Martensitic Transformation Of Rapidly Solidified TiNi Based Shape Memory Ribbons

Ti-Ni based shape memory alloys are being widely used in mechanical, clinicalmedicine and aerospace based on excellent shape memory effect （superelasticity）,biological compatibility, corrosion resistance and high damping capacity. In thepresent work, melt spinning technique was employed to prepare rapidly solidifiedribbon materials. The morphology of precipitated Ti₂Ni and the influence onmartensitic transformation were studied. We focused on the structural transitionkinetics and the following conclusions were obtained.（1） A dependence of phase transition kinetics on the cooling rate is found. Theaverage activation energy for martensitic transformation under lower cooling rates（0.5-5K/min） is about487kJ/mol, and within the cooling rate ranging7.5to15K/min,it decreases to162kJ/mol. When it transforms at higher cooling rates （20-30K/min）,the average activation energy is about55kJ/mol.（2） A time-depending effect is observed through DSC interrupted measurementsin TiNiFe alloy and it is proposed that this phenomenon originates from thermalfluctuations caused by the change of thermal history, and relates to the nucleation andgrowth of martensite. Our results show that the martensitic transformation of TiNiFealloy is athermal in nature.（3） Two types of Ti₂Ni particles were observed using high resolutiontransmission electron microscopy in Ti_51.5Ni_48.5, and a model was proposed based onthe microstructural observations. For the large Ti₂Ni particles （³00-500nm）, it issuggested that they are the production of L32（Ti）+TiNi Ti₂Ni peritectic reaction,where the subscript stands for the Ni content. However in case of small particles,novel structures composed of Ti-rich core and amorphous shell layer are observed,which are resulted from the rapidly solidified L（Ti）（Ti）+L24（Ti） andL24（Ti）（Ti）+Ti₂Ni divorced eutectic process. （4）4%tensile deformation was applied to Ti_51.5Ni_48.5melt-spun ribbons at roomtemperature （lower than the martensite finish temperature） and at420K （higher thanthe austenite finish temperature）, respectively, and the effect of deformation on themartensitic transformation was investigated. Differential scanning calorimetrymeasurements show that the phase transition temperatures change slightly withdeformation at420K. While in the case of deformation at room temperature, the backtransition temperatures strikingly increase18K, which is mainly attributed to theformation of high-density dislocations based on the high-resolution transmissionelectron microscopy observations. It is observed that the Ti₂Ni particles severelyimpede the dislocations during the deformation process.