Phase transition of Ni–Mn–Ga alloy powders prepared by vibration ball milling

This study investigated the phase transformation of the flaky shaped Ni–Mn–Ga powder particles with thickness around 1 μm prepared by vibration ball milling and post-annealing. The SEM, XRD, DSC and ac magnetic susceptibility measurement techniques were used to characterize the Ni–Mn–Ga powders. The structural transition of Heusler → disordered fcc occurred in the powders prepared by vibration ball milling (high milling energy) for 4 h, which was different from the structural transition of Heusler → disordered fct of the powders fabricated by planetary ball milling (low milling energy) for 4 h. The two different structures after ball milling should be due to the larger lattice distortion occurred in the vibration ball milling process than in the planetary ball milling process. The structural transition of disordered fcc → disordered bcc took place at ∼320 °C during heating the as-milled Ni–Mn–Ga powders, which was attributed to the elimination of lattice distortion caused by ball milling. The activation energy for this transition was 209 ± 8 kJ/mol. The Ni–Mn–Ga powder annealed at 800 °C mainly contained Heusler austenite phase at room temperature and showed a low volume of martensitic transformation upon cooling. The inhibition of martensitic transformation might be attributed to the reduction of grain size in the annealed Ni–Mn–Ga particles.

Research highlights
► The vibration ball milling with a high milling energy introduces the atomic disorder and large lattice distortion in the alloy during milling and makes the formation of disordered fcc structure phase in the alloy.
► The transition temperature and activation energy for disordered fcc → disordered bcc are ∼320 °C and 209 ± 8 kJ/mol, respectively.
► The alloy powders annealed at 800 °C for 1 h show a one-stage martensitic transformation with quite lower latent heat compared to the bulk alloy.