Martensitic Transformation And Mechanical Behavior Of Ni-Mn-Ga-Gd High-temperature Memory Alloys

Ni-Mn-Ga high-temperature shape memory alloys (HTSMAs) have attracted considerable attention due to their excellent thermal stability. However, the brittleness of Ni-Mn-Ga alloy greatly limits their applications. In this thesis, proper Gd content will be added into Ni-Mn-Ga alloy to refine grain size and improve the brittleness and shape memory effect (SME)The microstructure, martensitic transformation, mechanical behavior, SME and thermal stability have been investigated systematically by means of SEM, DSC, TEM, XRD and compressive tests.It is shown that the microstructure of Ni-Mn-Ga alloy is affected by Gd content and Ni content. When Gd content is no more than0.1at.%, the microstructure of Ni54Mn25Ga21-xGdx alloys is single phase of T martensite. When Gd content is more than0.1at.%, Gd(Ni,Mn)4Ga hard-brittle phase with hexagonal structure is formed. When Ni content is no more than57at.%, Ni53+yMn25Ga21.9-yGd0.1alloys display single T martensite microstructure. When Ni content is more than57at.%, γ phase with face-centered cubic structure is generated.The martensitic transformation temperatures are affected slightly by Gd addition, while, significantly rised by Ni addition. Transformation temperatures linearly increase with increase of Ni content, and then become stable when Ni content is more than57at.%. The transformation temperatures of Ni54Mn25Ga20.9Gd0.1alloy exhibit excellent thermal stability even after2000thermal cycles.The microstructure of Ni54Mn25Ga20.9Gd0.1alloy is single T martensite, and Tâ†'7M transformation occurs in the process of compression deformation. With the increase of compression deformation, the number of7M martensite increases and the number of T martensite decreases. When compression deformation is more than8%, only7M martensite can be observed in microstructure.7M martensite formed by compression deformation is transformed to parent after heating, and when the parent is cooled again, only T martensite with the same morphology and substructure to undeformed alloy can be observed.It is indicated that Gd doping can refines the grain of Ni-Mn-Ga, causing improvement of plastic. The compressive fracture strain of Ni54Mn25Ga21-xGdx alloys increases with Gd content increasing, and reaches the maximum of24.6%when Gd content is0.1at.%. With further increase of Gd content, compressive fracture strain decreases due to Gd(Ni,Mn)4Ga hard-brittle phase formation.Gd doping improves one-way SME of Ni-Mn-Ga alloy. The reversible strain of Ni54Mn25Ga21-xGdx alloys increases with the increase of Gd content and reaches a maximum value when Gd content is0.1at.%. With further increase of Gd content, reversible strain decreases. Complete recovery is obtained from the Ni54Mn25Ga20.9Gd0.1alloy with10%pre-strain, and reversible strain is7.5%. The two-way SME is observed in Ni54Mn25Ga21-xGdx alloys. Two-way SME firstly increases after decreases with increase of Gd content, and the largest value is obtained when Gd content is0.3at.%. Two-way SME of Ni54Mn25Ga20.7Gd0.3alloy increases from2.9%to3.5%after10times of shape memory cycle training.