Search

In this study, the effects of Ni content on the microstructure and mechanical properties of Al-5.0Cu-1Mn-based alloys were investigated. The results demonstrate that Ni alloying refines the grain size and enhances castability. At low Ni content, some of Ni atoms participate in forming Al7Cu4Ni at the grain boundaries. The rest Ni segregates to the interface between α-Al and AlCu3Mn2 phase, which inhibits AlCu3Mn2 growth. The amount of Ni-rich phase at grain boundaries increases significantly as Ni concentration increases at the expense of the decline precipitates. After T6 treatment, the lamellar Ni-rich phase spheroidized into discrete particles that were uniformly distributed along the grain boundaries. Ni-rich phase of particles can effectively pin grain boundaries and prevent grain rotation. The optimum mechanical property is achieved with 1% Ni addition, predominantly due to the synergistic strengthening of eutectic strengthening and precipitation strengthening. The ultimate tensile strength, yield strength and elongation reach 505 MPa, 348 MPa and 16.5% respectively, which far exceed the current commercial cast aluminium alloys. With increasing Ni concentration, the fracture mode of the alloy changes from ductile fracture to brittle-ductile mixed fracture, because the high amount of Ni-rich phase intermetallic compounds increase the sites for localized brittle fractures.

An electrochemical flow cell was constructed to permit the application of a mechanical scratch to the exposed material while monitoring the open circuit dissolution rates of the elemental alloy components. Elemental dissolution rates were determined where measured downstream using inductively coupled plasma atomic emission spectrometry (ICP-AES). The technique is demonstrated with Mg-Li(-Al-Y-Zr) alloy exposed to 0.01 M NaCl. A mechanical scratch was applied manually using a non-conductive and non-contaminating sapphire lancet without interrupting the on-line measurement. The ion dissolution rate from the newly exposed surface was significantly higher than the dissolution rate from the remaining surface area, corresponding to a high local current density. Notably, the increase in the dissolution rate from the scratched area occurred gradually after the scratch; then subsided within 200 s after the scratch, indicating that this particular Mg-alloy can repassivate by the formation of a surface film after a scratch was applied. This is the first reported use of an in situ scratch cell coupled with ICP-AES. Keywords: Magnesium, Magnesium‑lithium alloy, In situ, Electrochemistry, Online ICP