The effect of heat treatment at 500–655 °C on the microstructure and properties of mechanically alloyed Al–Ti–O based material

The microstructure, hardness, and tensile properties of a mechanically alloyed Al–0.35Li–1Mg–0.25C (wt.%)+7.5vol.%TiO2 material are reported, both as-extruded, and after heat treatments at 500–655 °C for times up to 750 h. Both X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed presence, as-extruded, of Al3Ti, cubic TiO (C–TiO), and monoclinic TiO (M–TiO) and, in addition, Al4C3, MgO, and TiO2 (brookite, anatase, and rutile) were identified by TEM. The Al3Ti dispersoids (displaying typically one of two orientation relationships with the α-Al matrix) were essentially spherical, as-extruded, and generally of size 90–150 nm, becoming more elongated and faceted at 655 °C, at which temperature the particle volume increased by a factor of 2.3 between 24 and 168 h exposure. Hardness increased by ∼4, 3, 10 and 23% in 504 h at 500 °C, and 24 h at 550, 600 and 655 °C, respectively, attributable mainly to further reduction of C–TiO and M–TiO to form additional Al3Ti. Al2O3 (mainly the α-variant) was an additional reaction product detected by XRD after 24 h at 655 °C. The starting grain size (0.49±0.18 μm) was unaffected by heat treatment for 24 h at 500 or 550 °C, and increased by only ∼20% during 168 h at 655 °C. Compared with 24 h at 655 °C, the hardness reduced by ∼4 and 78% after 168 and 750 h at this temperature, respectively. While heat treatment for 48 h at 500 °C had little effect on tensile properties, 48 h at 550 °C increased the room temperature yield strength and ultimate tensile strength by ∼7 and 13%, while the elongation to fracture was almost halved compared with the as-extruded condition. Fracture surfaces after room temperature testing, as-extruded, or following heat treatment for 48 h at 550 °C, were ductile, while cleavage fracture resulted from tests at 300 °C, following heat treatment for 48 h at 500 °C.