Microstructure, phase transformation and mechanical property of Nb-doped Ni–Mn–Ga alloys

• Microstructure, phase transformation and mechanical property were investigated.
• Addition of Nb in Ni–Mn–Ga alloy introduced a Nb-rich second phase.
• Martensitic transformation temperature was changed with increasing Nb content.
• Transformation enthalpy was decreased with increasing Nb content.
• The compressive strength was enhanced by the Nb-rich second phase.

This study investigated the microstructure, phase transformation and mechanical property of (Ni49.8Mn28.5Ga21.7)100-xNbx (x = 1, 3, 6, 9) alloys. The Nb1 alloy exhibited a single austenite phase at room temperature. With increasing Nb content for Nb3, Nb6 and Nb9, the alloy changed to a dual phase consisting of austenitic matrix and Nb-rich second phase with a hexagonal structure, and the amount of the second phase increased with the increase of Nb content. The martensitic transformation temperature and Curie temperature were changed and the transformation enthalpy was gradually reduced with increasing Nb content. The change of martensitic transformation temperature and Curie temperature was related to the introduction of Nb in the Ni–Mn–Ga structure that decreased valence electron concentration (e/a), increased unit cell volume and reduced magnetic exchange of the alloys. The decrease of transformation enthalpy was mainly attributed to the formation and increase of the Nb-rich second phase that reduced volume fraction of the matrix taking part in phase transformation. All the alloys presented a similar compression behavior with progressively fracturing characters (occurrence of several stress drops before complete fracturing). The fracture strength was slightly enhanced with increasing Nb content from Nb0 to Nb9, but the ductility has no apparent improvement.