Modification of the Properties of Ni-Mn-Ga Magnetic Shape Memory Alloys by Minor Addition of Terbium

摘要

Effect of terbium addition on the structure, phase constitution and hardness of the Ni_(49)Mn_(29)Ga_(22) alloy was studied. The Tb content varied in the range of 0 - 2 at.%. It was found that the Tb addition substantially refines the grain size, which dropped from 200 - 400 μm, for the Tb-free alloy, down to 30-50 μm for the 2 at.% Tb material. The terbium exhibited negligible solubility in the matrix phase and formed grain boundary layer. The mean composition of the boundary layer was: Tb - 16, Ni - 55, Mn - 7 and Ga - 22 at.%. The phase analysis revealed the presence of the following major phases in the alloys: Ni_2MnGa, Ni_3Ga. All the alloys studied exhibited martensitic structure at room temperature. The Tb addition did not affect the Curie temperature, which is consistent with the finding that Tb does not dissolve in the Ni_2MnGa phase. However, it was found that Tb addition changed the phase transformations temperatures. The A_s temperature (martensite-to-austenite transformation starting temperature) and M_s temperature (martensite-to-austenite starting temperature) grow slightly for low Tb concentrations and subsequently decrease for higher the Tb contents. The Tb containing alloys exhibited increased hardness, by about 40%, which was apparently caused by the grain refinement. No significant effect of the Tb addition on the magnetic shape memory effect was observed.