Effect of grain refinements on the microstructure and thermal behaviour of Mg–Li–Al alloy

Abstract

The light as-cast Mg–9Li–1.5Al alloys were manufactured and modified by 0.2 mass% Zr, commercial 0.2 mass% TiBor and 0.2 mass% AlSr master alloys. The thermal derivative analysis using UMSA platform was utilised to characterise a crystallisation process. Samples were heated up to 700 °C and then freely cooled down (~ 0.6 °C s−1) to ambient temperature in order to simulate the natural cooling of casts. Dilatometry study was used to identify changes in solid state. The relative elongation of unmodified and modified alloys was measured in the temperature range from 20 to 400 °C, with a heating rate 1.0 °C s−1. The effects of Zr, TiBor and AlSr content on the microstructure of analysed magnesium alloys were investigated. Evaluation of microstructure was identified by light microscope, scanning electron microscopy, X-ray diffraction and energy-dispersive X-ray spectroscopy. The results showed that the addition of TiBor reduced the grain size of Mg–9Li–1.5Al cast alloy from 930 to 530 µm, while the addition of AlSr master alloy reduced the grain size to 480 µm. Moreover, an addition of TiBor and AlSr simultaneously reduced the grain size to 430 µm. The addition of Zr causes a reduction in grain size to 630 μm. The addition of grain refinement causes changes in crystallisation process and variations in the coefficient of linear thermal expansion (CTE).