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 degreesC 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 alpha-Al matrix) were essentially spherical, as-extruded, and generally of size 90-150 nm, becoming more elongated and faceted at 655 degreesC, at which temperature the particle volume increased by a factor of 2.3 between 2.4 and 168 h exposure. Hardness increased by similar to4, 3, 10 and 23% in 504 h at 500 degreesC, and 24 h at 550, 600 and 655 degreesC, respectively, attributable mainly to further reduction of C-TiO and M-TiO to form additional Al3Ti. Al2O3 (mainly the a-variant) was an additional reaction product detected by XRD after 24 h at 655 degreesC. The starting grain size (0.49 +/- 0.18 mum) was unaffected by heat treatment for 24 h at 500 or 550 degreesC, and increased by only similar to20% during 168 h at 655 degreesC. Compared with 24 h at 655 degreesC, the hardness reduced by similar to4 and 78% after 168 and 750 h at this temperature, respectively. While heat treatment for 48 h at 500 degreesC had little effect on tensile properties, 48 h at 550 degreesC increased the room temperature yield strength and ultimate tensile strength by similar to7 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 degreesC, were ductile, while cleavage fracture resulted from tests at 300 degreesC, following heat treatment for 48 h at 500 degreesC. (C) 2002 Elsevier Science B.V. All rights reserved.