A hidden single-stage martensitic transformation from B2 parent phase to B19′ martensite phase in an aged Ni51Ti49 alloy

•A hidden single-stage B2 → B19′ martensitic transformation is found in aged Ni51Ti49 alloy.•B2 → B19′ occurs concurrently with two-stage B2 → R → B19′ MT, both form a multi-stage MT.•B2 → B19′ takes place in slightly stressed NiTi matrix with large Ni4Ti3 inter-particle space.•B2 → R → B19′ occurs in highly stressed NiTi matrix with small Ni4Ti3 inter-particle space.