Relationship between Rockwell hardness and pearlite lamellar spacing of hardened layer under different cooling rates for U75 V 60 kg m−1 heavy rail

In this study, the heat treatment processes for U75 V heavy rail were studied systematically. Further, a simple and convenient method for measuring pearlite average lamellar spacing in steel by electron probe analysis is introduced, which can make the results closer to the real pearlite average lamellar spacing. This method was used to measure the pearlite average lamellar spacing at different positions of rail head under different heat treatments. Furthermore, the relationship between pearlite average lamellar spacing and Rockwell hardness of U75 V rail steel was determined. This study will provide important theoretical guidance for the formulation of rail heat treatment processes and the measurement of pearlite average lamellar spacing in steel.     

Densification behavior and attrition resistance of porous Al2O3 beads prepared by dripping-ice templating

Porous Al₂O₃ ceramic beads with a small-scale lamellar pore structure were prepared by the dripping-ice templating method. The densification behavior of lamellar walls and the attrition resistance of porous Al₂O₃ beads were investigated. For the two-dimensional structural lamellar walls, the intrinsic pores shrunk, and the necking area grew during the sintering process, leading to the densification of lamellar walls and the shrinking of porous beads. In addition, the two-dimensional lamellar walls and three-dimensional Al₂O₃ bulk are compared and possibilities of elaborating of the discrepancy in grain size and pore morphology are discussed. Attrition resistance was determined by a grinding test. The dominant attrition mechanism of porous Al₂O₃ beads is abrasion. High sintering temperature led to a decrease in the attrition rate due to a wider necking area and denser pore walls.     

β单相区固溶温度对Ti-55531合金组织及性能的影响

采用OM、SEM及拉伸测试方法,结合系列的热处理工艺,研究了β单相区不同的固溶温度对Ti-55531合金片层组织参数及力学性能的影响规律。结果表明,β单相区不同温度固溶,再经相同的时效处理后,合金的组织均为片层状的次生α_s、残余β片和微量晶界α组成的片层组织,但不同温度固溶后合金的β晶粒尺寸改变,进而影响时效析出次生α_s片的含量及尺寸,最终导致合金力学性能的差异。固溶温度在830~900 ℃之间时,随固溶温度的升高,原始β晶粒尺寸增大,后续时效析出的次生α_s长、宽及长宽比均先增大后减小,数量越来越多,合金强度直线下降,塑性先降低后增加。固溶温度为860℃时合金对应的强塑性匹配最好。合金的断裂失效机制为以微孔聚集为主,沿晶开裂和穿晶断裂并存的混合断裂机制。     

Optimization,formation, and evolution of the photoinduced curing gradients and in-situ lamellar gaps in additive manufacturing of ZrO2 ceramics: From curing to sintering behaviors

The photoinduced curing gradients and in-situ lamellar gaps were related to the three-dimensional curing mechanism of stereolithography and researched in this work. Based on the photocuring mathematic theories presented by our team, photoinduced curing gradients were influenced by printing parameters such as laser power and frequency, scanning angle, and printing thickness. Among these curing gradients, the inadequate curing area was the main factor affecting the surface quality and physical properties of ceramics. The in-situ lamellar gaps were derived from the inadequate curing area. The lamellar gap size of green bodies was enlarged in brown bodies (sintering at 1170 ℃) and shrunk in sintered bodies (sintering at 1500 ℃). The correlations between the surface quality and printing parameters were clarified by establishing mathematical formulations of the lamellar gap size. Furthermore, anisotropic volume shrinkage, anisotropic surface topography, and density and hardness evolutions were associated with the wrapped particle density differences induced by the photoinduced curing gradients. Finally, the optimal printing parameters (75% of laser power, laser frequency of 150 kHz, scanning angle of 90º, printing thickness of 25 µm, and scanning speed of 2.5 m/s) were obtained by the orthogonal experimental analysis. This study provides a distinct understanding of the high-quality forming and extreme-performance manufacturing of stereolithography.     

Role of scanning speed on the microstructure and mechanical properties of additively manufactured Al2O3‒ZrO2

Melt-grown Al₂O₃–ZrO₂ eutectic (AZ eutectic) ceramics have attracted extensive attention for harsh environment applications. In this work, AZ eutectic ceramic is additively manufactured via one-step laser powder bed fusion (LPBF). The role of scanning speed on phase formation, crystallographic characteristics, microstructure evolution, and mechanical properties were systematically investigated. The as-fabricated specimens are mainly composed of α-Al₂O₃ and t-ZrO₂. Lower scanning speeds induced the formation of cellular structures consisting of randomly oriented ZrO₂. In contrast, nanometer eutectic lamellar structure with well-defined multiple crystallographic orientation relationships, for example, {10$\overline{1}$0} Al₂O₃ || {100} ZrO₂ and {0001} Al₂O₃ || {001} ZrO₂, occurred at higher scanning speeds. Both the cell size and lamellar spacing decreased with increasing scanning speed. With the microstructure refinement, the crack propagation mode changes from intergranular to transgranular fracture, leading to progressively enhanced fracture toughness with a maximum value of 7.76 MPa·m^1/2. The present work could shed light on tailoring the microstructure of LPBF AZ eutectic ceramic via varying processing parameters.     

高热稳定性α-氧化铁的宏量制备及其电化学性能研究

氧化铁(Fe2O3)是一种重要的n型半导体材料,被广泛应用于染料、废水处理、光催化和锂离子电池等领域。采用水热法合成了不同直径大小的片状结构的α-氧化铁,其中大尺寸的片状α-氧化铁在1000℃仍能保持原有的表观颜色和形态,证明了其具有高热稳定性,在油漆、染料等领域具有较大的应用潜力。研究了氢氧化钠与三氯化铁溶液浓度及其混合顺序对α-氧化铁材料性能的影响,并且分析了片状α-氧化铁的带隙、锂离子电池性能及粉体表观颜色与颗粒尺寸的依赖关系。结果表明,通过调整氢氧化钠溶液的浓度和氢氧化钠与三氯化铁的滴加顺序可以得到不同尺寸的片状α-氧化铁,α-氧化铁的颜色随着其颗粒尺寸的增大而加深,带隙随着颗粒尺寸的减小呈现上升趋势,并且纳米级颗粒相对于微米级颗粒会提高锂离子电池的实际容量。该研究有助于研发α-氧化铁的宏量制备工艺及发掘其在电化学、陶瓷釉料、颜料等方面的应用,对降低传统能源活动的碳排放、推动中国早日实现“双碳”的国家目标具有重要的意义。     

Ti-22Al-25Nb合金板条组织高温高周疲劳行为

研究了双尺寸板条组织的Ti-22Al-25Nb合金在650 ℃和700 ℃下的高周疲劳行为，采用升降法测试了合金的高温高周疲劳强度极限，当应力比R=-1，循环周次Nf=107次时，650 ℃和700 ℃的疲劳强度极限分别为470 MPa和400 MPa。对于双板条组织的Ti-22Al-25Nb合金，其疲劳裂纹既可萌生于试样表面，也可萌生于次表面，并且高周疲劳裂纹在次表面形核的试样具有更高的疲劳寿命。此外，研究发现双尺寸板条组织在高温高周疲劳损伤过程中以胞状析出的形式发生B2→β+O相变，形成组织中的不均匀区域，促使疲劳裂纹在此优先形核。     

Robust Full-Spectral Color Tuning of Photonic Colloids

Creation of color through photonic morphologies manufactured by molecular self-assembly is a promising approach, but the complexity and lack of robustness of the fabrication processes have limited their technical exploitation. Here, it is shown that photonic spheres with full-color tuning across the entire visible spectrum can be readily and reliably achieved by the emulsification of solutions containing a block copolymer (BCP) and two swelling additives. Solvent diffusion out of the emulsion droplets gives rise to 20–150 µm-sized spheres with an onion-like lamellar morphology. Controlling the lamellar thickness by differential swelling with the two additives enables color tuning of the Bragg interference-based reflection band across the entire visible spectrum. By studying five different systems, a set of important principles for manufacturing photonic colloids is established. Two swelling additives are required, one of which must exhibit strong interactions with one of the BCP blocks. The additives should be chosen to enhance the dielectric contrast, and the formation kinetics of the spheres must be sufficiently slow to enable the emergence of the photonic morphology. The proposed approach is versatile and robust and allows the scalable production of photonic pigments with possible future applications in inks for cosmetics and arts, coatings, and displays.     

Impact-Resistant Hydrogels by Harnessing 2D Hierarchical Structures

With the strengthening capacity through harnessing multi-length-scale structural hierarchy, synthetic hydrogels hold tremendous promise as a low-cost and abundant material for applications demanding unprecedented mechanical robustness. However, integrating high impact resistance and high water content, yet superior softness, in a single hydrogel material still remains a grand challenge. Here, a simple, yet effective, strategy involving bidirectional freeze-casting and compression-annealing is reported, leading to a hierarchically structured hydrogel material. Rational engineering of the distinct 2D lamellar structures, well-defined nanocrystalline domains and robust interfacial interaction among the lamellae, synergistically contributes to a record-high ballistic energy absorption capability (i.e., 2.1 kJ m⁻¹), without sacrificing their high water content (i.e., 85 wt%) and superior softness. Together with its low-cost and extraordinary energy dissipation capacity, the hydrogel materials present a durable alternative to conventional hydrogel materials for armor-like protection circumstances.     

预变形TA15合金脉冲电处理球化行为研究

开展了预变形片层组织TA15合金电脉冲处理实验，对片层组织形貌及位错、界面结构进行表征，深入分析预变形片层组织TA15合金在脉冲电流作用下的微观组织演变机理。结果表明，电脉冲促进TA15合金中的原子扩散和缺陷反应，从而使片层组织发生电致静态球化；且随着电流密度增大和通电时间的延长，片层组织球化率增大，平均晶粒尺寸先增大后减小。α相球化前期由末端迁移机制控制，后期受晶界分离和Ostwald熟化控制，而β相球化的主要机制是晶界分离。电脉冲处理后球化α相的新生界面强化导致材料硬度最大提升26.41%。