Ultra-fine grain size and exceptionally high strength in dilute Mg–Ca alloys achieved by conventional one-step extrusion

•Ultra-fine grain size (less than 1 μm) can be achieved in the Mg–Ca dilute alloy.•Yield strength of ∼290 MPa in the Mg–0.1 wt% Ca alloy can be achieved.•Excellent combined mechanical properties have been achieved.•Formation mechanism of ultra-fine grain size is clarified. We reported that the Mg–Ca dilute alloys can exhibit ultra-fine grain size (less than 1 μm) and the exceptionally high yield strength, e.g., ∼290 MPa in the Mg–0.1 wt% Ca alloy and ∼377 MPa in Mg–1.0 wt% Ca alloy, achieved by conventional one-step extrusion. Both increased activations of non-basal slips due to Ca addition and the dynamic partitioning of Ca atoms during extrusion are believed to play the critical role in facilitating formation of the submicron matrix grains. Multi-scale substructures, including the ultrafine recrystallized grains, the mono-dispersive Mg2Ca nano-phases, residual dislocations and Ca enriched sub-grain boundaries contribute to the high strength.