Annealing-induced phase transformations and hardness evolution in Al–Cu–Al composites obtained by high-pressure torsion

Abstract

Multilayered bulk Al–Cu–Al metal-matrix composite was fabricated by means of high-pressure torsion and subsequent annealing. The resulting composite had a heterogeneous structure consisting of ductile aluminum matrix and hard intermetallic inclusions with a gradient decrease in grain size and layer thickness when moving from the center to the periphery of the sample. Precipitation of $\hbox {Al}_{2}\hbox {Cu}$ intermetallic phase was revealed at the edge of the sample in the as-deformed state. Post-deformation annealing initiated the emergence of AlCu and $\hbox {Al}_{\mathrm {4}}\hbox {Cu}_{\mathrm {9}}$ intermetallic precipitates with increased hardness compared to strain-induced $\hbox {Al}_{\mathrm {2}}\hbox {Cu}$ particles. The growth kinetics of intermetallic compounds was obtained using precise X-ray phase analysis. It was found that the initial growth of intermetallic phases at temperatures 150–210 $^{\circ }$C depends on time $t^{1/2}$, indicating the bulk diffusion-controlled growth. The growth activation energy of Al$_2$Cu and AlCu phases was calculated to be 0.48 and 0.33 eV, respectively. The results obtained contribute to an understanding of the kinetics of annealing-induced growth of intermetallic phases and the corresponding evolution of strength characteristics in Al–Cu–Al composites. It was revealed that thermal treatment regimes resulting in enhanced mechanical properties are associated with moderate time and temperature of annealing, which allows avoiding partial dissolution of strengthening phases. The applied approach of phase kinetics analysis can become the basis for the development of new energy-efficient heat treatment modes of in situ Al-based composites allowing to govern their heterogeneity type and tailoring the mechanical properties of the material.