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 15° 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¯)γ//(002¯)M. A substructure of (1¯11¯) 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.