Roles of nanoscale precipitates and metastable austenite in determining strength and toughness of high-strength Nb-bearing steel

The effects of tempering temperature on the microstructure and mechanical properties of high-strength structural steel containing niobium were investigated to examine the roles of nanoscale precipitates and metastable austenite in determining the yield strength and toughness.After hot-rolling and quenching,three experimental steels were tempered at 590,630,and 670℃.During tempering,nanoscale Nb(C,N)precipitates were formed with the recovery of quenched martensite.The average diameters of Nb(C,N)precipitates increased from 5.4 to 8.2 nm as the tempering temperature was increased.Notably,reversed austenite with a volume fraction of 9%was formed at tempering temperatures up to 670℃.The yield strengths of steel containing tempered martensite tempered at 590 and 630℃ were 965 and 831 MPa,and the tensile strengths were 998 and 879 MPa,respectively.However,the steel comprising reversed austenite and tempered martensite tempered at 670℃ showed continuous yielding behavior,affording yield and tensile strengths of 610 and 889 MPa,respectively.The impact energy increased from 105 to 260 J at−60℃ with increasing tempering temperature.Reversed austenite improves low-temperature toughness by significantly increasing the crack propagation energy.

A novel Cu-doped high entropy alloy with excellent comprehensive performances for marine application

High entropy alloy(HEA)has attracted great interests as one of the promising multifunctional materials in marine applications.However,Cu as an effective biocide tends to form segregation in HEA,which could deteriorate corrosion and induce brittle fracture.Herein,we report a strategy to tailor the existing form of Cu in HEA from undesired large-scale segregation to uniform distribution with dispersed nanoscale precipitation,while retaining the unique structure characteristics of HEA.Eliminating Cu segregation improves toughness and avoids serious corrosion in the grain boundary.Uniform distribution with dispersed nanoscale precipitation of Cu further enhances the antifouling and lubricating abilities of Cu-doped HEA.Tailored AlCoCrFeNiCu_(0.5)HEA in this work has excellent comprehensive properties combining good mechanical properties,outstanding antifouling abilities,superior resistance to corrosion and wear.Furthermore,the corresponded mechanisms are discussed in terms of Cu-segregation-eliminated,nanoscale-Cu-precipitate-forming and comprehensive properties.