Effect Of Strengthening Strategies On Microstructure And Properties Of FeCrNiMn High Entropy Alloy

The high-entropy alloy is a new alloy material composed of a variety of main constituent elements.It has excellent characteristics unmatched by traditional alloys,such as high plasticity,high temperature resistance,wear resistance,and corrosion resistance,etc.FeCoNiCrMn high-entropy alloy is the earliest proposed high-entropy alloy with single-phase FCC structure,which can maintain excellent plasticity and fracture toughness from room temperature to liquid nitrogen temperature.However,this alloy contains expensive Co element,and its yield strength is even less than 200MPa under room temperature,which severely restricts its application and development as a structural material.This subject takes FeCrNiMn high-entropy alloy without Co as the object,studies the influence of cooling rate,heat treatment and Si element on its structure and mechanical properties,and provides guidance for the development of new high-entropy alloys with low cost and excellent comprehensive mechanical properties.FeCrNiMn high-entropy alloy samples with different cooling rates were obtained by casting ceramic molds?investment casting?and copper molds?metal mold casting?,and the effects of cooling rate on the structure and mechanical properties of the alloy were studied.The results showed:with different cooling rates,the alloys were all single-phase FCC solid solutions,and the structures were all dendritic morphologies.The content of Cr and Fe elements in the dendrite area was high,and the contents of the four elements in the interdendritic area were close to the theoretical value.After increasing the cooling rate,the grain refinement and segregation were improved,and the mechanical properties of the alloy were also improved.Compared with alloy samples cast at a low cooling rate,the size of dendrites in alloy samples cast at a high cooling rate was reduced from 200?250?m to 100?150?m,and the content of Cr element in the dendritic region was 50.96 at.%reduced to 38.28 at.%;The yield strength of the alloy increased from 269 MPa to 313 MPa,an increase of 16%,and the elongation decreased from 36.1%to 34.9%,only a decrease of 3%.This group of experiments confirmed that increasing the cooling rate of the alloy during solidification is an effective means to strengthen the FeCrNiMn high-entropy alloy.The FeCrNiMn high-entropy alloy in investment casting was heat-treated at700?1100?,and the influence of heat treatment process on the structure and mechanical properties of the alloy was studied.The results showed that compared with the as-cast FeCrNiMn high-entropy alloy,the dendrites generally decreased after heat treatment,and some Mn-rich and Ni-rich phases precipitated between the dendrites,and the overall mechanical properties of the alloy generally improved.Moreover,as the heat treatment temperature increases,the dendrite size decreases,the Mn-rich and Ni-rich phases increased,and the strength and plasticity of the alloy continue to increase.After heat treatment at 700?,900?,and 1100?,the dendrite size was reduced from 200?250?m of the as-cast alloy to 150?200?m,50?100?m,and 3?10?m respectively;newly precipitated“point like”Mn-rich and Ni-rich phases continues to increase,especially after heat treatment at 1100?,the phases were arranged in the matrix in the form of ridges;the yield strength of the alloy was 339 MPa,365 MPa,and 398 MPa,respectively.The alloy increased by 26%,35%and 47%respectively;the elongation of the alloy was 37.8%,49.6%and 60.2%respectively,which was increased by 5%,37%and 67%compared with the as-cast alloy.This group of tests confirmed that heat treatment is an effective means to strengthen the FeCrNiMn high-entropy alloy.Especially after heat treatment at 1100?,the strength and plasticity of the alloy are both increased to about 1.5 times of the as-cast state.Pottery mold?investment casting?was used to obtain FeCrNiMn Si_0.3 high-entropy alloy,and the alloy was heat-treated at 700?1100?to study the influence of Si element on the structure and mechanical properties of FeCrNiMn high-entropy alloy.The results showed that,after adding Si element to the FeCrNiMn high-entropy alloy,the dendrites in the alloy transform into equiaxed dendrites,and Cr-rich and Fe-rich phases precipitate in the equiaxed dendrite area Compounds such as Mn₃Ni₂Si and Mn₆Ni₁₆Si₇ were precipitated in the interdendritic region,the strength and corrosion resistance of the alloy were improved,but the elongation of FeCrNiMn Si_0.3 alloy was only 1.5%.After heat treatment at 700??1100?,the brittleness of FeCrNiMn Si_0.3high-entropy alloy has not been significantly improved,because the brittle Mn₃Ni₂Si and other compound phases in the alloy have not been completely eliminated during heat treatment.Nevertheless,the test confirmed that heat treatment can effectively eliminate the compound phases such as Mn₃Ni₂Si and Mn₆Ni₁₆Si₇ that caused brittleness,and this phase continued to decrease as the heat treatment temperature increased.This group of experiments shows that the addition of Si element is not an excellent method to strengthen the FeCrNiMn high-entropy alloy.Although the strength of the alloy is improved,it also causes the alloy to become brittle.However,by further adjusting the heat treatment process,it is expected to ameliorate the brittleness of FeCrNiMn Si_0.3 high-entropy alloy.