PDF《Design of High Temperature》

1 Design of HighTemperature Ti-Pd-Cr Shape MemoryAlloys with SmallThermal Hysteresis Deqing?Xue1,2 , Ruihao?Yuan1 ,Yumei?Zhou1 , Dezhen?Xue1,3 ,Turab?Lookman3 ,Guojun?Zhang1,2 , Xiangdong?Ding1 &

Jun?Sun1 The large thermal hysteresis (ΔT) during the temperature induced martensitic transformation is a major obstacle to the functional stability of shape memory alloys (SMAs), especially for high temperature applications.

We propose a design strategy for finding SMAs with small thermal hysteresis. That is, a small ΔT can be achieved in the compositional crossover region between two different martensitic transformations with opposite positive and negative changes in electrical resistance at the transformation temperature.We demonstrate this for a high temperature ternaryTi-Pd-Cr SMA by achieving both a small ΔT and high transformation temperature.We propose two possible underlying physics governing the reduction in ΔT.One is that the interfacial strain is accommodated at the austenite/martensite interface via coexistence of B19 and 9R martensites.The other is that one of transformation eigenvalues equal to 1, i.e., λ2?=?1, indicating a perfect coherent interface between austenite and martensite.Our results are not limited toTi-Pd-Cr SMAs but potentially provide a strategy for searching for SMAs with small thermal hysteresis. Shape memory alloys (SMAs) undergo a reversible martensitic phase transformation from the high symme- try austenite (A) to low symmetry martensite (M) phase upon the influence of temperature or stress field, giv- ing rise to the shape memory effect (SME) and superelasticity (SE), respectively1,2 . Both thermally induced and mechanically induced martensitic transformations involve hysteresis, i.e., the forward and reverse martensitic phase transformations do not coincide3,4 . The hysteresis is the macroscopic manifestation of the dissipated energy during a phase transformation and it is generally considered to originate largely from the strain incompatibility at the A/M interface, which gives rise to an energy barrier for the phase transformation3,5,6 . During the cyclic ther- mal or mechanical martensitic phase transformation, strain incompatibility introduces several irreversible pro- cesses, such as the generation of dislocations and microcracks, resulting in serious fatigue7C9 . The fatigue degrades physical, mechanical properties of SMAs, especially the SME and SE, and finally leads to failure7 . Therefore, the reversibility , the ability to pass back and forth through the phase transformation many times without degrada- tion of properties, is critical and extensive research has focused on reducing hysteresis in order to improve the reversibility of SMAs3,4,10C13 . In searching for thermoelastic SMAs with small thermal hysteresis (Δ? T), several different methodologies have been utilized. Experimentally, combinatorial synthesis of SMA thin films has been employed to screen the various compositions and select the best candidates10,11,13,14 . Very recently, an adaptive design strategy based on machine learning algorithms has been shown to effectively explore the compositional space to identify alloys with very small hysteresis15 . Theoretically, the geometrically non-linear theory of martensite (GNLTM) has been very useful in guiding the search for better alloys5,10 . The martensitic transformation can be described by the symmetric transformation matrix U, which maps the martensite lattice to the austenite lattice16,17 . The ordered eigenvalues of U, λ1?≤??λ2?≤??λ3, represent the presence of an invariant habit plane between austenite and martens- ite16,17 . The GNLTM provides the constraint, λ2?=??1, as means to reduce Δ? T, so that there is a perfect coherent interface (unstressed and untwinned) between austenite and martensite5 . Coupled with a combinatorial synthesis method, the GNLTM has led to the discovery of Ti-Ni-Cu and Ti-Ni-Cu-Pd systems with very small Δ?T10,14 . The