Martensitic Transformation And Strain Recovery Characteristics Of Aged Ti-Ni-Cu-Pd Narrow Hysteresis Shape Memory Alloys

Ti-Ni shape memory alloys possess a large recovery strain and stress.However,the transformation hysteresises are large in the vast majority of Ti-Ni alloys,which lead to slow response speed and low sensitivity as actuator materials.This does not meet the higher pursuits of sensitivity and response speed.In this paper,a new way is proposed to design narrow hysteresis shape memory alloys.That is to reduce the hysteresis by adjusting Pd content of off-stoichiometric Ti-Ni-Cu-Pd alloys,and improve shape memory effect by aging treatment.This article employs SEM,TEM,XRD,DSC,hardness tests and tensile tests to research the microstructures,martensitic transformations,aging precipitation behaviors and strain recovery characteristics of Ti-Ni-Cu-Pd alloys systematically.Research shows that with the increase of Pd content,the martensites in both Ti_50.5Ni_38-xCu_11.5Pd_x and Ti_49.5Ni_39-xCu_11.5Pd_x alloys are orthorhombic B19 structure.Due to a moderate amount of Pd addition,λ2 is close to 1,which leads to narrow hysteresis and high thermal cycle stability.The hysteresis of Ti_50.5Ni_33.5Cu_11.5Pd_4.5 alloy is only 4.8oC,and the change of transformation temperatures is less than 1oC after 5000 thermal cycles.TEM observations show that dislocations are extremely difficult to introduce in the process of thermal cycles,which is the main reason for the high transformation temperature stability in Ti-Ni-Cu-Pd alloys.DSC results show that there is obvious temperature memory effect in Ti_49.5Ni_34.5Cu_11.5Pd_4.5 alloy after incomplete thermal cycles,which means a separation of inverse phase transition peak in DSC curve.With the increase of incomplete thermal cycles,the separation of inverse phase transition peak becomes apparent.In the following complete thermal cycles,the temperature memory effect presents a repeatability.Through 1000 complete thermal cycles,it does not disappeart.This is totally different from the traditional shape memory alloys.The mechanism is the heterogeneous distribution of dislocations in martensites caused by the rare introduced dislocations during the incomplete thermal cycles,which is difficult to be disorganized by the few introduced dislocations in the complete thermal cyclings.Research shows that Ti2 Cu type second phases precipitate in Ti_50.5Ni_33.5Cu_11.5Pd_4.5 alloy.In the beginning stage of aging,fine granular second phases precipitate.With further increasing the aging time or temperature,the Ti2 Cu type second phases gradually grow to sheet-like morphology.After aging treatment,there is two-step martensitic transformation in this alloy.（Ni,Cu）2Ti type second phases precipitate in Ti_49.5Ni_33.5Cu_11.5Pd_5.5 alloy.Under low aging temperature,the precipitates are dispersed squares.Under high aging temperature,the size of the precipitates increases,presenting the shape of sheets and sphericities.When the aging temperatures are 600 oC and 650 oC,there are two-step and three-step martensitic transformations in this alloy,respectively.The multi-step phase transition attributes to a combination of compositional variation and stress field by the precipitation of Ti2 Cu and（Ni,Cu）2Ti type phases.Results of hardness tests and tensile tests show that the hardnesses and recoverable strains represent peak effect with increasing aging time or temperature.Aging at 500 oC for 1h,a large number of fine dispersed second phases precipitate.The strengthening of the matrix is strongest.And the shape memory effect is improved.When the tensile deformation is 5%,the strain recovery ratio is 97.8% in Ti_50.5Ni_33.5Cu_11.5Pd_4.5 alloy aged for 500oC/1h,which is 21% larger than that in solution-treated alloy.