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

Paper Number: 151012

151012 - The Role of Mechanical Loading in Phase Transitions and Twin Formation of Titanium Alloys 

In response to temperature and pressure changes, dual-phase titanium (Ti) alloys undergo different martensitic phase transformations, producing a rich martensite microstructure with internal twins. Mechanical loading is also known to impact martensitic phase transformation significantly. In this work, we integrate atomistic simulations with theoretical calculations to investigate the effect of mechanical loading on the martensitic phase transformation and martensite microstructure in dual-phase ti alloys.

We conduct molecular dynamics (MD) simulations to melt the initial structure containing one million Ti atoms at 2266 K and then quench it to 10 K, resulting in a metastable body-centered cubic (bcc)-Ti polycrystal that is stabilized by the fast quenching rate and included abundant grain boundaries and twin boundaries. Uniaxial loading was applied at a constant temperature of 600 K to observe the bcc-hexagonal close-packed (hcp) phase transition without thermal fluctuation interference. We also use the stable (bcc)-Ti-Nb single-crystal consisting of one million atoms, with exact amounts of Nb atoms randomly distributed in the Ti matrix and the [100], [011], and [01̄1] directions. Both structures are relaxed for 100 ps at 10 K and 0 Pa within the isothermal-isobaric ensemble. The Ti-Nb single-crystal is deformed in different loading directions at 10 K to study the effect of loading directions on phase transformation twinning. A wide range of strain rates, from 10⁸ to 10¹⁰ s⁻¹, is examined to see that the key twinning behavior remains consistent regardless of the applied strain rate. Finally, the common neighbor analysis (CNA) in OVITO is used to visualize the evolution of the microstructure.

Our MD simulation results and theoretical calculations of deformation gradients and transformation strains reveal the formation of {1011}, {1012}, {1121}, and {1122} twins in the hcp phase during different loading directions as a result of the bcc-hcp transformation during deformation of the metastable bcc-Ti polycrystal. Specifically, the {1011} and {1012} twins are found as the transformation twins, while the {1122} and {1121} twins are inherited from the initial {112} twin in the bcc phase. More importantly, aided by the calculation of the deformation gradient and transformation strain, we explained that the {1011} transformation twin is favored by [111]_bcc axis tension, and the {1012} transformation twin is favored by [001]_bcc axis compression.

Moreover, deformation of the bcc-Ti-Nb single crystal during different loading directions shows the copious occurrence of {112} twins involving either hcp or fcc intermediate phases. The hcp and fcc cases occur in Ti-Nb single-crystal during [100] compression and tension, respectively. Furthermore, by calculating the correspondence matrix, we identify the fcc and hcp cases as the normal deformation twin mode and the 1/2 atoms-shuffle mode, respectively. The twinning modes significantly influence twin–twin interactions and the final microstructure. Our theoretical calculation confirms that the selection of specific twin modes and variants is governed by the correlation between their deformation path and mechanical loading.

The results underscore the crucial role of mechanical loading in activating the specific twin modes in dual-phase titanium alloys, thereby providing a novel avenue for engineering twin microstructures through carefully designed thermomechanical processing techniques.

Presenting Author: Mehrab Lotfpour University of Nevada, Reno

Presenting Author Biography: Mehrab is a PhD student in Mechanical Engineering at the University of Nevada, Reno, working under the supervision of Doctor Lei Cao. He started his PhD in 2021, and his research area is multiscale microstrip analyses of materials with different crystal structures.

Authors:

Mehrab Lotfpour University of Nevada, Reno
Amir Hassan Zahiri University of Nevada, Reno
Jamie Ombogo University of Nevada, Reno
Lei Cao University of Nevada, Reno