Ligament size effect in strength of nanoporous gold in tensile and compressive loading

Abstract

Ligament-size-dependent strength in np-Au, strength increases with decreasing ligament size like as size effect in nanopillars, has been studied extensively. Many studies on ligament size effect were carried out with nanoindentations having the advantage of experimental convenience, but converting from hardness measured by nanoindentation to strength has been controversial. Although there have been a few studies related with ligament size effect investigated by tensile or compressive tests, systemic analysis on deformation of np-Au in both tensile and compressive loading is not fully carried out. In this study, we investigated the effect of ligament size of np-Au on tensile and compressive behavior. By preparing micro-scale samples without grain boundaries and micro-cracks that can be introduced in macro-scale samples, we could focus on effect of ligament size only. We prepared three coarsened np-Au samples with average ligament size of 56, 149, and 402 nm by free corrosion dealloying and post heat treatments. Tensile and compressive samples were prepared by focused ion beam (FIB) and the mechanical tests were performed using in-situ push-to-pull (P-to-P) devices for tensile testing, and with the nanoindenter installed with flat punch tip for compressive testing. We found ligament size effects in both tensile and compressive loading, but size effect exponents, n in σ = A𝑑 −𝑛 where  is the strength, A is the coefficient, and d is the ligament size, are different for tensile and compressive loading. We discuss the effect of loading mode on deformation and size effect in strength.