Alloying Effects On Microstructures And Mechanical Properties Of FeCoNiCr-based High Entropy Alloys

The concept of high-entropy aslloys (HEAs), a brandnew alloy design strategy, was proposed in the 1990s. Such alloys usually contain more than 5 components, and each constituent is in a near molar ratio but no larger than 35%. Due to the effect of high configuration entropy, HEAs tend to form simple solid-solution structure with severe lattice distortion, which in turn yields a variety of interesting features, including high phase stability and sluggish diffusion, and exhibit many unique properties.Nevertheless, research of HEAs is still at its initial stage, and many scientific questions urgently need to be addressed. First, previous studies in the HEA field mostly focus on compression and hardness properties while the tensile property, which is of great importance to practical applications, is far less concerned. Second, the composition-structure-property relationship in HEAs is not systematically studied and established. Lastly, due to their high phase stability and sluggish diffusion, HEAs have great potential to be utilized as high-temperature materials. However, relevant studies are really scarce.A prototype fcc (face-center-cubic) HEA system, i.e., FeCoNiCr(Mn), was selected as our starting point in this thesis. First, we explored alloying effects of Al on the microstructure and mechanical properties of the FeCoNiCrMn HEA. We found that addition of Al gradually induce the structure transtion from fcc to bcc (body-center-cubic). Formation of brittle B2 phase increased the tensile strength but decreased the ductility. Second, we studied high-temperature (i.e., from 1023 to 1123 K) flow behavior of the FeCoNiCrMn HEA. Based on the derived stress exponents, it was clear that the whole deformation process was diffusion dominated, whilst the microstructure of the fractured specimens indicated that the solid-solutioned matrix was not stable enough, and other strengthening mechanisms were needed. Thirdly, based on the above results, simultaneous addition of Ti and Al into the FeCoNiCr HEA was conducted, and a high density of nano ?' precipitates was formed and uniformly dispersed in the matrix. The room-temperature tensile strength was dramatically enhanced due to the pronounced precipitation hardening effect, along with respectable ductility and work hardening. Subsequently, effects of alloying additions and aging process on the precipitation behavior and tensile properties were systematically studied. The results indicated existence of strong phase competition between two main precipitations, i.e., the ?' phase and Heusler phase, and the optimal composition was determined as (FeCoNiCr)9?4TiAl4. Lastly, we investigated high-temperature flow and creep behavior of the optimized alloy, and found a shaply decrement in steady-state strain rate for 2 orders of magnitude, manifesting an outstanding particle strengthening effect even at elevated temperatures. However, the creep rupture lifetime was still not satisfatory, which was probably due to formation of the brittle Heusler phase and the weak grain boundaries.In addition, dynamic deformation behavior of the FeCoNiCrMn HEA at high strain rates under different loading modes, i.e., compression, tension and torsion, was preliminarily explored. It was found that fine-grained specimens (around 36?m) greatly stimulated twinning formation during deformation, and thus gave rise to outstanding mechanical performances such as high strength, large ductility and pronounced work hardening.