Effect of temperature on the structure and mechanical properties of SiC–TiB2 composite ceramics by solid-phase spark plasma sintering

SiC composite ceramics have good mechanical properties. In this study, the effect of temperature on the microstructure and mechanical properties of SiC–TiB₂ composite ceramics by solid-phase spark plasma sintering (SPS) was investigated. SiC–TiB₂ composite ceramics were prepared by SPS method with graphite powder as sintering additive and kept at 1700 °C, 1750 °C, 1800 °C and 50 MPa for 10min.The experimental results show that the proper TiB₂ addition can obviously increase the mechanical properties of SiC–TiB₂ composite ceramics. Higher sintering temperature results in the aggregation and growth of second-phase TiB₂ grains, which decreases the mechanical properties of SiC–TiB₂ composite ceramics. Good mechanical properties were obtained at 1750 °C, with a density of 97.3%, Vickers hardness of 26.68 GPa, bending strength of 380 MPa and fracture toughness of 5.16 MPa m^1/2.     

Anisotropic thermal expansion and ionic conductivity of a crystal-oriented,Mg2+-conducting NASICON-type solid electrolyte

Multivalent ion-conducting ceramics are required for the manufacture of high-safety, high-capacity rechargeable batteries. However, the low ionic conductivity of solid electrolytes and discrepancies in the thermal expansion between the battery components limit their widespread application. Furthermore, anisotropic thermal expansion in crystals during battery manufacturing and the charge-discharge cycles causes the formation of microcracks, which degrade the battery performance. The physical properties of ceramic materials with anisotropic crystal structures can be modified by varying the crystallographic orientation of their grains. In this study, a co-precipitation approach was used to synthesize an Mg²⁺-conducting (Mg_0.1Hf_0.9)_4/3.8Nb(PO₄)₃ solid electrolyte, and the grain orientation in the bulk sample was controlled using strong magnetic fields during the slip casting process. The results showed that inducing an orientation along the c-axis enhanced the apparent ionic conductivity of the bulk sample. It was also observed that (Mg_0.1Hf_0.9)_4/3.8Nb(PO₄)₃ crystal has a negative volumetric thermal expansion despite a positive linear thermal expansion along its c-axis. By adjusting the c-axis orientation of the grains, (Mg_0.1Hf_0.9)_4/3.8Nb(PO₄)₃ electrolytes with negative or positive linear thermal expansion coefficient have been produced. The findings of this study suggest that solid-electrolytes with negative, positive, or zero linear thermal expansion can be produced to create more compatible and higher-performance solid-state devices.     

Corrosion behaviour of an industrial shot-peened and coated automotive spring steel AISI 9254

A high-strength low-alloy steel, AISI 9254 (54SiCr6), is widely used for suspension spring production in the automotive industry. In this work, industrially manufactured zinc phosphate coated helical springs are subjected to detailed microstructural and surface analysis for better understanding of corrosion evolution. The material’s free corrosion potential and anodic/cathodic behaviour were investigated in NaCl solutions and corrosion propagation mechanisms were studied using potentiostatic polarisation on cross-sectional and external surfaces. The bulk material is fully martensite with uniformly distributed MnS inclusions, while the spring surface has a 2–3?μm mechanically deformed region introduced by shot-peening and a thin zinc phosphate coating. The corrosion open circuit potential of bulk material and shot-peened spring surface was about –0.7V_SCE without significant difference, while phosphated surface is more noble (more positive potential). MnS inclusions, stimulating the anodic attack in the steel, influence corrosion propagation and pit morphology to a large extent that can have an impact on the spring performance.     

Investigation of structural,optical, electrical,and magnetic properties of Fe-doped La0.7Sr0.3MnO3 manganites

In this work, the physical properties of nanocrystalline samples of La_0.7Sr_0.3Mn_1−xFe_xO₃ (0.0 ≤ x ≤ 0.20) perovskite manganites synthesized by the reverse micelle (RM) technique were explored in detail. The phase purity, crystal structure, and crystallite size of the samples were determined using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. All the samples had rhombohedral crystal structure and crystallite size increased with increase in Fe content in La_0.7Sr_0.3MnO₃. The scanning electron micrographs (SEMs) exhibited smooth surface morphology and nonuniform shape of the particles. The optical properties studied using UV-visible absorption spectroscopy revealed a decrease in the absorbance and optical band gap with an increase in Fe content in La_0.7Sr_0.3MnO₃ compound. The temperature-dependent resistivity measurements revealed semiconducting nature of x = 0 and 0.1 samples up to the studied temperature range, while a metal-to-insulator transition was observed at higher Fe doping. Magnetic studies revealed weak ferromagnetism in all the samples and a reduction in the maximum magnetization with an increase in Fe content. A close correlation between electrical transport and magnetic properties was observed with the doping of Fe ion in La_0.7Sr_0.3MnO₃ at Mn site. These results advocate strong interactions associated with the double exchange mechanism among Fe³⁺ and Mn³⁺ ions.     

Viscoelastic Behavior of Synthesized Liquid Soaps and Surface Activity Properties of Surfactants

In the present work, a rheological study of liquid soaps prepared from different mixture of surfactants as a function of surfactant type and concentration was performed. The curves of shear stress vs. shear rate and viscosity vs. shear rate were recorded at constant temperature, 294 ± 0.1 K. The surface activity properties were also studied. The results of the study showed that values of surface tension, γ, were in the range 31–40 mN m⁻¹ and the critical micelle concentration (CMC), was of the order 10⁻⁴ mol L⁻¹. The calculated maximum surface excess, Γ_max, varied from 2.40 to 3.66 μmol m⁻², while minimum area per molecule, A_min, varied from 41.1 (for amphoterics) to 81.4 Ų (for nonionic surfactants). The standard free energy of micellization, −29.8 and −29.3 kJ mol⁻¹ for anionic and amphoteric surfactants, respectively, were while values for nonionic surfactants varied between −31.8 and − 30.3 kJ mol⁻¹. The free energy of adsorption, was the lowest for amphoteric surfactants (−37.9 kJ mol⁻¹), followed by anionics (−40.4 kJ mol⁻¹) and nonionics (−43.34 to −46.84 kJ mol⁻¹), indicating that micellization process is spontaneous in the examined medium. The synthetized liquid soaps show pseudoplastic behavior and they achieved pipe flow. The results of this research indicate that flow behavior was affected significantly by the ionic charge of the surfactant and the ionic strength of the formulation, suggesting that the flow behavior could be changed by manipulating the choice of the surfactant and salinity. The pH value of all liquid soaps examined were weakly acidic, in the range of 5.0–6.4.     

等温退火对8030铝合金导线组织性能的影响

研究了不同等温退火工艺对8030铝合金导线组织及性能的影响。结果表明:等温退火前后合金均由α-Al基体和Al₆Fe相组成。在同一等温温度下,随着等温时间的延长组织逐渐趋于均匀化;同一等温时间下,随着等温温度的升高,组织趋于均匀化的时间缩短。经过等温退火处理后铝合金导线的导电率均有所提高,在470 ℃均匀化退火24 h后再经240 ℃等温4 h,合金导电率达到最高值57.21%IACS,比未经热处理试样的导电率提高了2.4%IACS。经过等温退火处理后铝合金导线的硬度及抗拉强度均有所降低,塑性大幅度提高。在470 ℃均匀化退火24 h后再经260 ℃等温8 h,合金的伸长率最高可达23.64%。热处理前后合金均为塑性断裂。     

强变形工艺对2519A铝合金组织与性能的影响

通过硬度测试、拉伸性能测试、透射电镜观察等分析手段研究了不同强变形工艺下2519A铝合金的力学性能与微观组织。结果表明,经50%的冷轧变形和165 ℃人工时效后,2519A合金的力学性能明显提高,其抗拉强度、屈服强度和伸长率分别为522 MPa、468 MPa和8.5%。而在冷变形前添加165 ℃×2 h预时效处理,合金的力学性能进一步提高,其抗拉强度、屈服强度和伸长率分别达到535 MPa、497 MPa和8%。预时效处理可以提高合金中θ′相的密度,使析出相分布更加均匀,有助于提高合金的力学性能。     

退火工艺对激光选区熔化成形Ti6Al4V合金组织及室温力学性能的影响

以Ti6Al4V球形粉末为原料,利用激光选区熔化成形方法制备了Ti6Al4V合金试样,采用光学显微镜、扫描电镜及力学性能测试等手段,研究了退火工艺对Ti6Al4V合金室温力学性能及组织的影响规律。结果表明: SLM成形沉积态Ti6Al4V合金室温抗拉强度超过1200 MPa,而平均断后伸长率仅为4.0%;在650 ℃下进行真空退火处理,合金的抗拉强度仍保持在1200 MPa左右,规定塑性延伸强度R_p0.2高于1150 MPa,但试样的断后伸长率<10%;而在750及800 ℃下进行真空退火处理,合金试样的抗拉强度降至1100 MPa左右,规定塑性延伸强度高于1050 MPa,伸长率达到甚至超过10%,材料的综合强韧性得到明显提升。随着真空退火加热温度和保温时间的增加,SLM成形Ti6Al4V合金原始β晶界逐渐变模糊,晶粒趋向于等轴化。与此同时,快速冷却转变的α′针状马氏体未出现明显地粗化。     

氮含量对HPD-1双相不锈钢组织及力学性能的影响

通过显微组织观察与力学性能测试研究了氮含量(0.08%~0.22％,质量分数)对HPD-1双相不锈钢硬度、拉伸性能、低温冲击性能及疲劳性能的影响。结果表明,氮含量变化可显著影响试验钢γ/α相比例,当氮含量由0.08％升高到0.22％,γ相含量由38.1％提高至56.5％。α相的硬度由273 HV10提高到343 HV10,γ相的硬度由238 HV10提高到299 HV10,试验钢强度明显提升。氮元素对两相比例和奥氏体相韧性的双重影响导致试验钢低温冲击性能呈先上升后下降的趋势;更高的氮含量抑制疲劳裂纹萌生与拓展,是影响HPD-1双相不锈钢室温疲劳性能的主要因素。撕裂棱是疲劳断口的显著特征。     

退火温度对HA/Ti-24Nb-4Zr复合材料组织与性能的影响

利用放电等离子烧结(SPS)技术制备了HA/Ti-24Nb-4Zr生物复合材料,研究了不同退火温度对复合材料显微组织和力学性能(抗压强度、屈服强度、屈强比、压缩弹性模量)的影响。结果表明,烧结态复合材料主要由β-Ti相、少量初生α-Ti相及HA相组成;随着退火温度的升高,复合材料基体中β-Ti相含量增多且晶粒逐渐长大,针状次生α-Ti相在晶界处和晶内不断析出,HA相结构和含量变化不大;与烧结态相比,不同退火温度处理后的复合材料强度和弹性模量先略微上升后下降,而塑韧性呈不断提高趋势;复合材料在850 ℃退火处理后,抗压强度、屈服强度、屈强比和压缩弹性模量值分别为1507 MPa、1270 MPa、0.84和42 GPa,塑韧性得到明显改善,作为生物医用植入材料具有潜在的应用前景。