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The study of the effect of pressure and temperature on the structural and mechanical properties of materials is very difficult for current experimental techniques. Here, we have synthesized the γ-Nb5Si3 compound with remarkable quality by high pressure and high temperature (HPHT) technology. The effects of high pressure and sintering temperature on its microstructure, mechanical properties, and oxidation resistance are discussed. The Vickers hardness and density of the γ-Nb5Si3 sample at 5.5 GPa and 1100 °C are 8.67 GPa and 5.73 g/cm3, respectively. It is shown that the HPHT behavior not only improves sample's compactness and microstructure, but also makes γ-Nb5Si3 to have a strong response to the Vickers hardness and related density. These results effectively demonstrate the functions of the HPHT technology, which accurately express the Vickers hardness, density, and thermal stability of γ-Nb5Si3 compounds. [ABSTRACT FROM AUTHOR]

In this paper, the mechanical properties, penetration characteristics, and deformation behaviors of FeNiCoCr high-entropy alloys (HEAs) annealed at different temperatures were investigated. The quasi-static and dynamic compression tests were conducted by a universal testing machine and Split Hopkinson Pressure Bar (SHPB) system, respectively. Furthermore, the penetration experiments of long rod projectiles (LRPs) of FeNiCoCr HEAs into semi-infinite steel targets launched by a small caliber ballistic gun with velocities ranging from 650 m/s to 1500 m/s were carried out, in comparison with that of steel LRPs. The microstructures of recovered projectiles were observed by the X-ray diffraction, optical microscope, and transmission electron microscope, which were used to further analyze the deformation behaviors of FeNiCoCr HEAs. The results showed that FeNiCoCr HEAs owned a single face-centered cubic structure. Annealing twins and grain refinement were detected, affecting the yield strength of the alloys. The strength of the alloy annealed at 600 °C was the highest and then decreased with the increasing annealing temperature. At higher strain rates, special adiabatic shear characteristics occurred in the alloys, while twins dominated the whole deformation process of the projectiles. Compared to other annealed alloys, the alloy annealed at 600 °C performed the best penetration performance. While the impact velocity had significant effects on the penetration performance of the alloy annealed at 1000 °C, the penetration performance of the alloy annealed at 850 °C on steel targets was almost the same as that of the steel LRPs. [ABSTRACT FROM AUTHOR]

Al/Ni composites are typical structural energetic materials, which have dual functions of structural and energetic characteristics. In order to investigate the influence of manufacturing methods on shock-induced chemical reaction (SICR) behaviour of Al/Ni composites, Al/Ni multi-layered composites with 3-5 cold-rolling passes and Al/Ni powder composites were obtained. Microstructural observation using scanning electron microscopy (SEM) and two-step impact initiation experiments were performed on the four Al/Ni composites. Furthermore, mesoscale simulations, through importing SEM images into the finite element analysis to reflect the real microstructures of the composites, were performed to analyse the particle deformation and temperature rise under shock compression conditions. The experimental results showed the distinct differences on the SICR characteristics among the four Al/Ni composites (i.e. by 3, 4 and 5 cold-rolling passes and powder compaction). The manufacturing methods provided the control of the particle sizes, particle distribution and the content of the interfacial intermetallics at scale of different microstructures, which ultimately affected the temperature distribution, as well as the contact between Al and Ni in Al/Ni composites under shock loading. As a result, the Al/Ni powder composites showed the highest energy release capacity among the four composites, while the energy release capability of Al/Ni multi-layered composites decreased with the growth of rolling passes. [ABSTRACT FROM AUTHOR]

The synthesis of high-quality silicide ceramic has been challenging mainly due to the high melting point of the silicide and the difficulty in achieving a uniform composition. This study reports the synthesis of dense and homogeneous niobium disilicon (NbSi 2) ceramics, accomplished with high-temperature and high-pressure (HTHP) technology. The synthesized bulk NbSi 2 ceramics were extensively characterized for their microstructures, phase stabilities, and oxidation resistance, along with the effects of pressure on their physical properties, and the results were then compared with first-principles calculations. The microstructures and relative densities of the sample were regulated by the slow-modulated HTHP sintering method, thus significantly influencing the physical properties and thermal stabilities of the NbSi 2 ceramics. The findings of the study provide valuable insights into the physical properties of silicide ceramics, th potentially paving the way for the further design and synthesis of multifunctional ceramic materials. [Display omitted] • The study synthesizes the dense (98.13%) niobium disilicon ceramics by high temperature and high pressure (HTHP) technology. • The TGA-DSC results verified that high pressure sintering at high temperature (1100 ° C) can effectively promote grain bonding and thus optimize the oxidation resistance. • From the atomic and electronic points of view, the interaction between Nb-Si atoms can indicate the physical properties of NbSi 2. [ABSTRACT FROM AUTHOR]

In this study, using MoSi 2 powder as starting material, the bulk MoSi 2 samples were sintered at 1000–1600 °C under 5.5 GPa in a DS6 × 14MN cubic press. Furthermore, their sintering behavior and mechanical properties were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM) and indentation tests. XRD results elucidate that MoSi 2 can still maintain its body-centered-tetragonal structure after the high pressure and high temperature (HPHT) sintering. SEM images imply the combination between grain growths and re-crystallization. Strong covalent bonding between the grains is crucial for improving the mechanical properties of samples. But re-crystallization results in a large number of pores between grains in the sintered body, so that the density and hardness was reduced. Indentation tests indicate that sintered MoSi 2 have hardness values of 13.6–15.0 GPa, which increase with increasing temperature and is harder than polycrystalline and single crystal MoSi 2 . The results demonstrate that the sintered MoSi 2 at 5.5 GPa/1300 °C possesses outstanding mechanical properties, including a high relative density ( 6.23 g / cm 3 ), Vickers hardness (15.0 GPa) and fracture toughness (10.7 MPa m 1/2 ) at applied load of 29.4 N. In addition, the oxidation resistance of the prepared samples is measured using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis. The results show that the oxidation resistance has been significantly improved from this work compared with polycrystalline MoSi 2 . [ABSTRACT FROM AUTHOR]

Abstract A series of pure HfB 2 ceramics have been prepared by sintering nano-grained powder using high-energy ball milling at 700–1600 °C and 5.5 GPa. The HfB 2 ceramics are characterized via various techniques for their residual stress, grain size, density, microstructures and defects, hardness, fracture toughness, thermal stability, and oxidation resistance. All properties strongly depend on the treatment temperature, but the exact manner of dependence for each property varies. The results identified that the HfB 2 ceramic sintered at a relatively low temperature of 1000 °C and 5.5 GPa – a bulk pure nano-grained composite for the first time – has the best overall performance. It has a relative density of 99.6%, a Vickers hardness of 26 GPa, a fracture toughness of 5.2 MPa m1/2, and excellent thermal stability and oxidation resistance at high temperatures. Additional strengthening and stabilizing effects are provided by microstructures and defects such as large-angle grain boundaries, stacking faults and twinning. Simultaneous high temperature and high pressure is an effective sintering route for HfB 2 ceramics with grain-size ranging from nanometer to micron. [ABSTRACT FROM AUTHOR]