Dynamic responses in shocked Cu-Zr nanoglasses with gradient microstructure

Shock is one of the physical processes that materials are most likely to suffer during applications, therefore the elusive shock properties of nanoglasses are unacceptable. Additionally, establishing gradient microstructure is a promising approach to optimize mechanics properties further. Here, shock characteristics of Cu64Zr36 nanoglasses with gradient microstructures are systematically investigated by molecular dynamics simulations in the particle velocity range of 0.5 to 5 km/s. Two types of gradient nanoglasses (GNGs) along the shock direction are prepared and analyzed, i.e., a negative gradient structure (S1) in contrast with a positive gradient structure (S2). The results show that the number of mechanically stable <0,0,12,0> and <0,1,10,2> atomic Voronoi polyhedra, which are typical building blocks of the amorphous structure in terms of Voronoi tessellation method, in grain interfaces is significantly less than that in grain interiors. As a result, the local free volume gradually changes along the shock direction by the designed gradient structure, which causes a significant impact on the shock wave profiles of shear strain, stress, configurational entropy, and temperature in the GNGs. However, due to a similar chemically-disordered feature in grain interiors and interfaces, the shock wave speeds of nanoglasses are not sensitive to grain sizes under the same shock strength, contrary to the usual shock wave speed mechanism in conventional polycrystalline. Thus, unlike traditional polycrystalline with grain size gradient, the indirect free-surface method of estimating spall strength is still applicable to the GNGs. Finally, the positive gradient structure results in lower temperature and free volume in the spall region, which causes the spall strengths of the S2 sample higher than those of the S1 sample.