Nucleation of Solid-State Phase Transformations: Nucleation of Solid-State Phase Transformations
Sponsored by: TMS Phase Transformations Committee
Program Organizers: Eric Lass, University of Tennessee-Knoxville; Sophie Primig, University of New South Wales; Keith Knipling, Naval Research Laboratory
Monday 8:00 AM
October 18, 2021
Room: B132
Location: Greater Columbus Convention Center
Session Chair: Eric Lass, University of Tennessee, Knoxville
8:00 AM Invited
Critical Nuclei at Hetero-phase Interfaces: Rongpei Shi1; Tae Wook Heo1; Brandon Wood1; Yunzhi Wang2; 1Lawrence Livermore National Laboratory; 2The Ohio State University
Two-step nucleation, wherein a metastable phase acts as a precursor for nucleating a thermodynamically stable phase, has been widely observed in many solid-state reactions. Among the advantages of two-step nucleation is that the stable phase may nucleate heterogeneously at the hetero-phase interface between the original and the precursory phases. Nevertheless, our understanding of HN at these interfaces remains incomplete. This deficiency stems from the discontinuity of the chemical potential across the hetero-phase interface, which profoundly affects the fundamental properties of the nucleus in a way that is not properly captured in existing models. Herein, we incorporate these effects to extend the classical nucleation theory to HN at hetero-phase interfaces and demonstrate that the nucleus shape along the minimum energy path is strongly size-dependent, and this additional degree- -of-freedom can result in the reduction of the critical nucleus volume and associated nucleation barrier by orders of magnitude relative to conventional predictions.
8:30 AM
Formation of the γ’’’-Ni2(Cr, Mo, W) Phase during Two-step Heat Treatment in Haynes® 244® Alloy: Thomas Mann1; Michael Fahrmann2; Michael Titus1; 1Purdue University; 2Haynes International
Several nickel alloys containing molybdenum and chromium have shown short range order (SRO) from cooling after a solution anneal heat treatment. This short range order includes DO22, DOa, and Pt2Mo type phases that can evolve into long range order (LRO) domains that increase the strength of the alloy. Haynes® 244® has shown this short range order to long range order transition resulting in the γ’’’-Ni2(Cr, Mo, W) phase, which significantly strengthens the alloy. Previous studies have shown the transformation to be a precipitation transformation at higher aging temperatures and a continuous ordering transformation at lower aging temperatures. This study investigates the use of a two-step aging treatment to rapidly age the alloy resulting in LRO of the γ’’’-Ni2(Cr, Mo, W) phase. The formation behavior and SRO to LRO transition were investigated using small-angle x-ray scattering and electron microscopy. Optimizing the aging heat treatment schedules and strength will be presented.
8:50 AM
Investigation of Nucleation Mechanisms Associated with the Formation of Coprecipitates in Ni-based Superalloys: Hariharan Sriram1; Semanti Mukhopadhyay1; Michael Mills1; Yunzhi Wang1; 1Ohio State University
Experimental studies have shown a wide variety of γ'/γ” coprecipitates in IN718-based alloys. Although the coprecipitate's growth and coarsening mechanisms have been investigated, the heterogeneous nucleation mechanisms of γ” on existing γ' that lead to different types of coprecipitates are yet to be understood. We analyze the individual and combined effects of concentration field and coherency elastic stress field associated with existing γ' at different sizes on nucleation of γ", leading to different coprecipitate configurations. The chemical driving force for nucleation is calculated using the CALPHAD databases, while the contributions from elastic interaction and anisotropic interfacial energies between different interfaces are quantified by comparing results from phase-field simulation and experimental characterization. Subsequently, an explicit nucleation algorithm is used in phase-field simulations to study nucleation and early stage of growth. The study may shed light on how to achieve desired coprecipitate microstructures for increased thermal stability and enhanced mechanical properties.
9:10 AM
Modeling Microstructure Evolution Using the Steepest-entropy-ascent Quantum Thermodynamic Framework: Jared Mcdonald1; Michael von Spakovsky2; William Reynolds2; 1Virginia Polytechnic Institute; 2Virginia Polytechnic Institute and State University
Sintering, grain growth and other coarsening phenomena are common in ceramic and metallic systems. Meso-/nano-scale techniques for modeling them include Discrete Element and Kinetic Monte Carlo methods. Unfortunately, these methods involve significant computational resources and possess limited scalability to distinct initial conditions. Additionally, they employ temperature-dependent rate equations in non-equilibrium conditions where temperature is not defined. An alternative approach that can predict all conceivable kinetic paths in a discrete system is the Steepest-Entropy-Ascent Quantum Thermodynamics. Utilizing this framework, a unique kinetic path for the evolution of each initial non-equilibrium state to stable equilibrium is found by solving an equation of motion in state space that takes the form of a series of ordinary differential equations. Results for microstructure evolution for three different initial conditions are given, demonstrating the phenomenological behavior of polycrystalline sintering, precipitate coarsening, and grain growth. Time evolutions of microstructural descriptors for these cases are presented as well.
9:30 AM
Observing the Solid-state Processes under Additive Manufacturing Conditions Inside the TEM: Sriram Vijayan1; Meiyue Shao1; Avantika Gupta1; Rohan Casukhela1; Joerg Jinschek1; 1The Ohio State University
Rapid thermal cycling and steep thermal gradients are the dominant process conditions during metal based additive manufacturing (AM) techniques such as powder bed fusion. These process conditions cause reduced partitioning of solute and thereby lead to the formation of metastable microstructures with an anisotropic, columnar morphology. To fabricate AM components with equiaxed microstructures, a careful understanding of AM process conditions is still needed. In this study, we describe the use of a MEMS-based heating stage to simulate ‘AM like’ processes inside a transmission electron microscope (TEM). This modified MEMS device enables us to study the microstructural response of the material under the combined influence of large thermal gradients (106 K/m) and rapid thermal cycling. We expose TEM lamellae, obtained from AM builds, to ‘AM like’ conditions in-situ to understand fundamental processes that contribute to the microstructural evolution of AM Ti-6Al-4V and Inconel 738 during the AM build process.