This study introduces a novel multi-material design strategy termed spatial metastability control (SMC) for enhancing the transformation-induced plasticity (TRIP) behaviors in ferrous medium-entropy alloys (MEAs). The approach utilizes a compositional heterostructure by integrating pure Fe into a metastable MEA, establishing a gradual phase stability across the MEA/Fe interface. This gradient forms a distinct transition region characterized by refined grain size, thermal martensite formation, an