In order to study the phase decomposition behavior in Mn solid solution, a
Mn-19.7Cu-7.9Ni-2Fe (at.%) alloy has been used, in which fcc → fct phase
transformation was postponed to as low as 150K, in both water quenched and slowly
cooled conditions. Shifted and splitted X-ray diffraction peaks are observed in the
slowly cooled sample, as the evidences for phase decomposition occurred during
slow cooling. Transmission electron microscopy (TEM) and high-resolution electron
microscopy (HREM) experiments have revealed a specific modulated microstructure
in the slowly cooled sample, which contained bumpy morphology and distinguished
striations. Corresponding to the HREM micrograph, extra reflections in electron
diffraction patterns of (100)* and (110)* zones confirm that a lattice modulation in a
period of only 3 lattice parameters has formed along <100> directions. In reference
to the variations of lattice parameter with Mn content in binary MnCu alloys, the