Effect of aging on martensitic transformation, microstructures and mechanical properties in off-stoichiometric Mn-Ni-Ga alloys

(Mn,Ni)4Ga-type γ precipitates were obtained in Mn53Ni25Ga22 alloys aged at 573-973 K. Aging greatly affects the transformation temperature by modifying the matrix composition and changing the unit cell volume. When the aging temperature is 573 K, needle-like γ precipitates 70-80 nm in length can be seen by transmission electron microscopy. Rotational twin martensite variants were discovered to be related with an angle rotation of 15o in the [1 1 1] direction. A crystalline orientation relationship between the tetragonal martensites and face-centered cubic γ phases was observed as [1 1 1]γ//[1 1 0]M and (022-bar)γ//(002-bar)M. A substructure of (1-bar 11-bar) type-I twins for γ phase was found in the 973 K/3 h aged sample in which the coarsened precipitates reveal a plate-like morphology. Compression tests show that a compressive strength of 1850 MPa with a fracture strain up to 25.6% can be achieved in 773 K/10 min aged samples due to a substantial dispersion strengthening from small γ precipitates.