Molecular dynamics simulations were employed to simulate the mechanical response and grain evolution in a Ni nanowire for both static and cyclic loading conditions at both 300 and 500 K for periods of 40 ns. The loading conditions included thermal annealing with no deformation, constant 1% extension (creep loading) and cyclic loading with strain amplitudes of 0.5% and 1% for 200 cycles. Under cyclic loading, the stress-strain response showed permanent deformation and cyclic hardening behavior. At 300 K, modest grain evolution was observed at all conditions within the 40 ns simulations. At 500 K, substantial grain growth is observed in all cases, but is most pronounced under cyclic loading. This may result mechanistically from a net motion of the boundaries associated with boundary ratcheting. There is a striking qualitative consistency between the present simulation results and the experimental observation of abnormal grain growth in nanocrystalline metals as a precursor to fatigue crack initiation.
Nanocrystalline metals offer significant improvements in structural performance over conventional alloys. However, their performance is limited by grain boundary instability and limited ductility. Solute segregation has been proposed as a stabilization mechanism, however the solute atoms can embrittle grain boundaries and further degrade the toughness. In the present study, we confirm the embrittling effect of solute segregation in Pt-Au alloys. However, more importantly, we show that inhomogeneous chemical segregation to the grain boundary can lead to a new toughening mechanism termed compositional crack arrest. Energy dissipation is facilitated by the formation of nanocrack networks formed when cracks arrested at regions of the grain boundaries that were starved in the embrittling element. This mechanism, in concert with triple junction crack arrest, provides pathways to optimize both thermal stability and energy dissipation. A combination of in situ tensile deformation experiments and molecular dynamics simulations elucidate both the embrittling and toughening processes that can occur as a function of solute content.
There has recently been a great deal of interest in employing immiscible solutes to stabilize nanocrystalline microstructures. Existing modeling efforts largely rely on mesoscale Monte Carlo approaches that employ a simplified model of the microstructure and result in highly homogeneous segregation to grain boundaries. However, there is ample evidence from experimental and modeling studies that demonstrates segregation to grain boundaries is highly non-uniform and sensitive to boundary character. This work employs a realistic nanocrystalline microstructure with experimentally relevant global solute concentrations to illustrate inhomogeneous boundary segregation. Furthermore, experiments quantifying segregation in thin films are reported that corroborate the prediction that grain boundary segregation is highly inhomogeneous. In addition to grain boundary structure modifying the degree of segregation, the existence of a phase transformation between low and high solute content grain boundaries is predicted. In order to conduct this study, new embedded atom method interatomic potentials are developed for Pt, Au, and the PtAu binary alloy.
It is well know that the addition of substitutional elements changes the mechanical behavior of metals, a effect referred to solid solution hardening. For body-centered-cubic (BCC) metals, screw dislocation play a key role in the mechanical properties. Here the detailed modification of the Peierls barrier for screw dislocation motion in Ta with W substitutional atoms is computing using density functional theory (DFT). A reduced order model (ROM) of the influence of W substitution on the Peierls barrier is developed. The mean field change in the Peierls barrier for a Ta10W alloy is determined and shown to be larger than anticipated based on simple elasticity considerations. The ROM could be used in future calculations to determine the local variability of the Peierls barrier and the resultant influence on the motion of screw dislocation in this alloy.