Research On Mechanical Properties Of High-Nitrogen FeMnCoCr High Entropy Alloys Based On Phase Proportional Control

As a newly-developed metal material,high entropy alloys,are usually composed of five or more elements.Mole fractions of each elements are almost the same.Although high entropy alloys have complex element composition,they are usually single-phase or doublephase solutions.High entropy alloys have advantages of high strength and toughness,wear resistance,corrosion resistance,radiation resistance and high hardness.They have great potential in military industry,aerospace and construction of nuclear power plants,etc.At present,microstructure control and material optimizing etc.of high entropy alloys need further researches.Recent research indicates that importing of bcc-structure α-martensite greatly enhanced yield strength of FeMnCoCrN high entropy alloy,thus shows a new approach to strengthening of low-stacking-fault-energy alloys which mainly have fcc structure.Since forming mechanism and its influence factors are not clear,this thesis systematically studies the nucleation and growth mechanisms and influences on mechanical properties of FeMnCoCrN high entropy alloy of α-martensite laths by doing researches on Fe46.8Mn30Co10Cr10N3.2high entropy alloy.Main conclusions are as follows:(1)The alloys are 60%-cold rolled,annealed at 750℃ in salt furnace for 3 min and cooled in different ways.When cooling approach changes from furnace-cooling to waterquenching,yield strength of alloys increase from 955 MPa to 1434 MPa.Over 616.8 MPa of the increment is caused by the increasing of α-martensite strengthening.This is attributed to the increasing of α-martensite density caused by decreasing of recrystallization ratio and increasing of dislocation density.(2)Fe46.8Mn30Co10Cr10N3.2 high entropy alloys are 30%、60%and 90%-cold rolled,annealed at different temperatures in salt furnace for 3 min and water-quenched.When recrystallization ratio increases a lot simultaneously,yield strength of the alloy become lower after increasing of cold-rolled-reduction.This is attributed to the inhibiting effect of rising of recrystallization ratio on α-martensite phase transition.(3)Fe46.8Mn30Co10Cr10N3.2 high entropy alloys are 30%-cold rolled followed by 870℃annealing,60%-cold rolled followed by 780℃-annealing and 90%-cold rolled followed by 700℃-annealing respectively.Three kinds of alloys with almost the same recrystallization ratio are made.Mechanical properties indicate that yield strength of alloy 90%-cold rolled followed by 700℃ annealing reaches 1552 MPa,768 MPa higher compared with alloy 30%cold rolled followed by 870℃-annealing.This is attributed to higher dislocation density of 90%-cold rolled alloy.Higher dislocation density is beneficial to nucleation of α-martensite.Dense α-martensite laths improve yield strength of the alloys obviously.