Effect of Carbon Nanotubes on Solution Treatment Temperature and Dissolution Characteristics of Precipitates in Al Alloy Produced by High-Energy Milling and Hot Extrusion

The present study discusses the effect of CNT on the solution treatment temperature for Al alloy (AA), i.e. Al–4.4Cu–0.5 Mg, Al alloy–CNT (AC) and Al alloy–Ni-coated CNT (ANC) composites fabricated by mechanical milling and hot extrusion. Differential scanning calorimetry indicated that dispersion of CNT in Al alloy increased the dissolution temperature of θ phase precipitates. X-ray diffraction (XRD) and scanning electron microscopy illustrated complete dissolution of θ phase in both AA and AC samples when solution-treated at 520 °C, whereas ANC sample exhibited the same behaviour at 500 °C itself. XRD patterns of ANC sample revealed the presence of an additional intermetallic Al7Cu4Ni phase, which remains undissolved even after the solution treatment at 540 °C. Utilisation of Cu atoms to form Al7Cu4Ni phase resulted in significant reduction in size and number density of θ phase in ANC. The hardness of AC and ANC samples was found to be higher compared to AA owing to dispersion strengthening by CNT and Al7Cu4Ni particles.