Phase Transformations and Microstructural Evolution: Microstructure and Precipitation I
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Yufeng Zheng, University of North Texas; Rongpei Shi, Harbin Institute of Technology; Stoichko Antonov, University of Science and Technology Beijing; Yipeng Gao, Jilin University; Rajarshi Banerjee, University of North Texas; Yongmei Jin, Michigan Technological University
Tuesday 8:30 AM
February 25, 2020
Room: 33B
Location: San Diego Convention Ctr
Session Chair: Yipeng Gao, Jilin University
8:30 AM
In-situ X-ray Diffraction Measurement during Deformation of Austenite above the Ae3 Temperature: Clodualdo Aranas1; Samuel Rodrigues2; Fulvio Siciliano3; John Jonas4; 1University of New Brunswick; 2Federal Institute of Maranhao; 3Dynamic Systems Inc; 4McGill University
Austenite has been shown to dynamically transform into ferrite during thermomechanical processing. The driving force is the softening that takes place during deformation while the energy obstacles consist of the free energy difference between austenite and ferrite, and the work of dilatation and shear accommodation. Phase transformation can only take place once the driving force is greater than the energy obstacles. In this work, microstructures of dynamically transformed ferrite from rolling simulations will be presented. The dynamic phase change was verified by performing hot deformation experiments using a Gleeble system equipped with a synchrotron light x-ray diffraction. The X-ray diffraction measurements were performed during and after deformation to accurately track the behavior of austenite. The results indicate that austenite transforms dynamically into ferrite, followed by reversion of ferrite to austenite phase during isothermal holding. This phenomenon is known to generate ultrafine grain microstructure, which translates to improved mechanical properties.
8:50 AM
Crystallographic Characteristics of γ'-Fe4N Formation Upon Nitriding of α-Fe: Helge Schumann1; Gunther Richter2; Andreas Leineweber1; 1TU Bergakademie Freiberg; 2Max Planck Institute for Intelligent Systems
Nitriding of iron and steel typically generates a nitride compound layer at the surface, leading to improvement of e.g. tribological properties. Especially the transformation of α-Fe to γ'-Fe4N is of interest, because this typically is the first stage of compound layer formation in low-alloy steel and cast iron. In the current study, γ'-Fe4N is generated at the surface of polycrystalline α-Fe plates and single-crystal α-Fe whiskers. In the whiskers quite large γ' plates are formed, with habit planes and orientation relationship to α as predicted requiring an invariant plane strain in combination with a lattice invariant shear already observed previously in Fe4N precipitates developing at low temperatures. In the case of the polycrystalline plates the shear strains associated with γ' cannot be accommodated like within the freestanding whiskers. Instead characteristic variant combinations develop consisting of 6 different orientation variants. Thereby, characteristic orientation variations occur during growth of the γ'.
9:10 AM
Atomic Structures and Processes in Al-Cu alloys: Matthew Chisholm1; Dongwon Shin1; Gerd Duscher2; Lawrence Allard1; Amit Shyam1; 1Oak Ridge National Laboratory; 2University of Tennessee-Knoxville
The highest strength aluminum alloys feature a dense distribution of precipitates formed by solid-state nucleation and growth. However, an understanding of atomic processes involved in this prototypical phase transformation is not complete. Using a combination of aberration-corrected electron microscopy and first principles calculations, the atomic structures and energetics at the interfaces between the aluminum matrix and 𝜃′-Al2Cu precipitates were examined. The atomic structures of these interfaces are critical in the development of the precipitate and in the evolving properties of the Al-Cu alloys. The atomic arrangements within the 𝜃′ precipitates and across the interphase boundaries will be presented to reveal several important features of how the precipitate grows. The interaction between Cu solute and the interfaces will be presented using results from Al-Cu alloys with a range of Cu contents. The role of the interfaces in elevated temperature strengthening and coarsening resistance will be discussed.
9:30 AM
Controlling Microstructure of Nanotwinned Cu by Tuning the Electroplating Temperatures and Chemical Additives: Kuan-Ju Chen1; 1National Chiao Tung University
We fabricate different microstructures of nt-Cu films by changing chemical additives and electroplating parameters. By tailoring the chemical additives and electroplating temperatures, we are able to electroplate Cu films with different grain sizes and twin densities. It seems like some impurities would significantly influence the growth of twins and the nucleation conditions, resulting in the different grain sizes. Also, we conduct the thermal stability experiments of the nt-Cu with different microstructures. Interestingly, the nt-Cu films with a smaller grain size exhibited a higher thermal stability which contradicts the classical theory.
9:50 AM
Morphological and Structural Instability of Iron-rich Precipitates in Cu-Fe-Co Alloys: Gilles Demange1; Kaixuan Chen2; Helena Zapolsky1; Renaud Patte1; Z.D. Wang2; 1University Of Rouen; 2School of Materials Science and Engineering, University of Science and Technology Beijing
Mechanical properties of metallic alloys strongly depend on their microstructure. In particular, the structure and morphology of precipitates lead to distinct strengthening effects. Recently, it was shown that in the fcc CuFeCo system, the shape of Fe-rich precipitates first undergoes an evolution from sphere to cuboid, and finally to petal-like [1]. When the temperature is decreased, the structure of iron-rich precipitates becomes bcc, and a complex internal structure is observed within precipitates. In this work, we used the standard phase field model (PFM) at the mesoscale, and the recently developed quasi-particle approach (QA) at atomic scale [2], combined with experimental observations, to elucidate this phenomenon during casting and aging process.
10:10 AM Break
10:30 AM
Iron-rich Microstructures in Post-Detonation Nuclear Debris: Timothy Genda1; Kim Knight2; Zurong Dai2; Bethany Goldblum1; Peter Hosemann1; 1University of California, Berkeley; 2Lawrence Livermore National Laboratory
The formation of radioactive fallout in nuclear detonations depends on the composition and thermal histories of the fireball, but the extent to which environmental materials impact formation processes is not fully understood. To investigate the impact of high iron content on fallout formation, we have analyzed previously unobserved iron-rich microstructures preserved in rapidly-quenched (seconds to minutes) mm-scale glassy fallout from a historical U.S. nuclear test. Autoradiography reveals correlations between long-lived radioactive species and iron rich (>30 wt%) rims where these microstructures are preserved. BSED and EDX characterization reveals a wide variety of crystalline dendritic features and textural evidence of liquid-liquid immiscibility, including 2-20 µm core/shell ‘amoeboids’ which resemble microstructures observed in microgravity immiscible alloy experiments, but have never been observed in silicate systems. Understanding how these microstructures evolved may provide novel means to constrain fallout formation conditions in an Fe-rich environment, informing improvements to first-principles based nuclear fallout formation models.
10:50 AM
Oscillated Cooling Method as an Alternative Crystal Growth Route to Control the Microstructure during Peritectic Solidification: Babak Alinejad1; Alberto Castellero2; Marcello Baricco2; 1Ibaraki University; 2Turin University
A new mechanism is developed for preventive porosity formation in the microstructure of CoSb3 phase during peritectic solidification. Peritectic solidification involves nucleation and growth of the primary phase and formation of second phase by the reaction of the remnant liquid phase with the primary phase. The dendritic growth results in trapping of liquid phase between dendritic arms, leading to a multiphase and because of contraction of liquid, the final structure has porosity. Microstructure analysis at different stages of solidification revealed that oscillated cooling could hinder spontaneous nucleation, improve homogenous nucleation, limit the number of primary grains and impose planar growth. These factors enhance melt flux during peritectic solidification and prevent porosity formation during phase transformation contraction. By utilizing this method, dense CoSb3 single phase was obtained directly from melt in one of the most complicated solidification systems (with two peritectic transformation) in single batch process without any post treatment.
11:10 AM Cancelled
Nanostructure of Fe0.65Cr0.35 Close to the Upper Limit of the Miscibility Gap: Frederic Danoix1; Alexander Dahlstrom2; Peter Hedstrom2; Joakim Odqvist2; Helena Zapolsky3; 1CNRS; 2KTH Royal Institute of Technology; 3Normandy University
The nanostructure of an Fe0.65Cr0.35 alloy has been investigated by atom probe tomography (APT) after high-precision thermal treatments in the temperature range 550-580°C, including the upper limit of the miscibility gap (MG). It is shown that, within the miscibility gap, the Fe-Cr solid solution decomposes into Fe-rich and Cr-rich regions through a spinodal decomposition mechanism. Approaching the upper limit of the miscibility gap, the amplitude of Cr concentration fluctuations is found to progressively decrease, whereas the main periodic length-scale of the decomposed ferrite grows exponentially. Here wo show that these results are in accordance with predictions of the classical mean field Cahn-Hilliard (CH) theory at the boundary of the unstable spinodal region. This supports the view of a flat MG between the unstable and stable domains in the central region of the phase diagram. Atomistic modelling based on atomic density function theory supports the experimental observations.
11:30 AM
Morphological Evolution Mechanisms in Phase-separating Polycrystalline Alloy Films Exposed to a Vapor Phase: William Farmer1; Rahul Raghavan1; Kumar Ankit1; 1Arizona State University
Morphological self-assembly during physical vapor deposition (PVD) of immiscible alloy films is typically influenced by a kinetic interplay of deposition flux and atomic diffusion. However, such films often develop surface grooves depending on the energy of the film/vapor interface, which modifies the surface evolution characteristics of the phase-separated domains with respect to inner regions of the film. Till date, most studies deal with substrate-directed spinodal decomposition while any effect of the free surface has been neglected. Here, we employ a ternary Cahn-Hillard approach to explore the influence of atomic diffusion along the surface, grain boundaries, and interphase interfaces on phase separation in polycrystalline alloy films. Our systematic evaluation of the scaling laws as a function of surface depth provides novel insights into the role of surface grooving and film thickness while serving as an important first step in comprehending morphological evolution in PVD of phase separating alloys.