Irradiation And Oxidation Behaviors Of AlCoCrFeNi System High Entropy Alloys

The study of multi-component alloys from the viewpoint of entropy has raised the exploration and recognition of materials science to a new level.In statistical physics,entropy is a measure of the degree of disorder in the arrangement of materials,corresponding to the "degree of disorder" of materials.Alloys with high mixing entropy and high contents of alloying elements tend to form high-entropy disordered phases,such as amorphous and solid-solution structures.Therefore,the entropy has the great influence on the "chaotic principle”of amorphous alloys and the "high entropy effect" of high entropy alloys.In the present work,the influence of entropy on the mirostructre and properties of muticompent alloys were studied.The microstructure evolution and performance of AlxCoCrFeNi high entropy alloys under 3 MeV Au+ ion irradiation and the oxidation behaviors of pure Ni and Ni-containing multi-component alloys at 800? were also investigated.The main findings are shown as follows:(1)The Al0.1CoCrFeNi,A10.75CoCrFeNi,and Al1.5CoCrFeNi high entropy alloys prepared by the vacuum arc-melting furnace have the same main phase composition as the phase diagram calculations.Increasing the Al content,the AlxCoCrFeNi system multi-component high entropy alloys have a transition from FCC to BCC structure.Al0.1CoCrFeNi is single phase FCC structure,while Al0.75CoCrFeNi is FCC + B2 structure,and Al1.5CoCrFeNi is A2 + B2 structure,which indicating that Al element can promote the stability of the BCC phase structure in high entropy alloys.In addition,we use the mixing entropy in multi-component alloys to measure the AlxCoCrFeNi system high entropy alloys,and the mixing entropy of the three alloys was 12.20,13.33 and 13.25 J/mol·K,respectively.When x is less than 1,it does not promote the formation of single phase disordered solid solution in AlxCoCrFeNi high entropy alloys by incerasing the entropy blindly.(2)The volume swelling and precipitation behavior of AlxCoCrFeNi high entropy alloys irradiated by 3 MeV Au+ ion irradiation at room temperature were investigated.The order of volume swelling after ion irradiation is FCC<FCC +BCC<BCC.which is inverse to the order of traditioinal radiation resistant materials with BCC<FCC.Moreover.Al0.1CoCrFeNi with a lower entropy of mixing exhibits greater phase stability against ion irradiation.No precipitate is observed even irradiated to?40 dpa.In contrast,numerous coherent precipitates with size?6 nm are present in A10.75CoCrFeN and Al1.5CoCrFeNi dual-phase alloys with higher mixing entropy.The average size of the precipitations increased with the irradiation dose.Meanwhile,the results of EELS show that for the Al1.5CoCrFeNi alloy,it was found that precipitation of A2 phase was enriched in Fe,while the precipitation of B2 was enriched in Co.(3)The microstructure evolutions of AlxCoCrFeNi high entropy alloys irradiated by 3 MeV Au+ ion irradiation at room temperature were systematically studied.For the single-phase Alo.1CoCrFeNi alloy,the TEM characterization show that it has the small and dense defects under low irradiation doses?1 dpa.With increasing the irradiation dose,the defect size gradually increases into dislocation lines or dislocation loops,indicating that the size of the defects increased,while their number density decreased.In addition,for the All.75CoCrFeNi and Al1.5CoCrFeNi high entropy dual-phase alloys,the size and density of defects in the disordered phase are smaller than that of the ordered phase.The formation energy and migration energy of defects in the disordered phase structure may be higher than that of the ordered phase,resulting in the suppression of the formation and movement of point defects,thereby exhibiting excellent resistance to irradiation defects,showing the degree of solid solution order is inversely proportional to the irradiation performance of multi-component alloys.(4)The microstructure evolutions of Al0.1CoCrFeNi irradiated by 3 MeV Au+ion irradiation at elevated temperatures from 250? to 650? were investigated.The interstitial loops with Burgers vectors b = 1/2<110>and b = 1/3<111>and stacking fault tetrahedrons were observed in Al0.1CoCrFeNi alloy,while no any voids were found.The loop size increases with the irradiation temperature.The growth and coalescence of small dislocation loops or clusters are the main reasons for the decreasing in the size of dislocation loops.Especially,at 650?.the interstitial dislocation loops or clusters and vacancies are recombined due to the recovery mechanism at high temperature,therefore,the density of defects is drastically reduced.In addition,the three-dimensional atom probe tomography(APT)characterization reveals that ion irradiation at elevated temperatures can induce an enrichment of Ni and Co as well as a depletion of Fe and Cr at dislocation loop from the viewpoint of eliminating or reducing the stress,which is that the smaller-sized atom Co/Ni tends to be riched in interstitial dislocation loops.while the larger Cr/Fe atom is depleted.(5)The oxidation behaviors of pure Ni and Ni-containing multi-component alloys with nearly equiatomic compositions at 800? were investigated.The order of oxidation resistance is FeNi<CoFeNi<Ni<CoCrFeMnNi<CoCrFeNi.indicating that the performance does not increase linearly as the entropy value increases,showing the nonlinear behavior.At the same time,the low-temperature impact toughness of AlxCoCrFeNi system multi-component alloys were investigated at 77 K,200 K,and 298 K.It was found that an inverse temperature dependence of impact toughness between 298 K and 77 K in CoCrFeNi and Al0.1CoCrFeNi alloys was observed,indicating that there is no ductile-brittle transition temperature in the temperature range of 298 K to 77 K,which is attributed to the capability of extensive nano-twinnings as well as ductile dimple fracture at 77 K.It should be noted that the impact energy of the CoCrFeNi alloy is 398 J at 77 K.