Martensite Transformation And Damping Capacity Of Grain Refined Ni-Mn-Ga-Gd High Temperature Shape Memory Thin Films

Ni-Mn-Ga alloy is a kind of promising high temperature shape memory alloy.But its polycrystalline brittleness restricts the development and application of Ni-Mn-Ga thin film.In this paper,grain-refined Ni-Mn-Ga-Gd high-temperature shape memory thin films with improved plasticity were obtained by using Gd doping and crystallization annealing of amorphous films.Scanning electron microscope,X-ray diffractometer,differential scanning calorimeter,transmission electron microscope,dynamic thermomechanical analyzer were employed to investigate the microstructure,phase transformation behavior,shape memory effect(SME)and damping behavior of thin films symmetrically.We focus on the effect of grain size on martensite substructure,SME and damping characteristics,and its physical mechanism is clarified.It is found that as-deposited film consists of amorphous matrix and about 5%fraction of the austenite phase nanocrystals.By adjusting the crystallization annealing process,thin films with mean grain size(GS)in the range of 126?1700 nm are obtained.When annealed at 450? for 1 min,the film is not completely crystallized,and the GS is 126 nm,mainly 7M martensite.When annealed at 550 to 750? and the annealing time is not over 5 h,the film is completely crystallized with GS between 179?832 nm,and the phase is mainly 7M.When annealed at 750? for 10 h,the GS of the film reaches 1700 nm,and the phase is dominated by NM martensite.Ni-Mn-Ga-Gd thin films with different GSs all undergo a single-step reversible martensitic transformation,and Ms can reach up to 370?.As the GS decreases,the phase transformation temperature gradually decreases,the stability of phase transformation increases.7M martensite variants have a(202)type I twinning relationship.The internal substructure is a periodically stacked<2 5>layer micro-twinned structure and some stacking fault with good coherence of variant boundary.NM martensitic variants have a(022)type ? twin relationship.The internal substructure of the variant is a stack fault and(202)type ? twins.A large number of dislocations can be observed at the interface between variants indicating a poor coherence.In the fully crystallized film,as the GS decreases,the yield strength and plasticity increase.The yield strength with a GS of 179 nm can reach 650 MPa,and the tensile fracture strain can reach 3.7%.Meanwhile the maximum fully recoverable strain(SME)increases first and then decreases.In the film with a GS of 257 nm,SME reaches the maximum value of 0.63%.After 20 thermal cycles under 500MPa,SME increased slightly from 0.63%to 0.65%while the change of transformation temperature is limited in 3?,which shows the best stability of the high temperature SME.With the increase of GS for the Ni-Mn-Ga-Gd thin films,the damping value at martensite state first increases then decreases.The peak value of tan?=0.046 is achieved in the film with a GS of 832 nm,and it can be maintained from-50? to 290?.In the film with a GS of 179 nm,a relaxation-type internal friction peak is appeared near 60?,and the peak value of tan? is as high as 0.08.With the increase of the vibration frequency,the peak damping value and the temperature corresponding to the peak increase,while the damping performance of martensite state for the films with GS over 179 nm does not change with frequency.With the increase of the strain amplitude,the relaxation type internal friction peak damping value and martensite state damping value both increase.When the amplitude strain reaches 1×10-3,both damping value tend to be stable.The effect mechanism of the grain size on the martensite substructure,SME and damping characteristics is as follows.In the film with small GS,in order to minimize the elastic strain energy,micro twins with periodic<2 5>layers are generated leading to the formation of 7M martensite.Amount of twin interface energy accumulates in the 7M martensite,which is in an unstable state thermodynamically.With the increase of the GS,the restriction to the motion of twinning dislocation is weakened,leading to coarsening of micro-twins under the driving of excessive twin interface energy,forming NM martensite with micro twin substructures of different thickness.The critical stress of reorientation and the yield stress decrease with the increase of GS.The difference between the two stresses reaches highest in the film with GS of 257 nm.When the applied stress is slightly lower than the yield stress during thermal cycles,the maximum volume fraction of favorable oriented martensite variant can be generated,thus achieving maximum SME in the film.With the increase of GS,the pinning effect on twinning dislocation from grain boundary is weakened.It is beneficial to the reverse movement of twin interfaces under alternating load,which improves the damping performance of martensite.When GS is further increased to 1700 nm,owing to the greatly decrease in amount of micro twin interfaces in NM martensite,the damping performance is greatly reduced.