Thermodynamic descriptions of quaternary Mg–Al–Zn–Bi system supported by experiments and their application in descriptions of solidification behavior in Bi-additional AZ casting alloys

Abstract

Abstract 16 Mg–Al–Zn–Bi quaternary alloys were utilized to measure the phase equilibria and transformation temperatures in the Mg-rich Mg–Al–Zn–Bi quaternary system by means of the X-ray diffraction, electron probe micro-analysis and differential scanning calorimetry techniques. The isothermal section at 400 °C and three vertical sections along Mg–8 wt%Al–0.75 wt%Zn–xBi, Mg–3.4 wt%Al–0.5 wt%Zn–xBi and Mg–6.9 wt%Al–2.3 wt%Zn–xBi in the Mg–Al–Zn–Bi quaternary system were constructed. Based on the literature data, the ternary Mg–Al–Bi and Mg–Bi–Zn systems were re-assessed using the CALculaiton of PHAse Diagram (CALPHAD) approach. The calculated phase equilibria agree well with the measured data. By directly extrapolating the constituent sub-ternary systems, the thermodynamic database for the Mg–Al–Zn–Bi quaternary system was developed. The remarkable consistency between the predicted phase equilibria and the presently measured data in Mg–Al–Zn–Bi quaternary system further demonstrated the accuracy and reliability of the established thermodynamic database. After that, by using the newly developed thermodynamic database, the growth restrict factors and the solidification curves in Bi-containing AZ series magnesium alloys were calculated and analyzed. It was confirmed that the grain size of AZ alloys can be refined with the addition of Bi, and the component Al had larger grain refinement effect than Bi. Besides, the amount of Bi had also noticeable effect on the solidification sequence of the AZ alloys.