θ′′′ precipitate phase, GP zone clusters and their origin in Al-Cu alloys

Abstract

Guinier-Preston (GP) zone is an important precipitate in Al-Cu alloys. In this work, atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and first-principles calculations are combined to study paired GP zones and clustered GP zones that form frequently in samples of an Al-1.74at%Cu alloy under-aged at 120 and 160 °C. In addition to the well-known θ″-Al3Cu phase that comprises two Cu monolayers (GP zones) separated by three {002} layers of aluminium, a hitherto unreported θ′′′-Al6Cu phase, comprising two GP zones separated by six {002} layers of aluminium, is also frequently observed. Statistical counting indicates that the Al6Cu phase forms with a number density just below that of θ″-Al3Cu. The prolonged ageing at 120 °C and 160 °C leads to the formation of clusters comprising a mixture of θ″ and θ′′′ phases. First-principles calculations reveal that the origin of the regular and irregular stacks of GP zones is the enhanced Al-Al bonds induced by a pre-existing GP zone, which triggers a new GP zone to preferentially form at the position three or six {002} Al planes away from the pre-existing GP zone. The enhanced Al-Al bonds can not only explain the experimental observations made in the present work, but also rationalize previously reported precipitation phenomena associated with GP zone, θ″ or θ′ precipitate phase. Data availabilityAll the relevant data are available from the corresponding author on request.