In-situ Synthesis and Characterization of Ceramic Reinforced Inconel 718 Coatings Using B4C-Inconel 718 Powders by Laser Directed Laser Energy Deposition

Inconel 718 has been widely used in aerospace, nuclear and marine industries due to excellent high-temperature mechanical properties and corrosion resistance. In recent years, laser directed energy deposition (DED) become a competitive method in the fabrication of Inconel 718 coatings. Compared with other surface coating processes, laser DED has the advantage of extremely fine-grained structures, strong metallurgical bonding, and high density. However, the hardness and wear resistance of Inconel 718 coatings still need to be improved. To further improve these properties, ceramic reinforced Inconel 718 coatings have been investigated. Compared with ex-situ ceramic reinforcements, the in-situ synthesized reinforcements have the advantage of refined ceramic particle size, uniform distribution, and low thermal stress. B4C was a preferable additive material to fabricate in-situ synthesized multi-component ceramic reinforced Inconel 718 coatings. The addition of B4C could form a large number of borides and carbides as ceramic reinforcements. In addition, the in-situ reactions between Inconel 718 and B4C could release heat during the fabrication, thereby promoting the melting of material powders. However, there are currently no investigations on the in-situ synthesis mechanisms, microstructure, and mechanical properties of laser DED fabricated B4C-Inconel 718 coatings. In this study, the effects of B4C on the properties of Inconel 718 coatings were investigated. Results show that Ni3B, NbB, and Cr3C2 phases were formed. With the addition of B4C, the size of Laves phase was refined and the porosity was decreased. The hardness and wear resistance of B4C reinforced coatings were improved by about 34% and 28%, respectively.

Microstructure and High-Temperature Performance of High K-Doped Tungsten Fibers Used as Reinforcement of Tungsten Matrix

Tungsten (W) fiber-reinforced tungsten (Wf/W) composite with ultra-high strength and high-temperature resistance is considered an attractive candidate material for plasma-facing materials (PFM) in future fusion reactors. The main component of Wf/W composite is tungsten wire, which is obtained through powder metallurgy and the drawing process. In this paper, high potassium (K)-doped tungsten wires with 98 ppm of K and 61 ppm of impurities are prepared using traditional and optimized processing technologies, respectively, and a comparative study with conventional K-doped tungsten wires with 83 ppm of K and 80 ppm of impurities is conducted. The high-temperature mechanical properties as well as the microstructure’s evolution of the prepared tungsten wires are investigated. The results show that the high-temperature performance of K-doped tungsten wires is improved by increasing the K content and by simultaneously reducing the impurities. By adopting small compression deformation and low-temperature processing technology, the high-temperature performance of high K-doped tungsten wires can be further improved. A microstructure analysis indicates that the excellent high-temperature performance is attributed to a combination of the small K bubble size, high K bubble number density, and long K bubble string, which are produced through optimization of the processing technology. A study on the processing technology and the performance of tungsten wires with a high K content and a high purity can provide important information regarding Wf/W composites.

Study of Crack Sensitivity of Peritectic Steels

By comprehensively considering both the high temperature mechanical properties and peritectic transformation during peritectic steel solidification, the strain εCth is proposed to evaluate the crack sensitivity of peritectic steels produced in the brittle temperature range in the present work. The zero ductility temperature (ZDT) and the zero strength temperature (ZST) of Fe–C–0.32Si–1.6Mn–0.01P–0.015S steel under nonequilibrium conditions by taking the effect of the peritectic transformation on the solute segregation into account were calculated by the CK microsegregation model (Clyne–Kurz model) and were compared with the measured data. The comparison results show that this model can well simulate the nonequilibrium solidification process of peritectic steel. Then, based on the calculation of the CK microsegregation model, the strain during the peritectic phase transformation in the brittle temperature range (ZDT < TB < LIT) was calculated under nonequilibrium conditions. The results show that the calculated strain is in good agreement with the actual statistical longitudinal crack data indicating that the strain can therefore be used to predict the crack sensitivity of peritectic steels effectively.

The Deformation Behaviors and Mechanism of the High-Temperature Mechanical Properties of a Ni48W35Co17 Alloy

Investigations of the plastic deformation mechanisms of Ni-W-based heavy alloys varying with increasing temperatures are very important for the development of hot forming processes and applications at elevated temperatures. In this study, the continuous variation of strength and plasticity of a novel Ni-W-based heavy alloy with increasing temperatures was investigated. The tensile strength of a Ni48W35Co17 sample at 600 °C was 471 MPa, which was 47% lower than that at 100 °C. A variation in an abnormal decrease in elongations at temperatures from 400 °C to 800 °C was found in this alloy. The elongation rate of the sample tensile at 600 °C was 19%, which was 73% lower than that at 100 °C. A microstructural analysis revealed that the number of twins in the sample tensile at a temperature higher than 600 °C increased considerably compared with the sample tensile at lower temperatures, indicating that the dislocation slips were suppressed during the high-temperature stretching process. The precipitates of the NiW phase were found in the 600 °C tensile sample, which was not clearly observed in the 400 °C tensile sample, suggesting that dislocation slips were affected by the formation of these precipitates. Moreover, the orientation relationship between the matrix and NiW phase was (200)Ni//(240)NiW and [001]Ni//[001]NiW. The tiny precipitated phase was the main reason for the plasticity decrease of the alloy with the temperature increase.

Fatigue failure warning method for needled ceramic matrix composite by acoustic emission monitoring

Ceramic matrix composite is a kind of mechanical engineering material with excellent high temperature mechanical properties, which has been widely used in aircraft propulsion system and thermal protection system. Therefore, it is of great significance to study the fatigue failure of needled ceramic matrix composite. In this investigation, based on the realtime acoustic emission (AE) monitoring of needled C/SiC ceramic matrix composite, the characteristics of AE energy during the fatigue damage process were obtained. In addition, considering the emission coefficient of AE energy and the threshold value of AE energy in single cycle, a method for judging the imminent fatigue failure of needled composite was proposed. By comparing the cycle of failure warning by proposed method with the experimental fatigue life, the proposed method can provide fatigue failure warning near and before fatigue failure.