Interstitial Strengthening in f.c.c. High-Entropy Alloys

NON-TECHNICAL SUMMARY The room-temperature solubility of small atoms such as carbon and boron in some traditional metals such as nickel and aluminum are very low and, thus, their solute strengthening effect in these metals is also very low. Recently, it has been shown that elements such as carbon and boron can have large solubilities in exciting new classes of alloys called high-entropy alloys and medium entropy alloys, and thus can significantly increase their strength. Such alloys are potentially useful in demanding engineering applications. This project will quantify the phenomenology of this strengthening effect and using advanced microstructural characterization techniques determine the underlying mechanisms of the strengthening. The project will be undertaken by the Principal Investigator, a Ph.D. student, and several undergraduates. For professional development, the graduate student will take both a teaching training course and ethics training. The public will be engaged both through outreach to local high schools for which some simple, inexpensive experiments will be developed that demonstrate some Materials Science phenomena, and at Science Pubs, in which community members and researchers have lively conversations about science topics. Research results will be published in refereed journals, presented at conferences, and archived in on-line databases. TECHNICAL SUMMARY The aim of the project is to understand both the phenomenology and micromechanisms of interstitial strengthening in f.c.c. high entropy alloys (HEA) and medium entropy alloys (MEAs). The working hypothesis is that interstitials have substantial effects on dislocation behavior (and, hence, mechanical properties) by changing slip from wavy to planar due to an increase in the friction stress (possibly due to short-range order, SRO), or a change in the stacking fault energy (SFE). To that end, the project will: determine the effects of interstitials on the stress-strain curves of single-slip-oriented single crystals, followed by post-mortem examination, using a TEM and an SEM, of the defect structure of crystals strained to various elongations; perform in situ deformation experiments in a TEM and an SEM, and use neutron diffraction to determine the dislocation behavior in detail; determine, using X-ray diffraction (XRD), STEM (including electron diffraction and X-ray spectroscopy), and atom probe tomography (APT), the effects of interstitials on the lattice (the size mismatch parameter, the modulus mismatch parameter, SFE, SRO) and their segregation to dislocations both before and after straining; and determine the exponents in the relationships between the increase in yield strength and both the interstitial concentration and the mismatch parameters. The work will be performed on the HEA/MEAs Fe40Mn40Co10Cr10 and CoCr0.25FeMnNi doped with various interstitials, and, time permitting, Fe40.4Ni11.3Mn34.8Al7.5Cr6. The work will involve synchrotron XRD measurements at the Argonne National Laboratory, and both in situ straining neutron diffraction studies and APT studies at the Oak Ridge National Laboratory. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.