Influence of length-scales on spatial distribution and interfacial characteristics of B4C in a nanostructured Al matrix

To provide fundamental insight into the influence of length scales on the spatial distribution and characteristics of ceramic/metal interfaces in nanostructured metal matrix composites, we studied an Al alloy reinforced with a broad size distribution of B4C particles, ranging from several nanometers to submicrometers. The B4C was incorporated into an ultrafme grained Al matrix using cryomilling and thermomechanical consolidation. The characteristics of the B4C/Al interface, namely the local chemistry and interfacial structure, were studied in detail using transmission electron microscopy (TEM) and atom-probe tomography. Results reveal significant differences in these characteristics as a function of particle length scale. A significant proportion (similar to 40%) of B4C nanoparticles was located intragranularly, i.e., within ultrafine grain interiors, whereas submicron B4C particles were surrounded by multiple Al grains, creating intergranular interfaces. Mg-O-N layers, Al2O3 nanoparticles and amorphous regions were observed at the intergranular interfaces, whereas most of the intragranular interfaces were semicoherent and free of other phases or segregation. By combining crystal structure modeling and high-resolution TEM, a specific orientation relationship was identified for the intragranular interfaces: (1-11)(Al)//(0 2 4)(B4C), 3.4 degrees angle between (0 0 2)(Al) and (0 0 3)(B4C), and 7.8 degrees angle between (2-20)(Al) and (0 2 1)(B4C). Mechanisms related to length scale effects on the formation of intragranular versus intergranular interfaces and corresponding structures and chemistries are discussed, as well as the implications of these interface characteristics on strength and ductility. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.