Nanowires have a wide range of applications in nanotechnology and have a great technical importance due to their unique properties and structure. The material properties of the nanowires are significantly influenced by the surface atoms due to the high surface to volume ratio at the nanoscale. A fundamental understanding of the surface effects at the nanoscale is necessary to study the characteristics and properties of the nanowires.
In this work, molecular dynamics simulations are used to demonstrate the surface stress driven phase transformations in titanium nanowires. The surface-induced stress plays a critical role for the phase transformation of nanowires, and when combined with factors such as temperature and surface orientations, can lead to the so-called pseudoelasticity or shape memory effects in Ti. The study is focused on the behavior of HCP metal nanowires and structural transformations at the nanoscale.
Transformations leading from two initial HCP orientations of ⟨11-20⟩ and ⟨1-100⟩ of the Ti nanowires are discussed. Ti nanowires with an HCP α phase and initial orientation of ⟨11-20⟩ in the axial direction, spontaneously transforms to a BCC phase with an axial orientation of ⟨100⟩ with {110} surfaces, on relaxation at 0 K. Upon heating at a higher temperature, the BCC phase transforms to an FCC phase, which on tensile loading and subsequent unloading shows shape memory effects between FCC orientations. On the other hand, Ti nanowires with HCP phase and initial orientation of ⟨1-100⟩ in the axial direction, spontaneously transforms to an FCC phase with an axial orientation of ⟨100⟩ with {-110}/{001} surfaces, on relaxation at 0 K. In this case, a stacking fault is formed in the axial direction and as a result, no further transformation is possible upon heating or tensile loading. However, the stacking fault can be avoided by using an odd number of atomic layers but no significant transformation is noticed upon heating or tensile loading.
The transition pathways are also studied by the nudged elastic band (NEB) method using the initial and final states. The NEB method is used to find the minimum energy paths for the phase transformations. A set of intermediate images between the initial and final states are interpolated linearly by using the quenched velocity Verlet (QVV) algorithm to find the minimum energy path (MEP) of the transformations. The MEP for the phase transformations of HCP ⟨11-20⟩ and HCP ⟨1-100⟩ nanowires to BCC ⟨100⟩/{110} and FCC ⟨100⟩/{-110}{001} nanowires respectively have been plotted. The MEPs for the spontaneous phase transformation at 0 K for smaller width nanowires, follow a lower energy path compared to the initial potential energy. But nanowires with a larger cross-sectional width require additional thermal energy for the phase transformation to be complete, and the resulting activation energy barrier can be found using the NEB method.