Phase Transformation And Thermal Cycling Stability Of NiMnCuGa High-temperature Shape Memory Alloys

NiMnCuGa,whose martensitic transformation temperature can be adjusted to be above 500?,is a new type high temperature shape memory alloy?HTSMA?developed from NiMnGa alloy.As known that,thermal cycling stability is an important parameter for the practical application in HTSMA.In this thesis,the effects of Cu content and heat treatment on the thermal cycling stability of Ni₅₆Mn_25-x Cu_x Ga₁₉?x=0,2,4,6?HTSMAs were extensitvely studied.By using optical microscopy,XRD,SEM and TEM,the microstructure before and after thermal cycling was studied.The relationship between the microstructure and the thermal cycling stability was discussed.The results show that Cu content has a significant impact on the thermal cycling stability of Ni₅₆Mn_25-x Cu_xGa₁₉?x=0,2,4,6?alloys.Ni₅₆Mn₂₅Ga₁₉ and Ni₅₆Mn₂₃Cu₂Ga₁₉exhibit the best thermal cycling stability.Ni₅₆Mn₂₁Cu₄Ga₁₉ alloy shows the medium thermal cycling stability.The worst one is Ni₅₆Mn₁₉Cu₆Ga₁₉ alloy.With the increase of thermal cycles,transformation temperatures of Ni₅₆Mn₂₁Cu₄Ga₁₉ and Ni₅₆Mn₁₉Cu₆Ga₁₉decrease gradually.Microstrucutral observations show that,if the content of Cu exceeds2at.%,?phases will occur in the matrix.The amount and size of?phases would increase with increasing Cu content.It is suggested that the formation of?phases results in the decrease of thermal cycling stability in the as-annealed Ni56Mn21Cu4Ga19 and Ni56Mn19Cu6Ga19.In Ni₅₆Mn₂₁Cu₄Ga₁₉ alloy,when aging at 450?,transformation temperatures increase firstly and then remain unchanged.When aging at 600?,transformation temperatures decrease firstly and then keep stable.For Ni₅₆Mn₁₉Cu₆Ga₁₉ alloy,the influence of aging on transformation temperatures is more compelx.For Ni₅₆Mn₂₁Cu₄Ga₁₉,aging at 450?reduces the thermal cycling stability.In contrast,aging at 600?improves the thermal cycling stability significantly.As to Ni₅₆Mn₁₉Cu₆Ga₁₉,aging at 450?and600?both lead to the improvement of thermal cycling stability.For both of these alloys,increasing aging time will increase the thermal cycling stability.As confirmed that,the amount of?and??phases has a negative effect on the thermal cycling stability.Transformation temperatures of Ni₅₆Mn₂₁Cu₄Ga₁₉ ribbon are increased by annealing at 700?,800?and 900?.Annealing at 800?and 900?leads to larger changes in transformation temperatures than annealing at 700?.The ribbon annealed at 700?exhibits better thermal cycling stability than that annealed at 800?and 900?.The transformation peaks of the former keep stable even after seven thermal cycles.For the ribbons annealed at 800?and 900?,transformation peaks disappear after two cycles.No second phases are found in the ribbon annealed at 700?.However,the formation of second phases is found in the ribbons annealed at 800?and 900?.The amount and size of second phases would increase with increasing annealing tremperature and duration,which considerably weakens the thermal cycling stability.