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CoCrNi medium‐entropy alloy (MEA) is a promising material for aviation and aerospace applications due to its excellent strength and ductility. However, its low surface hardness and poor wear resistance pose challenges for its tribological performance. To address this issue, electrochemical boronizing is explored as a potential solution, offering advantages such as a fast boronizing rate and low cost. This study demonstrates that electrochemical boronizing can significantly enhance the surface hardness and self‐lubrication performance of CoCrNi MEA, as well as retain impressive mechanical performance. Results show that a boronized layer with a thickness of about 50 μm is formed on the surface in 45 min. The boronized layer is mainly composed of Co2B, Ni2B, CrB, CoB, and Ni3B. After boronizing, the surface hardness of the sample increases by about 9 times, and the ultimate tensile strength and uniform elongation can reach 97% and 73% of the original sample, respectively. Compared with the original alloy, the friction coefficient and wear rate of boronized MEA are reduced by 38% and 85% under deionized water conditions, respectively. X‐ray photoelectron spectroscopy analysis reveals the generation of boron‐containing tribofilm on the wear scars, which significantly reduces friction and wear during the friction process. [ABSTRACT FROM AUTHOR]

Boronized alloys with high surface hardness has been revealed advance in enhancing the anti-wear performance of critical mechanic components. However, its application in industrial production is greatly limited due to the poor mechanical properties with crystallographic orientated brittle borides. In the present study, we demonstrated that high ductility and high strength were achieved for the boronized AlCoCrFeNi 2.1 eutectic high entropy alloy (EHEA) with the formation of nano-sized borides. The AlCoCrFeNi 2.1 EHEA was treated with electrochemical boronizing method, and its phase formation was studied by x-ray diffraction (XRD), electron back-scattered diffraction (EBSD) and focused ion beam-transmission electron microscope (FIB-TEM). Results show that the nano-grained of FeB, NiB and Cr 3 B 4 with size of about 135.5 nm were formed on the boronized layer with non-preferred crystallographic orientation of the phase. The surface hardness of boronized AlCoCrFeNi2.1 EHEA reached to about 18 GPa, being about 4 times higher than that of the original alloy. Further investigation by tensile testing shows that the tensile strength of boronized samples is about 1.2 GPa with a respectable ductility of about 16.04% (± 0.49) tensile ductility to failure. Meanwhile, extension test and digital image correlation (XTDIC) test shows that the boronized layer failed when the tensile ductility is about 5.1%. The disordered nano-grained in the boronized layer benefit in preventing the formation and propagation of cracks, which improved the ductility of the boronized AlCoCrFeNi 2.1 EHEAs. [Display omitted] • The nano-sized borides of FeB, NiB and Cr 3 B 4 with grain size about 135.5 nm was generated on the AlCoCrFeNi 2.1 EHEA surface. • The elongation and strength of boronized EHEA reach 16.04% and 1197 MPa, and the surface hardness reaches 21 GPa. • The fracture of the brittle boronized layer occurred at the strain of 5.1%. • The crystallographic non-oriented nano-sized borides significantly inhibition crack generation and propagation. [ABSTRACT FROM AUTHOR]

The as-cast AlCoCrFeNi 2.1 eutectic high-entropy alloy (EHEA) has high fracture strength and ductility, which displays an excellent application prospect in key structural materials. However, its low surface hardness and poor wear resistance at room temperature limit the actual applications in tribology. Here we showed the enhancement of surface strengthening and self-lubrication properties of AlCoCrFeNi 2.1 EHEA by a powder-pack boronizing treatment. SEM and XRD analysis indicate that a double affected layer with a thickness of about 42 μm was formed on the surface at 900 °C for 4 h, which were mainly consisted of Ni 2 Si and FeB. The surface hardness of the alloy was significantly increased from 396 HV to 1108 HV after the treatment. Tribological results show a remarkable self-lubrication property of the boronized EHEA. The friction coefficient of boronized EHEA under the water condition was reduced by 21%, and the corresponding wear volume was decreased by 65%. EDS and XPS analysis demonstrate the formation of silicon/boron-containing tribofilm on the wear scar, which benefits in the achievement of self-lubrication properties with friction and wear reduction under the water condition. Our research provides a general strategy with the powder-pack boronizing method to strengthen AlCoCrFeNi 2.1 EHEA and enhance self-lubrication properties, which would certainly boost its industry application. • An impressive enhancement of surface hardness was achieved after boronizing. • The affected layer showed a double-layer structure, which was mainly Ni 2 Si and FeB. • Boronized EHEA showed the remarkable self-lubrication property in water. • The formed silicon/boron lubrication film benefited in self-lubrication property. [ABSTRACT FROM AUTHOR]

Fretting wear is one of the main factors restricting the fastener service reliability for the high-temperature titanium alloys. Enhancing the high-temperature fretting wear resistance of titanium alloys with laser cladding coatings is an effective method. Thus, in this work, a self-lubricating composite coating was designed and fabricated on the surface of TC21 titanium alloy with laser cladding. Special attentions were made to the effect of the fractions of the hard and self-lubricating phases on high-temperature fretting resistance. In addition, the contribution of laser cladding power was also concerned. The primary phases of the self-lubricating composite coatings are α-Co, CaF 2 , WC, TiC and Cr 23 C 6 , and the reasons for the formation of TiC and Cr 23 C 6 are investigated from a thermodynamic point. The prepared composite coatings are dense and uniform. The crystallographic characteristics and the volume fractions of the hard phases (WC, TiC and Cr 23 C 6) and self-lubricating phase (CaF 2) were revealed, and the effect of grain size on the coating properties was analyzed. The hardness of the coatings were all maintained around 700 HV 0.2 , which was significantly higher than that of the substrate. The composite coating significantly improves the friction and wear resistance at 500 ℃. The hard phase was also found to be the most resistant to friction and wear. The frictional performance of the coating gets better when the hard phase and the self-lubricating phase increase. However, the hard phase contributes more to the high-temperature friction and wear resistance than the self-lubricating phase and the wear volume is 1/40 that of substrate. For the laser cladding power, the higher power can lead to a higher fraction of CaF 2 but a lower fraction of hard phase, causing a decline in COF but an increase in wear volume. The wear mechanisms of the coating and the substrate were analyzed, the subsurface stress distribution state of the wear scars were further analyzed. The current results present new insights on the anti-wear design strategy to the composite coatings consist of hard and self-lubricating phase. [Display omitted] • The high-temperature fretting wear volume is reduced about 40 times. • The strategy to the addition of the hard and lubricating phase were revealed. • The hard phase has a greater impact on the high-temperature fretting wear. • This work guide for the protection of the titanium alloys. [ABSTRACT FROM AUTHOR]

• Enhanced wear-resistance were achieved by reducing the content of Nb in TiZrHfNb. • The wear loss is related to the subsurface instabilities during friction. • Deformation-induced grain refinement and phase transformation enhance wear-resistance. A sliding enhanced wear-resistance can be significantly achieved by reducing the content of Nb element in TiZrHfNb refractory high entropy alloys (RHEAs). The wear loss reduced by half from TiZrHfNb to TiZrHfNb 0.4. The wear loss is directly related to the subsurface instabilities induced by plastic deformation during friction. The origins can be attributed to two factors: i) plastic deformation induced grain refinement; ii) deformation-induced BCC to HCP phase transformation. [ABSTRACT FROM AUTHOR]

A series of TiZrV 0.5 Nb 0.5 Al x refractory high entropy alloys were prepared by arc-metling, aiming to solve the problems of high density, poor oxidation and wear resistance of refractory high entropy alloys at elevated temperature. The microstructure, mechanical and tribological properties were investigated. The results indicate that the increase in Al addition could promote precipitation of Laves phase in the BCC matrix, thus regulation the mechanical properties. The tribology behavior is strongly dependent on the temperature. Below 400 °C, the wear mechanism is dominated by mechanical wear. In this case, the TiZrV 0.5 Nb 0.5 Al 0.25 alloy with high strength and good ductility at the same time, exhibits optimal wear resistance. When the temperature increases above to 600 °C, the coefficient of friction and the corresponding wear rate dramatically decrease with Al addition, which could be attributed to the preformed formation of dense and stable oxide glazed-layer before wear. Accordingly, the main wear mechanism changes from mechanical wear to oxidation wear. This study provides an effective strategy for improving wear resistance and oxidation resistance of refractory high entropy alloys in a wide temperature range. • Al-containing alloys hardly wear at 600 °C and 800 °C. • The high temperature worn surface of Al-containing alloy has a dense oxide layer. • The Al0.25 alloy has unique needle-like Laves phase and the best wear resistance. [ABSTRACT FROM AUTHOR]

The fretting behaviors of a metastable β titanium alloy with three kinds of microstructures were studied. Special attentions have been paid to the underlying wear mechanism mediated by fretting conditions and microstructural features. The wear volume is more sensitive to the stroke amplitude rather than the normal loading. The mixed abrasive-adhesive wear is dominated in all fretting samples. The microstructural features make a significant contribution to the difference in the wear resistance and subsurface microstructure stability. Due to a large difference in the grain size and the ability of plastic deformation in adjacent grains, the SM (single-phase microstructure) sample is more favorable to initiation and propagation of cracks. The DM (duplex microstructure) and BM (basketweave microstructure) samples, however, have a high resistance ability to fretting wear and subsurface microstructure stability. This can be attributed to the large amounts of the β/α interfaces and the dispersed orientation distribution of α precipitates. The present work offers valuable experimental data for the prediction of fatigue life after fretting. • The fretting behaviors are evidenced to connect with coupling effect of microstructural features and fretting conditions. • The fretting wear volume is more sensitive to the stroke amplitude rather than the normal loading. • The microstructural origins of fretting wear mechanisms are illustrated systematically. • Microstructure features dependence on the subsurface microstructure stability is revealed in detail. [ABSTRACT FROM AUTHOR]

The wear behaviors of γ-TiAl alloy with two typical microstructures, i.e., the lamellar colony microstructure (denoted as the HIPed sample) and the fine equiaxed microstructure (denoted as the heat-treated sample), were thoroughly studied during dry sliding test to resolve the microstructural feature dependence of wear mechanisms. Special efforts have been made to the subsurface stability during sliding by using electron backscatter diffraction (EBSD) techniques. It is demonstrated that both cases present a similarity in the friction coefficients but a disparity in the wear loss. The lower wear loss was achieved in the case of HIPed sample. This can be attributed to the various directions in the local strain induced by plastic deformation during sliding, the crystallographic structure and the coherent interface in the lamellar colony, leading to high resistance ability to the wear. For the case of heat-treated sample, the high plastic deformation ability in the α 2 grains along the surface and the equiaxed morphology generate to high ability to plastic deformation and sliding during friction, contributing to the high wear loss. However, the heat-treated sample presents a high subsurface structure stability according to the local misorientation analysis. The present results provide new information on wear mechanisms of γ-TiAl alloy and may offer a method to enhance its wear resistance by tailoring the microstructural features. • The wear loss in a dry sliding of a γ-TiAl alloy is evidenced to connect with microstructural feature. • Special efforts have been made to the subsurface stability during sliding by using EBSD techniques. • The lower wear loss was achieved in the case of the lamellar colony microstructure. • The fine globular microstructure presents a high subsurface structure stability. [ABSTRACT FROM AUTHOR]

Hot deformation is an important procedure of metastable β titanium alloy for manufacturing. And it is essential to improve the homogeneity of deformation microstructure in titanium alloy. In this work, the stress relaxation processing accompanied by hot compression in a metastable β titanium alloy is designed to resolve the related mechanisms of stress relaxation by examining the microstructure evolution and improve the wear-resistance. The results demonstrated that the stress relaxation processing leads to the modification and evolution in morphology, size, and volume fraction of α precipitates. The α precipitates distribute randomly in β matrix in a fine and equiaxed shape after stress relaxation. The elimination of deformation bands and formation of sub-grains are the mediated mechanism in the β phase during stress relaxation. The stress relaxation makes a significant contribution to reduce the wear loss and improve the microstructure stability during friction. The lower wear loss can be attributed to the formation of fine β sub-grains and the dispersed crystallographic orientations of α precipitates. This work provides an effective method to microstructure refinement and a valuable solution to enhance the wear-resistance by tailoring the microstructural features. • Stress relaxation results in modifications in morphology, size, and volume fraction of α precipitates. • The elimination of deformation bands and formation of sub-grains is stress relaxation mechanism. • The stress relaxation process makes a significant contribution to wear reduction. • This work contributes to microstructure refinement and enhanced wear-resistance. [ABSTRACT FROM AUTHOR]