Effects Of Ceramic Addition On Microstructure And Mechanical Properties Of Laser Melting Deposited CrMnFeCoNi High-entropy Alloys

Laser Melting Deposition(LMD)technique provides a more powerful tool to obtain High-Entropy Alloys(HEAs)components with more homogeneous composition in face centered cubic(FCC)system represented by CrMnFeCoNi HEA.However,the low yield strength of LMD-fabricated FCC system HEAs is still a roadblock for their practical application.Therefore,new fabrication and modification methods are still required to further improve the strength of HEA printing components.In this thesis CrMnFeCoNi HEA-based composites with different addition content of Ti C and Ti N ceramics are prepared by LMD process.The effects of ceramic addition on the microstructure and mechanical properties of CrMnFeCoNi HEAs are studied by means of scanning electron microscopy(SEM),Transmission Electron Microscopy(TEM),energy dispersive spectrometer(EDS),X-ray diffraction(XRD)and mechanical tensile test.The results are as follows:The results of XRD analysis indicate that,due to high-entropy effect,CrMnFeCoNi HEAs consist of FCC single-phase solid solution crystal structure and the CrMnFeCoNi HEA-based composites with Ti C and Ti N creamic addition are composed of FCC matrix phase and Ti C/Ti N ceramic phase,by means of LMD.The CrMnFeCoNi HEA samples prepared by LMD with a low laser power(800 W)show obvious preferred orientation along the direction of(200),and the preferred orientation can be reduced and the isotropy can be improved by increasing laser power and adding Ti C ceramic phase.The results of SEM and EDS tests show that,the addition of 2.5wt.%and 5wt% Ti C and 2.5wt% Ti N ceramic particles can be uniformly distributed in the HEA matrix while the Ti N ceramic particles will occur agglomeration in the HEA matrix when the addition amount reaches 5wt%.The results of microstructure observation show that the CrMnFeCoNi HEAs fabricated at a low LMD power is mainly composed of coarse columnar grain structure,and the increasing of LMD power and Ti C addition amount can promote columnar-to-equiaxed transition(CET)and refine the grain size in the HEAs.The HEA-based composites with Ti N addition mainly composed of elongated columnar grain structure.The results of static mechanical tensile tests at different temperatures show that the addition of both Ti C and Ti N ceramic phases can effectively improve the tensile strength of CrMnFeCoNi HEA matrix.Further SEM analysis of tensile fractures show that the addition of Ti C and Ti N ceramic pashe hinder the expansion of the slip bands in the deformation process of the HEA composite,thus improving the tensile strength of the composite.The above results indicate that the distribution of Ti C is more uniform in the HEA matrix and thus the obvious strengthening effect on CrMnFeCoNi HEAs compared with Ti N.In conclusion,we have successfully prepared CrMnFeCoNi HEA-based composites reinforced by ceramic particles by LMD,and the resultant composite has the characteristics of uniform structure and composition,fine grains and dispersion of ceramic strengthening phase.The present work provides a new method and an important approach for strengthening the mechanical properties of FCC system HEAs,and thus has important research significance.