An investigation into the thermal stability and grain growth kinetics of al 5083-B(4)C cryomilled nanocomposites

Metal-matrix nanocomposites are known for high strengths, improved thermal properties and good wear resistance. However, it is critical for these materials to retain their nanostructures upon exposure to high temperatures during processing and applications. Nanocomposite powders of Al-B4C have been produced via cryogenic milling (cryomilling) of B4C reinforcements in a matrix of Al 5083. The effect of B4C reinforcements, as well as second-phase cryomill-generated dispersoids (specifically Al nitrides) on grain growth in nanocrystalline Al was investigated by cryomilling powders in liquid N2 and liquid Ar for different durations. Chemical analysis indicated higher N content for powders with longer exposures to liquid N 2 during cryomilling. Powders were also characterized by XRD, SEM and TEM. Average grain sizes were calculated by established XRD-based methods while also confirmed by TEM. Thermal stability of the composite was investigated in the form of bulk SPS samples, as well as in powders, via isochronal and isothermal annealing to temperatures up to ∼0.86-0.96 Tm for durations as long as 24 hours. Grain growth in SPS-consolidated samples showed irregular trends, an observation that was attributed to the inhomogeneity in microstructures after sintering. However, the results were indicative of higher thermal stability for samples with higher N content. Grain growth patterns in annealed cryomilled powders showed consistent trends for all grain size calculation methods. Average grain sizes increased with time and temperature and tended to stabilize almost after the fourth hour of annealing. XRD-based calculations showed the main decrease in microstrain also occurring during the first four hours of annealing. The 24-hour cryomilled powder showed superior thermal stability and retained its nanostructure even after 24 hours of annealing at 550°C, due to high contents of nitrides and other secondary cryomill-generated dispersoids that are known to retard grain growth by pinning grain boundaries. In addition, the presence of boron carbide reinforcements in the cryomilled powder was found to improve thermal stability of the Al 5083 matrix, when compared to plain Al 5083 cryomilled powder studied in the literature. Mechanisms of grain growth were investigated by kinetic studies according to the conventional grain growth exponent model and Burke's model. The results showed two thermally activated grain growth regimes: a low temperature regime of T<450°C with an activation energy of Q ≅ 14 kJ/mol where stress relaxation was the dominating mechanism for grain growth, and a high temperature regime of T>450°C with Q ≅ 57-75 kJ/mol where it appears that grain boundary diffusion dominates. Grain growth data for high temperatures of 500°C and 550°C revealed a novel two-step characteristic in the isothermal grain growth pattern of the cryomilled powder.