Microstructures And Mechanical Properties Of FeCoCrNi-based High-entropy Alloys Fabricated By Selective Laser Melting

Due to the "high mixing entropy" and "hysteretic diffusion" effects,the mechanical properties of multi-elemental high-entropy alloys(HEA)are better than those of traditional alloys.However,crystal structure has an important influence on the mechanical properties of high-entropy alloys.For example,Face Centred Cubic(FCC)has the characteristics of good plasticity and low strength,while Body Centered Cubic(BCC)has the characteristics of high strength and poor plasticity.By changing the content of Al in the high entropy alloy,the crystal structure can be transformed,and then the proportion of FCC phase and BCC phase can be adjusted to obtain high entropy alloy of excellent comprehensive mechanical properties.However,the traditional manufacturing methods(such as casting,powder metallurgy,etc.),lack the ability to control the microstructures and mechanical properties,and form complex structures,resulting in the problems of coarse structure,severe segregation and poor mechanical properties.Selective laser melting(SLM)additive manufacturing is an advanced manufacturing technology,which takes into account the requirements of precise manufacturing and high performance.It can control the microstructures of local area through certain process strategies and methods,and produce products with complex structure and specific performance,which cannot be achieved by other manufacturing technologies.In this paper,taking FeCoCrNi-based high-entropy alloys as the research objects,by adjusting the mixing ratio of equiatomic FeCoCrNi(Al content is 0at.%)and FeCoCrNi Al(Al content is 20 at.%)high-entropy alloy powders,used SLM technology to fabricate FeCoCrNi-based high-entropy alloys with different Al content of 0at.%,11 at.%,13 at.%,15 at.%,20 at.%.The microstructures and mechanical properties of high-entropy alloys controlled by material composition and SLM process parameters were studied,and the relationship between composition,process,microstructures and properties and its influence law were explored.The main research contents and conclusions have been drawn as follows:(1)Firstly,the process optimization of FeCoCrNi-based high entropy alloys of different Al content fabricated by SLM was studied.Through the single track,single layer and block process optimization experiments of FeCoCrNi(Al content is 0at.%)and FeCoCrNi Al(Al content is 20 at.%),the highest density of FeCoCrNi and FeCoCrNi Al were 99.95% and 99.53% respectively,and the corresponding optimal process parameters were: the laser power was 300 W,the scanning speed was 700mm/s,the scanning distance was 0.09 mm.FeCoCrNi-based powders with different mixing ratios of FeCoCrNi and FeCoCrNi Al were fabricated by using the optimal process parameters.The Al content in the mixed powders were 11 at.%,13 at.%,15 at.%,and the highest density were 99.61%,99.73% and 99.42%,respectively.The results showed that the optimization of SLM process parameters was reasonable,and the different content of Al in FeCoCrNi-based high-entropy alloys has little effect on SLM process parameters.(2)On the basis of process optimization,the crystal structure evolution of FeCoCrNi-based high-entropy alloys of different Al content fabricated by SLM were studied,and the effect of Al content on the crystal structure of FeCoCrNi-based high entropy alloys was obtained by X-ray diffraction.The results showed that when Al content is 0at.% and 11 at.%,it is FCC phase structure,when Al content was 13 at.%,it was FCC + BCC dual-phase structures,when Al content was 15 at.% and 20 at.%,it was disordered BCC(A2)phase and ordered BCC(B2)phase structures.In this study,the composition of Al transformed from FCC phase to BCC phase was in the range of 11 at.%-15 at.%,which was less than that reported in the traditional casting method of 11.2at.%-18.4 at.%.This was due to the rapid melting and solidification characteristics of SLM,which had a certain inhibitory effect on the diffusion of Al.(3)On the basis of crystal structure studies,the microstructure evolution of FeCoCrNi-based high-entropy alloys of different Al content fabricated by SLM was studied.The microstructures and composition distribution of each sample were observed by scanning electron microscope.The results showed that there are unique sub-grain structures such as cellular,elongated cellular,acicular and granular in the samples.The reason can be explained according to the non-equilibrium convection theory and the results of energy spectrum analysis: during the manufacturing process of SLM,the cooling rate is very high,so the hexagonal convection pattern generated by Bernard convection at the edge of the micro-molten pool and the strip convection pattern generated from the center to the edge of the molten pool were retained in the final microstructures,forming cellular and elongated cellular microstructures respectively;at the same time,the change of Al content led to the transformation of cellular structure to acicular microstructures.When the Al content was15 at.%,spinodal decomposition occurred,and the braided microstructures with light and dark staggered arrangement was formed.When the Al content further increased to 20 at.%,it turned into nano granular microstructures.(4)On the basis of microstructures research,the effect of different Al content on the hardness of FeCoCrNi-based high-entropy alloys fabricated by SLM was studied.The Vickers hardness of each sample was tested by microhardness tester.The results showed that the Vickers hardness of FCC phase and BCC phase in the samples increased with the increase of Al content.The nanoindentation hardness of different microstructures in each sample was further tested by nano-indentation instrument.The results showed that the nanoindentation hardness of different microstructures morphologies,such as cellular,elongated cellular,acicular,grain boundary and granular,were corresponding to the results of Al content in microstructures morphologies by energy spectrum analysis.The more Al content in the microstructures,the corresponding hardness was higher.The reason of the above phenomenonare was related to the lattice distortion strengthening effect caused by the different degree of interstitial solution strengthening of Al.(5)On the basis of the above research,the effect of SLM process parameters and material composition on the tensile properties of FeCoCrNi-based high entropy alloys at room temperature were studied.Firstly,the effect of SLM process paraeters on room temperature tensile properties of FeCoCrNi was studied.The results showed that the high entropy alloy fabricated with the optimal SLM process parameters not only obtained the highest density,but also had the best room temperature tensile properties,which verified the effectiveness of SLM process optimization.On this basis,the room temperature tensile properties of FeCoCrNi-based high entropy alloys of different Al contents were studied by using the optimal process parameters.The results showed that with the increase of Al content,the proportion of FCC phase decreased,while the proportion of BCC phase increased.Therefore,the yield strength and tensile strength of the sample increased significantly,while the strain gradually decreased.When the Al content was 13 at.%,best combination of comprehensive tensile properties at room temperature was obtained,the yield strength,the tensile strength,and the strain were 844±9MPa,1269±30MPa,4.09±0.20%,respectively.Through the analysis of fracture morphologies,it can be seen that when Al content was 0at.% and 11 at.%,the fracture showed ductile fracture,when Al content was 13 at.%,the fracture showed quasi cleavage fracture of brittle fracture and ductile fracture,and when Al content was 15 at.% and 20 at.%,the fracture showed brittle cleavage fracture.