Alloy Behavior and Design Across Length-Scales: An SMD Symposium Honoring Easo George: Point Defects and High Temperature Materials
Sponsored by: TMS Structural Materials Division, TMS: High Temperature Alloys Committee, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Michael Mills, Ohio State University; George Pharr, Texas A&M University; Robert Ritchie, University of California, Berkeley; Muralidharan Govindarajan, Oak Ridge National Laboratory
Monday 2:00 PM
March 20, 2023
Room: Cobalt 502B
Location: Hilton
Session Chair: Govindarajan Muralidharan, Oak Ridge National Laboratory
2:00 PM Invited
Strong Effects of Tiny Compositional Changes on Structural and Functional Material Properties: Gunther Eggeler1; 1Ruhr-Universität Bochum
In materials technology it is not uncommon to come across cases, where tiny compositional changes strongly affect structural and functional properties. A few examples from different fields are presented. Thus, it is shown how segregation of S/Sb to grain boundaries in Fe/Cu-based alloys promotes the formation of creep cavities. The segregation of alloy elements to planar faults in ordered intermetallic systems is discussed. It is then shown how small variations in Ni-concentration in a NiTi shape memory alloy affect phase transition temperatures and how the uptake of small quantities of C and O during ingot metallurgy of NiTi shape memory alloys affects phase transformation temperatures and structural fatigue lives. And finally, a case study is presented which shows how the creep behavior of a Re-free Ni-base single crystal superalloy changes when its low W content is reduced by a factor of 2. Implications for alloy design are discussed.
2:30 PM Invited
Some Unusual Aspects of the Deformation Behavior of FeAl: Ian Baker1; 1Dartmouth College
This presentation will discuss the work of the author with Easo George over a 17 year period on the mechanical properties of the B2 compound FeAl. Two dramatic features of the mechanical properties of FeAl will be emphasized, environmental embrittlement and the yield anomaly (increasing strength with increasing temperature), including the effects of boron and aluminum content on these phenomena. Both the room temperature ductility and yield strength of FeAl are affected by the presence of water vapor, which reacts with the aluminum to produce embrittling atomic hydrogen. Thus, the elongation to failure depends strongly on strain rate. A model, developed by George and Baker, for the yield anomaly based on vacancy strengthening will be outlined along with a comparison of predictions of the model with experiments Lessons drawn from the work on FeAl will be applied to current studies on high entropy alloys.
3:00 PM Invited
Solid Solution Hardening Effects on Thermal Stability and Mechanical Properties of Nanostructured Ni-enriched CrMnFeCoNi High Entropy Alloys: Tom Keil1; Enrico Bruder1; Karsten Durst1; 1Technical University Darmstadt
This work focuses on the saturation grain size as well as grain growth and decomposition tendencies of Ni-enriched subsystems of the CrMnFeCoNi Cantor alloy (Ni20, Ni60, Ni92 and Ni100)) after heat treatment for RT to 900°C with an emphasis on a transition from dilute solid solutions to high entropy alloys. The influence of solutes on the defect storage, microstructural stability and mechanical properties studied via Nanoindention will be discussed. After High Pressure Torsion saturation grain sizes in the nanometer regime are found for the HEA compositions, whereas the grain refinement scales with solid solution strengthening contribution. All alloys exhibit a two-stage coarsening behavior with marginal grain growth below a certain temperature and typical diffusion driven growth above, followed by decomposition tendencies for the HEA type alloys. Annealed alloys show a pronounced change in the pile-up behavior around nanoindentations, indicating a strong change in the work hardening behavior (Hardening via annealing).
3:30 PM Break
3:50 PM Invited
Deformation Mechanisms in Compositionally Complex Polycrystalline CoNiCr-based Superalloys: Steffen Neumeier1; Andreas Bezold1; Mathias Goeken1; 1Friedrich-Alexander-Universität Erlangen-Nürnberg
To improve the mechanical properties for high temperature applications of fcc medium-/high-entropy alloys (MEA/HEA) based on CoNiCr, compositionally complex alloys have been developed that are additionally strengthened with L12 structured precipitates. In this work, deformation mechanism maps are presented for such polycrystalline CoNiCr-based superalloys with high precipitate fractions. Despite their chemical differences, the dominant deformation mechanism in all four investigated alloys transitions from shearing by APB-coupled dislocation pairs to shearing under superlattice stacking fault formation with increasing temperatures and/or decreasing strain-rate. However, the regions of the dominant deformation mechanism varies between the different alloys. Additionally, microtwinning is observed at even higher temperatures and/or lower strain-rates but only in the alloys, in which Al and W are substituted for Ti and Ta. Accordingly, the influence of segregation processes and the role of small compositional changes on the occurring deformation mechanisms are discussed and potential guidelines for future alloy design are given.
4:20 PM Invited
Heat-Resistant Cr-Alloys, Microstructure, Oxidation and Creep: Uwe Glatzel1; Mathias Galetz2; Anke Silvia Ulrich1; 1University Bayreuth; 2DECHEMA Forschungsinstitut
A two phase alloy of Cr, bcc, solid solution matrix strengthened by a Cr3Si, A15 ordered, intermetallic phase was investigated with the goal of achieving an alloy with properties "Beyond Nickel-Based Superalloys". These Cr-Si alloys may enable higher working temperatures and lower densities. In addition, Cr is the only refractory metal which intrinsically builds an oxide scale suitable for protecting the alloy from oxidation. Si improves the oxide scale formation even further as well as a high temperature strength by precipitation of an intermetallic phase Cr3Si. We observed that the addition of further alloying elements such as Mo, Ge, and Pt leads to an improvement in creep properties (Mo), oxidation resistance (Ge), and nitridation resistance (Pt) by maintaining the two-phase microstructure. Creep tests were conducted at 980°C and higher and reveal promising results. The microstructures, scales, and reaction products were investigated by XRD, SEM, EPMA and image analysis.