Study On The Micro-structural Adjustment And Damping Capacity In Ti-Ni Shape Memory Alloy Cold-drawn Wires

Ti-Ni shape memory alloys attracted much attention due to its interesting properties,i.e.shape memory effect,superelasticity,good biocompatibility and excellent damping performance.A lot of damping devices that developed based on its high damping performance have been widely used in transportation,construction,machinery,submarines and other fields for vibrations and noise reduction.The internal friction（IF）of the Ti-Ni shape memory alloy reaches about 0.05.To further explore the microstructural mechanism of the damping behavior in SMAs and improving the damping capacity of the materials is always the target of many researchers.In this study,the dynamic analysis,static mechanical properties and phase transformation behaviors associated with the microstructural evolution in the Ti-Ni shape memory alloy wires in the heat treatment process have been systematically investigated by transmission electron microscope（TEM）,scanning electron microscope（SEM）,EBSD,differential scanning calorimetry（DSC）,X-ray diffraction（XRD）,dynamic mechanical analyzer（DMA）.The experiment results showed that phase transformation of Ti_49.3Ni_50.7 alloy can be achieved by one step,two steps or more steps phase transition process.The original samples occurred B2-R one step transformation,while B2-B19’one step transformation was exhibited in the solution treated samples.A B2-R-B19’two steps phase transformation was observed as a low temperature（250°C）aging treatment was carried out.The Ni₄Ti₃ precipitates can form in Ti_49.3Ni_50.7 alloys by aging at450°C.As for a short time aging,it undergoes a B2-R-B19’two step transformation around the grain boundary region,while a B2-B19’transformation happened in the grain interior.When the aging time increased,the Ni₄Ti₃ particles are evenly distributed across without being affected by grain boundary.Thus,the whole sample undergoes B2-R-B19and B2-B19’three stage transformation on cooling process while two stage B19’-R-B2 transformation appeared on the heating process.The stress-strain curve of the TiNi wires generally includes four stages:elastic stage before stress induced phase transformation,stress induced martensitic transformation platform,elastic stage of the martensite phase and the unloading stage.Through the tensile test under different temperature conditions,the stress strain curves have the different of elastic strain,stress values and the different slope of stress-strain curve.The tensile results also show that when the strain of the material reached 10%,the recoverable strain can reach 94%.The damping properties results show that,the―no precipitates R phase‖was obtained through low temperature aging which has an ultrahigh damping value(Q^-1≈0.4)in a wide temperature range（-40°C-35°C）.This work can supply not only the engineering application value but also give a guideline for obtaining high damping materials.The R phase with no precipitation（or other defects）through controlling the microstructure will be a necessary condition for a high damping material.Two criterions should be obeyed for obtaining high damping capacity:First,take full advantage of the R phase with the smallest twinning shear（s=0.0265）,which makes the twin interface easily moving.Second,avoiding introducing obstacles such as precipitates and dislocations is essential for high damping.The excellent damping behavior of Ni-Ti alloy is related to the structure of R-phase without precipitates and an appropriate amount of hydrogen.