Understanding the effect of bimodal microstructure on the strength–ductility synergy of Al–CNT nanocomposites

The research work presents a novel methodology to fabricate Al–CNT nanocomposites possessing excellent amalgamation of strength and ductility. A unique combination of processing techniques comprising of ball milling and spark plasma sintering was utilized for consolidating the samples. High-energy ball milling of as-received Al powder and subsequent addition of CNT into the milled Al powder led to a substantial enhancement in the mechanical properties of the resultant composites. Al compact comprising of as-received microcrystalline powder displayed a tensile strength of 105 MPa, which increased to 161 MPa for Al compact with ball-milled nanocrystalline powder. The respective values of ductility reduced from 41.9 to 6.6%. Adding 0.5 wt% CNT in the later compact led to a further upsurge in the tensile strength to 217 MPa; however, the nanocomposite exhibited a further reduction in the ductility to 4.7%. To address this problem, bimodal microstructure was introduced in Al–CNT nanocomposites by using a mixture of nanocrystalline and microcrystalline Al powders (10, 20, 30, 40 and 50 wt%) as the matrix. Al–CNT nanocomposite containing 30 wt% microcrystalline Al powder exhibited a remarkably improved strength–ductility synergy showing 149% higher yield strength and 78% higher tensile strength than the Al compact with microcrystalline grained matrix and 130% higher elongation than the Al–CNT nanocomposite with nanocrystalline grained matrix. Furthermore, to assess the effectiveness of bimodality and CNT reinforcement, experimentally determined value of yield strength of Al30M70N–CNT was assessed with respect to the theoretically calculated value. It was found that the experimentally obtained yield strength of the nanocomposite was about 94% closer to the predicted value endorsing the effectiveness of the designed fabrication route.