Shape-performance Integrated Design And Strengthening Mechanism Of Precipitation Strengthened FeNiCoCr-based High-entropy Alloys

The emergence of high-entropy alloys has greatly expanded the composition design space of metallic materials.By adjusting the composition and content of the main elements,alloy systems with various unique mechanical,physical,and chemical properties can be developed,providing more new options and ideas for breaking through the performance limit bottleneck of traditional structural materials.Fe Ni Co Cr HEAs are the most typical class of HEAs with excellent structural stability,mechanical properties,and corrosion resistance,but like other FCC structural alloys,Fe Ni Co Cr HEAs applications are limited by the low room temperature yield strength.Therefore,how to further improve the yield strength of Fe Ni Co Cr HEAs has become a hot topic of research.In this paper,Fe Ni Co Cr-based high entropy alloys were prepared by low-speed ball milling+hot pressing sintering method,and the effects of Si C,and Al/Ti dopants on the microstructure and properties were investigated respectively.The specific conclusions are as follows.The Fe Ni Co Cr high entropy alloys with different Si C content（0 wt%,2.5 wt%,5 wt%,and 7.5 wt%）were successfully prepared by the hot press sintering technique.The main matrix phase of the alloys is the FCC phase,and Si C reacted with Cr to form Cr₇C₃ secondary phase with an orthogonal crystal structure,and its content increased with increasing the Si C content.It is observed that the alloy with 7.5 wt%Si C content formed Si-rich amorphous Si O₂ particles at the grain boundaries,while Ni-and Si-rich L1₂-structured Ni₃Si nanoparticles are precipitated within the crystal.When the Si C content is less than 5 wt%,the yield strength of the HEA increased little,and the strengthening mechanism is mainly associated with the strengthening of Cr₇C₃ particles.With a further increase in the Si C content to 7.5 wt%,the HEAs show a yield strength of 891 MPa,respectively,which is 87.6%higher than that of the HEA without Si C.At the same time,the compressive strain still reaches 28%.Since the strengthening of Si O₂ particles is predominant,the main strengthening mechanisms for the alloy are attributed to thermal mismatch and Orowan strengthening caused by the Si O₂ particles at the grain boundaries.The（Fe Ni Co Cr）_100-x-yAl_xTi_y high-entropy alloys were successfully prepared by the hot-pressure sintering technique.The matrix phases of these alloys are FCC structure,among which（Fe Ni Co Cr）₉₃Al₅Ti₂,and（Fe Ni Co Cr）₉₅Al₅Ti₅ alloys form nano-precipitation phase with L1₂-type inside the FCC matrix grains.The nanoparticles in（Fe Ni Co Cr）₉₃Al₅Ti₂ alloy are irregularly rounded spheres with an average size of 8 nm and a volume ratio of 12%,and the nanoparticles in（Fe Ni Co Cr）₉₅Al₅Ti₅ alloy are regular rounded squares with an average size of113 nm and a volume ratio of 48%.In addition,there are some minor phases:bulk Cr-richσphase and Fe-rich BCC structured phase in（Fe Ni Co Cr）₉₃Al₅Ti₂ alloy;bulk Cr-richσphase and a small amount of fine（Ni,Al,Ti）-riched phase and Al-rich phase formed at grain boundaries in（Fe Ni Co Cr）₉₀Al₅Ti₅ alloy;Cr-richσphase,Ni₃Ti phase,and Ti-rich phase in（Fe Ni Co Cr）₉₅Ti₅ alloy.The（Fe Ni Co Cr）₉₀Al₅Ti₅alloy has the highest true yield strength and ultimate true compressive strength of 948 MPa and1684 MPa,respectively,and maintains 46%of the true compressive strain.The high strength mainly originated from the high-density L1₂-type nanoprecipitation phases within the crystal.The（Fe Ni Co Cr）₉₀Al₅Ti₅ alloy was quenched at 850°C and quenched at 1150°C,respectively.Both heat treatments significantly reduce the yield strength of the alloy,and the 1150℃quenching causes the largest reduction but significantly improves the plasticity of the alloy.Quenching at 850°C led to the growth of the intracrystalline nano-L1₂ precipitated phase into equiaxed grains with a size of more than 10 mm and the formation of a small amount of lamellar HCP structured phase.Quenching at 1150°C results in the complete dissolution of the intracrystalline nano-L1₂ precipitated phase.