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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]

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]

TiZrHfTa 0.5 refractory high-entropy alloy (RHEA) shows great strength and ductility trade-off at ambient temperature, and has a lot of potential for use at elevated temperature. In this work, the effect of B element addition on the wear properties at different temperatures was thoroughly investigated. The results revealed that 1 at% B addition sample presents the best tribological properties at elevated temperatures, and borides exhibit a morphology evolution from blocky to acicular with the increasing B content. The wear rates at ambient and elevated temperatures successfully decrease with the increasing B content, which is ascribed to the increasing hardness, the solution strengthening, and the pinning effect at the grain boundaries induced by the second phase. In addition to the B content, the wear test temperature also significantly affects wear properties. With the analysis of the wear behavior and worn surface, it is demonstrated that the lowest wear rate at 600 ℃ is attributed to the continuous and densified oxide layer, while the serious wear at 400 ℃ results from the uncovered matrix and loose oxide layer. This work offers a new method to improve the wear properties of RHEAs at ambient and elevated temperatures. • The addition of B element for TiZrHfTa 0.5 RHEA results in the preferable wear performance at RT, 400 ℃ and 600 ℃. • The wear performance at 600 ℃ for four types of samples present the lowest wear rate. • The continuous and densified oxide layer formed at 600 ℃ contributes to the lowest wear rate. • The loose oxide layer formed in 400 ℃ leads to more metal fragments and more serious wear. [ABSTRACT FROM AUTHOR]

The CoNi equiatomic alloy was undercooled and spontaneously solidified using the glass fluxing method. The dendrite growth behaviors were recorded by a high-speed high-resolution camera. Three dendrite growth stages (i.e., the power law growth stage when undercooling Δ T <120 K, the plateau growth stage when 120 K<Δ T <135 K and the linear law growth stage when Δ T >135 K) were found and well predicted by a dendrite growth model. For the as-solidified microstructures, the average grain size increases with undercooling, indicating that unique grain "coarsening" instead of common spontaneous grain refinement occurs. Such a grain "coarsening" phenomenon was ascribed to stress accumulation upon rapid solidification. Furthermore, a plenty of sub-grains were observed within the abnormal coarse grains. From the electron back-scattering diffraction results, the angle difference between neighbouring sub-grains was less than 6°. The current work might be helpful for not only theoretically understanding dendrite growth but also practically controlling the non-equilibrium microstructures. • Rapid solidification of CoNi equiatomic alloy was studied using the glass fluxing method. • Three growth stages were observed and well predicted by the dendrite growth model. • Sub-grains within the coarse grains were identified by electron back-scattering diffraction. • A unique grain "coarsening" phenomenon for the as-solidified microstructures was reported. [ABSTRACT FROM AUTHOR]