Martensitic Transformation in Fe-Mn-Si Based Shape Memory Alloys

The critical driving force for martensitic transformation fcc ( gamma ) yields hcp ( epsilon ) in ternary Fe-Mn-Si alloys increases with the content of Mn and decreases with that of Si. Thermodynamical prediction of M//s in ternary Fe-Mn-Si alloys was established. The fcc ( gamma ) yields hcp ( epsilon ) martensitic transformation in Fe-Mn-Si is a semi-thermoelastic and the nucleation process does not strongly depend on soft mode. Nucleation occurs directly through an overlapping of stacking fault rather than pole mechanism, and it is suggested that stacking fault energy (SFE) is the main factor controlling nucleation. Based on the phenomenological theory of martensite crystallography, a shuffle on (0001)//h//c//p plane is required when d//1//1//1 does not equal d//0//0//0//2. The derived principal strain in Bain distortion is smaller, i, e., more reasonable than the values given by Christian. Alloying elements strengthening the austenite, lowering SFE of gamma phase and reducing T//N** gamma temperature may be beneficial to shape memory effect of Fe-Mn-Si based alloys. Accordingly, Fe-Mn-Si-RE and Fe-Mn-Si-Cr-N (or Fe-Mn-Si-Ni-Cr-N) are worthy to be recommended as shape memory materials with improved shape memory effect. (Edited author abstract) 48 Refs.

Effects of second phases on mechanical properties and martensitic transformations of ECAPed TiNi and Ti-Mo based shape memory alloys

TiNi and Ti-based shape memory alloys were processed by equal channel angular pressing （ECAP） at 673-773 K along Bc route to obtain ultrafine grains for increasing the strength of parent phase and improving the functional properties. The effects of both thermodynamically stable and metastable second phases on the mechanical properties and martensitic transformations of these alloys were investigated. It is found that thermodynamically stable Ti2Ni phase has no effect on martensitic transformation and superelasticity of Ti-rich TiNi alloy, thermodynamically stable α phase is harmful for ductility of Ti-Mo-Nb-V-Al alloy, but metastable Ti3Ni4 phase is effective for R phase transformation, martensitic transformation and superelasticity of Ni-rich TiNi alloy. The mechanisms of the second phases on the martensitic transformations and mechanical properties were discussed.

Performance of a base isolator with shape memory alloy bars

A new and innovative base isolation device is introduced in this paper based on extensive research carried out by the authors and their co-workers. A prototype of the device was built and experimentally tested on the shaking table. The new base isolation device consists of two disks, one vertical cylinder with an upper enlargement sustained by three horizontal cantilevers, and at least three inclined shape memory alloy （SMA） bars. The role of the SMA bars is to limit the relative motion between the base and the superstructure, to dissipate energy by their super-elastic constitutive law and to guarantee the re-centring of the device. To verify the expected performance, a prototype was built and tested under sinusoidal waves of displacement of increasing frequency with different amplitudes. It is shown that the main feature of the proposed base isolation device is that for cyclic loading, the super-elastic behavior of the alloy results in wide load-displacement loops, where a large amount of energy is dissipated.

EFFECT OF DYNAMIC RECRYSTALLIZATION ON SHAPE MEMORY EFFECT IN Fe－Mn－Si BASED ALLOYS

Improvement of shape, memory effect (SME) in Fe-Mn-Si based alloys has been investigated, compared with that by conventional 'training' treatment. It is found that SME in Fe-Mn-Si alloy can be greatly improved by ausforming and 3.8%recovery strain and 2.2% complete recovery strain can be reached by ausforming at 973 K when dynamic recrystallization has just occurred. The mechanism for the improvement of SME is proposed.

SHAPE MEMORY EFFECT OF POLYCRYSTALLINE Fe-Mn-Si ALLOYS

The influence of quenching temperature on the shape memory effect(SME)of the Fe-Mn-Si polycrystalline alloys and their martensitic transformation temperature have been studied.The SME of the hot-rolled specimen may be remarkably enhanced by selecting quenching temperature of 600—800℃.It has been shown that SME is influenced not only by the distribution of ε-phase morphology but also by its pre-existence.

Properties and Application of Iron-based Shape Memory Alloy

The properties of FeMnSiCrNi shape memory alloy were investigated. The results show that the best shape memory effect of Fe14Mn6Si9Cr5Ni alloy is 85%. The transformation amount of the ε→γ transformation is not complete after heating the alloy to 1000 K, As and Af points drop with increased transformation enthalpy (ΔH γ→ε ) by thermal cycling and increased prestrain. The alloy shows also good creep and stress relaxation resistance. In addition, the alloy having a tensile force of 20 kN and a sealing pressure of 6 MPa can satisfy requirements for possible industrial application on pipe joints.

THE FAILURE MECHANISM OF REVERSE SHAPE MEMORYEFFECT IN A Cu-BASE ALLOY

The interior structural evolution accompanying reverse shape memory effect (RSMEin a Cu-Zn-Al alloy was studied by means of transmission electron microscopy. It was found that RSME is closely related to bainitic transformation in this alloy during the isothermal reaction at moderate temperatures. At a given temperature and a certain external constraint stress, the shape memory effect depends mainly on the aging time.During the early stage, the shape memory effect enhances with the increase of reactiotn time. Then it will decrease gradually apon further aging. If the alloy is overaged, the stacking faults of bainite will disappear gradually by the motion of partial dislocations through which long range diffusion of solute atoms takes place, giving rise to the deterioration of RSME. When all the bainite transforms to α phase, RSME will lose completely.

ON THE STRAIN JUMP IN SHAPE MEMORY ALLOYS—A CRYSTALLOGRAPHIC-BASED MECHANICS ANALYSIS

The strain jump across the Austenite-Martensite (A-M) interface in single crystal Cu-14wt%Al-4.12wt%Ni Shape Memory Alloys (SMAs) under uniaxial tension was studied in this paper. A crystallographic-based mechanics analysis on the formation and microstructure of the interface was performed. By using the high sensitive Moiré interference technique, the full-field deformation patterns during the transformation process were successfully recorded. The orientation of the habit plane (A-M interface) and the magnitude of the shape strain were determined precisely from the Moiré fringe patterns. The theoretical predictions on the habit plane normal and the shape strain were compared with the measured results and good agreements were obtained.

Effect of texture on internal friction of Cu-Al-Mn based shape memory alloys

Shape memory alloys (SMAs) are well-known as high damping materials. Recently,we have reported that the Cu-Al-Mn SMAs show a high internal friction as well as the other SMAs such as Ni-Ti, Cu-Zn-Al SMAs. Since the Cu-Al-Mn-based SMAs possess an excellent cold-workability, a strong recrystallization texture can be formed by thermomechanical treatment. The present authors demonstrated that the texture control is considerably effective for improving the SM properties and especially, in the Cu-Al-Mn-Ni SMAs with a strong {112}<110> recrystallization texture, a large pseudoelastic (PE) of about 7% can be obtained. Such a texture control is expected to enhance the damping capacity in the Cu-Al-Mn-based SMAs as well as PE. In this study, the effect of texture on the internal friction in Cu-Al-Mn and Cu-Al-Mn-Ni SMAs was investigated by tensile testing mode of Dynamic Mechanical Spectrometer (DMS). In sheet specimens with the {112}<110> texture, value of dynamic damping tanφ depends on the loading direction and an excellent damping capacity of tanφ= 0.1 can be obtained at the rolling direction in the martensite phase condition. Moreover, these Cu-Al-Mn-based SMAs show a high tensile strength over 600 MPa in the martensite phase. The present Cu-Al-Mn-based SMAs should have a great potential as the damping materials.

Martensitic transformation and shape memory effect in ausaged Fe-Ni-Si-based alloys

It is well known that the morphologies of the α’ martensite formed from the γ phase in ferrous alloys are classified into five types of lath, butterfly, （225）A type plate,lenticular and thin-plate. Among those α’ martensites, only the thin-plate martensite,which is characterized by containing a high density of transformation twins, has a potential of exhibiting a perfect shape memory （SM） effect.Recently the present authors found in Fe-Ni-Si alloys that the thin-plate martensite is formed by the introduction of fine and coherent γ’-（Ni,Fe）3Si particles with a L12 ordered structure in the austenite matrix due to ausaging. In the present study, the SM properties of the ausaged Fe-Ni-Si alloys with the thin-plate martensite are investigated by a conventional bending-test. The effects of the addition of Co to the Fe-Ni-Si alloys on the martensitic transformation and the SM properties are also investigated. It is shown that while the ausaged Fe-Ni-Si ternary alloys exhibit an imperfect SM effect due to reverse transformation from stress-induced thin-plate martensite to austenite, the SM properties are improved by the addition of Co. An almost perfect SM effect is confirmed in the Fe-Ni-Si-Co alloys by heating to 1 100 ℃ after deformation at -196 ℃.

Shape-memory effect in Co-Ni single crystal

The thermal shape-memory effect at room temperature for Co-32%Ni(mass fraction) magnetic shape memory alloy of single crystal was presented. When compressing the sample along the direction at room temperature, strain can be recovered to some extent during later heating and the recovery rate varies with the pre-strain. But no obvious recoverable strain can be obtained along other crystal directions. For the thermal-mechanical training of the sample along , the recovery strain decreases obviously during the second round of compress and nearly no recovery happens after the third round of compress. A possible mechanism based on reversible motions of Shockley partial dislocations was proposed.

快速凝固对Fe-Mn-Si系形状记忆合金组织和性能的影响

研究了快速凝固对Fe-Mn-Si系形状记忆合金组织和性能的影响。结果表明,快速凝固由于大大细化了组织,从而导致合金屈服强度提高,同时,快速凝固试样内部容易产生较多的α′马氏体,这种预存的马氏体也有助于提高基体的强度,从而抑制不可逆塑性变形的产生,让更多的变形由应力诱发ε马氏体来承担,和轧制试样相比,形状记忆效应约提高13%～200%;快速凝固试样由于晶粒细小,晶界多,可供ε马氏体形核的位置也较多,所以ε马氏体长得比较细小,也容易发生交叉穿越的现象。

Fe-Mn-Si-Cr-Ni形状记忆合金约束下相变的电阻原位分析

通过同步测量约束加热和冷却过程中合金的电阻率和回复应力与温度的关系,对Fe-Mn-Si-Cr-Ni形状记忆合金约束下的转变过程进行了详细研究,结果表明,变形约束加热后的冷却过程中,回复应力随温度降低而增加,当回复应力增加到大于合金的屈服强度时,将首先发生塑性变形;然后随温度的进一步降低,回复应力到达应力诱发ε马氏体相变的临界应力时,回复应力将诱发ε马氏体相变,导致回复应力随温度的降低而下降,塑性变形和应力诱发ε马氏体相变都将显著松弛回复应力,降低合金冷却到室温时的回复应力,建立了合金加热和冷却过程中回复应力的方程,提出了该记忆合金管接头成分设计原则。

淬火温度对Fe-Mn-Si-C合金形状记忆性能的影响

研究淬火温度对Fe-Mn-Si-C合金的形状记忆性能的影响,发现在700℃淬火可以获得最佳的形状记忆性能。通过对合金光学显微组织、透射电镜组织的观察并结合奥氏体晶粒尺寸、形状回复率、层错几率等性能的测量,系统地讨论了不同淬火温度下,热诱发马氏体数量、Ms点、空位浓度、层错能等对形状记忆性能的综合影响,进一步探讨淬火温度对形状记忆性能影响的微观机制。

Fe-Mn-Si系形状记忆合金中热诱发ε_M的单元结构及生长特性研究

用透射电子显微镜观察了Fe-17M-10Cr-5Si-4Ni合金中热诱发ε马氏体(εM)的组成结构,探讨了εM生长特性,实验表明,合金中较大尺寸的热诱发εM具有由生长取向大致相同的单元以首尾相接方式组成的结构特点,层错带平行分布的体系中εM具有促发形核、接续长大的生长特性。

时效工艺对Fe-Mn-Si-Cr-Ni系合金形状记忆效应的研究

分别对两种成分的Fe-M n-Si-Cr-Ni系合金进行不同工艺的时效处理,通过金相观察、电镜分析、硬度和形状记忆回复率的测试表明,由于时效沉淀相的析出,强化了奥氏体基体,即使不通过热机械训练,也可获得较明显的形状记忆效应。加入少量Nb和C的合金提高形状记忆效应更为明显。

Fe-Mn-Si形状记忆合金fc(γ)→hcp(ε)马氏体相变温度的热力学预报

借助于Ｆｅ－Ｍｎ－Ｓｉ合金层错几率Ｐｓｆ的Ｘ射线测量，计算了Ｐｓｆ与合金成分的关系，得到Ｆｅ－Ｍｎ－Ｓｉ三元系的１／Ｐｓｆ表达式．结合层错形核的热力学模型，经回归得Ｆｅ－Ｍｎ－Ｓｉ合金ｆｃ（γ）→ｈｃｐ（ε）马氏体相变的临界相变驱动力和Ｐｓｆ的关系式，进而得到临界相变驱动力与合金成分的关系．借助Ｐｓｆ建立了成分与相变驱动力之间的关系，结合有关热力学分析计算得到的Ｆｅ－Ｍｎ－Ｓｉ合金γ和ε两相Ｇｉｂｂｓ自由能曲线，预报了Ｆｅ－Ｍｎ－Ｓｉ三元系合金的ｆｃｃ（γ）→ｈｃｐ（ε）马氏体相变的温度．

铁基形状记忆合金-变摩擦复摆隔震连续梁桥地震反应分析

针对传统摩擦复摆隔震桥梁支座位移过大、自复位能力不足等问题,利用变摩擦机制和形状记忆合金的超弹性特性,研发了一种新型铁基形状记忆合金-变摩擦复摆隔震支座(iron-based shape memory alloy-double-concave variable friction pendulum bearing,Fe-SMA-DVFPB)。构建其本构模型,并确定其等效分析模型。提出Fe-SMA-DVFPB隔震桥梁基于位移的抗震设计方法,以实际工程为背景,分别设计了应用不同类型支座的隔震桥梁,并对用不同类型支座隔震的桥梁抗震性能进行对比分析。研究表明:相比传统摩擦复摆支座隔震桥梁,Fe-SMA-DVFPB隔震桥梁支座相对位移最大减小量为23.79%,残余位移最大减小量为93.56%;应用Fe-SMA-DVFPB隔震支座能更有效地控制支座相对位移与残余位移;Fe-SMA-DVFPB隔震桥梁支座相对位移与残余位移减小量明显大于墩底弯矩和剪力增加量。Fe-SMA-DVFPB能够进一步提升桥梁的震后可恢复能力。

激光焊对Fe-Mn-Si系形状记忆合金焊缝区组织和性能的影响

研究了激光焊对Fe-Mn-Si系形状记忆合金焊缝区组织和性能的影响,结果表明,激光焊焊缝区组织显著细化,大大提高基体的强度,同时,由于激光焊焊缝过冷度大,容易产生较多的层错,这些层错有利于应力诱发ε马氏体形核,从而显著提高焊缝区的形状记忆效应,和母材相比,提高幅度达40%-178% 左右;在模拟油田介质情况下,激光焊焊缝耐腐蚀性能高于母材,这是因为,激光焊焊缝组织细小均匀,在腐蚀过程中产生钝化现象,同时也降低了腐蚀电池的电位差,从而降低了合金的腐蚀速度,提高耐腐蚀性能.

Fe-Mn-Si基形状记忆合金耐磨性的研究

通过滑动摩擦磨损试验研究了一种Fe-Mn-Si基形状记忆合金的耐磨性,并借助扫描电镜(SEM)观察,X射线衍射(XRD)等手段研究了其摩擦磨损过程的相变机制.研究发现油摩擦时,Fe-Mn-Si基形状记忆合金具有良好的耐磨性,磨损机制为磨粒磨损.XRD分析和SEM观察表明,油摩擦时Fe-Mn-Si基记忆合金可通过应力诱发→马氏体相变及其相变伪弹性降低应力集中,抑制塑性滑移变形,避免微裂纹的形成和扩展提高其耐磨性.