The Preparation And Mechanical Behavior Of Dual-Phase High Entropy Alloy Films

Alloys have been widely used as structural and functional materials.Recently,high entropy alloys(HEAs)have attracted considerable attention.High entropy alloys is defined as solid solution alloy that contains five or more elements with concentration range from 5 to 35 atomic percent.Previous studies have demonstrated that high-entropy alloys featured a lot of promising properties which traditional alloys are incaomparable with,such as high strength,high hardness,high wear resistance and corrosion resistance,excellent radiation resistance and so on.Due to their superior properties,high-entropy alloys have great potentials in application prospects of films and coatings such as wear-resistant and corrosion-resistant protective coatings.Therefore,as a kind of structural materials with excellent properties,the strengthening mechanism of high-entropy alloy films have been the focus of attention.It is a great challenge to design and fabricate high hardness high entropy alloy films with unique structure.Two kind of HEAs have been selected in this thesis,one is the CoCrCuFeNi which is the typical 3d transition HEA with FCC structure and the other is the MoNbTaW which is the typical refractory HEA with BCC structure.The CoCrCuFeNi/Al and MoNbTaW/Al nanolayered films were prepared by dc magnetron sputtering and their were annealed for one hour at 550?.XRD and TEM were applied for microstructural characterization and the nanoindentation tests were performed to obtain the hardness of these films.In this thesis,we present a tunable alloying strategy to prepare high entropy alloy thin films with dual-phase structure.The relationships between hardness,microstructure and size effect of dual-phase high-entropy alloy were studied and the strengthening mechanisms were discussed.The main conclusions are given below:1.Interface alloying is an effective method to produce dual-phase HEA films.when the as-deposited multilayers were annealed at 550 ?,A1 atoms diffused into the CoCrCuFeNi layer,and then a dual-phase AlxCoCrCuFeNi formed.The strengthening in CoCrCuFeNi/AI dual-phase high-entropy alloy showed obvious size effect.At ?=2 and ?=10 nm,the HEA films achieved maximum hardness of 10.4 GPa.2.The relative large atomic radius of Al lead to severe lattice distortion,resulting in a stronger solid solution strengthening behavior in the dual-phase HEA.Thus,the high hardness of CoCrCuFeNi/Al dual-phase HEA films is mainly attributed to the solid solution strengthening and the phase interfacial strengthening.3.Compared with CoCrCuFeNi/Al,MoNbTaW/Al has a completely different strengthening mechanism.Unlike the FCC/FCC interface between CoCrCuFeNi/Al,the BCC/FCC interface between MoNbTaW/Al has greater restrict to dislocation movement,so the strengthening effect of interface in MoNbTaW/Al is more significant.After annealing,the strength of films increases obviously due to the notable solid solution strengthening caused by Al.There is a critical size in the strengthening of MoNbTaW/Al.When the thickness of MoNbTaW is 50 nm,the strengthening by heterogeneous interfaces and the solid solution strengthening both reach their maximum,thus the hardness of the films reaches highest 14.8 GPa.