**1. Introduction**

Developing strong and ductile magnesium alloys has been one of the main research areas in the last twenty years, as they exhibit great potential in aerospace, military, new energy automobile, medical instrument and electronic applications [1–5]. High strength magnesium alloys with ultimate tensile strength higher than 400 MPa have already been successfully prepared with the addition of abundant rare-earth (RE) elements, owing to the intense second phase (precipitate) strengthening effect [6–8]. However, the RE addition impairs the lightweight advantage of magnesium alloys, and increased their cost as well, making them difficult to use for cost-sensitive industry applications [9–11]. Therefore, design and preparation of reinforced RE-free magnesium alloys have recently regained people's attention.

Mg-Al based (AZ series) alloys are one of the most commonly used commercial magnesium alloy series, whose strengthening precipitate is mainly Mg17Al12 phase [12–14]. Mg17Al12 possesses a relatively low melting point (730 K) and tends to be softened above 473 K, thereby exhibiting a weak strengthening effect, especially at high temperatures [15,16]. In the last few years, the Mg-Al-Ca (-Mn) alloys, which were employed as typical heat-resistant casting magnesium alloys formerly, have been researched for high strength wrought Mg alloys. Ultrahigh strength characteristics involving tensile yield strength exceeding 400 MPa have already been obtained for high-Ca-content Mg-Al-Ca-Mn alloys [17–21]. However, excessive Ca addition intrinsically deteriorated the ductility of Mg alloys, whose elongations were always lower than 5%, even after remarkable grain refinement [17–20]. To balance the strength and ductility, Ca content should be decreased in these alloys.

With the increase of Ca content in Mg-Al-Ca alloys, Mg17Al12 phase turns into three laves phases in sequence, namely, Al2Ca (C15), (Mg,Al)2Ca (C36) and Mg2Ca (C14) phases, respectively [17]. Among these intermetallic compounds, Al2Ca phase possesses the highest melting point, approximately 1352 K [22]. Studies of the first-principles calculation also proved that Al2Ca phase exhibits the best thermal stability for the three laves phases [23]. Moreover, Al2Ca phase could form with low or moderate Ca addition, which might facilitate a maximum combination of strength and ductility. So far, most Al2Ca-containing magnesium alloys were prepared by hot extrusion, and they usually exhibited remarkably improved strength [24–26]. Although Al2Ca network phases were crushed after extrusion, large particles remained and the broken particles were not uniformly distributed within a α-Mg matrix. As a consequence, local stress concentration was easy to generate at the junctions of brittle second phase particles and α-Mg matrix during a tensile test, resulting in nucleation and propagation of microcracks at an early time [19]. Therefore, it is essential to develop an effective method to refine Al2Ca phases and increase their dispersibility, in order to improve the ductility of these high strength alloys.

Our previous studies have already shown that the multi-pass equal channel angler pressing (ECAP) is effective to refine large Al-Si particles, and network-shaped second phases in magnesium alloys [27–33]. As for Al2Ca containing alloys, no attempt of ECAP processing has been reported. Therefore, in the present work, we prepared an Al2Ca-dominated Mg-3.7Al-1.8Ca-0.4Mn (wt %) alloy first, and then systematically investigated its microstructural evolution and mechanical properties under an industrial-scale ECAP processing at three different processing parameters. By refining and dispersing Al2Ca second phase particles, we successfully prepared a high strength and high ductility Al2Ca-containing magnesium alloy block.
