Atomic-scale investigation of the interface precipitation in a TiB₂ nanoparticles reinforced Al–Zn–Mg–Cu matrix composite

The effects of nanosized reinforcement particles on precipitation reactions in age-hardenable Al alloy matrix composites have been largely unknown. In this work, an Al–Zn–Mg–Cu matrix composite reinforced with uniformly distributed TiB₂ nanoparticles was successfully produced. The solid-soluted, peak-aged and overaged materials were then characterized, at the atomic scale using (high-resolution) scanning transmission electron microscopy, to provide a fundamental insight into the interface precipitation. Our results demonstrated that the facetted TiB₂ nanoparticles have a significant impact on the precipitation in matrix areas adjacent to the TiB₂/Al interfaces. The interfaces after solid-solution treatment are tightly-bonded and oxide-free. The TiB₂ particles and Al matrix display two orientation relationships (ORs): the well-reported [21¯1¯0]_TiB2//[101]_Al, (0001)_TiB2//(1¯11)_Al (OR1) and the new [21¯1¯0]_TiB2//[101]_Al, (011¯0)_TiB2//(111¯)_Al (OR2). The interface precipitates (i.e., interphase) having the size of several tens of nanometers were formed after ageing and were determined to be (Zn_1.5Cu_0.5)Mg phase. Their formations were only related to the initial OR1 and OR2 where the mutual ORs between the TiB₂, interphase and Al matrix were further developed. Periodically spaced misfit dislocations were revealed at the semi-coherent TiB₂/Al interfaces, which are generally considered beneficial to the heterogeneous precipitation. They not only reduced nucleation energy barrier, but also acted as short-circuit diffusion paths for transporting solute atoms and vacancies, accelerating growth rate. However, the growth of interphase at the interface parallel to close-packed {111} Al planes was suppressed by the ultra-low accommodation factor. In addition, such an interface precipitation reduced the mismatch of the TiB₂/Al interface, increasing the overall coherency and being potential for effective interface strengthening.