Fabrication Of Rapidly Solidified And Multi-step Constrained Aged NiTi Shape Memory Alloy And Its Phase Transformation And Functional Fatigue Behavior

Recently,there has been an increasing trend in research and application of NiTi alloys with two-way shape memory effect?TWSME?.As an extrinsic functional property,the TWSME of NiTi alloys can only be enabled by certain"training"treatments,in which constrained aging treatment is an important"training"method for securing TWSME.For obtaining excellent TWSME and phase transformation temperatures higher than room temperature,a relatively long aging time is needed during the"training"of NiTi alloys by conventional constrained aging treatment.This means a low"training"efficiency of the constrained aging treatment.Therefore,to promote further application of NiTi alloys with TWSME,it is very necessary to improve the"training"efficiency by innovating the constrained aging treatment.Noteworthily,NiTi alloys are usually subjected to long-term thermal-mechanical loading/unloading cycles in practical applications,which leads to repeated occurrence of martensitis transformation and reverse transformation in NiTi alloys.Under such circumtances,significant shifts in functional properties of NiTi alloys,such as martensitic phase transition temperature,recoverable strain and shape memory output stress,will be induced by the martensitic transformation cycling.Such shifts of functional properties usually occur before emergence of obvious structural damage and can result in the functional failure of NiTi alloys,that is,functional fatigue.At present,there is still a lack of understanding of the functional fatigue and underlying mechanism of constrained aged NiTi alloys with TWSME.Therefore,it is necessary to carry out a systematic study on this issue,so as to provide guides for the design of NiTi two-way memory alloys.In this study,aiming at overcoming the shortcomings of conventional constrained aging treatment,such as lower transformation temperature than room temperature during high-temperature constrained aging,long aging time during medium-temperature constrained aging for an optimized TWSME and abnormal TWSME induced by low-temperature constrained aging,a novel two-step constrained aging treatment?500°C/1 h+300°C/39 h?is proposed based on the principle of thermodynamics and kinetics of precipitation in solid solution.By using two-step constrained aging treatment,the Ni₅₁Ti₄₉ alloy can be"trained"to have excellent TWSME with high efficiency.After two-step constrained aging treatment,Ni₅₁Ti₄₉ alloy possesses a shape recovery ratio up to 96.4%in full martensitic state.Compared with conventional one-step constrained aging treatments,the two-step constrained aging treatment can effectively increase R-phase transformation start and finish temperatures of the Ni₅₁Ti₄₉alloy,which are increased to 60.2°C and 40.9°C,respectively.Thus,as-trained Ni₅₁Ti₄₉ two-way shape memory alloy is suitable for the application with working temperature above room temperature.Lenticular coherent Ni₄Ti₃ precipitates with an average length of 149.1±14.3 nm were introduced into the NiTi matrix,which enables the Ni₅₁Ti₄₉ alloy to have excellent TWSME.Relationships among the constrained aging treatment,microstructure,martensitic transformation behavior and TWSME were revealed.On the basis of above two-step constrained aging treatment,a three-step?multi-step?constrained aging treatment?300°C/1 h+500°C/1 h+300°C/18 h?is proposed.By using this novel multi-step constrained aging,the total constrained aging time is shortened to 20 h,yet the alloy has excellent TWSME in terms of shape recovery rate r?r?,r?i?and r?m?being 42.6%,59.8%and 90.1%respectively.The microstructural characterization results show that the average length and number density of preferential-oriented coherent Ni₄Ti₃ precipitates are150.3±38.4 nm and 123.8±9.6?m^-2,respectively.The evolution of microstructure is studied and discussed in detail.The above results are theoretically meaningful for the development of high-efficiency training processes of NiTi two-way shape memory alloys.Further,a functional fatigue testing system was set up for exploring the functional fatigue of constrained aged Ni₅₁Ti₄₉ two-way shape memory alloy.The Ni₅₁Ti₄₉ alloy was subjected to thermal cycling fatigue,constant tension-thermal cycling fatigue and limited strain-thermal cycling fatigue,respectively.Results show that constrained aged Ni₅₁Ti₄₉ two-way shape memory alloy has an excellent functional fatigue resistance,which is mainly attributed to excellent microstructural stability.Moreover,the functional fatigue property of the Ni₅₁Ti₄₉ alloy underwent different aging treatments was systematically studied by thermal cycling test.It was found that the Ni₅₁Ti₄₉alloy subjected to high-temperature aging treatment?500°C or 550°C?shows poor thermal cycle stability.With the increase of thermal cycling number,temperature of R?B19?transformation decreases gradually;the two overlapped exothermic peaks of the Ni₅₁Ti₄₉ alloy subjected to aging treatment of 550°C/50 h are actually contribubed by three stages of martensitic transformations?a multi-step martensitic transformation,MMT?.The MMT consists of a single-step B2?B19?martensitic transformation and a two-step B2?R?B19?martensitic transformation.Based on above results,a mechanism related to medium-scale heterogeneity of coherent stress field is proposed to understand the observed MMT behavior.Additionally,the mechanism of the influence of aging treatment on microstructure,martensitic transformation behavior and functional fatigue performance was revealed.Finally,considering the poor functional fatigue performance of solution-treated NiTi alloys,the influences of alloy composition on thermal cycling stability of martensitic transformation in nearly-equiatomic ratio NiTi alloys were systematically studied.For solution-treated Ti-rich NiTi alloys,the alloy composition change has little effect on the thermal cycling stability;while for solution-treated Ni-rich NiTi alloy,the thermal cycling stability is significantly improved with the increase of Ni content.For Ni-rich NiTi alloys with low Ni content?50.0⁵0.6 at.%?,thermal cycling leads to one-step martensitic transformation of B2?B19?changing to two-step transformation of B2?R?B19?.The quenched-in defects?QIDs?in Ni_50.9Ti_49.1 alloy are stabilized by lowering the maximum temperature of thermal cycling(T_max).Stablized QIDs play an important role in solid solution strengthening and improving lattice compatibility between B2 phase and B19'martensite.Thus,the thermal cycling stability of Ni_50.9Ti_49.1 alloy is improved by stabilized QIDs.This method provides an inspiring insight to improving functional fatigue performance of the NiTi alloy.The results obtained in this study offer important theoretical guidance for the accurate regulation of the microstructure and properties of NiTi alloys and for evaluation of their functional reliability.