The Formation of L-Phase Precursor Originated from Cu-Containing Guinier–Preston Zone in Al–Mg–Si–(Cu) Alloys

Abstract L-phase precursors were discovered in AA6061-T8 aluminium alloy using high-resolution scanning transmission electron microscopy. Precipitates with a lateral size of approximately 2 nm form after cold rolling and artificial ageing. These fine L-phase precursors are characterized by Cu-rich column(s) that form single or multiple C sub-units as the structural core. Based on the arrangement of C sub-units, two distinct types of L-phase precursors are observed, each exhibiting different initial structural configurations. Type I precursors initiate from a single C sub-unit, which subsequently develops into an in-plane mirror-symmetric structure in cross section. Growth of Type I precursors proceeds through further addition of Cu atoms that substitute the Si atoms in the precursor’s periphery. Type II precursors are characterized by two Cu columns that form two C sub-units in a rotationally symmetric configuration. The formation of both types of the L-phase precursor begins with Cu substitution for Si within the Guinier–Preston (GP) zones during early stages of precipitation. In a specific 2 β ″ type GP zone, Cu substitution follows a well-defined pattern. Density functional theory (DFT) calculations indicate that Cu substitution behaviour in 2 β ″ type GP zones is following the pathway which overcomes the minimum energy barrier, subsequently evolving into an L-phase precursor with a characteristic Cu distribution pattern. These findings establish a direct link between atomic-scale Cu substitution behaviour and the early-stage evolution of Cu-rich precipitates in 6xxx aluminium alloys.