The current work investigates how the interactions among constituent elements in high entropy alloys or complex concentrated alloys (HEA/CCAs) can lead to lattice instability and local chemical ordering which in turn affects the microstructure and properties of these alloys. Using binary enthalpies of mixing, the degree of ordering in concentrated multi-component solid solutions was successfully tailored by introducing Cr, Al and Ti in a CoFeNi HEA/CCA. CoFeNi was selected as the base alloy to achieve a close to random solid solution as indicated by the near-zero binary enthalpies in CoFeNi alloy system. The room temperature tensile properties of these alloys with varied degree of ordering follow a consistent trend where yield stress increased with degree of ordering. This novel approach provides a new alloy design strategy to obtain controlled ordering tendencies and consequently targeted mechanical properties. Further studies on specific alloys have been conducted to utilize this ordering tendency in attaining precipitation strengthening. For this purpose, Al, Ti and Ni were selected to promote ordering and Co, Fe, and Cr were chosen to strengthen the solid solution matrix. In Al0.25CoFeNi HEA/CCA, the ordering tendency between Al and Ni results in a competition between two long-range ordered phases, L12 and B2. While homogenous L12 precipitation takes place at an annealing temperature of 500oC, heterogeneous B2 precipitation occurs at 700oC. At 600oC, this competition between L12 and B2 phases results in a novel nano-lamellar microstructure. The alternating lamellae are mainly FCC and BCC based whose morphology is similar to pearlite in steels. However, the FCC lamella is made up of FCC and L12 phases and the BCC lamella is made up of BCC and B2 phases. A different thermomechanical processing route can be used to obtain the same phase composition but distributed in a nano-grained fashion. This nano-grained microstructure exhibits the best strength-ductility combination in this alloy. Thermomechanical processing can also be used to engineer the transformation pathway of L12 from homogenous to discontinuous precipitation. The homogenous and discontinuous L12 precipitation has been investigated in two different alloys namely, Al0.2Ti0.3Co1.5CrFeNi1.5 and Al0.3Ti0.2Co0.7CrFeNi1.7. While discontinuous precipitation (DP) is generally considered deleterious to mechanical properties, the results from this study suggests that microstructures with DP perform better compared to homogenous L12 up to 500oC. However, beyond 500oC, microstructures with homogenous L12 appears to perform better than discontinuously precipitated FCC+L12 microstructure.
| Identifer | oai:union.ndltd.org:unt.edu/info:ark/67531/metadc1833470 |
| Date | 08 1900 |
| Creators | Dasari, Sriswaroop |
| Contributors | Banerjee, Rajarshi, 1972-, Mishra, Rajiv S., Srivilliputhur, Srinivasan Gopalan, Scharf, Thomas W., Fraser, Hamish L. |
| Publisher | University of North Texas |
| Source Sets | University of North Texas |
| Language | English |
| Detected Language | English |
| Type | Thesis or Dissertation |
| Format | xii, 217 pages : illustrations (chiefly color), Text |
| Rights | Public, Dasari, Sriswaroop, Copyright, Copyright is held by the author, unless otherwise noted. All rights Reserved. |
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