Seminar series: Prof. Michael Kaufman, Colorado School of Mines

Thursday 13th February - Origin of Diffuse Intensities in Electron Diffraction and Relation to Short Range Order
Interpreting diffuse intensities in electron diffraction patterns can be challenging in samples with high atomic-level complexity, as often is the case with multi-principal element alloys. For example, diffuse intensities in electron diffraction patterns from simple face-centered cubic (fcc) and related alloys have been attributed to short-range order (SRO), medium-range order (MRO), or a variety of different {111} planar defects, including thin twins, thin hexagonal close-packed layers, relrod spiking, and incomplete ABC stacking. Here we demonstrate that many of these diffuse intensities, including 1/3{422} and ½ {311} reflections in ⟨111⟩ and ⟨112⟩ selected area diffraction patterns, respectively, are due to reflections from higher-order Laue zones from the matrix material and are not due to SRO or MRO. We explain these intensities and show that our calculated intensities of projected higher-order Laue zone reflections as a function of deviation from their Bragg conditions match well with the observed intensities, proving that these intensities are universal in fcc materials. Finally, we provide a framework for determining the nature and location of diffuse intensities that could indicate the presence of SRO or MRO.

Thursday 27th February - Tailoring the Deformation Mechanisms in Non-equiatomic Co-Cr-Fe-Ni-Mn Medium/High Entropy Alloys (M/HEAs) to Achieve Enhanced Mechanical Behavior
Thermodynamic calculations (e.g. using CALPHAD) were used to identify three non-equiatomic CoCrFeMnNi M/HEAs with different deformation characteristics, namely, twinning induced plasticity (TWIP) and transformation-induced plasticity (TRIP). The three alloys - Co15Cr15Fe50Mn10Ni10, Co20Cr20Fe40Mn10Ni10, and Co25Cr25Fe30Mn10Ni10 - were produced by vacuum induction melting, cast into a copper chill mold, homogenized, thermomechanically processed, recrystallized, characterized, and tested in tension at 25 °C and -100 °C. The annealed and deformed samples were characterized by a combination of electron backscattered diffraction (EBSD), high-energy synchrotron X-ray diffraction (HE-SXRD), and transmission electron microscopy (TEM). Increasing the Co and Cr concentrations is shown to result in lower stacking fault energy (SFE), increased recrystallization rates and lower grain growth rates resulting in finer grain size in the Co25Cr25 alloy, where the highest strength-toughness was observed. Due to the decrease in SFE with increasing Co and Cr, a transition in the dominant deformation mechanisms from TWIP (Co15Cr15) to TWIP/TRIP (Co20Cr20) to TRIP (Co25Cr25) was observed. This work sheds light on the development of novel FCC M/HEAs in the CoCrFeMnNi system, by identifying alloys that exhibit an optimal strength-ductility/toughness balance.

Thursday 13th March - Design, Production, and Characterization of Advanced Multi-Principal Element Alloys
Multi-principal element alloys (MPEAs) or High Entropy Alloys (HEAs) have received significant attention over the past two decades. The multicomponent approach to alloy design incorporates large, often equiatomic fractions of multiple elements, rather than basing the composition on a single elemental solvent. Because the resulting compositional landscape is massive and largely unexplored, progress in this area requires high-throughput experimental methodologies and computational models to effectively design alloys with desired properties. In this presentation, some of the high-throughput techniques developed to date will be described with specific emphasis on the “Effective Atomic Radii for Strength” (EARS) and the “Strength PredIction by NAno Hardness” (SPINAcH) methodologies developed to predict solid solution strengthening in single-phase MPEAs. Using the EARS methodology, a ternary Cr-Ni-Co alloy that is over 50% stronger than one of the strongest Cr33.3Ni33.3Co33.3 HEAs from the CrMnFeNiCo family, while retaining equivalent ductility. We show that the strength is maximized when roughly half of the atoms in the alloy are large and the other half are small implying that the strongest compositions are not necessarily equiatomic since the strength is controlled by lattice distortion rather than configurational entropy. By combining this methodology with CALPHAD methods, it is possible to conduct more efficient high-throughput evaluations and performance-driven alloy design.