Experimental measurement and thermodynamic evaluation of the Mg + Cu + Sr ternary system

Abstract Isothermal sections of the Mg + Cu + Sr ternary system at 300 and 400 °C were investigated using sixteen key samples. The equilibria relationship and phase composition of each sample were deterimined by scanning electron microscopy (SEM) equipped with an energy-dispersive spectroscope (EDS). A new phase (named Mg23-xCu77Srx) was found at the first time in the both isothermal sections. The crystal structure information within nineteen major diffraction peaks of the new phase Mg23-xCu77Srx were identified according to the present XRD results. Moreover, the solid solubility range of Sr in the compound Mg23-xCu77Srx was found to be 6.3 ≤ x ≤ 8.4 at. % with a constant value of about 77 at. % Cu. Six and seven three-phase equilibria regions in the Mg + Cu + Sr ternary system were determined at 300 °C and 400 °C, respectively. The maximum solid solubility of Mg in the terminal solution fcc was found to be 6.8 at. % at 300 °C and 6.4 at. % at 400 °C, respectively. The solid solubility limits of the compounds Mg38Sr9, Mg23Sr6 and Mg2Sr were determined to be 2.9 at. %, 2.7 at. % and 2.2 at. % at 300 °C, respectively. Furthermore, the solid solubility limits of the compounds Mg38Sr9, Mg23Sr6 and Mg2Sr were determined to be 3.0 at. %, 1.6 at. % and 2.8 at. % at 400 °C. The isopleths of the Mg + Cu + Sr ternary system were analyzed combined with DSC measurement and CALPHAD method. Thermodynamic modelling of the Cu + Sr binary system and Mg + Cu + Sr ternary system have been carried out by the CALPHAD technique. The liquid solution was described using the modified quasi-chemical model in the pair approximation (MQMPA). The compound energy formalism (CEF) model was used for the solid solution phases. Thermodynamic evaluation of the Mg + Cu + Sr ternary system was carried out for the first time using the present experimental data. The obtained thermodynamic database of Mg + Cu + Sr ternary system will provide a favourable support for the Mg-based biodegradable implant development.