Mechanical Properties of CoCrFeNiTi High Entropy Alloy Prepared by SLM

Contributor: Wang Xing Ideal Zhang Hang

Contributor: State Key Laboratory of Mechanical Manufacturing Systems Engineering

High entropy alloy (HEA) is an alloy composed of five or more elements with an equimolar or near equimolar concentration between 5% and 35%.  HEA has high corrosion resistance, oxidation resistance, wear resistance and excellent mechanical properties.  However, the inherent complexity of HEA makes it difficult to produce uniform alloy parts, especially large parts.  By using conventional methods such as arc melting or induction melting, multiple physical remelting is required to ensure uniformity of HEA. In addition, it is difficult to process these alloys into parts with complex geometric shapes.  Therefore, people are paying more and more attention to AM methods, such as selective laser melting (SLM) and electron beam melting (EBM).

Tadashi Fujieda et al. of Hitachi Materials Research and Development Group Technology Center studied the preparation of CoCrFeNiTi-based high entropy alloy by selective laser melting (SLM) and electron beam melting (EBM). The study found that the higher cooling rate during SLM resulted in fine and uniform microstructure and no visible segregation (as shown in Figure 1). In EBM samples, Ni and Ti separated and formed spherical morphology.  The columnar grain of SLM sample is weaker than that of EBM sample, the grain tends to be equiaxed, and the crystal anisotropy of SLM sample is also weaker than that of EBM sample.  In addition, the grain size of SLM sample is much smaller than that of EBM sample.  (as shown in fig. 2) this is due to the difference in the ratio of thermal gradient (g) to solidification rate (r) of the molten pool. the G/R of SLM is much lower than EBM, and the solidification rate of SLM is higher.

The tensile property of SLM sample is far better than that of EBM sample, and the ductility is improved. The elongation at break of SLM sample is six times that of EBM sample.  This is because the SLM sample is homogeneous in composition and does not contain any Ni3Ti intermetallic compound with spherical microstructure, which is the origin of fracture.