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Boronizing treatment has been considered as a promising strategy to enhance the surface hardness, self-lubrication and wear resistance of high entropy alloy (HEA), endowing the advancements of HEA in engineering applications. However, the anti-friction, anti-wear and corrosion resistance properties of the boronized HEA in sulfuric acid and seawater environments remain unclear. Here, the self-lubrication and corrosion resistance of the boronized CoCrFeNiMn HEA in these corrosive media were studied to assess the applicable environments. Firstly, a boronized layer with thickness of approximately 25 μm on the CoCrFeNiMn HEA surface was fabricated using the powder-pack boronizing treatment. Tribological assessments demonstrated exceptional self-lubricating properties of boronized HEA in various liquid conditions, with over 30 % reduced friction coefficients and over 80 % decreased wear rates. XPS analysis confirmed the formation of a boron-containing tribofilm, enhancing self-lubrication and reducing friction and wear in liquid environments. However, electrochemical tests indicated that the corrosion resistance of the boronized HEA was significantly affected by the liquid medium. The impedance value (|Z|) of boronized HEA decreased to 4.6 % of that of the original HEA in 3.5 wt% NaCl solution, revealing poor corrosion resistance. Conversely, an enhancement of corrosion resistance was observed in 0.05 M H 2 SO 4 solutions with the increase of the |Z| value by 8 times. Therefore, for applications requiring service safety, boronizing HEAs are more suitable for acidic environments, but not in seawater environments, offering valuable insights for the industrial application of CoCrFeNiMn HEA in reducing friction and wear. • The COF of boronized HEA is over 30 % lower than original HEA in liquid conditions. • The tribofilm (composed of H 3 BO 3 and B 2 O 3) significantly reduce friction and wear. • Boronized HEAs are more suitable for acidic environments, but not in seawater. [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 boronizing treatment has been an effective way to strengthen and enhance the lubrication performance of alloys. However, the traditional power-pack boronizing method has been limited due to its time-consuming. Improving the boronizing efficiency is highly demanded for the strengthening and industry application of the high-entropy alloys (HEAs). In this study, the electrochemical boronizing strategy was applied to improve the surface hardness and self-lubrication performance of AlCoCrFeNi 2.1 eutectic HEA (EHEA). The microstructure, phase formation and hardness of boronized alloys were studied. Results confirm the high efficiency of electrochemical boronizing to form a boronized layer with a thickness of about 35 μm in 30 min. The surface hardness of the boronized AlCoCrFeNi 2.1 HEA reached to about 18 GPa due to the presence of NiB, FeB and Cr 3 B 4 in the boronized layer. Tribological tests reveal that the self-lubrication property of AlCoCrFeNi 2.1 HEA in water condition was significantly enhanced by the electrochemical boronizing treatment, e.g., the friction and wear reductions reach to about 25% and 80%, respectively. Lubrication mechanism investigation by X-ray photoelectron spectroscopy (XPS) analysis showed the formation of a H 3 BO 3 tribofilm on the sliding surface during the friction process, which effectively reduce friction and wear. This study provided a novel and fast approach for surface hardening and self-lubrication enhancement of HEA systems using the electrochemical boronizing strategy. • A boronized layer was obtained on the AlCoCrFeNi 2.1 alloy surface in 30 min. • The surface hardness was about 4 times higher than that of the original EHEA. • The self-lubrication performance of boronized EHEAs in the water is excellent. • The lubrication enhancement was due to the in-situ formation of H 3 BO 3 tribofilm. [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]

The CoNi, CoFeNi and CoCr x FeNi (x = 0.2, 0.6, 1) alloys were undercooled and in-situ observed. Sluggish dendrite growth was found for the CoCrFeNi alloy, e.g., the maximum dendrite growth velocity decreases by nearly two orders from the CoNi (78 m s−1) to the CoCrFeNi (0.83 m s−1) alloy. It was shown that it is the Cr element and its adding amount but not the number of components that plays a more decisive role in sluggish dendrite growth. The current work might be helpful for understanding rapid solidification, thereby further controlling the microstructures and modulating the mechanical properties of high entropy alloys. Image 1 • A metastable BCC phase was identified in the undercooled CoCrFeNi alloy. • Sluggish dendrite growth was found with the increase of the number of components for the equiatomic alloy melt. • It is not the components number but the properties and its adding amount that play important roles in sluggish growth. • The sluggish dendrite growth could be ascribed to the interaction between the interface and component atoms. [ABSTRACT FROM AUTHOR]