Shape memory and related phenomena

Fe-Mn-Si alloys are shape memory alloys which make use of the γ→e stress-induced martensitic transformation. In this study, we report the effects of alloying additions on the shape memory effect (SME) of these alloys. It was found that the Ms temperature, the Neel temperature (TN) and the volume of stress-induced martensite govern the SME. Through the optimization of these factors we found that new alloy systems such as Fe-28Mn-6Si-5Cr, Fe-20Mn-5Si-8Cr-5Ni and Fe-16Mn-5Si-12Cr-5Ni alloys could exhibit good SME along with good corrosion resistance. And it was also found that the thermomechanical treatment which improved the SME in Fe-Mn-Si base system was also effective to improve the SME of these new systems.

Shape memory effect in metallic glasses

Shape memory effects in Ni55Fe18Ga27 single crystal grown along the [001] direction by the Czochralski method was studied. It was found that deformation of [001] single crystal in the martensite state was realised via reorientation of 10 M martensite and stress-induced transformations: 10 M → 14 M → L10. On unloading, the reverse L10 → 14 M → 10 M transformations occurred and a large unelastic strain recovered. On heating, the oriented 10 M martensite transformed to the L21 austenite phase and the shape memory effect was observed. An increase in preliminary strain resulted in an increase in the shape memory effect value to 4.6%. The [001] Ni55Fe18Ga27 alloy single crystal demonstrated transformation plasticity and shape memory effects on cooling and heating under stress however, an increase in stress decreased the values of these effects. This was caused by stress-induced martensite appearing in the sample during loading in the austenite state, which decreased the volume of the austenite phase that could undergo the martensitic transformation on cooling. The [001] Ni55Fe18Ga27 alloy single crystal demonstrated a two-way shape memory effect and its value depended on the residual strain in a non-monotonic way and the maximum recoverable strain was 0.7%.

Shape memory alloys (SMAs) belong to a group of functional materials with the unique property of “remembering” the shape they had before pseudoplastic deformation. Such an effect is based on crystallographic reversible thermo-elastic martensitic transformation. There are two crystal phases in SMAs: the austenite phase (stable at high temperature) and the martensite phase (stable at low temperature). Austenite to martensite phase transformation can be obtained by mechanical (loading) and thermal methods (heating and cooling). During martensitic transformation, no diffusive process is involved, only inelastic deformation of the crystal structure. When the shape memory alloy passes through the phase transformation, the alloy transforms from high ordered phase (austenite) to low ordered phase (martensite). There are two types of martensite transformations. First is temperature-induced martensite, which is also called self-accommodating (twinned) martensite. The second is stress-induced martensite, also called detwinned martensite. The entire austenite to martensite transformation cycle can be described with four characteristic temperatures: M_s – martensite start temperature, Mf – martensite finish temperature, As – austenite start temperature, and Af – austenite finish temperature. The main factors influencing transformation temperatures are chemical composition, heat treatment procedure, cooling speed, grain size, and number of transformation cycles. As a result of martensitic transformation in SMAs, several thermomechanical phenomena may occur: pseudoelasticity, shape memory effect (one-way and two-way SME) and rubber-like behavior. Pseudoelasticity occurs when the SMA is subjected to a mechanical loading at a constant temperature above Af. The second thermomechanical behaviour that can be observed in SMA is the shape memory effect (SME), mainly one-way SME, which is the most commonly used SME. When the sample is subjected to a mechanical loading, the stress reaches a critical value and the transformation of twinned martensite into detwinned martensite begins and finishes when the loading process is finished. When the loading-unloading process is finished, the SMA presents a residual strain recoverable by alloy heating, which induces the reverse phase transformation. As a result, the alloy recovers to its original shape. In this paper, a review of thermomechanical properties of shape memory alloys and general characteristics of martensite transformations is shown.

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

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.

In this study, the property improvement of shape memory effect (SME), pseudoelasticity (PE) and stress-strain (σ-e) cycling of Ti49.3Ni50.7 and Ti50Ni50 shape memory alloys (SMAs) is investigated. Ti49.3Ni50.7 SMA aged at 300℃×100h and 400℃×8h can reach the maximal precipitation-hardening with the hardness of the former being higher than that of the latter. Tensile test indicates that the specimen aged at 300℃×100h has better SME/PE and σ-e cycling properties than that aged at 400℃×8h. Cold-rolling effect on the property improvement is studied on Ti50Ni50 SMA. Experimental results show that the degree of cold-rolling lower than 20% is insufficient to strengthen the SMAs to improve their properties, such as the σ-e cycling stability and the recoverable storage energy in σ-e curve. If the annealing of cold-rolled specimen is over, the SMAs’ properties can also be deteriorated. At the same time, the σ-e cycling test indicates that, after 20th cycles, both R-phase and B19’ martensitic transformations are depressed due to the dislocations pile-up during the cycling, and the B2→R transformation is more depressed than R→B19’ one. In this study, the maximal PE strain induced by stress-induced martensite (SIM) is found to be lower than ~7% and the plasticity deformation occurs if the strain is higher than 7% which will deteriorate the SMAs’ PE property. For the strain rate (e ) effect on the property improvement of Ti50Ni50 SMA, in the e range of 2.5×10-4s-1~1.0×10-2s-1, the σ-e cycling with higher e will be more beneficial to the forward SIM transformation, instead of the reverse SIM transformation during the cycling.

Abstract: The specific behaviour of shape memory alloys (SMA) is due to a martensitic transformation [Shape Memory Materials. Cambridge University press, Cambridge]. This transformation consists mainly in a shear without volume change and is activated either by stress or temperature. The superelastic behaviour and the one‐way shape memory effect are both due to the partition between austenite and martensite. The superelastic effect is obtained for fully austenitic SMA: loaded up to 5% strain, a sample recovers its initial shape after unloading with a hysteretic loop. The one‐way shape memory effect is obtained when a martensitic SMA, plastically deformed, recovers its initial shape by simple heating. Superelasticity and one‐way shape memory effect are useful for several three‐dimensional applications. Despite all these phenomena are well known and modelled in 1D, the 3D behaviour, and especially the one‐way shape memory effect, remains quite unexplored [Mater. Sci. Res. Int., 1 (1995) 260]. Actually, the development of complex 3D applications requires time‐consuming iterations and expensive prototypes. Predictive phenomenological models are consequently crucial objectives for the design and dimensioning of SMA structures. Therefore, a series of 2D proportional and non‐proportional, isothermal and non‐isothermal tests have been performed. This database will be used to build a phenomenological model within the framework of irreversible processes.

The shape memory effect (SME), superelasticity (SE), and cyclic deformation behavior of two-phase α/β brasses have been investigated at various temperatures, using tensile tests andin situ optical microscopic observations. The morphology and characteristics of the (thermoelastic) martensitic transformation and the mechanism of the SME are similar to those for single-phase β-brass, but the amount of irrecoverable strain is larger in the two-phase alloys due to plastic deformation of the α particles. After unloading and heating, the slipbands in the discrete a particles remain, whereas the martensite almost disappears; thus, the higher the volume fraction of α particles, the larger the amount of irrecoverable strain. The deformation behavior of alloy A at temperatures above the martensite start (Ms) temperature (with 26 pct α phase) is dominated by deformation of the α phase, so complete SE cannot be obtained after cyclic deformation, both at room temperature and at -40 °C. While in alloy B (containing 15 pct α phase), the deformation behavior is dominated by the formation of stress-induced martensite (SIM). The α particles are deformed before SIM formation on loading at room temperature, but on the contrary, SIM forms before the α particles are deformed on loading at -40 °C (>Ms). Complete SE can be obtained in alloy B after cyclic deformation at room temperature to a given strain but does not occur at -40 °C because the a particles are deformed along with the growth of pre-existing SIM under larger strain during cycling at this temperature.

Hydrogen’s effect on the shape memory effect (SME) of [1¯17]-oriented Ti49.7-Ni50.3 (at.%) alloy single crystals, with a B2–B19′ martensitic transformation (MT), was studied after being electrolytically hydrogenated at a current density of 1500 A/m2 for 3 h at room temperature under isobaric tensile deformation. It was shown that, under the used hydrogenation regime, hydrogen was in a solid solution and lowered the elastic modulus of B19′ martensite. The hydrogen in a solid solution increased (i) the yield strength σ0.1 of the initial B2 phase by 100 MPa at Md temperature, (ii) the σ0.1 of the stress-induced B2–B19′ MT by 25 MPa at Ms temperature, and (iii) the plasticity of B19′ martensite relative to the hydrogen-free crystals. At the same level of external stresses, the SME in the hydrogenated crystals was greater than that in hydrogen-free crystals. At external tensile stresses σex = 200 MPa, the SME was 4.4 ± 0.2% in the hydrogenated crystals and 1.8 ± 0.2% without hydrogen. Hydrogen initiated a two-way SME of 0.5 ± 0.2% at σex = 0 MPa, which was absent in the hydrogen-free crystals. The physical reasons leading to an increase in the SME upon hydrogenation are discussed.

On the significance of C and Co on shape memory performance of Fe–Mn–Si–Cr–Ni shape memory alloy

Microstructure, martensitic transformation and superelasticity of Ti 49.6Ni 45.1Cu 5Cr 0.3 shape memory alloy

Microstructure, superelasticity and shape memory effect by stress-induced martensite stabilization in Cu–Al–Mn–Ti shape memory alloys

Shape memory alloys (SMA) have been an extensively used material for actuators in micro-electromechanical systems (MEMS) because actuation force and displacement are greatest in SMA amongst many actuator materials [1]. Of the alloys currently available for SMA actuators, the most popular system is Nitinol (or NiTi) due to its good oxidation resistance, reversible martensitic transformation, broad range of transformation temperatures (from -100 - 100 °C), and specific power density [2]. Current commercially available SMA wire has easily achieved no-load strain of 5% with medium gage SMA wires demonstrating an axial force capacity of 2 Newtons or more. While the potential use of SMA materials in a thermal-electric motor has been documented beginning in the 1980's, there are a number of new allows and fatigue-resistant materials that may lead to more general designs with a wide range of motions and applications. Shape memory alloys are a special type of material that exhibit two unique properties, pseudo-elasticity and shape memory effect (SME). SMA undergoes SME because of martensitic or diffusionless transformation where each atom has a slight displacement, creating observable changes throughout the structure as the allow changes states. This alloy has the ability, once heated, to return to its parent austenite phase where it exists at higher symmetry. Upon cooling, the material returns to one of many lower symmetry martensitic phases. This thermal cycle is shown in Figure 1. [3,4]. It is even possible for many variants of martensite to be present in the same material. Pseudo-elasticity is a rubber-like flexibility that allows the SMA to be contorted for a variety of purposes. Once contorted, the application of heat will cause the alloy to undergo martensitic transformation. Upon completion of the cycle, the alloy will have returned to its original shape. The development of SMA-based electromechanical devices delivers traditional mechanical motion with non-traditional methods. Rather than electromagnetic components rotating about a central axis to produce power, the rotary SMA motor utilizes contracting elements, and mush as spark ignition rotary engine, it can be designed to produce angular motion. Motion is accomplished with sequenced electrical signals sent across each element mounted between an eccentric crank. Rotary motion is produced during the power portion of the cycle for specific SMA elements under the application of an electrical signal. Based on this concept, our team developed a demonstration model with four active elements. We have demonstrated rotary motion of the device for an extended period of time, and we believe that macro-scale models can reduce the concept substantially and perhaps to the MEMS level.

Martensitic transformations in the Fe-(24-30)Ni-(5-8)Si (mass%) alloys have been investigated by means of optical and transmission electron microscopy, differential scanning calorimetry and hardness-testing. The Ms temperature is decreased by Si addition and the morphology of martensite is mainly lenticular in the unaged specimens. However, a pronounced decrease in the Ms temperature and a change in the martensite morphology from the lenticular to the thin plate type are observed on ausaging at 400°C. The increase in austenite hardness, the decrease in the Ms temperature and the increase in the tetragonality of martensite after ausaging at 400°C are clarified as due to the formation of nanoscale particles of γ'-(Ni,Fe) 3 Si with the L12 structure during ausaging at 400°C. The Fe-Ni-Si alloys that form thin plate martensite show the shape memory effect, which arises from the reverse transformation of stress-induced martensite to austenite. Precipitation hardening of the austenite phase by fine y' particles during ausaging improves the degree of shape recovery.