Order–disorder transition of vacancies from the full- to the half-Heusler structure in Ni2−xMnSb alloys

•L21/C1b ordering transition in Ni2−xMnSb (x = 0–1) alloys was firstly confirmed.•Ordering of vacancies in C1b phase could be detected by STEM-HAADF method.•L21/C1b ordering temperatures of Ni2−xMnSb alloys were determined by DSC.•Phase diagram in NiMnSb–Ni2MnSb pseudo-binary system have been established.

The thermal transformations and crystal structures in the ordered bcc phases appearing in Ni2−xMnSb alloys have been investigated. Thermo-analyses on NiMnSb(x = 1) show three λ-shaped peaks suggesting a magnetic and chemical ordering transitions at approximately 450, 865 and 965 °C. With decreasing x, the first and second peak temperatures gradually decrease, while the third one increases. By Z-contrast imaging technique, it was confirmed that the second peak corresponds to the order-disorder transition of vacancies from the L21 to the C1b structure.