Study on the Micro-Mechanism of the Relaxation Behavior in Ti-Ni-Based Shape Memory Alloys

Author:Zuo Shun Gui

Supervisor:jin xue jun


Degree Year:2016





Besides the shape memory and superelastic effects,Ti-Ni-based shape memory alloys exibit high damping capacity which can be used to reduce vibrations and noise in building and transportation vehicles,etc.The internal friction(IF)of the Ti-Ni shape memory alloy is about 0.02 which results from the abundance of boundaries in martensite phase.To improve the damping capacity of the material further is always the seeking target of many researchers.Compared with the transformation IF peak,the relaxation IF peak is stable at constant temperature,which is more important for practical engineering applications.It is hopeful to improve the damping capacity of Ti-Ni-based shape memory alloy further by introducing the relaxation IF.Besides,investigating the relaxation IF is helpful in understanding the effect of defects on phase transition,mechanical and structure evolution behavior.There are several relaxation IF peaks in Ti-Ni-based shape memory alloys,such as IF related to the interaction between twin boundary and hydrogen(PTWH),IF attributed to“strain glass freezing”in non-transforming alloys.PTWH is a broad peak though its broadening mechanism is still unclear,besides,there is still a lack of systematic research on how to control the intensity and the temperature of PTWH.For the“strain glass freezing”peak,its mechanism remains controversial.Some researchers think that it should be interpreted as relaxational premartensitic anelasticity rather than“strain glass freezing”.Apart from these,the relaxation behavior of Ti-Ni-based shape memory alloys at high temperature is rare reported.Thus,the aim of this work is to study the micro-mechanism of the above-mentioned relaxation behavior in Ti-Ni-based shape memory alloy by internal friction and microstructure characterization.The main contents and results are as follows:1.The effect of hydrogen on PTWH in Ti50Ni30Cu20 alloys was investigated,the broadening mechanism of the PTWH was discussed based on microstructure observation,then the effect of hydrogen on the phase transition and the tensile behaviors was studied.Results show that the Ti50Ni30Cu20 alloys manifested stable and high damping capacity near ambient temperature when the CH was near 30 ppm,the maximum IF reached 0.16 for PTWH.The intensity and the temperature range of PTWH decreased for specimens with excessive levels of hydrogen.We also found that the hydrogen content of the alloys decreased with the increasing annealing temperature in open air,which was accompanied by the decrease of the intensity of PTWH.No PTWH could be observed when the alloy was annealed in open air above 993 K.The TEM results show that there were several kinds of twin such as{011}compound twins and{111}typeⅠtwins in the alloys.The interaction between these multiple types of twins and hydrogen are main reasons for the broadening of PTWH.B19-phase transition and related tensile properties in Ti50Ni30Cu20 shape memory alloy doped with hydrogen were also investigated.Results show that the presence of hydrogen suppressed B2-B19 transformation by decreasing the transformation enthalpies of B2-B19phase transition in Ti50Ni30Cu20 alloy,which was similar to the effect of hydrogen on B2-B19’transition in Ti-Ni superelastic alloy.Besides,the detwinning stress increased and the elongation of the Ti50Ni30Cu20 alloys decreased with the elevation of hydrogen levels.The morphological properties of the fracture surfaces suggested that the fracture mode changed from a semi-brittle fracture to a brittle intergranular fracture with increased hydrogen content.Further,it was found that aging treatment at room temperature shifted the martensitic transformation temperature(MTT)to a lower temperature.These alterations in MTT increased with the increment of hydrogen content.After aging treatment,the Ti50Ni30Cu20alloy exhibited strong thermal cycling stability.Based on our analysis,these effects were caused by the combined activities of different hydrogen states,including those of hydrides and solid-solution hydrogen.2.In the second section,the effect of hydrogen on relaxation behavior of Ti50Ni44Fe6alloy at low temperature was investigated.Ti50Ni44Fe6 alloy was thought to be a typical alloy undergoes“strain glass transition”.Results show that there was no“strain glass freezing”peak in de-hydrogenated Ti50Ni44Fe6 alloy,but this peak appeared in alloys containing some hydrogen.This relaxation peak can be fitted both by Vogel-Fulcher and Arrhenius relation.The activation parameter of the relaxation peak fitted by Arrhenius relation is determined:Ea=0.58±0.02 eV,τ0=1.27×10-1818 s.The hydrogen also decreased the IF value below the anelastic IF peak which implied a pinning effect of hydrogen to the nanodomains at this temperature.It is proposed that the relaxation behavior in Ti50Ni44Fe6 alloy result from the interaction between hydrogen and nanodomains rather than from“strain glass freezing”.3.In this section,the high temperature(room temperature-900 K)relaxation behavior in Ni50.3Ti29.7Zr20 shape memory alloy was investigated at first,then the effects of aging treatment on the internal friction and phase transition behavior were studied.Results show that an internal friction peak(P1)located at about 787 K for 0.5 Hz,it’s noteworthy that peak P1 happens to locate in the temperature range of the H phase precipitation.For the solution treated alloy,peak P1 was superimposed on a transitory IF,this transitory IF disappeared after thermal cycling or annealing.The activation parameter of peak P1 was also determined:Hact=1.48±0.02 eV,τ0=7.6×10-10 s,r2=1.34(β=1.2).The broadening factorβof P1suggested that peak P1 is caused by point defects rather than dislocations.In binary Ti-Ni SMA,an IF peak similar to P1 was observed but at a lower temperature with decreased intensity.The above characteristics implied that P1 is caused by a short distance diffusion mechanism of Ni inside NiTi-based alloy(e.g.the Zener mechanism).The in-situ IF measurement during isothermal treatment show that the dynamic modulus increased with the prolonging of the isothermal time,which implied an precipitation hardening effect,and the higher the isothermal temperature,the less time the IF reach a steady value.After aging treatment at 500°C,the H phase distribute homogeneously in the alloy.These precipitates have a spindle shape and strengthen the alloys significantly.The Ms of the alloy increased to 132°C after aging for 24 h,which make the alloy have the potential to be used as a high temperature shape memory alloy.Besides,the aging treatment lead to a decrease of the hysteresisΔT and an increase of the thermal stability.