Study On Transformation Characteristics And Mechanical Properties Of TiNiFeCu(Nb) Shape Memory Alloys

TiNi-based shape memory alloys（SMAs）possess excellent functional properties,such as shape memory effect,superelasticity,and biocompatibility,making them highly attractive for high-performance applications.Previous studies have shown that by controlling the grain size of TiNi alloy to the nanoscale,the thermal and stress-induced martensitic phase transformation behavior or path can be altered,leading to enhanced performance.However,the influence of alloying elements on the phase transformation behavior of nanocrystalline TiNi-based alloys remains unclear.For instance,Fe and Cu are key elements for regulating the phase transformation behavior of TiNi-based alloys,with the former decreasing the critical transformation temperature and often inducing the R phase,while the latter promoting the B19 phase and enhancing the geometric compatibility of the transformation.Therefore,exploring the synergistic effects of nanocrystallization and alloying in TiNi-based alloys may enrich the fundamental data and theories of shape memory alloys,leading to a deeper understanding and control of the thermal and force-induced martensitic phase transformation behavior of alloys and supporting the development of high-performance alloys.This study employed conventional metallurgical methods such as melting,forging,severe cold drawing,and annealing to design and prepare TiNiFeCu alloys and nanocrystalline TiNiFeNb alloy wires with different grain sizes（ranging from 20 nm to～1900 nm）（Nb assisted in regulating the B2-R phase transformation behavior）.Advanced testing methods,such as in-situ synchrotron XRD,DSC,TEM,etc.,were used to systematically study the microstructure and thermal-induced martensitic phase transformation behavior（especially the phase transformation paths containing R or B19 phases）.The mechanical properties and stress-induced martensitic phase transformation behavior were systematically characterized using uniaxial tension,in-situ synchrotron XRD,and other methods.The study elucidated the influence of alloy element content and distribution,grain size,lattice compatibility,etc.,on the thermal and stress-induced phase transformation and mechanical behavior of the alloy.The main research results are as follows:（1）It was found that the thermal-induced phase transformation paths of TiNiFeCu alloys exhibit a significant grain size effect.For Ti₅₀Ni₄₂Fe₂Cu₆ alloy,when the grain size is larger than 200 nm,the transformation path is a two-step transformation of B2→B19→B19’.When the grain size is between 83 nm and 200 nm,the transformation path changes to a three-step transformation of B2→ R→ B19→ B19’.When the grain size is smaller than 24 nm,the transformation path becomes a two-step transformation of B2→R→B19.For Ti₅₀Ni₄₀Fe₂Cu₆ alloy,when the grain size is larger than 83 nm,the transformation path is a two-step transformation of B2→R→B19.When the grain size is smaller than 24 nm,the transformation path becomes a transformation of B2→R.（2）It was found that nanocrystallization significantly reduces the formation temperature of B19 phase in TiNiFeCu alloy,but cannot completely suppress its formation,which is different from the complete suppression of B19’ phase in nanocrystalline TiNi-based alloys.The addition of Fe element has a significant inhibitory effect on the formation of both B19 and B19’ phases.In Ti₅₀Ni₄₀Fe₂Cu₆ alloy,the formation of B19’ phase cannot occur,and the temperature at which B19 phase appears is much lower than that in Ti₅₀Ni₄₂Fe₂Cu₆ alloy subjected to the same heat treatment temperature.Moreover,the production of B19 phase can be completely suppressed by preparing nanocrystalline Ti₅₀Ni₄₀Fe₂Cu₆ alloy.（3）Mechanical tests showed that both nanocrystalline Ti₅₀Ni₄₂Fe₂Cu₆ and Ti₅₀Ni₄₀Fe₂Cu₆ alloys exhibit superelasticity in the temperature range of 213K to 323K and 93K to 223K,respectively.The average stress-hysteresis of nanocrystalline Ti₅₀Ni₄₂Fe₂Cu₆ alloy is only 180MPa,which is smaller than most TiNi-based alloys.Based on the calculation results of synchrotron X-ray diffraction,the "cofactor conditions" λ₂,XⅠ,and XⅡ of B2 phase and B19’ phase in Ti₅₀Ni₄₂Fe₂Cu₆ alloy at 298K are 0.9763,0.9821 and 1.0269,respectively,which are closer to 1 than those of other Cufree TiNi-based alloys undergoing B2 to B19’ phase transformation.This indicates that Cu element improves the lattice compatibility between B2 phase and B 19’ phase,resulting in small stress-hysteresis in the alloy.（4）In nanocrystalline TiNiFeNb alloys,the transformation temperature of B2→R decreases with increasing Nb content,and the intensity of the R phase diffraction peaks also decrease with increasing Nb content.When the Nb content is 1 at.%,the alloy shows no diffraction peak change during cooling,but the resistance curve abnormally increases,suggesting the possible occurrence of nanodomains in the alloy.The formation of nanodomains may be caused by the fluctuation of alloy composition induced by Nb doping,which leads to the fluctuation of phase stability in the alloy.The region where B2 phase is stable inhibits the R phase transformation in the metastable region.（5）The nanocrystalline TiNiFeNb alloys exhibit superelasticity in the temperature range of 98K to 298K.Among them,Ti_49.8Ni₄₇Fe₃Nb_0.2,Ti_49.6Ni₄₇Fe₃Nb_0.4,and Ti_49.3Ni₄₇Fe₃Nb_0.7 alloys exhibit R-orientation plateaus in low-temperature tension,and the length of the R-orientation plateau increases as the temperature decreases.In addition,as the Nb content increases,the length of the R-orientation plateau decreases at the same tensile temperature.When the Nb content is increased to 1 at.%,the R-orientation plateau is not present in the entire tensile temperature range.