Band 115, Heft 6

In this study, the effects of Mo additions on the microstructure, hardness and bending strength of TiC–Fe cermets manufactured by spark plasma sintering were investigated by scanning electron microscopy, X-ray diffraction and mechanical properties tests. Thermodynamic calculations were utilized to analyze the evolution of the microstructure. The results show that the conventional core/rim structure was not formed during the solid-phase sintering of TiC–Fe cermets. The heat-treatment process and reasonable Mo addition can effectively enhance the overall performance of the alloys. With the addition of minor Mo, the hardness of the sintered cermets improved significantly, which was attributed to the solid solution strengthening mechanism of Mo in Fe binder. However, excessive Mo exceeded the solubility range of Fe binder and precipitated in the form of M 6 C and Mo 2 C phases, which damaged the bending strength of the heat-treated cermets.

Due to their special structure and many excellent properties, porous Si 3 N 4 ceramics have a wide range of potential applications as a new structural and functional integrated material. In the present study, porous Si 3 N 4 ceramics with excellent bending properties and satisfactory porosity were prepared by the gel-casting method. The variations in porosity, bending strength, fracture morphology and phase composition of the porous Si 3 N 4 ceramics with different weight fractions of La 2 O 3 –MgO sintering additive were investigated. The results showed that the density of porous Si 3 N 4 ceramics increased linearly with the increase of sintering additive content. When the content of sintering additive increased from 2.5 wt.% to 10 wt.%, the porosity of porous ceramics decreased from 30 % to 10 %. With the increase of sintering additive content, the bending strength of the samples first increased and then decreased. When the sintering additive content was 5.0 wt.%, the average bending strength of the samples reached a maximum of 240.88 MPa, and scanning electron microscopy and X-ray diffractometry results showed that β-Si 3 N 4 particles with well-developed pores and a maximum aspect ratio of 5.8 were formed in the porous silicon nitride ceramics. This study provides a reference for the preparation of porous Si 3 N 4 ceramics with good bending properties and porosity.

Surface hardening techniques of steel are of great practical interest for applications in various industrial sectors. Boriding is one of the most economical choices. However, the chief deterrent to the widespread application of the technique is the difficulty in attaining a thick, dense boride layer. To solve this problem, a paste boriding process was performed on the surface of AISI 410 steel by introducing 6 wt.% CeO 2 addition into the boriding agent. The microstructure of the boride layer is comprised of (Fe, Cr)B and (Fe, Cr) 2 B, which lie in the external and internal layers of the boride layer, respectively. CeO 2 addition makes it possible to prepare a thick, dense boride layer on the surface of the steel substrate, thereby improving both wear and corrosion resistance of the steel. The catalysis mechanism of the rare earth element Ce can be ascribed to three aspects. First, CeO 2 addition can take part in the chemical reactions involved in the boriding process to produce more active boron atoms. Second, Ce can facilitate the adsorption of active boron atoms onto the surface of the steel through preventing the formation of iron oxides on the steel’s surface. Third, Ce can diffuse into the surface of the steel and generate severe lattice distortion due to large atomic size, thereby promoting the boron diffusion. These results provide a high-quality, low-cost pathway for the surface hardening of steel in practical industrial applications.

Double compression tests were performed on vanadium–titanium microalloyed steels with different nitrogen contents by using a Gleeble-3800 thermo-mechanical simulator to study the softening behaviors of the deformed austenite during different time intervals between the two passes. The static recrystallization fractions were calculated by the stress offset method and static recrystallization diagrams for the tested steels were obtained. The effects of deformation temperature and interval time on the softening behaviors were analyzed. Especially, the effect of nitrogen on the softening behaviors of the tested steels is discussed in detail. The results showed that the softening behaviors of the tested steels with various nitrogen contents are different. As far as the steel with low nitrogen content is concerned, the softening fraction increases monotonically with increasing time interval, and higher temperature can promote the static recrystallization. However, with more nitrogen added into vanadium–titanium microalloyed steel, precipitated particles of vanadium titanium carbonitride can be observed in the tested steel at the temperature of 850 °C or 800 °C, which leads to the formation of plateaus on the softening curves. An increase in nitrogen content in the steel is favorable for vanadium titanium carbonitride precipitation, which leads to a stronger prohibition of static recrystallization and a longer plateau on the softening curves. Moreover, the precipitated particles in the tested steel will not play an inhibition role in static recrystallization until the nitrogen content in the steel reaches a critical value.

The effects of addition of Er and Zr on the aging precipitation phase and mechanical properties in cast Al–Cu–Mn–Si–Mg alloy were studied. The addition of Er and Zr can refine the as-cast grains. The average grain size of as-cast alloy containing Er and Zr is 140.60 μm, while the average grain size of as-cast alloy without Er and Zr is 168.54 μm. After T6 heat treatment, the yield strength of the alloy containing Er and Zr reached 334.3 MPa, while the yield strength of the alloy without Er and Zr was only 284.3 MPa. After aging the strengthening precipitates of both alloys contain θ′ phase, Q′ phase and T phase. In the alloys containing Er and Zr the Al 3 (Er, Zr) phase with L 12 structure precipitated during the solution process, which reduced the diameter of the θ′ phase during subsequent aging. After T6 heat treatment, the precipitation strengthening contributed 67.09 % and 61.31 % to the total strength of the alloy with Er, Zr and without Er, Zr, respectively.

The phase relations of the R 2 Fe 14 B–Ce 2 Fe 14 B ( R = Nd, Pr) pseudobinary systems have been investigated by using X-ray powder diffraction, scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and differential thermal analysis. The X-ray powder diffraction results show that continuous solid solutions were formed in this R 2 Fe 14 B–Ce 2 Fe 14 B ( R = Nd, Pr) pseudobinary systems, and all ( R , Ce) 2 Fe 14 B solid solutions belong to the tetragonal structure with space group P 4 2 / mnm . With the increase in Ce content, the lattice parameters a , c , the peritectic reaction temperatures of the ( R , Ce) 2 Fe 14 B ( R = Nd, Pr) solid solutions decrease linearly, and the cell volume V varies in the form of the regression line. Based on the X-ray powder diffraction results and differential thermal analysis data, the quasi-binary phase diagrams of the R 2 Fe 14 B–Ce 2 Fe 14 B ( R = Nd, Pr) systems have been established.

Zn 2 SiO 4 :Gd 3+ powder was obtained by the sol–gel preparation technique and then characterized by using Fourier-transform infrared spectroscopy and X-ray diffraction techniques. The sample exhibited photoluminescence in the ultraviolet spectral zone with a maximum emission wavelength of 314 nm. Ultraviolet emission in the range of 305–320 nm is also called ultraviolet-B emission. This UV range emission is used in lamps for skin treatment and phototherapy. The electron paramagnetic resonance spectrum recorded for the Zn 2 SiO 4 :0.015Gd 3+ phosphor revealed g values of 3.46, 2.64, and 2.20, confirming the presence of Gd 3+ ions in the small ligand field. Furthermore, Gd 3+ ions attain tetrahedral symmetry in the prepared host compound.

In this study, the translaminar fracture toughness of carbon fiber laminated composites with different layer sequences was investigated experimentally and numerically for different crack directions. In the numerical study, first of all, the critical stress intensity factor was determined by using the M -integral method. Three-dimensional model and M -integral analysis were achieved in the ANSYS finite element package program. The non-local stress fracture criterion was used to in order to find failure curves of the materials. Then, in order to find the crack propagation directions numerically, the solid model was transferred to the LS-DYNA program and progressive failure analysis was performed. Fracture toughness decreased by 9.92 % with the change of crack angle from 15° to 90°. As the fiber angle changed from 0° to 45°, it decreased by 9.17 %. The biggest error between the experimental and numerical study results was found at α = 45°, with a rate of 12.3 %.

In recent years, the risk of blast attacks on underwater tunnels, important transportation routes, has increased due to the growing prevalence of vicious regional conflicts and global terrorist activities. In this work, we investigate the blast impact behavior of underwater tunnels filled with honeycomb core and covered by solid panels. The core is composed of 3D auxetic materials with high energy absorption compared to conventional honeycombs. In order to improve the stiffness of the auxetic structures, a pair of crossed rods is introduce to each cell. The relative densities of 3D auxetic structures are derived theoretically. The coupling effects of the geometrical parameters on the relative density are investigated. Then the deformation patterns of the underwater tunnels at different blast heights are analyzed. The kinetic energy and absorbed energy are discussed for tunnels with 3D auxetic materials and solid materials. Results show that tunnels composed with 3D reinforced auxetic structures can absorb much more energy than solid ones. Moreover, localized damage is observed which means greater chance of survival and smaller repairs after extreme impact. Finally, a stiffness-improved 3D reinforced auxetic structure is presented to enhance the tunnel’s strength and stability further.