Mechanical properties of interphase boundaries (IB), stability of defects and microstructure in heavily deformed binary eutectics (Al-Sn, Zn-Sn, Pb-Sn, Cd-Sn, Bi-Sn) have been investigated at room temperature. Experiments were carried out on atomically clean surfaces of alloys and on bimetallic joints with clean interface. It has been shown that after severe deformation the phases are strengthened and relaxation processes occur mainly on the boundaries in all eutectics. For superplastic eutectics with low interphase boundary energy the intensive development of the diffusion – controlled processes of self- healing, sintering, segregation and enveloping were observed. These diffusion processes, directed to restoration of a contact, are the reason of softening of interphase boundaries and superplastic viscous flow. It has been shown that for the eutectics with high IB energy (Al−Sn, Zn−Sn) the interphase boundary sliding leads to the formation of narrow IB cracks with sharp angles. Diffusion healing of micropores and cracks on these IB does not develop at room temperature and deformation defects are stable. Such deformed structure of IB defines low temperature brittleness of Zn-Sn, Al-Sn eutectics and hydrogen brittleness of Al-Sn.