Microstructural Characteristics and Mechanical Properties of CNT/Ni Coated CNT–Dispersed Al Alloys Produced by High Energy Ball Milling and Hot Extrusion

The effect of dispersion of carbon nanotubes (CNT) and Ni coated CNT in Al alloy (AA) on the evolution of microstructure and resultant mechanical properties are investigated. Al alloy (Al–4.4Cu–0.5Mg), Al alloy–CNT (AC) and Al alloy–Ni coated CNT (ANC) composites were produced from elemental powders by mechanical milling (MM) followed by hot extrusion. Unlike CNT–containing Al or Al alloy milled powders, Al₄C₃ phase formed during milling of ANC powder. The formation of Al₄C₃ is due to the dissolution of Ni present on CNT surface in Al alloy during MM, thereby, creating defects on CNT that promoted Al₄C₃ formation. AC and ANC samples exhibited larger size and lower number density of θ′ precipitates when compared to AA. The larger size of θ′ precipitates is due to the presence of ultra–fine grains, reduced quenched–in vacancies and higher dislocation density that promoted the nucleation resulting in enhanced precipitation and growth kinetics. The strengths of AC and ANC samples are higher when compared to CNT-free AA, and ANC exhibited highest yield strength of 567 MPa. However, the ductility of ANC sample is not improved due to the presence of Al₇Cu₄Ni phase, which promoted crack initiation.