Application of novel constrained friction processing method to produce fine grained biomedical Mg-Zn-Ca alloy

•A novel friction-based processing method, constrained friction processing (CFP), is applied to produce potential Mg implants.•Grain size was reduced from over 1 mm to around 4 µm within 3 s by a single process cycle.•CFP increased the compressive yield strength by 350% and ultimate compressive strength by 53%.•Material flow in CFP is driven by severe shear deformation and affected by heat input.•Microstructure evolution is revealed by “stop-action” technique.