Microstructures And Deformation Mechanisms Of Fcc Phase High Entropy Alloys

Author : Carlyn R. LaRosa
Publisher :
ISBN 13 :
Total Pages : 136 pages
Book Rating : 4.:/5 (133 download)

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Download or read book Atomic-scale Deformation Mechanisms and Phase Stability in Concentrated Alloys written by Carlyn R. LaRosa and published by . This book was released on 2021 with total page 136 pages. Available in PDF, EPUB and Kindle. Book excerpt: High entropy alloys are a relatively new class of multicomponent alloys which exhibit a remarkable combination of strength, ductility, and fracture toughness. There are two general classes of high entropy alloys: 3d-transition metal high entropy alloys and refractory element high entropy alloys. The former has garnered considerable interest for applications at lower temperatures, while the latter is typically developed for high-temperature applications. Understanding the deformation mechanisms operative in these materials is central to improving their mechanical performance. Here, we present our efforts to answer some of these questions. First, we show that the exceptional mechanical properties of CrCoNi, a ternary derivative of the widely studied CrMnFeCoNi high entropy alloy, are the result of a magnetically-driven phase transformation. While previously thought to have a single-phase fcc crystal structure, we have shown the existence of a nano-structured hcp phase which is unique to CrCoNi among the CrMnFeCoNi parent high entropy alloy and its derivatives. We also provide a mechanistic picture for the formation of this phase transformation and show that it can be formed both homogeneously and heterogeneously. We also demonstrate how dislocation interactions with existing phase boundaries can produce both strength- and ductility-enhancing mechanisms. Next, we present our newly developed bond energy model for the prediction of stacking fault energies in concentrated alloys, such as high entropy alloys. We introduce the conceptual basis of our model, explain how to determine the requisite bond energies, and apply our models to four different concentrated alloys. We show that our model can predict stacking fault energies for the intrinsic stacking fault, extrinsic stacking fault, twin formation, and hcp formation which are in good agreement with the values calculated using density functional theory, despite a small fraction of the computational cost. We discuss how to further expand our model to provide additional utility. Finally, we apply MonteCarlo and Multi-Cell Monte Carlo methods for phase prediction in refractory high entropy alloys and refractory superalloys. We apply the Monte Carlo method for the phase prediction of AlMoNbTi and AlHfNbTi, which have both been shown experimentally to have the B2 crystal structure. While the predictions for the former were in accordance with previous reports, the latter was shown to be unstable in the bcc crystal structure. We subsequently apply the Multi-Cell Monte Carlo method for phase prediction of a Al5.9Nb23.5Ta23.5Ti23.5Zr23.5 refractory superalloy, which has a two-phase A2+B2 microstructure. Phase separation was predicted, and the compositions of the two phase were similar to those reported experimentally. However, while the Nb- and Ta-rich simulation cell crystallized in a disordered A2 phase, as expected, the Zr-rich simulation cell was unstable. The instability of this Zr-rich phase and theAlHfNbTi alloy would suggest the inclusion of the anharmonic vibrational energy is an important component for alloys containing group 4 elements, which are unstable in the bcc crystal structure in their elemental forms. We discuss future benchmarking efforts and provide recommendations for future studies of phase prediction to develop new two-phase refractory superalloys.