Interfaces and Phase Transformations: Interfaces and Phase Transformations II
Sponsored by: TMS Phase Transformations Committee
Program Organizers: Arun Devaraj, Pacific Northwest National Laboratory; Matthias Militzer, University of British Columbia; Matthew Steiner, University of Cincinnati; Mohsen Zaeem, Colorado School of Mines; Yufeng Zheng, University of North Texas
Wednesday 8:00 AM
November 4, 2020
Room: Virtual Meeting Room 37
Location: MS&T Virtual
Session Chair: ARUN DEVARAJ, Pacific Northwest National Laboratory; yufeng Zheng, University of Reno Nevada
8:00 AM Invited
Study of Strain Rate and Temperature Dependent Behavior of Pseudo-morphic bcc Mg within the Mg/Nb Nanocomposites: Manish Jain1; Rajaprakash Ramachandramoorthy1; Marko Knezevic2; Nenad Velisavaljevic3; Nathan Mara4; Irene Beyerlein5; Johann Michler1; Siddhartha Pathak6; 1Empa-Swiss Federal Laboratories for Materials Science and Technology; 2University of New Hampshire; 3Argonne National Laboratory; 4University of Minnesota; 5University of California, Santa Barbara; 6University of Nevada Reno
In this work, interface strain engineering of Mg with Nb was exploited to pseudo-morphically transform hexagonal closed packed (hcp) Mg into a stable body centered cubic (bcc) Mg at ambient pressures. The adjacent Mg/Nb interfaces were spaced within a few nanometers forming multilayered Mg/Nb nanocomposite. Transmission electron microscopy (TEM) and X-Ray diffraction (XRD) studies reveal that in physical vapor deposition (PVD) deposited Mg/Nb multilayers at lower thicknesses of ~5 nm, Mg undergoes an interface strain induced phase transition from hcp to bcc. We performed in situ SEM micro-pillar compression tests at strain rates from 0.001/s to 1000/s and temperatures from -90 oC to 225 oC on Mg (bcc)/Nb 5nm/5nm and Mg (hcp)/Nb 20nm/20nm nanolaminates to compare the responses of hcp vs. bcc Mg under such extreme loading conditions. Results from these tests are analyzed in terms of the measured activation energies and activation volumes from sub-micrometer sized Mg/Nb multilayer nanocomposites.
8:30 AM
Shear-mediated Interfacial Structure Evolution in Nanoscale FCC Gold: Scott Mao1; Zhengwu Fang1; 1University of Pittsburgh
Grain boundary (GB), as the most common interfacial structure in polycrystalline solids, plays a vital role in the deformation of nanocrystalline metals. Up to date, there are several theoretical models proposed to characterize the GB structures, and to interpret the underlying mechanisms of GB evolution (migration or transformation). However, the exact mechanism of GB evolution at atomic-scale remains elusive due to the technical limits. By performing in-situ transmission electron microscopy study, we successfully made gold bi-crystals with different GB structures, such as symmetrical tilt GB (STGB) and asymmetrical tilt GB (ATGB). By applying shear stresses on the as-fabricated bi-crystals, the sequential atomic-scale observations on the GB evolution were obtained. The results show that STGB migrates without changing its structure while the ATGB gradually transforms into STGB during shearing. Disconnection-mediated mechanisms are proposed to interpret the phenomena. Such observations provide important guideline for exploiting the atomistic mechanisms of interfacial structure evolution.
8:50 AM
Phase Transformations in High-Temperature Industrial Applications: An Experimental and Computational Study of Nitridation in Commercial Austenitic Stainless Steel: Alice Young1; Milo Kral1; Catherine Bishop1; 1University of Canterbury
Service conditions in methanol production plants create a driving force for nitrogen uptake in austenitic stainless steel piping, resulting in formation of internal nitrogen-rich precipitates. Such precipitates severely reduce the ductility of the material, increasing the likelihood of unexpected failure. Understanding the factors influencing phase transformation during the nitridation process is a critical first step towards being able to anticipate such failures. In this work, nitridation of the widely used Alloy 800H was studied at service-relevant temperatures 800-1000°C. Both experimental and computational approaches were used. Phase identification and characterisation of precipitate-matrix interface morphology and crystallography was conducted using both scanning and transmission electron microscopy. In conjunction, a finite-difference model for nitridation of 800H was developed. All phase stability and diffusion data was calculated using Thermo-Calc and DICTRA. Experimental observations were used to inform incorporation of key thermodynamic and kinetic factors affecting transformation rates, such as precipitate morphology and nucleation barriers.
9:10 AM
Microstructure and Mechanical Properties of Pre-aged Thermo-Mechanically Processed NiTiHf Shape Memory Alloy: Faith Gantz1; Nathan Ley1; Jessica Rider1; Jordyn Ward1; Drew Forbes2; Marcus Young1; 1University of North Texas; 2Fort Wayne Metals Research Products Corp.
Shape memory alloys (SMAs) are multifunctional materials with high energy density, providing a high power-to-weight ratio as actuators in various applications. NiTiHf presents a cost-effective solution for high temperature SMA actuators, i.e. those that exhibit transformation temperatures above ~115 °C. In this study, a Ni50.3Ti29.7Hf20 (at.%) HTSMA was industrially produced then hot-extruded. Some samples were hot-rolled to 25% reduction in thickness, then a portion of the extruded and hot rolled samples were subjected to conventional aging (3hrs between 450 and 750 °C), while the other portion was subjected to pre-aging (12hr at 300 °C) plus conventional aging. The processing and aging treatments were designed to control the H-phase precipitate nucleation and growth and to determine the coherency between the matrix and the nano-precipitate H-phase interface, respectively. The alloy maintained stable phase transformation properties between pre-aged and conventional aging treatments for the extruded and hot-rolled conditions.