Molecular dynamics (MD) studies on phase transformation and deformation behaviors in FCC metals and alloys

Citation

Qi, Yue (2001) Molecular dynamics (MD) studies on phase transformation and deformation behaviors in FCC metals and alloys. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9NXP-E603. https://resolver.caltech.edu/CaltechETD:etd-09172008-112120

Abstract

This thesis focused on the phase transformation and deformation in face center cubic (FCC) metals and alloys. These studies use the new quantum modified Sutton-Chen (QMSC) many-body potentials for Cu, Ni, Ag, and Au and for their alloys through simple combination rules. Various systems and processes are simulated by standard equilibrium molecular dynamics (MD), quasi-static equilibrium MD and non-equilibrium MD (NEMD), cooperated with different periodic boundary conditions. The main topics and their outlines are listed as the following: 1) Melting, glass formation, and crystallization processes in bulk alloys: Using cooling rates in the range of 2*10[superscript 12] to 4*10[superscript 14]K/s, we find that CuNi and pure Cu always form an FCC crystal while Cu[subscript 4]Ag[subscript 6] always forms a glass (with Tg decreasing as the quench rate increases), which confirms the role of size mismatch in glass formability and validates the accuracy of the force field. 2) The size effects in melting and crystallization in Ni nano clusters, ranging 100 to 8007 atoms: We find a transition from cluster or molecular behavior below ~500 atoms to a mesoscale nanocrystal regime (with well-defined bulk and surface properties and surface melting processes, which leads to T[subscript m,N] = T[subscript m,bulk] - α N[superscript -1/3]) above ~750 atoms. Cooling from the melt leads first to supercooled clusters with icosahedral local structure, then for N>500 the supercooled clusters transform to FCC grains, while clusters with N<500 form icosahedral structures. 3) The deformation behavior of metallic nanowires of pure Ni, NiCu and NiAu alloys, under high rates of uniaxial tensile strain, ranging from 5*10[superscript 8]/s to 5*10[superscript 10]/s: These nanowires are too small to sustain dislocations; instead we find that deformation proceeds through twinning and coherent slipping mechanisms at low strain rate, and amorphization at high strain rate. We find that critical strain rate, beyond which the crystal transformed into glassy state, for NiAu (13% size mismatch) is 100 times slower than that for NiCu (2.5% size mismatch). Thus the critical strain rate also depends on the glass formability. 4) The calculation of the 1/2<110> screw dislocation in nickel (Ni): From a quadrupolar dislocation system with 3-D periodic boundary conditions, we found the screw dislocation dissociated into two partials on {111} planes, and the core energy is 0.5 eV/b. We also studied motion and annihilation process of opposite signed dislocations with different configurations of dissociation planes. On two intersecting or parallel dissociation planes, a cross-slip process is captured and the energy barriers is 0.1eV/b in our simulations. 5) Friction Anisotropy at Ni(100)/(100) interface: We carried out a series of NEMD simulations for sliding of Ni(100) interfaces under a constant force. We found that the clean, flat, and incommensurate interface has a very small static friction coefficient, as analytical theory predicted. However surface roughness can increase the static friction on the incommensurate interfaces dramatically, and increase the friction on the commensurate interfaces to a lesser extent. The dynamic frictional coefficients are comparable to the experimental values and show the same anisotropic behavior, thus explaining the difference between theory and experiment.

Thesis Files