Temperature evolution in shape memory alloy during loading in various conditions

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.

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.

Architected material analogs for shape memory alloys

Stress-induced martensitic transformations and shape memory at nanometer scales

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

The basic characteristics of the martensitic transformation of TiNi shape memory alloys are described. They include the crystal structures of the parent and martensite phases, the recoverable strain associated with the martensitic transformation, the transformation temperatures, the temperature and orientation dependence of deformation behaviour, etc. Shape memory and superelasticity related to the martensitic transformation are also explained. Introduction The shape memory effect (SME) and superelasticity (SE) are associated with the crystallographically reversible nature of the martensitic transformation which appears in shape memory alloys (SMAs). Such a crystallographically reversible martensitic transformation has been named “thermoelastic martensitic transformation”. The name originates from the characteristic of the martensitic transformation in shape memory alloys, i.e., the total free energy change associated with the thermoelastic martensitic transformation mainly consists of two thermoelastic terms, chemical free energy and elastic energy, while the total free energy change associated with the conventional martensitic transformation, which appears in steels for instance, consists of the energy of interfaces and plastic deformation in addition to the two thermoelastic terms. Therefore, the interface between transformed and untransformed regions moves smoothly according to the temperature variation so that the transformation temperature hysteresis is small, from several to several tens of degrees K, compared with those of steels that are several hundreds of degrees K. The characteristic that plastic deformation does not occur in the thermoelastic martensitic transformation is one of the necessary factors for the perfect shape recovery upon the reverse transformation in shape memory alloys. The martensitic transformation itself is not a new phenomenon.

Molecular dynamics simulation of phase transformation and wear behavior of Ni35Al30Co35 high temperature shape memory alloy

It is illustrated, that our method [1-3] (developed for the separation of pressure dependence of the elastic and dissipative energy terms) can also be applied for the determination of stress-dependence of the non-chemical (elastic and dissipative) free energy contributions to the martensitic phase transformations. For this the results of Tanaka et al. [4], on the effect of constant applied tensile stress on martensitic phase transformations, in Ti-51at%Ni shape memory alloys (up to 400 MPa) have been used. Supposing that the volume derivatives of the elastic energies differ from zero at the martensite start (and austenite finish) as well as at the martensite finish (and austenite start) temperatures, it was shown that the dissipative energy terms were approximately zero for the B2-R transformation. For the R-B19' transition these energies are not negligible and are of the order of 1 x 10 7 J/m 3 . The stress dependence of the elastic energy terms of the B2-R and R-B19' transitions have also been determined from the original data. The results are consistent with the effect of hydrostatic pressure on the martensitic transformation measured in a similar alloy [21.

Shape memory alloys (SMAs) are those that can return to their initial shape after deformation under a stimulus, such as temperature or stress. They are capable of recovering deformations of up to 8%. Generally, the martensitic transformation is reversible in nature and the shape memory alloys exhibit two unique characteristics, super-elasticity effect (SE) and shape memory effect (SME), depending on whether these properties/responses are brought on by stress and temperature, respectively. Since the shape memory alloys undergo full cycling, they transform from austenite to martensite at temperatures between martensite finish and austenite finish. However, partial cycling refers to heating above the austenite start temperature but below the austenite finish temperature followed by cooling to below the martensite finish temperature. The phase transformation is partial before it is complete, consequently, only smaller amounts of the phases undergo a phase transition. Based on the operating temperature window and the transformation temperatures of the alloy, partial cycling can be divided into three categories. This chapter discusses the various types of cycling, i.e., thermomechanical, thermal, and partial cycling behavior of nickel-titanium-based shape memory alloys.

Shape memory alloys (SMAs) are well known for their unique shape memory effect (SME) and superelasticity (SE) behavior. The SME and SE have been extensively investigated in past decades due to their potential use in many applications, especially for smart materials. The unique effects of the SME and SE originate from martensitic transformation and its reverse transformation. Apart from the SME and SE, SMAs also exhibit a unique property of memorizing the point of interruption of martensite to parent phase transformation. If a reverse transformation of a SMA is arrested at a temperature between reverse transformation start temperature (A s) and reverse transformation finish temperature (A f), a kinetic stop will appear in the next complete transformation cycle. The kinetic stop temperature is a ‘memory’ of the previous arrested temperature. This unique phenomenon in SMAs is called temperature memory effect (TME). The TME can be wiped out by heating the SMAs to a temperature higher than A f. The TME is a specific characteristic of the SMAs, which can be observed in TiNi-based and Cu-based alloys. TME can also occur in the R-phase transformation. However, the TME in the R-phase transformation is much weaker than that in the martensite to parent transformation. The decrease of elastic energy after incomplete cycle on heating procedure and the motion of domain walls have significant contributions to the TME. In this paper, the TME in the TiNi-based and Cu-based alloys including wires, slabs and films is characterized by electronic-resistance, elongation and DSC methods. The mechanism of the TME is discussed.

NiTinol Shape Memory Alloys (SMA) are becoming one of the ideal choices for biomedical industries due to their unique properties such as Shape Memory Effect (SME), Super Elasticity (SE) and Biocompatibility. In the process of making complicated biomedical implants, welding processes play a vital role. In this work, an attempt was made to study the effect of heat input and Post Weld Heat Treatment (PWHT) on the TIG-welded NiTinol SMA. TIG welding was carried out on 1-mm thick NiTinol sheets. With increase in heat input, there was a significant variation in Phase Transformation Temperature (PTT) of welded samples. The variation in PTT is attributed to the formation of intermetallic phases such as Ti2Ni, Ni3Ti and NiTiO3 and coarse grain formation. Electron Back Scattered Diffraction (EBSD) analysis on the weld revealed that the average grain size of parent material was increased from 9.92851[Formula: see text][Formula: see text]m to 48.292345[Formula: see text][Formula: see text]m after the welding process. The PWHT was carried out on the best weld characteristic sample. PWHT did not produce significant effect on PTT. Austenite start and finish temperature slightly decreased after PWHT, whereas slight drift towards the positive side was noticed in martensite start and finish temperature.

On the multiplication of dislocations during martensitic transformations in NiTi shape memory alloys

Hybrid Polymeric Composites (HPC) structural materials pose a challenge of developing microcracks and delaminations under impact and dynamic loads. This paper presents the development and testing of an Intelligent Hybrid Polymeric Composite (IHPC) beam embedded with Ni-Ti Shape Memory Alloy (SMA) with crack growth retarding ability. Upon heating to austenite finish temperature (Af), Ni-Ti SMA wire contracts as a result of detwinned martensite-austenite phase transformation. The contraction of the SMA was utilized to stiffen and retard crack growth in the IHPC beam, hence resulting to an increase of mode I fracture stress intensity factor (KIC). The SMA wire specimens were aged at 250°C, then prestrained at 3% in order to stabilize austenite start (As) and austenite finish (Af)transformation temperatures. The values of As and Af for Ni-Ti SMA were determined. The IHPC and Polymeric Virgin (PV) notched beams were fabricated from epoxy resin. A four point bending test was performed on the beams to determine the effect of actuated Ni-Ti SMA on mode I fracture stress intensity factor KIC. The test was done at two temperatures, at T1 (below As) and at T2 (Af). Results showed that actuation of the Ni-Ti SMA increased the value of KIC for IHPC beams at T2 by 189% over the value of KIC for PV beams at T1. Actuation of the Ni-Ti SMA increased the value of KIC for IHPC beams at T2 by 41% over the value of KIC for IHPC beams at T1. Results showed that at T1 the loaded PV and IHPC beams fractured with unsteady crack propagation, while at T2 the loaded IHPC beams fractured with steady crack propagation. An increased value of KIC and steady crack propagation at T2 indicated that the SMA improved the crack retarding ability of the HPC beam.   Key words: Intelligent hybrid polymeric composite, shape memory alloy, stress intensity factor, crack growth, crack retardation.

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

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