Continuous Phase Transformations: Session II
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Chemistry and Physics of Materials Committee, TMS: Phase Transformations Committee
Program Organizers: Jessica Krogstad, University of Illinois at Urbana-Champaign; Gregory Thompson, University of Alabama; Matthew Steiner, University of Cincinnati; Janelle Wharry, Purdue University
Monday 2:00 PM
March 15, 2021
Room: RM 55
Location: TMS2021 Virtual
Session Chair: Janelle Wharry, Purdue University; Matthew Steiner, University of Cincinnati
2:00 PM Invited
Effect of Bulk and Local Phase Transformation on Deformation Mechanisms: Maryam Ghazisaeidi1; Mulaine Shih1; Edwin Antillon2; 1Ohio State University; 2Naval Research Lab
Motivated by high entropy alloys (HEAs), we first study the dislocation behavior in the composition-dependent metastable fcc alloys. Transitioning from fcc to hcp stable phases in fcc alloys corresponds to a change from positive to negative average stacking fault energy (SFE). Using Molecular Dynamics simulations, we study the effect of average versus “local” SFE on the dislocation stability and its connection to the decorrelation force to break the partial dislocations from equilibrium in these alloys. Next, using our newly developed Multicell Monte Carlo method for phase prediction, we study the formation of local phases at dislocations and boundaries and their subsequent effect on deformation in model alloys. Our results provide a framework for a broader consideration of the effect of bulk and local phase transformations on deformation mechanisms in metallic alloys.
2:30 PM
Cross-Slip and Work-Hardening in Short-Range Ordered FCC Alloys: Anas Abu-Odeh1; Mark Asta1; 1University of California, Berkeley
Planar-slip, or dislocation motion localized to a few slip planes, can be observed in many concentrated substitutional FCC alloys. This is largely attributed to short-range ordering (SRO) and the slip-plane softening effect, where the first dislocation in a planar-slip region forms a diffuse anti-phase boundary (DAPB), which causes subsequent dislocations to glide with less resistance. This study focuses on incorporating the effect of SRO and the DAPB on cross-slip energetics and planar-slip. Using Ni-10%Al as a model alloy, atomistic modeling will be used to derive dislocation mobility laws as a function of ordering and temperature, as well as cross-slip activation barrier distributions as a function of order and DAPB presence. These will be input into models for correlated cross-slip and screw dislocation annihilation, two mechanisms which have the potential to increase work-hardening. This extends previous understanding of SRO effects on mechanical properties beyond that of traditional DAPB strengthening.
2:50 PM Invited
Structural Transformations Driven by Irradiation in the High Defect Density Limit: Sergei Dudarev1; Daniel Mason1; Peter Derlet2; 1UK Atomic Energy Authority; 2Paul Scherrer Institut
Transferable interpretation of structural data is a major challenge in nuclear materials science. Quantitative models are restricted to exposure not exceeding a tiny fraction of dpa, whereas in the high dose limit, consistent analysis is difficult and interpretation relies on temperature scaling, rate theory or cluster dynamics. These models, valid in the low defect density approximation, do not treat the microscopic stresses driving structural transformations. Although intuitively it appears likely that a material might eventually reach an equilibrium state, statistically invariant with respect to the generation of new defects, the structure of such a state is not known. We find that the dynamic equilibrium state is characterised by strong fluctuations of stress and strain while largely retaining crystalline order. The defect content saturates at a fraction of a percent, and microstructure exhibits anisotropic response to external applied stress. Simulations predict unusual structural signatures of irradiation-induced transformations, well correlated with observations.
3:20 PM
Magnetically Driven Short-range Order in the CrCoNi System: Flynn Walsh1; Robert Ritchie1; Mark Asta1; 1Lawrence Berkeley National Laboratory
The presence, nature, and impact of chemical short-range order in CrCoNi are all topics of current interest and debate, particularly in the context of the system’s role as a model for related high-entropy alloys. First-principles calculations reveal that its origins are fundamentally magnetic, with repulsion between like-spin Co-Cr and Cr-Cr pairs that is complemented by the formation of a magnetically aligned sublattice of second-nearest neighbor Cr atoms. Serving primarily to relieve magnetic frustration, this behavior does not precede any known long-range ordered state and may in fact contribute to the metastability of the solid solution. Ordering models following these principles are used to explain anomalous experimental measurements concerning both net magnetization and atomic volumes across a range of compositions, demonstrating the effects of both short-range order and magnetic interactions.
3:40 PM
Short-range Clustering and Ordering Evolution of Ni-22Cr-13Mo Alloy: Po-Cheng Kung1; Jessica Krogstad1; 1University of Illinois at Urbana-Champaign
The ability to predict short-range clustering (SRC) and ordering (SRO) structures enables mechanical properties tailoring in Ni alloys by precipitation hardening and glide plane softening. In present work, a short-range structure was found in the quenched Ni-22Cr-13Mo alloy (Hastelloy C22). The additional weak electron diffraction spots caused by this structure have been reported in another Ni-Cr alloy and referred as a consequence of SRO formation. However, in-depth diffraction pattern analysis and atomic resolution scanning transmission electron microscope image show that the weak diffraction in Hastelloy C22 is caused by Mo-rich discs lying in {111} planes. After annealing at 300 °C for 34 days, these disordered clusters grew larger and could eventually become the precursor of Ni2(Mo,Cr) SRO, which has been reported in this alloy, if prolong the annealing time or increase the temperature. The continuous evolution of these clusters at high temperature is demonstrated by in-situ electron diffraction experiment.
4:00 PM
Thermodynamics of the Invar Transition: Phonons vs. Magnetism: Stefan Lohaus1; Pedro Guzman1; Brent Fultz1; 1California Institute of Technology
The invar effect, where the volume remains unchanged with increasing temperatures, is characterized by a continuous magneto-volume transition with competing thermodynamic effects. A change in magnetism from a ferromagnetic high spin state to a low spin state with temperature or pressure is accompanied by a decrease in volume, which counteracts the usual thermal expansion. For a fundamental thermodynamic understanding of this effect, the interaction between phonons, electrons and their spins was investigated. We performed nuclear inelastic X-ray scattering and synchrotron Mössbauer experiments at several pressure and temperature conditions across the invar transition, to measure the phonon spectrum and the magnetic evolution in Ni-Fe invar. These experiments allow us to quantify the vibrational and magnetic entropies and investigate their effect on the free energy of invar. The magnetic and vibrational entropies have opposing effects on the phase transition, helping to explain the unchanging volume and the thermodynamics behind the invar effect.
4:20 PM Invited
Intrinsic Phase Stability and Continuous Phase Transformations in TiAlZrN Ultra-hard Nitride Coatings: Vahid Attari1; Raymundo Arroyave1; 1Texas A&M University
Transition metal nitrides (TMNs) are generally characterized with high hardness, oxidation resistance, and relatively high melting points. Among many TMNs, we study Ti$_{1-x-y}$Al$_x$Zr$_y$N alloys which are inherently unstable for a wide range of composition and temperature and transform continuously into base cubic nitride compounds i.e. AlN, TiN, and ZrN. Also, during later stages of evolution, the c-AlN transforms to w-AlN. This transition is associated with strain localization in the form of the lattice misfit between wurtzite and rock salt phases. We have developed a phase-field model to study the nanostructure of these metastable TMNs under elasto-chemical conditions and we have further looked at the inertial roots of the AlN cubic to wurtzite transition at strain space. Results will be Presented (i) on the elasto-chemical interactions in the nanostructure, and (ii) on the tendency of the cubic to wurtzite transition in composition space along with experimental data where possible.