exhibit the same orientation as the parent colonies. Besides, the degree of α-segregation and the type of β phase will affect the local characteristics of the refined lamellar colonies.
查看更多>>摘要：? 2022 Acta Materialia Inc.Significant benefits of either heterogeneous microstructures or deformation-induced martensitic transformation (DIMT) in metallic materials stem from their superior strain hardening and tensile properties. Herein, we present an unprecedented strain hardening behavior at 77 K in a ferrous medium-entropy alloy comprising a metastable face-centered cubic (FCC) bimodal microstructure with different grain sizes. Pre-straining yields fine body-centered cubic (BCC) martensites, and subsequent heat treatment causes reverse martensitic transformation from BCC to FCC, affording the bimodal FCC microstructure. Furthermore, the yield strength is enhanced due to the presence of submicron FCC grains and geometrically necessary dislocations (GNDs) that are generated during the pre-straining. Profuse GNDs in reversely transformed FCC promote DIMT. Moreover, when true strain exceeds 0.2, widespread DIMT in the interior of the remaining FCC drastically increases the strain hardening rate and consequently delays necking. The outstanding tensile properties derived from this thermomechanical process are because of the DIMT and hetero-deformation-induced strengthening.
查看更多>>摘要：? 2022 The AuthorsThis work provides unambiguous evidence for the occurrence of icosahedral quasicrystal (iQC) enhanced nucleation during selective laser melting of gas atomized commercially-pure Ni powders. This solidification mechanism, which has only been recently reported in a few alloys and has to date never been observed in pure metals, consists on the solidification of grains of the primary phase on the facets of iQCs formed due to the presence of icosahedral short range order in the liquid. The occurrence of iQC enhanced nucleation has been inferred from the observation in the SLM processed pure Ni samples of an excess fraction of partially incoherent twin boundaries and of clusters of twinned grain pairs sharing common 〈110〉 five-fold symmetry axes. This work further evidences that additive manufacturing methods may constitute an invaluable tool for investigating the fundamentals of solidification and for the design of unprecedented grain boundary networks.
查看更多>>摘要：? 2022Eutectic growth of Ni-Si alloy with a composition of Ni44Si56 was observed in situ during directional solidification. Lamellar structure was evident behind the solidifying interface, which indicates that the two intermetallic phases, NiSi and NiSi2, generated a regular eutectic. The lamellae spacing was approximately 9.8 μm at a growth rate of 149.6 μm s?1. The phase boundaries migrated rapidly during the cooling stage and the lamellae enlarged significantly, which suggests that the lamellae spacing measured after the solidification may not be accurate for obtaining the relationship between the growth rate and the lamellae spacing. The in situ observation provides a promising way to obtain the exact eutectic spacing of Ni-Si eutectics.
查看更多>>摘要：? 2022 Acta Materialia Inc.Developing high-performance lead-free ceramics for capacitive applications has become more and more critical owing to the increasing concern about environmental protection and electronics industry. Here, a dual defect manipulation strategy was proposed to boost energy storage performance in CuO-modifided Sr2NaNb5O15-based lead-free tungsten bronze relaxor ceramic. Induced defect dipoles and reduced oxygen vacancy concentration generate a pinched polarization hysteresis loop and high breakdown strength, thus giving rise to a high recoverable energy density of 4.17 J/cm3 and efficiency of 89.5% at 350 kV/cm, together with an ultrafast discharge speed of 55 ns and a high power density of 118.4 MW/cm3. This proposed strategy is generalizable for realizing high capacitive performance in tungsten bronze-based dielectrics and other lead-free ceramics that benefit from dual defect manipulation.
查看更多>>摘要：? 2022Optical materials have been extensively used in thermometers and anti-countering for their non-contact, advantageous rapid, and high precision properties. Herein, a double-perovskite Ca2LaNbO6:Pr3+ color-tunable phosphor was synthesized. The emissions (blue, green, red1, and red2) from Ca2LaNbO6:Pr3+ phosphor have different temperature-dependent properties because of the thermal quenching channel generated by the intervalence charge transfer state between Pr3+ and Nb5+ ions. They can be used to realize four fluorescence intensity ratio models (Green/Blue, Green/Red2, Red1/Blue and Red1/Red2) to measure temperature with high absolute and relative sensitivities. Moreover, using the Ca2LaNbO6:Pr3+ phosphor and the carbon dots with green afterglow to create anti-counterfeiting patterns, the multi-mode anti-counterfeiting under different portable lamps (254 nm and 365 nm) was realized. Therefore, the prepared multifunctional phosphor indicated a potential application in both optical temperature sensing and anti-counterfeiting simultaneously.
查看更多>>摘要：? 2022 Acta Materialia Inc.The feasibility was investigated of fabricating super wear-resistant hard B4C composites by spark plasma sintering (SPS) at ultra-low temperature (i.e., 1400 °C) using very high proportions of MoSi2 aids (i.e., 35–50 vol.%). It is shown that with 40 vol.% MoSi2 aids already sufficient Si transient liquid phase is formed in situ during SPS (by the reaction B4C+2MoSi2→SiC+2MoB2+3Si) to achieve the ultrafast full densification of B4C at 1400 °C, the Si melting point, simply by pore filling, particle rearrangement, and liquid spreading. Importantly, it is also shown that the resulting B4C composites are hard (i.e., ≈23 GPa) and super wear-resistant (i.e., ～107 (N?m)/mm3), attributes both deriving from the composites' quadruplex-particulate (i.e., B4C plus β-SiC, β-MoB2, and MoSi2), fine-grained (i.e., <1 μm), fully-dense microstructure. Thus, this work opens a new avenue for the present and future lower-cost fabrication of novel B4C composites for use in contact-mechanical and tribological applications.
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