Alloys, with six different compositions in the system Mg-Ca-Zn were produced by melt spinning. The aging behavior of alloys was investigated by measuring the changes in microhardness after isochronal aging and the microstructure was analyzed by means of Transmission Electron Microscopy(TEM), Energy Dispersive X-ray Spectrometry (EDS), Scanning Transmission Electron Microscopy (STEM) and Scanning Electron Microscopy (SEM). All six compositions in the as-solidified condition show a difference in microstructure between the wheel contact side (zone A) and the free surface side (zone B) as a result of the differences in the solidification rate across the ribbons. One of the alloys was chosen to be more deeply investigated in this work as it exhibited grain boundary films and presented the highest peak hardness among the low Ca alloys. The comparison between the two microstructural zones in this alloy can aid in understanding of the phase transformation steps during cooling with a model which is proposed here.