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

Paper Number: 150077

150077 - Influence of Non-Uniaxial Bending on Twinning and Phase Transformation in Molybdenum Nanowires 

Body-centered-cubic (bcc) metallic materials, especially Molybdenum (Mo), are used in a significant number of applications due to their excellent mechanical, thermal, and electrical properties. Mechanical properties, such as twinning and phase transformation, are pivotal mechanisms for enhancing the strength and ductility of metals. However, these phenomena traditionally have been studied under uniaxial loading, with limited focus on non-uniaxial conditions. This study employs LAMMPS to conduct molecular dynamics (MD) simulations to investigate the effects of non-uniaxial loading on twinning and phase transformation in Mo nanowires (NWs). We introduce novel bending mechanisms: mono-directional bending (along the x-axis) and bi-directional bending (along both the x and y axes). All NWs examined possess a rectangular cross-section with dimensions of 3 nm × 3 nm × 15 nm. Additionally, larger structures with dimensions of 10 nm × 10 nm × 50 nm were simulated to assess size-dependent effects.

Our atomistic simulations reveal a phase transformation sequence from bcc to face-centered cubic (fcc) to re-oriented bcc in both [1 0 0]- and [1 1 0]-oriented Mo NWs under both bending modes. A metastable fcc phase under bi-directional bending is observed between bending deformation from 10° to 23°, corresponding to 13% bending deformation of the original Mo NWs. Additionally, a stable multilayer fcc phase has been observed in [1 1 0]-oriented NWs under mono-directional bending. Bi-directional bending induces the formation of tetra-twin boundaries (TTBs) within {1 1 2} slip systems, whereas mono-directional bending activates {1 1 0} slip systems without TTB formation. This finding suggests that while the metastable fcc phase is essential for bcc reorientation, it is insufficient for TTB formation, which requires multi-directional bending to twist the {1 1 0} plane into the {1 1 2} plane. As a result, the nucleation of TTBs and phase transformation facilitates secondary elasticity and substantial uniform plastic deformation, which are critical for developing ductile yet robust Mo nanostructures applicable across various engineering fields.

In parallel, an atomic experiment has been proposed. Bending experiments of [100]-, [110]-, and [111]-oriented Mo cantilevers were conducted inside a scanning electron microscope (SEM). These micrometer-sized and notched cantilevers were fabricated with a focused ion beam (FIB) from a [110]-oriented single-crystal Mo bulk sample, followed by short-time electropolishing to remove the damage introduced by the high-energy ion beam exposure of FIB milling. This process was expected to activate the phase transformation by decreasing the density of defects in the cantilevers. The structures below the notch were checked in a transmission electron microscope (TEM) and compared with the simulation results.

As a cutting-edge technological material, Mo warrants extensive investigation. This study significantly advances the understanding of the mechanical behaviors of Mo and bcc materials, proposing a novel twinning structure under non-uniaxial loading. The elucidation of these phenomena holds profound implications for understanding the mechanical behavior of bcc Mo NWs under diverse loading conditions, thereby advancing the frontiers of materials science and nanotechnology.

Presenting Author: Sicheng Qian University of Rochester

Presenting Author Biography: I am Sicheng Qian, a senior majoring in mechanical engineering and minoring in electrical computer engineering at the University of Rochester. My research interest is in computational nanomaterial systems, focusing on the mechanical behavior of nanostructures under high pressure and deformation.

Authors:

Sicheng Qian University of Rochester
Afnan Mostafa University of Rochester
Feitao Li Technion - Israel Institute of Technology
Eugen Rabkin Technion - Israel Institute of Technology
Niaz Abdolrahim University of Rochester