Grain growth in Ni-Mn-Ga alloys

The influence of annealing temperature and time on grain growth in polycrystalline Ni-Mn-Ga samples near the stoichiometric composition Ni2MnGa was investigated. Grain growth was only observed for compositions with a Ni content below 50 at.%. The existence of constitutional vacancies as a possible origin for the different grain growth behaviour was excluded by positron annihilation spectroscopy (PAS). In order to activate grain boundary motion and hence grain growth in Ni50Mn29Ga21 the samples were annealed and deformed in situ in compression up to various strain levels. A sharp threshold to initiate grain growth is observed between 8% and 10% of compression strain.