The Role Of Carbon And Titanium In Grain Refinement And Room Temperature Tensile Properties Of Cast CrFeCoNi High Entropy Alloys

As a promising engineering material,CrFeCoNi system high entropy alloys?HEAs?have attracted extensive attention worldwide.Their cast alloys are of great importance because of their great formability of complex components.However,the columnar crystals produced during the solidification process of CrFeCoNi high entropy alloys are not only large in size,but also have obvious preferred orientation,which seriously affects their mechanical properties.thus the solidification structure of as-cast alloys should be amended to meet the needs of engineering applications.Taking into account the CrFeCoNi high entropy alloy casting process under normal conditions,this research were carried out by alloying method to optimize the solidification structure,resulting in the transformation of columnar to equiaxed crystals and further grains refinement,thereby the mechanical properties were improved.Here,we used C and Ti to refine the as-cast grains of a type of high entropy alloy CrFeCoNi.The alloys with various addition of C or Ti were produced by arc melting and drop casting.The solidification macrostructure and microstructure were characterized by using optical microscopy,scanning electron microscopy and transmission electron microscopy.The phase structure was examined by X-ray diffraction and the mechanical properties were tested by tensile testing rig.Pseudo-binary phase diagrams of CrFeCoNi-C system was calculated by ThermoCalc software with the database of TCFE7,and then the mechanism of grain refinement was analyzed.The cast HEAs transform from coarse columnar grains into equiaxed grains with the C level increased to?2 at.%and the size of equiaxed grains are further decreased with increasing C addition.It was revealed that the interdendritic segregation of Cr and C results in grain boundary precipitation of M₂₃C₆ carbides.The grain refinement is attributed to the effect that carbon generates additional constitutional supercooling.The yield stress and tensile strength at room temperature are increased with increasing C addition.The CrFeCoNi-xTi high-entropy alloy solidification structure changes significantly with the addition of Ti.Ti-Ni enrichment regions was first formed on the grain boundary with increasing the Ti content.When Ti addition reached up to a certain amount,the reticulated second phase Ni₃Ti formed at grain boundaries.When the content of Ti is above 2.5 at.%,solidification structure changed to equiaxed grain structure.When the Ti content is 2.5 at.%and 5 at.%,the yield stress and tensile strength at room temperature are increased due to the grain refinement.Compared with the matrix,the yield strength of CrFeCoNi-2.5 at.%Ti and CrFeCoNi-5 at.%Ti alloy increased by 58.4%and 173.5%,and the tensile strength increased by 30.1%and 84.6%,respectively.