Microstructural evolution and mechanical properties of cast high-Li-content TiB2/Al-Li-Cu composite during heat treatment

The microstructural evolution and mechanical properties of cast high-Li-content TiB2/Al-Li-Cu composite during heat treatment were systematic investigated by a combination of various techniques, including transmission electron microscopy (TEM) with HAADF-STEM mode, electron backscatter diffraction (EBSD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The coarse secondary phases, mixed with clusters of relatively larger TiB2 particles, distributed along grain boundaries were observed in as-cast state. And most of them dissolved into the α-Al matrix after a two-stage solution treatment. The grain size of the studied composite exhibited excelled thermal stability owing to effective pinning grain boundaries by the stable particles at the elevated temperature, resulting in no significant increase after solution treatment. The studied composite showed a strong age-hardening response with the peak-hardness was obtained in about 160 h at 175 °C. TEM results showed much fine δ′ precipitates were uniformly distributed within grains after quenching and it coarsened with the increasing age duration. Fine plate-like θ′ and T1 precipitates were observed in the under-aged (UA, aged for 8 h) condition. The rate of precipitation and growth in its diameter was much greater than its coarsening rate, resulted in the large aspect ratio and high number density of T1 and θ′ precipitates were visible in the peak-aged (PA) condition. Finally, a slight decline in hardness was observed in the over-aged (OA) condition due to the apparent coarsening of θ′ and T1 precipitates. This precipitation behavior was further verified by DSC. After appropriate heat treatment, a good comprehensive mechanical properties could be obtained in the composite.