Transition Properties Of Shape Memory Alloys At Extreme Conditions:High Doping Level And Small System Size

Shape memory alloys are widely utilized in the field of sensor,actuator,and medical devices.These important applications of shape memory alloys are due to their shape memory effect and superelasticity.Previous studies have revealed that the fascinating properties of shape memory alloys are originated from martensitic transformation.In practice,shape memory alloys are usually modified to match their working conditions.Two kinds of modification are important nowadays: the first one is doping point defects,which is used for adjusting the martensitic transformation properties and superelasticity;the second one is reducing system size,which is used for implanting shape memory alloys into Micro-Electro-Mechanical Systems?MEMS?.Although these two kinds of modifications broaden the potential applications of shape memory alloys,it is found that the martensitic transformation becomes quite different.On one hand,for shape memory alloys with doping,previous results show that when the content of doping defect is sufficient,the martensitic transformation vanishes.Instead,strain glass transition appears.Even though it is proved that the strain glass transition is a new kind of transition and dominated by kinetic factors,the effect of doping point defects on the crossover from martensitic transformation to strain glass transition is still unclear.On the other hand,for shape memory alloys with small size,it is found that when the size of shape memory alloys is smaller than a critical value,the martensitic transformation vanishes.Such size effect greatly undermines the possible applications of shape memory alloys in nanoscale.However,the origin of system size on martensitic transformation is still unclear.Considering the great impact of doping point defects and reducing system size on martensitic transformation,the present work aims to elucidate the role of doping point defects and reducing system size on the transformation properties of shape memory alloys.Firstly,the transformation properties of Ti_50-xNi_50+x alloys with different excess-Ni?acts as point defect?content are studied experimentally.The crossover from martensitic transformation to strain glass transition with doping point defect is revealed.It is found that when the defect content is low,martensitic transformation still appears.But the transformation temperature decreases dramatically.Moreover,quasi-dynamic disordered nanodomains can be found above the transformation temperature.When the Ni content is sufficient?x>1.3 at.%?,strain glass transition occurs instead of martensitic transformation.The quasi-dynamic disordered nanodomains frozen into frozen-disordered state below the freezing temperature.In addition,a new phase diagram of Ti-Ni alloys is established.Such phase diagram enables us to understand the role of point defect:?i?reducing the thermodynamic driving force for the formation of martensite,and?ii?creating random local stress which prefers the premartensitic nanostructure and strain glass.Based on the two-fold role of point defect,we establish a physical picture which shows microscopically the crossover from martensitic transformation to strain glass transition.Our work enables a simple explanation for several long-standing puzzles,such as the anomalous negative temperature coefficient of electrical resistivity in Ni-rich Ti-Ni alloys and the vanishing of transition latent heat with increasing Ni content.Secondly,the composition dependence of strain glass transition of Ti_50-xNi_50+x alloys?x>1.3 at.%?is systematically studied.It is found that the strain glass transition is closely related to the defect content in strain glass regime.With increasing defect concentration,the strain glass becomes dilute,which is similar to the dilute behavior of spin glass.The dilute behavior of strain glass results from the reducing of volume fraction and size of nanodomains.These results indicate that the point defect also plays a two-fold role in strain glass regime:?i?further reducing the thermodynamic driving force for the formation of martensite,and?ii?creating random local stress.Moreover,such a systematical study enables us to propose a physical picture for the dilute behavior of strain glass transition.Thirdly,the martensitic transformation of free standing nanoparticles is systematically studied by molecular dynamics simulations and analytical models.It is found that with reducing particle size,the transformation temperature is reduced greatly.Below a critical size?1.5 nm for present study?,the martensitic transformation disappears.At the same time,the transformation latent heat gradually vanishes.In order to understand the size effect of martensitic transformation,we study the microstructure of martensite particles.It is found that the martensite particles are core-shell structure composed a martensite core covered by a near-parent-phase shell.The core-shell structure of martensite particles enables us a clear microscopic explanation on size effect.With reducing particle size,the near-parent-phase shell gradually dominates the transformation properties of nanoparticles,leading to the reducing of transformation temperature.Moreover,a Landau-type model,which can predict the relation between transformation temperature and particle size,is proposed.It is found that the Landau model is well consistent with simulation results.Fourthly,the effect of surface conditions on martensitic transformation is studied.The transformation of free-standing and non-transforming layer coated nanoparticles are compared.It is found that with coating non-transforming layer,the critical size of vanishing martensitic transformation can be increased by about 10 times.Moreover,the transformation kinetics is also affected by surface conditions.For free surface nanoparticles,a heterogeneous nucleation mechanism dominates the martensitic transformation.For coated nanoparticles,a homogeneous nucleation mechanism becomes dominant.Thus,our results provide a practical guidance on designing of shape memory alloys in nanoscale.Finally,We perform molecular dynamics simulations to investigate the response of shape memory alloys to applied stress at the nanoscale.Simulation results show that shape memory alloy nanoparticles below the critical size not only demonstrate superelasticity but also exhibit features as absence of hysteresis,inelastic distortion of continuous nature,and high blocking force.Atomic level investigations show that this nonhysteretic superelastic behavior results from a continuous transformation from parent phase to martensite phase under external stress.Our results potentially broaden the application of shape memory alloys to the nanoscale.They also suggest a method to achieve nonhysteretic superelasticity with conventional shape memory alloys.In conclusion,the effects of point defect and system size on martensitic transformation are systematically studied by experiments,molecular dynamics simulations,and analytical models.The present work enables physical pictures of martensitic transformation with doping point defect and reducing system size.Based on this work,the role of point defect and system size is discussed.Moreover,new properties of shape memory alloys at the nanoscale is discovered.Therefore,the present work will play an important role in the application of shape memory alloys in high doping level and nanoscale.