Mechanical in situ CT investigations on cruciform members of square-celled Mg-PSZ reinforced TRIP-steel honeycomb structures are presented. Furthermore, test series on a pure TRIP-steel and MMC bulk material with 10 vol% Mg-PSZ as well as macro-scale compression tests on honeycomb structures are performed under quasi-static loading. The measured stress–strain curves of the cruciform columns are related to their buckling and failure behaviour. The mechanical and structural characteristics of the cell wall node elements are compared to the damage mechanisms of the complete honeycombs. Differences in the damage patterns between the pure TRIP-steel and the MMC are demonstrated. The phase transformation effect in the Mg-PSZ particles is pointed out based on EBSD and EDX measurements.