Ti50Ni50及Ti49.3Ni50.7鈦鎳形狀記憶合金變態及機械性能之研究

Aichi Institute of Technology, 1247 Yachigusa, Yakusa-cho, Toyota, 470-0392, Japan; tobushi@aitech.ac.jp Mechanical characteristics and temperature changes related to stress-induced martensite transformation developing in TiNi shape memory alloy have been presented. Exothermic martensite forward and endothermic reverse transformations have been recorded with use of three kinds of infrared cameras, including very fast and sensitive Therma-Cam Fenix DTS. It was found that the temperature distribution on the surface of the specimens was uniform during straining below the austenite start temperature, while investigating shape memory effect, whereas bands of higher temperature corresponding to localized martensitic transformation were recorded during the process carried out above the alloy austenite finish temperature. The shape memory effect (SME) and superelasticity (SE) are the main phenomena which appear in a shape memory alloy (SMA), depending on the test temperature T. They are controlled by two material parameters: the austenite finish (Af) and the austenite start (As) temperatures. If T is higher than Af, the SE appears, and if T is lower than As, the SME appears. The behavior is caused by the stress-induced reversible martensite transformation (MT) which takes place during the SMA loading and unloading. In the case of SE, almost complete reverse transformation occurs during the SMA unloading Figs 4-9. In the case of SME, quite significant residual strain is observed after the SMA unloading (Figs 2,3). The strain related to the martensitic phase disappears, if the specimen is heated after the unloading above the Af temperature. The energy storage and the energy dissipation due to the SE in SMA are very large and the recoverable stress and strain are quite large compared to the traditional metals. The described properties enable the SMA many applications, e.g., as damping elements, driving force of actuators or main parts of heat engines. The MT in general can be induced by variation in temperature or stress, so the SMA behavior depends on the thermomechanical loading conditions (1-4). The main point studied in the paper is the homogeneity of the martensite transformation process, carried out in various conditions, since the homogeneity usually assures higher reliability of the SMA applied systems.

Microstructural and fractographic observations were systematically done on Fe-30Mn(6-x)Si-xAl (x=0, 1, 2 and 4 mass %) alloys. Optical and transmission electron microscopic observations and X-ray diffractions revealed that the deformation mode continuously shifts from the stress induced fcc/hcp martensitic transformation to the mechanical twinning of the fcc austenite as the Al content increases. It was also clarified by the scanning electron microscopic observations that the microstructural change depending on the Al content is accompanied by the change in the fracture mode from the quasi-cleavage fracture to the ductile fracture. INTRODUCTION Fe-Mn-Si-based shape memory alloys (SMAs) exhibit the shape memory effect (SME) associated with fcc (γ-austenite) / hcp (e-martensite) martensitic transformation [1]. A recoverable strain obtained in a typical Fe-Mn-Si SMA: e. g. Fe-30Mn-6Si (hereinafter compositions are shown in mass%), was reported about 2% in the solution treated condition [2]. This value can be increased to about 4% by so-called the training treatment [3, 4] and fine dispersion of precipitates such as NbC carbides [5-7], etc. One drawback of the alloy was its poor ductility of about 30%. In contrast to this, it was recently reported that the Fe-30Mn-3Si-3Al TWIP (Twinning Induced Plasticity) steel exhibits the ultra-high ductility as much as about 90% [8], but this alloy shows no significant SME. The composition of the Fe-30Mn-3Si-3Al TWIP steel is such that a part of Si in the Fe30Mn-6Si SMA is replaced by Al. In order to systematically investigate the effect of the Al content on the SME and TWIP effect, the present authors prepared four kinds of FeMn-Si-Al alloys by gradually varying the amount of Al substituting Si: i. e. Fe-30Mn(6-x)Si-xAl (x=0, 1, 2 and 4). The following two conclusions were drawn as a result [9]: i) the alloys with x=0 and 1 exhibited similar SME, but no recognizable SME was observed for the alloys with x>2, ii) the ductility linearly increased with increasing the amount of Al. The above-mentioned changing tendencies may originate from the continuous change in the deformation mode from the stress-induced γ → e martensitic transformation to the mechanical γ twinning. However, there has been no systematic study on the effect of the Al content on the deformation and fracture modes between the SMA and the TWIP. In the present paper, microstructural observations using optical microscopy (OPM) and transmission electron microscopy (TEM), phase identification using X-ray diffraction (XRD) and the fractographic observations using scanning electron microscopy (SEM) were carried out to clarify the effect of the Al content on the deformation mode and the corresponding fracture mode of the alloys. EXPERIMENTAL In this paper, hereafter the Fe-Mn-(6-x)Si-xAl (x=0, 1, 2 and 4) alloys are referred to as Al-0,Al-1,Al-2,Al-3 using mass % of Al. The specimens were prepared by vacuum induction melting. After hot forging and rolling at 1270K, the specimens were subjected to solution treatment at 1270K for 3h followed by water quenching. The OPM observations were performed on the samples, which were mechanically and electrolytically polished to obtain smooth surfaces and then extended by about 3%, using a differential interference microscope. The phase constitutions and internal microstructures in the deformed specimens were investigated with a RINT 2500 X-ray diffractometer and with a JEOL 2000FX II transmission electron microscope, respectively. The specimens for TEM observation were carefully prepared to avoid the formation of stress-induced martensite and reverse transformation on heating, using a chemical polishing solution of hydrogen peroxide and hydrofluoric acid mixed in the ratio of 10: 1. The specimens were finally subjected to electropolishing using acetic acid and perchloric acid mixed in the ratio of 20:1 at room temperature, to obtain the TEM foils. Fracture surfaces were examined on the specimens fractured at room temperature with a Hitachi S-3100 scanning electron microscope. RESULTS AND DISCUSSION DEFORMATION BEHAVIOR Figures 1 (a) to (d) show the OPM images observed on the specimens of Al-0 to Al-3, respectively, deformed by tensile strain to 3%. Some grains seen in the figures involves anneal twins. Anneal twin boundaries on {111}γ planes are indicated by arrows in the OPM photos. In each of parent and twin crystals, there are surface striations. It should be noted in Figs. 1(a) to (d) that the width and interval of the striations inside crystals becomes smaller with increase in the Al content. It has been widely accepted that the striations appeared in the Fe-Mn-Si SMAs are formed by the stress-induced γ → e martensitic transformation, while those in the FeMn-Si-Al TWIP steels are due to the mechanical γ twinning. It is inferred from the variation in the microstructures from Figs. 1(a) to (d) that the deformation mode should continuously change from the stress-induced e martensite to the mechanical γ twins, when the Al content is increased from 0 mass % to 3 mass %. Our previous result [9] showing the linear change in the ductility depending on the Al content also supports this speculation. However, it is difficult to distinguish these two deformation products by OPM observations, because both have plate shapes on the {111}γ habit. Figure 1: Deformation microstructures observed by optical microscopy on the specimens of (a) Al-0, (b) Al-1, (c) Al-2, and (d) Al-3. The observations were performed at the tensile strain of about 3%. The phase identification by means of the XRD was, therefore, performed to investigate semi-quantitatively the dependence of the amount of the e phase on the Al content. It was revealed that the intensity of peaks from the e phase relative to that of peaks from the γ phase gradually decreases with increasing the Al content, though not presented here. However, it is impossible to investigate the amount of the mechanical γ twins by the XRD. In order to confirm the existence of the e phase and the γ twins, the electron diffraction pattern analysis using TEM was employed. Figure 2(a) shows the bright field image taken in the Al-0. The plates observed in Fig. 2(a) were identified as the e phase by the corresponding electron diffraction pattern shown in Fig. 2(b). The incident beam is parallel to [011]γ // [21 1 0]e. The diffraction pattern in Fig. 2(b) clearly shows the well known features of the γ → e transformations: i) the S-N orientational relationship between the γ and e crystals, ii) the streaks along γ directions due to small thickness of the e plates. The streaks run in two directions: i. e. and . The (11 1) and (111 ) traces nominal to the corresponding streaks are seen in Fig. 2(a). Figure 2(c) and (d) show an example of the mechanical γ twins (γTM) observation in the Al-3. The zone axes of the diffraction pattern are [011]γ // [011]γTM. A lamella structure consisted of nano-sized twins and the retained austenite is formed in the specimen, being consistent with the previous results in the literature [10]. After a number of careful observations, a very small amount of the e plates were also found even in the Al-3, although it was undetectable in the XRD profile. 200 111 111 MT 200 MT

Stress-induced martensitic transformations and shape memory at nanometer scales

In this study, the properties of shape memory effect (SME), pseudoelasticity (PE) and stress-strain (σ-e) cycling exhibited in Ni-rich Ti48.7Ni51.3 and Ti48.4Ni51.6 shape memory alloys (SMAs) are investigated. These SMAs are solid-soluted(SS) at 900℃x 1 hr, water quenched, and then aged at 250℃~500℃ for various time. The hardnesses of as-SS Ti48.7Ni51.3 and Ti48.4Ni51.6 SMAs are 362HV and 386HV, respectively. The maximum hardness for SS and aged specimens is 415 HV for Ti48.7Ni51.3 SMA aged at 300℃x10 hrs, and is 455HV for Ti48.4Ni51.6 SMA aged at 350℃x3 hrs. In the early aging, the curve of hardness at room temperature v.s. aging time for Ti48.7Ni51.3 SMA aged at 250℃ has two maxima, but that for Ti48.7Ni51.3 SMA aged at 300℃~500℃ and that for Ti48.4Ni51.6 SMA aged at 250℃~500℃ have only one maximum. From DSC tests of Ti48.7Ni51.3 and Ti48.4Ni51.6 SMAs, B2↔R transformation mainly appears in specimens aged at 250℃~350℃, but B2↔R↔B19’ transformation occurs in specimens aged at 400℃~500℃. The tensile tests indicate that, in specimens of Ti48.7Ni51.3 and Ti48.4Ni51.6 SMAs aged at 500℃x10 hrs, their SME, PE and σ-e cycling properties are better than other aging conditions due to Ti3Ni4 precipitates grow larger and the resistance for B19’ transformation is less.

Mechanical behaviour of NiTiNb shape memory alloy wires– strain localisation and effect of strain rate

TiNi shape memory alloy (SMA) specimens have been subjected to tension carried out at various strain rates. The goal was to investigate a nucleation and development of the stress-induced martensitic transformation by infrared (IR) and acoustic emission (AE) techniques. Therefore, both the infrared radiation and acoustic emission data were recorded using a fast infrared camera and acoustic emission set-up, respectively. It has been shown that the initial, macroscopically homogeneous transformation initiates in the elastic stage of the deformation even before the stress-strain curve knee and formation of the localized transformation bands. It has also been found that the homogeneous transformation occurs at similar stress level for all strain rates applied, while the localized martensitic transformation depends on the strain rate. Nucleation and development of the localized transformation bands, detected by the infrared camera, were confirmed by acoustic emission technique. The differences between the IR and AE activities were recorded during the TiNi SMA loading and unloading process, manifesting different dynamics of the stress-induced martensitic forward and reverse transformation. K e y w o r d s : shape memory alloy, TiNi, superelasticity, martensitic transformation, tension test, acoustic emission

利用TEM和拉伸实验研究了时效工艺对Ti-50.8Ni-0.3Cr（原子分数,%）形状记忆合金（SMA）显微组织和超弹性的影响.随时效时间（t_（ag））延长,300℃时效态Ti-50.8Ni-0.3Cr SMA的Ti3Ni4析出相呈细小颗粒状,400℃时效态合金的析出相由颗粒状逐渐变为针状,500℃时效态合金的析出相由针状逐渐变为粗片状.时效温度对析出相形态的影响比t（ag）显著.随t_（ag）延长,300和400℃时效态合金的抗拉强度（σb）先增大后趋于稳定,σb（500℃）先减小后趋于稳定,且σb（400℃）〉σb（300℃）〉σb（500℃）.300和400℃时效态合金的超弹性优于500℃时效态合金.随t（ag）延长,该合金的应力诱发马氏体相变临界应力逐渐减小,300℃时效态合金的超弹性能耗（△W）降低,400℃时效态合金的△W升高,500℃时效态合金的△W先升高后降低.

Stress-induced martensitic transformation behavior of a Ti–Ni–Hf high temperature shape memory alloy

The stress-induced martensitic transformation behavior and the microstructure of stress-induced martensite (SIM) in a Ti{sub 36}Ni{sub 49}Hf{sub 15} high temperature shape memory alloy (SMA) have been investigated using tensile tests and transmission electron microscopy (TEM) in this paper. It shows that, compared with TiNi SMAs, ther is no stress plateau in the stress-strain (S-S) curve as the TiNiHf alloy deformed in austenite. The martensitic transformation enthalpy is calculated to be - 106.84 cal/mol. The martensite variants mainly show preferential oriented morphologies for the TiNiHf alloy deformed to 8% at 523 k. The substructure of SIM and deformed SIM in the present alloy is (001) compound twin. Martensite variants are (011) type I twin related. Further increasing the deformation temperature and strain, the preferential oriented SIMs gradually develop to martensite variants with variant-crashed / variant-intersected morphologies. The inexistence of stress plateau in the S-S curve of the TiNiHf alloy may mainly result from the dislocation slip during stress-induced martensitic transformation. (orig.)

Low-energy tensile-impact behavior of superelastic NiTi shape memory alloy wires

The deformation properties of TiNi shape-memory alloy subjected to strain control and stress control were investigated experimentally. The results obtained are summarized as follows. (1) In the case of a full loop, the stress-strain curves under stress-controlled conditions are similar to those under strain-controlled conditions with high strain rate. The overshoot and undershoot do not appear at the start points of the stress-induced martensitic transformation in these curves. (2) In the case of subloop under stress-controlled conditions, temperature decreases and therefore the strain increases owing to the martensitic transformation at the early stage of the unloading process. At the early stage in the reloading process, temperature increases and therefore the strain decreases owing to the reverse transformation. (3) In the case of subloop under stress-controlled conditions, the starting stresses of the martensitic transformation and the reverse transformation in the loading and unloading processes coincide with the transformation stresses under strain-controlled conditions with low strain rate, respectively. (4) The deformation behaviours for a subloop under stress-controlled conditions are prescribed by the condition for progress of the martensitic transformation based on the transformation kinetics. (5) The deformation behaviors subjected to cyclic loading under stress-controlled conditions at constant temperature are also prescribed by the conditions for progress of the martensitic transformation.

In the case of the NiTi alloy, some research works reported that the stress-strain curve differs significantly between tension and compression. It is considered that the difference in stress-induced martensitic transformation behavior may be observed under different loading modes in Fe-based shape memory alloy (Fe-SMA), which is less expensive compared with the widely-used NiTi alloy. In this study, both tensile and compressive tests of Fe-28Mn-6Si-5Cr shape memory alloy, which is one of Fe-SMA, are conducted under quasi-static and impact deformation at various strain rates. During deformation, it is attempted to evaluate the stress-induced martensitic transformation by measuring the volume resistivity as well as temperature for increasing a reliability of the alloy. Furthermore, from the results of volume resistivity at each strain rate, the strain rate sensitivity of stress-induced martensite under tensile and compressive tests is investigated. Additionally, the difference in stress-induced martensitic transformation under tension and compression is studied.

Composites containing NiTi shape memory alloy (SMA) long-fiber, short-fibers or Ti long-fiber in a Polycarbonate (PC) matrix have been fabricated by the injection molding technique. Also, prestrained SMA long-fiber/Epoxy matrix composites have been fabricated. The fracture behavior and thermo-mechanical deformation behavior are examined; (1) Fracture behavior – uniaxial tensile tests up to fracture for SMA long-fiber and short-fiber composite (SMAC). (2) Thermomechanical deformation behavior – tensile loading–unloading tests for Pseudoelastic (PE) long-fiber/PC matrix composites. Several thermo-mechanical loading tests for Shape Memory Effect (SME) long-fiber/PC matrix and SME long-fiber/Epoxy matrix composites were used. The obtained results are as follows: (1) The stress–strain relation up to the final fracture of the Shape Memory Alloy Composites (SMACs) showed the repeated up-and-down of the stress which corresponds to the necking of the specimen, fiber fracture, and matrix fracture. The strain for the initiation of necking and the strain for the fiber or matrix fracture in the SMACs were higher than those in the Ti composite. This is attributed to the unique stress–strain relations accompanied by the stress-induced martensitic transformation of the SMA fibers. (2) The SMAC containing PE fiber and PC exhibited the pseudoelastic-like deformation under tensile loading–unloading. (3) The SMAC containing SME fiber and PC exhibited the large contraction by heating after tensile loading–unloading, but the compressive residual stress in the matrix expected in this process was not remarkable. However, compressive residual stress in the matrix may become greater by embedding prestrained fiber in the matrix.

Mechanisms for influence of post-deformation annealing on microstructure of NiTiFe shape memory alloy processed by local canning compression

Phase field study of the microstructure evolution and thermomechanical properties of polycrystalline shape memory alloys: Grain size effect and rate effect