**2. Materials and Methods**

The raw materials for preparation of studied Mg-3.7Al-1.8Ca-0.4Mn (wt %) alloy were pure Mg, pure Al, Mg-30 wt % Ca and Mg-10 wt % Mn master alloys. The designed alloy was prepared by semi-continuous casting method, with an ingot diameter of 90 mm. Then large cuboid samples with dimension of 50 mm × 50 mm × 100 mm were cut from the center of the ingot for further industrial-scale ECAP. To explore the influence of processing parameters on refinement of Al2Ca-containing alloys, three ECAP routes were proposed, i.e., 4 passes at 623 K, 12 passes at 623 K, and 12 passes at 573 K. The ECAP die employed was a self-design rotary-die (RD) with die angle of 90 ◦ and outer arc angle of 0◦, which was described detailedly in our previous work [33]. Before ECAP, the sample was inserted in the die and both were heated and kept at a set temperature for 20 min within an induction heating furnace. Then multi-pass RD-ECAP were performed successively by an automatic control system without intermediate heating. This ECAP process is time-saving, and the total processing time for 12 passes was less than 15 min.

Metallographic specimens were then cut from the cast and ECAP specimens, mechanically grinded by #80, #400, #1000 and #2000 SiC abrasive papers, polished and etched with a 4 mL nitric acid and 96 mL ethanol mixed solution. Then, microstructure characterizations of the alloys were carried out by the X-ray diffractometer (XRD, Cu-Kα, Bruker D8, DISCOVER, Bruker Corporation, Karlsruhe, Germany), optical microscope (OM, Olympus BX51M, Olympus, Tokyo, Japan), and a scanning electron microscope (SEM, Sirion 200, FEI Company, Hillsboro, OR, USA) equipped with an X-ray energy dispersed spectrometer (EDS, Genesis 60S, FEI Company, Hillsboro, OR, USA). To further identify various phases, observation of the transmission electron microscope (TEM, Tecnai G2, FEI Company, Hillsboro, OR, USA) was conducted. The TEM samples were prepared by mechanical grinding and ion thinning. To reveal the grain size distributions of ECAP alloys, the electron back-scatted diffraction (EBSD) analyses were conducted (SEM, Hitachi S-3400N, Hitachi, Tokyo, Japan). Moreover, the average particle sizes were measured by counting at least 100 particles in three SEM images through the Image-J software (Image 1.48, NationalInstitutes of Health, Bethesda, MD, USA). To evaluate the mechanical properties, tensile tests were performed via a CMT5105 electronic universal testing machine (MTS, Shenzhen, China) with a ram speed of 0.5 mm/min at room temperature. The dumbbell shaped specimens with gauge dimension of 7.5 mm × 2 mm × 2 mm were cut from the center section of ECAP samples with the loading direction parallel to the extrusion direction. For each processing situations, three tensile specimens were employed.
