Microstructure and mechanical properties of aluminum matrix composites with bimodal-sized hybrid NbC-B4C reinforcements

Abstract

Aluminum (Al) is an earth-abundant metal recognized with superior properties for vital applications in the aerospace and transportation industries. Structural components of Al exhibit poor performance due to its inherent low mechanical strength. In this work, the mechanical properties of Al are enhanced by reinforcing with bimodal micron-sized Niobium Carbide (NbC) and nano-sized Boron Carbide (B4C) ceramic particles. Al-NbC-B4C hybrid composites have been synthesized via ball milling followed by cold compaction and microwave sintering. NbC micro-reinforcement composition has been kept fixed (5 wt%), while B4C nano-reinforcement composition varied from 0.5 wt%, 1.0 wt%, 1.5 wt%, and 2.0 wt%. XRD patterns revealed the high crystallinity with no detected new phases formed in the sintered composites. TEM micrographs presented the microstructure evolutions with uniform distribution of (micron + nano) hybrid bimodal-sized ceramic reinforcements in the Al matrix. FE-SEM micrographs and corresponding elemental mapping demonstrated the homogeneity in the elemental distribution of synthesized Al-NbC-B4C composites through the ball milling and microwave sintering processes. Roughness values and AFM images showed the formation of insoluble secondary phases dispersed in the Al matrix enhancing its surface resistance towards localized plastic deformations. Al-5 wt%NbC-2.0 wt%B4C composite has exhibited an ultrahigh improvement in the mechanical properties compared to pure Al. It showed enhancements in microhardness (46%), nanohardness (54%), and Young’s modulus (31%). It also showed high ultimate compression strength of 328 MPa and a low engineering failure strain of 0.64. FE-SEM compressive fractography confirmed the strengthened dispersion hardening effect from bimodal-sized ceramic particles obstacle multi-length cracks and resisting fracture failure.