Processing, microstructure and ageing behavior of in-situ submicron TiB2 particles reinforced AZ91 Mg matrix composites

In this paper, in-situ submicron TiB2 particles (2.5 wt.%) reinforced AZ91 matrix composites are processed by the master alloy method combined with an optimized SHS technique. The effect of TiB2 particles on the microstructure, ageing behavior and mechanical properties of Mg matrix composites is investigated. The results reveal that only TiB2 phase is in-situ formed in the Al-TiB2 master alloy and exhibits the size distribution of 100 nm-700 nm. Compared with the unreinforced AZ91 alloy, the grain size and morphology of Mg17Al12 phase in as-cast composites are much refined. The ageing process of composites is accelerated and the peak-aged time of composites decreases from 42 h to 22 h significantly with the introduction of TiB2 particles. Large numbers of fine Mg17Al12 in composites preferentially precipitate at grain boundaries and near TiB2 particles regions. In addition, the hardness, yield strength, ultimate tensile strength and elongation of as-cast composites are improved by 16.1%, 53.0%, 26.3% and 25.6%, respectively. After T6 heat treatment, the strength of composites is increased further due to large amounts of submicron Mg17Al12 phase, and the age hardening efficiency of composites is higher than that of AZ91 alloy.