Boundaries and interfaces in ultrafine grain composites

The present study was motivated by two questions. First, what are the characteristics of grain and phase boundaries in a nanostructured material containing multiple phases? Second, what is the influence of these interfaces on mechanical behavior? Accordingly, a three-constituent Al 5083/B4C ultrafine grain (UFG) composite, consisting of a coarse grain (CG) phase (1-2 μm), an UFG phase (100-200 nm) and B4C particles (∼0.7 μm), was selected for study. Interest in this particular Al 5083/B4C system stems from its hierarchical architecture, which comprises multiple scales, as well as from a reported yield strength of 1145 MPa. The associated grain boundaries (GB) and interfaces were investigated by transmission electron microscopy (TEM), high-resolution TEM, energy dispersive X-ray spectroscopy and electron energy loss spectroscopy methods. The role of high/low-angle GB, equilibrium and non-equilibrium GB within and between the CG and UFG regions, twin boundaries, twist transition boundaries and impurity segregation at GB in strengthening mechanisms is discussed.