Phase Transformations and Microstructural Evolution: General Topic II
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Rongpei Shi, Harbin Institute of Technology; Yipeng Gao, Jilin University; Fadi Abdeljawad, Lehigh University; Bharat Gwalani, North Carolina State Universtiy; Qi An, Iowa State University; Eric Lass, University of Tennessee-Knoxville; Huajing Song, Los Alamos National Laboratory
Thursday 2:00 PM
March 18, 2021
Room: RM 57
Location: TMS2021 Virtual
Session Chair: Rongpei Shi, Lawrence Livermore National Laboratory
2:00 PM
Microstructural Evolution and Deformation Behavior during Uniaxial Compression of Al-Si Alloys: Tingkun Liu1; Matthew Olszta1; Bharat Gwalani1; Changyong Park2; Arun Devaraj1; 1Pacific Northwest National Laboratory; 2Argonne National Laboratory
In-situ synchrotron X-ray diffraction of Al-Si was performed using a diamond anvil cell (DAC) to explore the microstructural evolution and deformation of alloys under uniaxial compression. The results show the transformation of Si follows Si-I Si-XI Si-V in Al-4Si, while the Si-I phase directly transforms to Si-V in Al-1Si during compression. The Si-III phase was observed after the pressure released. Site-specific transmission and scanning transmission (S/TEM) analysis of the surface in contact with the diamond anvil was also performed, showing unique morphologies including a narrow region of amorphous aluminum oxide interspersed with nano-dimension crystalline Al and Si. Most interestingly, a complex intermediate interface was discovered between the faceted Si-I and the interior Si-III in Al-1Si. It is comprised of a small fraction of Al nanocrystals and a majority nanocrystalline Si-I. A minor fraction of the Al/Si interface is composed of the Si-III grains contacting the Al matrix.
2:20 PM
Suppression of Samson Phase Formation in Al-Mg Alloys by Boron Addition: Ramasis Goswami1; 1Naval Research Laboratory
Al-Mg alloys, particularly Al 5083 and Al 5456, are widely used for structural applications because of their unique properties, such as lightweight, high strength and good weldability. However, structural parts manufactured with these alloys fail catastrophically as a result of the formation of Al3Mg2, known as Samson phase, at grain boundaries. Here we demonstrate that the Samson phase formation is suppressed in Al 5083 by alloying with B, which traps most of Mg in solid solution as AlMgB2 phase. This decreases the supersaturation level of Mg in Al matrix, which is a driving force for the formation of Samson phase in Al 5083. We observe Cu-rich precipitates, instead of the Samson phase, at grain boundaries upon extended annealing at 150C. This is a significant finding as it provides new insight as to how to minimize the longstanding problem of stress corrosion cracking in Al-Mg alloys.
2:40 PM
Transformations in Amorphous Environments near "Critical" Temperatures: Deep Choudhuri1; 1New Mexico Institute of Mining and Technology
Structural changes during crystallization and vitrification in dilute face-centered-cubic (FCC) alloys was investigated using model Al-Sm alloys. Molecular dynamics simulations were performed to study the solidification behavior of Al-1at.%Sm and Al-5at.%Sm at 10^10, 10^11 and 10^12 K/s cooling rates. Two structural features were identified from these simulations. In case of Al-1at.%Sm, we learn that, near the melting point, liquid phase manifested pockets of unique transitional structures comprising triangular arrangements in near-parallel layers that encapsulated a FCC-HCP coordinated core. We defined such a structure as the pre-critical nucleus, which is contained within an otherwise predominantly uncoordinated amorphous liquid phase. However, within the range of cooling rates employed, Al-5at.%Sm manifested only amorphous structure after solidification. The liquid structure in the transitional state contained temperature dependent icosahedron clusters that manifested as double-peak in the radial distribution function. Near the Tg Al-5at.%Sm achieves additional local ordering via the formation of inter-penetrating icosahedral frameworks.
3:00 PM
Crystallographic Transitions in Compositionally Complex Alloy Thin Films: Daniel Goodelman1; Andrea Hodge1; 1University of Southern California
This presentation will highlight the effect of changing elemental compositions on the crystallography of compositionally complex alloys (CCAs). Magnetron sputtering will be employed to synthesize a combination of elements with varied crystal structures. This technique allows for easily tunable conditions such as operating power, working pressure, and working gas, among others, that influence the composition of each of the alloying elements present. Preliminary results indicate that there is a transition from a BCC to a FCC crystallographic structure when altering the composition of each of the alloying elements. The goal of the project is to see how varying the composition of these elements influences the overall crystallographic structure of the sputtered CCA, and to explore the non-equiatomic composition space of the sputtered CCA using high-throughput techniques.
3:20 PM
Porous Graphite Fabricated by Liquid Metal Dealloying of Silicon Carbide: Gina Greenidge1; Jonah Erlebacher1; 1Johns Hopkins University
Conventional liquid metal dealloying (LMD) is a processing technique whereby one component of an alloy is selectively dissolved in a melt at high temperatures. Microstructural evolution in this process is quite complex and often leads to a bicontinuous topologically-connected two-phase microstructure. We used LMD to prepare porous graphite by dealloying silicon carbide in molten germanium. SiC further complicates phase evolution in LMD by adding a chemical transformation wherein the hybridization of carbon changes from sp3 to sp2. The dealloying depth, concentration profile and length scales of the dealloyed microstructure were examined and we introduce here a quantitative kinetic model for the interface velocity and steady-state concentration profile in the dealloyed layer. Our observations are consistent with rate-limiting kinetics in the germanium side of the interface due to a spatially varying diffusion rate associated with the development of the carbon phase.
3:40 PM
Analysis of Dendrite Fragmentation from Microgravity Solidification Experiments: Zachary Thompson1; Tiberiu Stan1; Peter Voorhees1; Nathalie Mangelinck-Noёl2; Henri Nguyen-Thi2; 1Northwestern University; 2Aix Marseille Univ, Université de Toulon, CNRS, IM2NP, Marseille, France
Dendritic two-phase microstructures frequently form during the processing of metallic alloys. These microstructures can undergo several changes during solidification, including fragmentation of secondary and tertiary dendrite arms. These fragments can have a large effect on materials properties of the solidified part, notably by acting as nucleants for the formation of equiaxed grains that can lead to a columnar-to-equiaxed transition (CET). However, fragment formation can be difficult to quantify as fragments often reattach to a different portion of the primary dendrite. Fragments can also remelt due to interdendritic fluid flow. This work focuses on experiments performed in microgravity aboard the International Space Station to reduce the complicating effect of convection. The samples were returned to Earth and serial sectioned to create 3D image data, which are then used to measure the extent and distribution of fragment formation. This work produces benchmark data for understanding fragmentation and its effect on the CET.