Session: 16-01-01: Government Agency Student Poster Competition

Paper Number: 150414

150414 - Novel Methods for Phase-Transformation-Assisted Twinning in Molybdenum Nanomaterials: Simulation and Experimental Study 

In pursuit of designing stronger, more resilient, and advanced Molybdenum (Mo)-based systems for numerous engineering applications, such as fusion reactors, flexible nanoelectronics, aerospace alloys, and other nanotechnologies, we systematically investigated the factors contributing to deformation mechanisms in Mo under various loading conditions,  including phase-transformation (PT)-assisted twin formation, dislocation slips, and classical twins. We report new and timely findings from systematic and comprehensive analyses of orientation-dependent deformation mechanisms in single-crystal Mo nanowires (NWs) and bulk systems, polycrystals with random and textured grains, and bimetallic Cu/Mo interfaces, primarily based on molecular dynamics (MD) simulations. Twinning can be defined as coordinated shear movements of atoms that require overcoming a large energy barrier, and hence achieving high stresses is a pre-requisite for facilitating such atomic rearrangements. Phase transformation, bimetallic interfaces with misfit dislocations, and surfaces associated with large stress concentration areas can act as potential routes for twin formations. Firstly, we report that the application of uniaxial tensile or compressive loading induces PT-assisted twins (i.e., bcc₁→fcc→twinned-bcc₂) in Mo NW single crystals loaded along <100> and <110> directions, while <111> NWs fail by activating {110} slip systems. Additionally, a correlation of deformation mechanisms in NWs with bulk Mo structures is also established, highlighting the role of metastable fcc phase and associated stress in triggering PT-assisted twins across multi-dimensional Mo-based systems. Our study then delves into the detailed mechanisms of PT paths, such as the Bain→Pitsch followed by tensile <100> NWs during their bcc₁→fcc→bcc₂ transitions. To validate MD results, Density Functional Theory (DFT) calculations were also carried out to generate ‘energy-per-atom vs lattice parameter’ plots. These 2D energy landscape plots capture the phase transitions and reveal all stable, metastable, and unstable crystal phases—bcc₁, fcc, and bcc₂—across a wide range of lattice parameter values. Despite minor differences in energy values, DFT calculations are well in agreement, within statistical limits, with MD-computed values in terms of detecting phase transitions (i.e., bcc₁→fcc→twinned-bcc₂) and reorientation (i.e., bcc₁→bcc₂) across all the NW cases studied.

Next, to achieve more uniform and homogeneous PT-assisted twinning, evenly distributed large-stress concentration areas are needed throughout the structure, and for that reason, we extend our investigations of deformation mechanisms to polycrystalline Mo structures, both randomly oriented and textured grains, in the hopes of achieving more uniformly-distributed PT-assisted twins. The study focuses on probing the role of mean grain size, crystallographic orientation, and associated stress in triggering uniform phase transformation and twin formation in Mo-based polycrystalline systems. 

Finally, 20 nm thick Cu/Mo films are deposited on sapphire substrates, employing the magnetron co-sputtering method to investigate the effects of interfacial stress on PT-assisted twins. The resulting morphology of a micro-sized Cu particle with many Mo nanoparticles underneath is found after the solid-state dewetting of thin films. The Cu/Mo interfaces are characterized by scanning transmission electron microscopy (STEM), and two kinds of orientation relationships (ORs)—[1-21]_Cu || [01-1]_Mo, (111)_Cu || (011)_Mo and [01-1]_Cu || [010]_Mo, (111)_Cu || (10-1)_Mo—are observed. To comprehend even further, atomistic models of the STEM-observed semi-coherent Cu/Mo interfaces are generated wherein misfit dislocations are identified and characterized using various metrics—lattice disregistry, interfacial energy, elastic strain, and von Mises stress. The morphology of Mo particles indicated the coalescence of smaller Mo nanoparticles driven by the evaporation of Cu particles. A semi-quantitative kinetic model based on the surface and interface self-diffusion of Mo is formulated to recognize this drag force. Additionally, size- and temperature-dependences of the interfacial energy are incorporated into the analysis to study the stability of the interfaces. The strain distribution around the interface area is obtained by performing the geometric phase analysis (GPA) method on atomic-resolution STEM images, and the qualitative values from GPA agree well with those obtained from atomistic simulations. Our findings hence establish Mo-based systems, oriented distinctly, as potential nanomaterials with enhanced ductility and strength that can be particularly appealing for mechanical applications.

Presenting Author: Afnan Mostafa University of Rochester

Presenting Author Biography: An international graduate (1st year Ph.D.) student working on an NSF-funded project on deformation mechanisms, i.e., phase-transition-induced twinning, in Molybdenum systems.

Authors:

Afnan Mostafa University of Rochester
Linh Vu University of Rochester
Feitao Li Technion - Israel Institute of Technology
Aditya Dey University of Rochester
Hesam Askari University of Rochester
Eugen Rabkin Technion - Israel Institute of Technology
Niaz Abdolrahim University of Rochester