Structure And Shape Memory Effect Of Ni-mn-ga-gd High Temperature Shape Memory Alloys

The structure, shape memoy effect of Ni54Mn25Ga21-xGdx(x=0,0.1,0.2,0.5,1,2) high temperature shape memory alloys have been investigated by means of SEM,TEM,XRD,DSC,compressive tests.The effects of Gd addition on element, martensitic transformation temperature, the mechanism and shape memoy effect for that have been revealed.The results of experiment show that the crystal of Ni-Mn-Ga-Gd alloys can be apparently refined by increasing Gd content, and with the increase of Gd content, the grain size decreases. Ni-Mn-Ga-Gd alloys consists of the Ni-Mn-Ga matrix and the second phase. With the increase of Gd content, the second-phase grows and its volume fraction increases gradually, and mainly along the grain boundaries. The exprimental results show that, Ni-Mn-Ga-Gd alloys undergo one-step thermoelastic martensitic transformation during the process of heating and cooling. With the increase of Gd content, the transformation temperature first increased and then decreased, then increased. The main reason for this result is the content of Ni and Mn in the matrix increased with Gd content changed little, while the Ga content was changed greatly by the impact of Ga content to make the transformation temperature of Ni-Mn-Ga-Gd alloys have fluctuations. The type of martensites in Ni-Mn-Ga-Gd alloys have not a greater impact with increasing Gd content, The matrix of alloy is always non-modulated tetragonal martensite.Compression test results show that the an appropriate Gd addition can significantly improve the mechanical properties of Ni-Mn-Ga alloys. Both the compression strength and strain have a strong dependence upon the Gd content. When the Gd content is 1at.%. With the increase of Gd content , the hardness of matrix increased gradually, and the hardness of second-phase is higher than the matrix. The compressive strength and strain reach the maximum value. The fracture type of Ni-Mn-Ga alloy changes from intergranular fracture of Ni-Mn-Ga alloy to transgranular cleavage fracture with increasing Gd content.Gd addition can improve the shape memory effect of Ni-Mn-Ga alloys.The maximum of shape memory effect is attained in Ni₅₄Mn₂₅Ga₂₀Gd₁ alloys. 100% shape memory recovery ratio is achieved in Ni₅₄Mn₂₅Ga₂₀Gd₁ alloy when compress with 3% strain, and the shape memory recovery strain is 1.8%. The maximum value of recoverable shape memory strain is achieved in Ni54Mn25Ga20Gd1 alloy with pre-strain of 4%, and the shape memory recovery strain and recovery ratio are 1.9% and 87.5%, respectively. The two-way shape memory effect is observed in Ni₅₄Mn₂₅Ga₂₁-xGdx alloys.