Microstructure And Mechanical Properities Of CoCrFeNiW_x High Entropy Alloys

CoCrFeNi system high entropy alloys exhibited excellent plasticity but relatively low strength,which has been one of the limiting factors for the application of CoCrFeNi system high entropy alloys.Tungsten has the largest atomic radius among Co,Cr,Fe,Ni,and W,and shows a relatively low mixing enthalpy with other four elements.In addition,tungsten may react with Fe,Co at high temperatures to generate topological-close-packed ? phase.Tungsten was also widely used as a solid solution strengthening element in nickel-based and cobalt-based superalloys.In order to improve the strength of CoCrFeNi high entropy alloy,the strengthening method of introducing brittle and hard TCP phases into the CoCrFeNi high-entropy alloy by adding transition elements was proposed.W element can promote the formation of ?phase in CoCrFeNi high entropy alloys.However,the influence of W element on the microstructure and properties of CoCrFeNi high-entropy alloy has not been studied in depth.Consequently,in this paper,CoCrFeNiW_x high-entropy alloys were prepared via arc melting,and the microstructure and properties of the as-cast,as-annealed,as-forged plus annealing treatment samples were systematically studied.With the increase of W content?x=0-0.6?,the microstructure of the as-cast alloys varies from single-phase solid solution?x=0,0.2?to FCC+?FCC+??hypoeutectic structure?x=0.4?and ?+?FCC+??hypereutectic structure.CoCrFeNi,CoCrFeNiW_0.2 and CoCrFeNiW_0.4 alloys exhibited excellent plasticity,their strength increases with the increase of tungsten content,in contrast,the plasticity decreases?x=0,0.2,0.4?.CoCrFeNiW0.6 alloy possesses large-size ?-phase dendrites and shows brittle fracture during tensile experiment,exhibited low tensile strength.The as-cast alloys were subjected to 1200?/3h annealing.After annealing,? phase precipitates with a diameter of?1?m emerged at the FCC phase where containing supersaturated tungsten.These particles can effectively impede dislocation gliding and cause the second phase strengthening effect.? phase precipitated from FCC phase and causes the decrease of W element,weakening the solid solution strengthening effect.Combining effect of the two factors above lead to the higher strain hardening rate of the as-annealed CoCrFeNiW_0.2 alloy.Its plasticity and strength were simultaneously improved?UTS=545.6MPa,EL=60.47%?compared with the as-cast alloy;CoCrF eNiW_0.4 alloys contain rod-shape eutectic ? phase.As-annealed CoCrF eNiW_0.4 alloy shows dispersive annealing induced ? phase precipitates in FCC dendrites,and its strength increases with plasticity decreases?UTS=970.9MPa,EL=10.50%?compared with as-cast alloys,plastic FCC phase can surpress cracks propagation in? phase.For the as-cast TCP phase strengthened CoCrFeNi high-entropy alloy proposed until now,the TCP phases in CoCrFeNi-based HEAs generally are intermetallic compounds with higher formation temperature in the binary phase diagram of composition elements'pairs.Moreover,the ? phase precipitation ability of tungsten-containing FCC phase can be evaluated by???value.The???value decreases with ? phase precipitation.As-forged samples+500?/1.5h heat treatment of CoCrFeNiW_x alloy?x=0.2,0.4?exhibited annealing induced hardening effect,which may have relation with dislocations generated during the cold deformation.During low temperature annealing process,W atoms may segregated around dislocations and increased the strength of the FCC phase.The as-forged samples+?800?/0.5h+700?/1h?heat treatment of CoCrFeNiW_0.4 alloy showed submicron W-rich intermetallic compound in FCC region.The nucleation rate of these precipitates may have a relation to the local dislocation density of the FCC phase.In addition,W-doped as-forged alloys showed remarkable recrystallization resistance.