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Additive manufacturing of an equimolar CoCrNi medium entropy alloy (MEA) by selective laser melting (SLM) was investigated, emphasizing its microstructure, properties, and metallurgical defects. It was found that SLM sample density exhibited a non-monotonic relation with their volume energy density (VED); the density first increased but then decreased while the input VED was gradually increasing. A maximal relative density of 98.9 pct was accessible at a VED of 83.3 J/mm3. X-ray diffraction indicated that the printed CoCrNi MEA exhibited an FCC single-crystal structure where the lattice constant decreased with the increasing VED owing to the evaporation of the Cr element in the SLM process. The average grain size gradually increased with increasing VED, irrespective of whether viewed from the top or the side of the printed part. Similarly, the residual stress increased with increasing VED, which produced more microcracks and deteriorated the tensile preformation of the printed samples, although the yield strength (630 MPa) showed no apparent difference at different VEDs. Owing to the ultrafine cell structure, the mechanical property of the SLM printed sample was twice that of cast or wrought CoCrNi MEA.

Dense boron carbide bearings with a central through hole were fabricated by hot-press sintering. When the sintering temperature was above the critical grain growth temperature, an increase in the applied pressure at 2100 °C caused a distinct abnormal grain size region. Two grain growth mechanisms were explored; normal grain growth (NGG) is controlled by a grain boundary diffusion mechanism whereas the abnormal grain growth (AGG) pattern abides by the ‘Bose–Einstein’ grain growth law.

The interdiffusion kinetics of Cu-Ni system and the influence of porous structure on atomic diffusion kinetics during spark plasma sintering (SPS) were investigated. The interdiffusion coefficient of the Cu-Ni system annealed under SPS is much higher than that without current when the temperature exceeds 700 °C. By comparing the interdiffusion behaviors between Ni powder/Cu foil interface and Ni foil/Cu foil interface, the interdiffusion rate at the foil/powder interface was found to be significantly higher than that at the foil/foil interface during SPS. The diffusion process at the Ni powder/Cu foil interface shows two clearly identified stages: a high diffusion rate at the initial stage with high porosity and a slower diffusion rate at the mid-late stage with low porosity. The diffusion coefficient at the initial stage is nearly 5.2 times higher than that of the mid-late stage, demonstrating that the diffusion rate at the foil/powder interface decreases with the gradual reduction of porosity during the SPS densification process. The porous structure leads to an extremely high local current density at the neck area, which results in a high density of crystal defects at the diffusion interface and subsequently accelerates atomic diffusion.

Spark plasma sintering (SPS), as a novel powder consolidation method, has great potential in the production of high entropy alloys (HEAs). The heating rate is an important parameter in SPS, and understanding its effect is a critical issue for the purpose to optimize microstructure and promote mechanical properties of HEAs. The AlCoCrFeNi was adopted as a representative HEA in this study. The phase compositions, microstructural textures, and mechanical properties were investigated in this study in detail. It was found that the FCC phase formed at BCC grain boundaries while the σ phase precipitated at the interface of B2/BCC no matter of the heating rate. The texture varied at different heating rates as the texture {110} in B2/BCC was characterized at the heating rate of 150 °C/min while the complex texture {051} in FCC was observed at the heating rate of 200 °C/min. The Vickers hardness and wear resistance decreased with increasing of the heating rate. Furthermore, the compressive strength and corresponding plastic strain reached their highest 2752.0 ± 48 MPa and 26.5 ± 1.3% at the heating rate of 150 °C/min.

Whether selective laser melting (SLM) printed AlMgScZrMn alloy possesses more excellent performances than other processing routes was unknown. In the study, the microstructures and mechanical properties of AlMgScZrMn alloys fabricated by selective laser melting (SLM), spark plasma sintering (SPS) and cast were comparatively investigated. SLM printed sample shows columnar and equiaxed duplex grains with average grain size about 7 μm, which is coarser than SPS consolidated sample with finest equiaxed grain (4 μm) but smaller than casting sample with coarse equiaxed grain (25 μm). SLM printed sample shows the highest tensile strength (394 ± 5 MPa) and elongation (10.5 ± 1.2%) than SPS sample (231 ± 3 MPa, 0.59 ± 0.21%) and cast sample (188 ± 2.3 MPa, 3.7 ± 0.58%), although SPS sample has the finest equiaxed grains. For SPS sample, previous particle boundary (PPB) with oxide inclusion significantly deteriorates ductility. For cast sample, the coarse equiaxed grain with intergranular eutectic leads to inferior strength. In addition, the average dimension of Al3(Sc,Zr) particle in SLM sample is about 1–10 nm, which is much finer than SPS sample (0.5–0.8 μm) and cast sample (1–2 μm). The present study will provide a basis for the future industrial applications of the AlMgScZrMn alloy.

A densification creep model was proposed to analyze the densification mechanism involved in spark plasma sintering (SPS) of boron carbide containing low-melting-point sintering aid. This model incorporated the effect of the sintering aid on relative density as well as the actual internal stress during SPS of boron carbide ceramics. The internal stress bias caused by sintering aid was considered and a more accurate effective stress was used to investigate the kinetics during SPS. The proposed model explains why an increase of sintering aid amount decreases the sintering activation energy and promotes densification.

The traditional theory attributes the rapid densification of spark plasma sintering (SPS) to the thermal effect of electric current. However, the nonthermal effects of electric current are still not well understood, despite its proven significance. In this study, the mechanisms by which nonthermal effects contribute to the rapid densification of tungsten powder during SPS were investigated. Electromigration was considered as an important nonthermal effect of current, and an electromigration-enhanced creep model incorporating the contribution of temperature, pressure, and electric current on densification was constructed. The kinetics model agreed well with the experimental data and was applied to evaluate the contribution of electromigration during SPS quantitatively. Although the contribution of electric current is relatively minor compared with the temperature and pressure, it is a driving force to be reckoned with in densification.

Equiatomic CrMnFeCoNi high entropy alloy (HEA) was firstly used as sintering aid for preparation of B4C ceramics by spark plasma sintering (SPS) at temperature ranging from 1500 to 2100 °C. The mechanical properties of B4C-5 vol.% HEA ceramics as well as densification mechanisms during the SPS process were investigated. B4C-5 vol.% HEA ceramic sintered at an optimized low temperature of 1950 °C obtained an attractive combination of properties, including a relative density of 99.27%, a microhardness of 3427 Hv0.2 and a rupture strength of 619 MPa, which was 37% higher than the highest strength of pure B4C ceramic. The densification mechanism during the SPS was elucidated by the stress exponent n and the apparent activation energy Qd. At low effective compaction stresses (n = 1, Qd = 519.5 ± 82.6 kJ/mol) in the temperature range of 1700–1950 °C, the densification was controlled by grain-boundary sliding. At high effective compaction stresses (n around 2.5 and 5.5), the densification mechanism is transformed to dislocation climb, which was verified by transmission electron microscopy.

The effect of electric current on the atomic diffusion of W-Ti system using spark plasma sintering (SPS) was investigated. The interdiffusion coefficient of W-Ti system during SPS was observed to be higher compared to that in the traditional sintering. The electromigration effect was observed in W-Ti system and an improved kinetics model was applied to reveal the role of electric current in atomic diffusion kinetics. It was found that the interdiffusion coefficient increased with increasing current density, while the activation energies were constant. This phenomenon is mainly because the atomic moving velocity is accelerated by the electron wind force in the direction of electron flow, but the diffusion mechanism remains unchanged, resulting in the invariability of activation energy of binary substitution-diffusion. Additionally, the interdiffusion coefficient is independent of the pulse pattern but strongly relies on the effective current density, implying the main acceleration mechanism of pulse current remains to be electromigration.

Although laser solid forming (LSF) has a wide range of applications in material manufacturing, the technique has been severely limited due to the solidification cracks during rapid laser melting/solidification. In this paper, an LSF technology assisted by friction stir processing (FSP) is proposed to eliminate the LSF crack through the FSP thermomechanical coupling effect, with the Ni–16Cr–8Fe alloy as a representative material. By FSP-assisted LSF, the cracks at the top surface of the Ni–16Cr–8Fe alloy layer were eliminated. Meanwhile, the severe plastic deformation layers can be observed and gradient grains with tens of nanometers to tens of micrometers from the top surface to the inner. The LSF-printed dendrite microstructure was transformed into nanocrystals or nanotwins in the deformation zone with twining as the main deformation mechanism. The grain refinement results in the increase of hardness about 30%.

Theoretical understanding in the innate function of electric current on the spark plasma sintering of amorphous powder is very limited. We proposed an improved isothermal model considering particle size evolution in this work to study the effect of current mode on shrinkage of amorphous Fe 41 Co 7 Cr 15 Mo 14 Y 2 C 15 B 6 powder. It revealed a reduction in the activation energy from 71.1 ± 1.6 kJ·mol −1 under direct current to 55 ± 2.0 kJ·mol −1 under pulse current, representing a resultant acceleration of densification by adjusting current mode. The enhancement of consolidation is attributed to increase of electric density induced by pulse current.

Micron-sized boron carbide (B 4 C) powders were subjected to spark plasma sintering (SPS) under temperature ranging from 1700 °C to 2100 °C for a soaking time of 5, 10 and 20 min and their densification kinetics was determined using a creep deformation model. The densification mechanism was interpreted on the basis of the stress exponent n and the apparent activation energy Q d from Harrenius plots. Results showed that within the temperature range 1700–2000 °C, creep deformation which was controlled by grain-boundary sliding or by interface reaction contributed to the densification mechanism at low effective stress regime ( n = 2, Q d = 459.36 kJ/mol). While at temperature higher than 2000 °C or at high stress regime, the dominant mechanism appears to be the dislocation climb ( n = 6.11).

Although laser deposition has found its widespread applications in materials manufacture, this technology has been significantly limited by the associated metallurgical defects, such as pores, brittle phase precipitation along grain boundary during the laser melting/solidification process. In this paper, we used a novel laser deposition technology assisted by friction stir processing (FSP) to eliminate these common metallurgical defects. A Fe–Cr–Si alloy was selected as a representative material. Severe plastic deformation caused by FSP has refined the coarse solidification microstructure into ultrafine grained or even nano-grained microstructure on the top surface of the Fe-Cr-Si samples. Meanwhile, the large and interconnected Fe-Cr intermetallics precipitates were broken down to nanoparticles. The refinement of both primary grains and intermetallic particles results in an increase of microhardness from 640 HV to above 700 HV and a decrease of wear rate from 4.8 × 10−14 to 3.2 × 10−14 mm3/m.

The diffusion behavior of Al in FeCoCrNiMox high-entropy alloys (HEAs) at temperatures ranging from 773 to 873 K with and without pulse current was studied in this paper. The diffusion coefficients D was determined based on the Fick’s law and corresponding pre-exponential factor D 0 , as well as activation energy Q D . It was found that the D under pulse current dramatically increased about hundreds of times in contrast with that without current. In addition, the D 0 and Q D were determined to reduce from 1.369 - 0.324 + 0.424 × 10 - 13 to 5.016 - 1.984 + 3.284 × 10 - 15 m2/s, and from 39.7 ± 1.8 to 21.6 ± 3.4 kJ/mol respectively with the Mo contents increasing from Mo0 to Mo0.2 without pulse current. The introduction of pulse current into the diffusion couples dramatically promoted the D 0 at all the diffusion couples. However, the pulse current was ascertained to obviously increase the Q D in contrast to that of diffusion couples without pulse current.

Although laser cladding has find its widespread application in surface hardening, this technology has been significantly limited by the solidification crack, which usually initiates along grain boundary due to the brittle precipitation in grain boundary and networks formation during the laser rapid melting/solidification process. This paper proposed a novel laser cladding technology assisted by friction stir processing (FSP) to eliminate the usual metallurgical defects by the thermomechanical coupling effect of FSP with the Ni-Cr-Fe as representative coating material. By the FSP assisted laser cladding, the crack in laser cladding Ni-Cr-Fe coating was eliminated and the coarse networks of laser cladding coating was transformed into dispersed nanoparticles. Moreover, the plastic layers with thicknesses 47–140 μm can be observed, with gradient grain refinement from substrate to the top surface in which grain size reached 300 nm and laser photocoagulation net second phase crushed in the layer. In addition, cracks closed in the plastic zone. The refinement of grain resulted the hardness increased to over 400 HV, much higher than the 300 HV of the laser cladding structure. After FSP, the friction coefficient decreased from 0.6167 to 0.5645 which promoted the wear resistance.

Rapid densification associated with spark plasma sintering has been well documented; however theoretical understanding is still very limited. In this work we proposed a densification creep model to explicitly elucidate the effect of current density on densification kinetics, by taking electromigration into account. This new model was then validated experimentally. It is found that an increased current density decreases densification activation energy (energy barrier), thus accelerating powder densification. In essence, the rapid densification is caused by direct current that accelerates mass transport and increases dislocation mobility.

Al0.3CoCrFeNi high-entropy alloy (HEA) was additively manufactured by laser melting deposition (LMD) with emphasis on its microstructure and tensile properties. The LMD as-built HEA features a fiber texture aligned toward the build direction. The dislocation loops are observed with a high density of dislocations neighboring to them in the LMDed samples. The partials dislocation glides induced by thermal stress leading to disordered-ordered transformation during LMD. Besides, the precipitated B2 particles are induced by thermal stress during cooling or thermal exposure during remelting. The cracks easily initiate and propagate along with the phase boundaries between the matrix FCC and precipitated B2 phases. Such cracks damage the tensile properties resulting in poor ductility in the tensile tests. Subsequent annealing facilitate the further precipitation of the B2 and σ phase, which introduces hardening and leading to the increase of the Vickers hardness and wear resistance of the annealed samples. Nevertheless, the annealing induces dislocation recovery, which reduces the required critical shearing stress for plastic deformation. As a consequence, the yield stress decreases from 476.9 ± 6.4–373.5 ± 6.8 MPa after annealing.

The densification kinetics of boron carbide (B4C) during multi-stage spark plasma sintering was studied. The densification mechanisms were analyzed according to the stress exponent n and the apparent activation energy Qd using a creep deformation model. The results showed that the densification mechanisms were controlled by viscous flow and grain boundary diffusion at the low effective stress with initial temperature range of 1600–2000 °C, while the dominant mechanism is the dislocation climb at the effective stress regime with final temperature of 2100 °C and the multi-stage sintering can reduce the apparent activation energy. Meanwhile, the scheme of multi-stage sintering can obtain nearly theoretical dense B4C and avoid grain growth. Therefore, the basic mechanical properties suggesting a good combination of high hardness (37.63GPa) and bending strength (539.86 MPa) was obtained by the multi-stage sintering.

The strong electric current in spark plasma sintering (SPS) often gives sintered materials unique microstructures. In the present study, it is found that the Co3W grains formed at the W/Co bonding interface preferentially grow along [2 1 ¯ 1 ¯0] (or its equivalent orientations) in the current direction during SPS, which is significantly different from that without current. We propose that the resistance anisotropy of grains under electromigration leads to anisotropic atomic diffusion, inducing the preferential growth of grains under current. This work may provide an approach and a theoretical foundation for the preparation of materials exhibiting directional growth.

As Unmanned Aerial Vehicle (UAV) play a progressively important role in both military and civilian affairs, the requirements on its dependability and validity are higher and higher. In order to meet the application requirements of the UAV downlink communication system, we did the theoretical research and analysis based on the basic principles of transmission and reception of differential frequency hopping (DFH) communication systems. In this paper, for a case where the sampling frequency, hopping speed, frequency hopping bandwidth, and frequency hopping points is constant, a theoretical study about the performance analysis of the differential hopping communication systems over Rician fading channels with different fan-out coefficients, full-band jamming, partial-band jamming, and comb spectrum jamming is confirmed. On the whole, the larger the fan-out coefficient, the stronger the anti-jamming comprehensiveness of the differential frequency hopping communication system.

Al0.7CoCrFeNi high-entropy alloy (HEA), as a promising material, has attracted growing interest. This study aimed to investigate the effect of phase transformation in non-equilibrium Al0.7CoCrFeNi HEA on its properties and densification kinetics during spark plasma sintering (SPS). We found that the BCC-to-FCC phase transformation causes a grain size reduction from 5.66 to 3.10 μm, while decreases the Vickers hardness from 372.5 ± 4 to 328.4 ± 3 HV. The decreased Vickers hardness reduces the wear resistance with the volume wear rate increasing from 1.583 × 10−4 to 2.108 × 10−4 mm3·N−1·m−1. Besides, the BCC-to-FCC phase transformation improves the ductility from 2.5 ± 0.4 to 8.4 ± 0.3%. The further precipitation of a θ phase observed in the sample sintered at 1100 °C improves the ultimate tensile strength from 856.1 ± 13 to 1053.7 ± 8 MPa, although the ductility decreases from 8.4 ± 0.3 to 7.3 ± 0.3%. Interestingly, we also observed that the BCC-to-FCC phase transformation reduces densification kinetics, because it decreases the diffusion coefficient.

For improving the capacity of the communication, the OFDM technology is used widely. The anti-jamming performances of OFDM system are analysed with the full-band interference, partial-band interference and comb spectrum interference under the rice channel. With the same parameters that the sampling frequency, the number of IFFT points, the cyclic prefix, and so on, when the numbers of subcarriers are 128, 256, and 512, respectively, the anti-jamming performances of OFDM system are simulated. The result of simulation shows that the anti-interference performance is obviously superior when the number of subcarriers is 128.

For recognizing the MIMO DUSTC signals without a priori knowledge, a blind recognition method is presented. By estimating the number of transmitting antennas, separating the mixed signals, and recognizing modulation, the unitary matrices S is estimated by statistics and DUSTC signals are recognized. The simulation results showed that the method was effectively to recognize DUSTC signals with the SNR no less than 4dB.