Wear-resistant CoCrNi multi-principal element alloy at cryogenic temperature

Traditional high strength engineering alloys suffer from serious surface brittleness and inferior wear performance when servicing under sliding contact at cryogenic temperature.Here,we report that the recently emerging CoCrNi multi-principal element alloy defies this trend and presents dramatically enhanced wear resistance when temperature decreases from 273 to 153 K,surpassing those of cryogenic austenitic steels.The temperature-dependent structure characteristics and deformation mechanisms influencing the cryogenic wear resistance of CoCrNi are clarified through microscopic observation and atomistic simulation.It is found that sliding-induced subsurface structures show distinct scenarios at different deformation temperatures.At cryogenic condition,significant grain refinement and a deep plastic zone give rise to an extended microstructural gradient below the surface,which can accommodate massive sliding deformation,in direct contrast to the strain localization and delamination at 273 K.Meanwhile,the temperature-dependent cryogenic deformation mechanisms(stacking fault networks and phase transformation)also provide additional strengthening and toughening of the subsurface material.These features make the CoCrNi alloy particularly wear resistant at cryogenic conditions and an excellent candidate for safety–critical applications.

Tribological mechanisms of slurry abrasive wear

Abrasive wear mechanisms—including two-body and three-body abrasion—dominate the performance and lifespan of tribological systems in many engineering fields,even of those operating in lubricated conditions.Bearing steel(100Cr6)pins and discs in a flat-on-flat contact were utilized in experiments together with 5 and 13 μm Al_(2)O_(3)-based slurries as interfacial media to shed light on the acting mechanisms.The results indicate that a speed-induced hydrodynamic effect occurred and significantly altered the systems'frictional behavior in tests that were performed using the 5 μm slurry.Further experiments revealed that a speed-dependent hydrodynamic effect can lead to a 14% increase in film thickness and a decrease in friction of around 2/3,accompanied by a transition from two-body abrasion to three-body abrasion and a change in wear mechanism from microcutting and microploughing to fatigue wear.Surprisingly,no correlation could be found between the total amount of wear and the operating state of the system during the experiment;however,the wear distribution over pin and disc was observed to change significantly.This paper studies the influence of the hydrodynamic effect on the tribological mechanism of lubricated abrasive wear and also highlights the importance to not only consider a tribological systems'global amount of wear.

On the origin of plasticity-induced microstructure change under sliding contacts

Discrete dislocation plasticity(DDP)calculations are carried out to investigate the response of a single crystal contacted by a rigid sinusoidal asperity under sliding loading conditions to look for causes of microstructure change in the dislocation structure.The mechanistic driver is identified as the development of lattice rotations and stored energy in the subsurface,which can be quantitatively correlated to recent tribological experimental observations.Maps of surface slip initiation and substrate permanent deformation obtained from DDP calculations for varying contact size and normal load suggest ways of optimally tailoring the interface and microstructural material properties for various frictional loads.

Tribological performance and microstructural evolution ofα-brass alloys as a function of zinc concentration

Tailoring a material's properties for low friction and little wear in a strategic fashion is a long-standing goal of materials tribology.Plastic deformation plays a major role when metals are employed in a sliding contact;therefore,the effects of stacking fault energy and mode of dislocation glide need to be elucidated.Here,we investigated how a decrease in the stacking fault energy affects friction,wear,and the ensuing sub-surface microstructure evolution.Brass samples with increasing zinc concentrations of 5,15,and 36 wt%were tested in non-lubricated sphere-on-plate contacts with a reciprocating linear tribometer against Si3N4 spheres.Increasing the sliding distance from 0.5(single trace)to 5,000 reciprocating cycles covered different stages in the lifetime of a sliding contact.Comparing the results among the three alloys revealed a profound effect of the zinc concentration on the tribological behavior.CuZn15 and CuZn36 showed similar friction and wear results,whereas CuZn5 had a roughly 60%higher friction coefficient(COF)than the other two alloys.CuZn15 and CuZn36 had a much smaller wear rate than CuZn5.Wavy dislocation motion in CuZn5 and CuZn15 allowed for dislocation self-organization into a horizontal line about 150 nm beneath the contact after a single trace of the sphere.This feature was absent in CuZn36 where owing to planar dislocation slip band-like features under a 45°angle to the surface were identified.These results hold the promise to help guide the future development of alloys tailored for specific tribological applications.

A wear-resistant metastable CoCrNiCu high-entropy alloy with modulated surface and subsurface structures

Sliding friction-induced subsurface structures and severe surface oxidation can be the major causes influencing the wear resistance of ductile metallic materials.Here,we demonstrated the role of subsurface and surface structures in enhancing the wear resistance of an equiatomic metastable CoCrNiCu high-entropy alloy(HEA).The CoCrNiCu HEA is composed of a CoCrNi-rich face-centered cubic(FCC)dendrite phase and a Cu-rich FCC inter-dendrite phase.Copious Cu-rich nano-precipitates are formed and distributed uniformly inside the dendrites after tuning the distribution and composition of the two phases by thermal annealing.Although the formation of nano-precipitates decreases the hardness of the alloy due to the loss of solid solution strengthening,these nano-precipitates can be deformed to form continuous Cu-rich nanolayers during dry sliding,leading to a self-organized nano-laminated microstructure and extensive hardening in the subsurface.In addition,the nano-precipitates can facilitate the formation of continuous and compacted glaze layers on the worn surface,which are also beneficial for the reduction of the wear rate of CoCrNiCu.The current work can be extended to other alloy systems and might provide guidelines for designing and fabricating wear-resistant alloys in general.