采用机械合金化方法制备CoCrMoNbTi难熔高熵合金粉末,并通过激光熔覆技术成功制备出CoCrMoNbTi高熵合金涂层.研究了球磨时间对合金粉末组织形貌的影响,并利用X射线衍射仪、扫描电子显微镜和能谱仪等分析了高熵合金粉末和涂层的微观结构.结果表明,随着球磨时间的增加,单质金属的衍射峰按其熔点由低到高陆续消失.粉末微观形貌随球磨时间变化明显,粉末由原始状态被挤压成片状,片状粉末逐渐焊合在一起形成扁平状粉末颗粒.在球磨时间达到 40h时,粉末实现完全合金化,此时粉末形貌趋于球形且得到了极大的细化,粉末中各元素分布均匀,形成了稳定的单相体心立方固溶体结构.CoCrMoNbTi难熔高熵合金激光熔覆层成形质量良好,主要由体心立方固溶体和少量Cr2Nb、Co2Ti化合物组成,树枝晶组织细小致密.

In this study, CoCrMoNbTi, CoCrMoNbTi(B4C)0.2 and CoCrMoNbTi(SiC)0.2 high-entropy alloy coatings are prepared by laser cladding. These three coatings show significant differences in terms of microstructure and mechanical properties. In terms of microstructure, the Pure-HEA exhibits a uniform microstructure distribution, and the coating consists of two BCC phase-solid solution structures (BCC1 and BCC2) with different lattice constants. SiC-HEA exhibits a denser microstructure than Pure-HEA, and both coatings exhibit a typical dendritic structure. In contrast, the microstructure of B4C-HEA transformed from dendritic crystals to fully equiaxed crystals, and the grain refining effect can even up to two orders of magnitude. Both B4C-HEA and SiC-HEA are composed of BCC1, BCC2 and TiC phases, where TiC is obtained by the in-situ reaction. The effects of B4C and SiC on the mechanical properties of the high-entropy alloy are evaluated in terms of microhardness and wear resistance. The results show that B4C has the best strengthening effect on the alloy properties, the microhardness of the coating increases from 666.2 HV0.5 to 886.9 HV0.5. The room temperature wear resistance of the alloy is enhanced, and the frictional coefficient and wear rate are significantly reduced. Moreover, the wear mechanism is transformed from adhesive wear to abrasive wear due to the addition of B4C. This work provides insight into the application of ceramic particle-reinforced high-entropy alloys.

CoCrMoNbTi high entropy alloy coating with boron carbide (B4C) addition was prepared on titanium alloy substrate via laser cladding. The influences of B4C content on solidification microstructure, solid phase transformation and structure as well as the resultant mechanical properties of the high entropy alloy were investigated. With B4C addition, the original dendritic columnar grains transform into equiaxed crystal structure in a finer and more uniform manner. By increasing the content of B4C, the solid phase evolution follows three steps: BCC1 + BCC2 → BCC1 + BCC2 + TiC → BCC1 + BCC2 + TiC + TiB, and the mechanical properties of the coating section vary correspondingly. The nano-hardness (H) and Young's modulus (E) of the CoCrMoNbTi(B4C) 0.6 coating were enhanced to 10.75 GPa and 240.35 GPa, compared to the original CoCrMoNbTi coating (H = 6.83 GPa and E = 168.88 GPa). This work provides a practical way of in-situ strengthen of the high-entropy alloys via laser cladding with carbides and borides addition.

Refractory high-entropy alloys (RHEAs) exhibit outstanding softening resistance and thermal stability at elevated temperatures. Unfortunately, poor ductility at room temperature has remained the critical issue for their processability and practical application. In this study, an original-type fabrication method of RHEA was proposed, using multi-walled carbon nanotubes (MWCNTs) to enhance the alloy prepared via laser melting deposition (LMD) technology. The processing optimization, microstructure evolution and mechanical properties were systematically investigated for LMD processing of CNTs/CoCrMoNbTi0.4 RHEA. The results have shown that CNTs/CoCrMoNbTi0.4 RHEA have a polycrystalline structure (BCC, HCP, and TiC). As the optimal LMD-processing parameters of laser linear energy density of 3.6 J/mm were applied, owing to the formation of high densification and an ultrafine microstructure, the fully dense LMD-processed alloy exhibited high microhardness of 1015 HV0.5, fracture strength of 2110.5 MPa, and fracture strain of 2.39%. The solid solution strengthening and load transfer are considered as the main strengthening mechanisms occurring simultaneously during compressive tests at room temperature, leading to excellent mechanical properties of LMD-processed CNTs/CoCrMoNbTi0.4 RHEA, which explores the potential application of RHEAs.

The AlCrFeMnNi high-entropy alloy (HEA) coatings were deposited on 316L stainless steel through laser cladding technology that employs pre-alloyed HEA powders. The microstructure, phase composition and hardness of the laser-cladded coatings prepared at different laser scanning speeds were investigated. The wear behavior and properties of the HEA coatings at different temperatures (from room temperature to 600 ?C) was comprehen-sively evaluated using a pin-on-disc test. It was demonstrated that the laser-cladded AlCrFeMnNi HEA coatings exhibited dense and uniform microstructures consisting of a single BCC solid solution phase. A columnar-to-equiaxed transition structure feature was observed in the molten pool of the laser-cladded coating. The coat-ings possessed better high-temperature wear performance due to the protective effect of oxide films on the wear track. The coating at 400 degrees C exhibited the best wear resistance, which had the lowest friction coefficient and wear rate of 0.48 and 1.246 x 10-4 mm(3)/N . m, respectively. At room temperature, the main wear mechanism of the coating was abrasive wear. However, the predominant wear mechanism at high temperatures was oxidation and adhesive wear.

To determine the potential of an AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) as a structural material, its laser beam welding (LBW) performance was evaluated, and the microstructure and mechanical properties of weld joint were studied. A fully penetrated, defect-free joint was obtained, in which the fusion zone (FZ) exhibited a eutectic lamellar microstructure containing FCC(L1(2))/BCC(B2) solid solution phases. The FZ contained refined columnar grains, which grown with the preferential 111 orientation induced by the rapid cooling during LBW. The tensile strength of the FZ was superior than that of the base metal (BM), which was attributed to grain refinement and higher dislocation density. LBW is a suitable process for joining AlCoCrFeNi2.1 EHEAs.

The application scope and market demand for additive-manufactured high-entropy alloys (AM HEAs) have broadened of late. However, a long-standing problem associated with AM HEAs is their limited ductility. In this study, a dual-phase AlCoCuFeNi HEA, consisting of body-centered cubic (BCC) solid solution matrix with uniformly dispersed face-centered cubic (FCC) structured precipitates, was fabricated by selective electron beam melting (SEBM). SEBM involved a preheating process can enable the formation of Cu-rich FCC phases with needle-like and spherical morphologies, as well as nanotwins that in-situ precipitated from the metastable BCC(B2) matrix. The compressive strength and ductility of the SEBM HEA were superior to those of the HEAs processed by selective laser melting (SLM) AM technique. Furthermore, using selective electron beam remelting (SEB-RM) during SEBM could result in a higher relative density, finer microstructure, and enhanced compressive properties. Particularly, the SEB-RM sample exhibited a better compressive strength of 2572 MPa, a yield strength of 870 MPa, and a strain of 18.3%. The improved mechanical properties of SEBRM samples could be ascribed to the refined grains and the formation of FCC precipitates, mostly along the grain boundaries. This provides new insights into the dual-phase HEAs-fabricated via a combination of the SEBM additive manufacturing process and selective electron beam remelting-that exhibit in-situ strengthening. (c) 2021 Elsevier B.V. All rights reserved.

An approach of laser remelting (LR) during laser melting deposition (LMD) process was applied to improve the forming quality and mechanical properties of the AlCoCuFeNi high entropy alloys (HEAs). In particular, the effect of laser remelting on the surface morphology, phase, microstructure and hardness of the parts were investigated using LSCM, SEM, XRD and Vickers microhardness tester. The results show that both LMD and LMD+LR samples dominantly consisted of a body-centered-cubic (BCC) and face-centered cubic (FCC) solid solution phases. There are many splash particles on the surface of the LMD part, which is attributed to the serious balling effect. The analysis shows that the LR process can improve the surface quality and microhardness of the LMD HEA samples due to the elimination of balling defects (microcracks and porosity).

In situ carbides (TiC/Cr7C3) reinforced CoCrMoNbTiC0.2 high-entropy alloy coatings were prepared on the Ti-6Al- 4V titanium alloy substrate by laser melting deposition technology. Effect of the laser power on the surface morphology, phase consistent, microstructure and microhardness were investigated. The results show that the coatings were composed of a simple BCC solid solution and a small amount of TiC and Cr7C3 carbides. The in-situ MC (TiC/Cr7C3) carbides were evenly distributed in the BCC matrix. The laser power has a significant impact on the forming quality and mechanical properties of the coatings. As the optimal laser power of 1500 W were applied, the coating mostly free of defects exhibited a fine dendritic microstructure. With the increasing laser power, the microhardness of the coatings was first increased and then decreased gradually. The highest microhardness of the coating (1500 W) was up to 650 HV0.5, which was 2 times higher than that of the substrate. The excellent mechanical properties of the coatings were attributed to the synergetic effects of the second phase strengthening, solid solution strengthening and fine microstructure.