(299c) Molecular Dynamics Simulation of the Sintering of Titanium/Aluminum Core/Shell Nanoparticles

Bimetallic core/shell nanoparticles (NPs) yield intriguing microstructures after being processed into final products. Direct metal laser sintering (DMLS) and selective laser sintering (SLS) are two powder fusion processes for metallic NPs, with the latter being the preferred additive manufacturing technique for these materials. Among bimetallic NP candidates for SLS, Ti/Al core/shell NPs have found much interest lately. However, not much is known about their sintering states during this process. To fill this knowledge gap, we performed a series of classical molecular dynamics (MD) simulations to roughly mimic the conditions of SLS for Ti/Al core/shell NPs. We further investigated the role of Ti core volume fraction (3, 12, and 30%) on the resultant uniaxial tensile properties of the sintered NPs during various sintering states. For this purpose, we created a “chain” model from five single thermally equilibrated Ti/Al core/shell NPs with weak neck connections by solid-state sintering at 298 K. The chains were then heated to 800 K with slow and fast heating rates of 0.04 K/ps and 0.2 K/ps, respectively. Next, they were relaxed at this high temperature (to yield stronger NP-to-NP neck connections), where after they were cooled back to 298 K with a cooling rate of 0.08 K/ps. They were then relaxed at this temperature for different periods of 1, 4, and 10 ns. To determine the characteristics of the heat sintering process, we measured the mean square displacement (MSD) of the atoms and squared radius of gyration from the trajectory data. MSD as a function of temperature indicates the extent of sintering and the quality of the final sintered products. We further investigated the structural evolution of the atomic configurations during the sintering process. Overall, heating at a rate of 0.04 K/ps yields lower neck-formation temperatures for all NP core volume fractions. Also, on average, an increase in the Ti core volume fraction of the NPs from 3% to 30% causes a drop in the neck-formation temperatures at both heating rates. In a follow-up procedure, we subjected the sintered NP products to uniaxial tension with strain rates of 0.001, 0.01, and 0.1 %/ps along the x-direction. For all cases of core volume fractions, the Young’s moduli were determined to be within 40 GP, indicating that Ti core volume fraction has a negligible effect on the elastic tensile properties of the Ti/Al core/shell NP chain products. Our results further indicate that a larger Ti core volume fraction yields stronger chain structure and, hence, a higher tensile strength. The effect of heating rate on the tensile strength of the final products with larger core volume fractions is more pronounced. Also, high strain rates result in an improvement in the tensile strength and ductility of the final products. It is expected that insights gained from this work will promote the SLS process of bimetallic core/shell NPs.