Effect of Small γ Precipitates on the Two-way Shape Memory Effect in a Cu–Zn–Al Alloy

After the treatment for the Stabilization of Stress-Induced Martensite (SSIM) in Cu–Zn–Al alloys, it was found that the small γ precipitates in the β austenite are ellipsoidal with a large strain field oriented in the same direction, while in the martensite, the γ precipitates changed their shape from ellipsoidal to spheroidal, which relaxed the strain field. To check whether the strain field of the γ precipitates is available to produce thermoelastic martensitic transformation, in situ observations with a heating sample holder in TME were performed. It was found that after heating above the A_s temperature, the spherical γ precipitates in the martensite recovered their strain field and elliptical shape. During cooling, the strain field of the γ precipitates disappeared again. This means that the strain fields of the γ precipitates trained by the SSIM method play an important part in the thermoelastic martensitic transformation that presents the two-way shape memory effect.     

Ni–Mn–Ga shape memory alloys development in China

Study on Ni–Mn–Ga ferromagnetic shape memory alloys recently keeps active. Intermartensitic transformation was found. Magnetic field enhanced phase transformation strain was discovered, and achieved up to 4%. Fifteen percent super high strain induced by variant reorientation under stress was obtained in non-modulated martensite. Six percent large magnetic field induced strain was achieved, and the temperature dependence was investigated in 5-layered martensite single-variant Ni–Mn–Ga alloys. Several other systems of magnetic shape memory alloys and high temperature shape memory alloy Ni–Mn–Ga are also reviewed.     

Internal friction behaviour of Ni–Mn–Ga

The internal friction (IF) behaviour of shape memory alloys (SMA) is characterised by an IF peak and a minimum of the elastic modulus during the martensitic transformation (MT), and a higher IF value in the martensitic state than in parent phase. The IF peak is considered to be built of three contributions, the most important of them being the so-called “transient” one, existing only at non-zero temperature rate. On the other hand, the ferromagnetic Ni–Mn–Ga system alloys undergoes a MT from the L2₁ ordered parent phase to martensite, the characteristics of the transformation depending largely on the e/a ratio of the alloys. Indeed, a variety of transformation sequences, including intermediate phases between parent and martensite and intermartensitic transformations, have been observed for a wide set of studied alloys. Furthermore, the IF and modulus behaviour during cooling and heating these alloys show specific characteristics, such as modulus anomalies, strong temperature dependence of the elastic modulus, temperature dependent internal friction in martensite, and, as a general trend, a low transient contribution to the IF. In the present work, the IF and modulus behaviour of several Ni–Mn–Ga alloys will be reviewed and compared to that observed for “classical” systems like Cu- or NiTi-based shape memory alloys.     

Characteristics of Ti–Ni–Pd shape memory alloy thin films

Ti–Ni–Pd thin films were deposited by RF magnetron sputtering. Microstructure and phase transformation behaviors were studied by X-ray diffraction (XRD), by transmission electron microscopy and by differential scanning calorimeter (DSC). Also tensile tests and the internal friction characteristics were examined. Annealing at 750 °C followed by subsequent annealing at 450 °C resulted in relatively homogeneous microstructure and uniform martensite/austenite transformation. The results from DSC showed clearly the martensitic transformation upon heating and cooling, the transformation temperatures are 112 °C (M* peak) and 91 °C (M peak), respectively. The transformation characteristics are also found in strain–temperature curves and internal friction–temperature curves. The film had shape memory effect. The frequency had no effect on the modulus, but the internal friction decreased with increasing frequency.     

The effect of α1 plates on the martensitic transformation and shape memory effect in a β Cu–Zn alloy

The martensitic transformation and shape memory effect (SME) in a β Cu–Zn alloy containing various amounts of α₁ plates have been investigated. The results showed that the characteristic temperature of martensitic transformation decreased as the isothermal aging time increased. The crystal structure, twin relationships between martensite variants, and characteristics of martensitic transformation of the β Cu–Zn alloy were not affected by the existence of α₁ plates. However, the α₁ plates were distributed homogeneously in the parent phase and functioned as grain boundaries, hindering the progress of martensite variants, and reducing the effective grain size of the parent phase and the size of self-accommodating plate groups formed upon cooling. In addition, the strain recovery due to the SME decreased as the isothermal aging time increased (the quantity of α₁ plate increased) and/or imposed prestrain increased. Nevertheless, the SME mechanism in the β Cu–Zn alloy containing α₁ plates was not affected by the presence of the α₁ plates.     

Quantitative microstructural characterisation of Fe–30Ni alloy after martensitic transformations by means of stereological and magnetic methods

In the Fe–30Ni alloy investigated a martensitic transformation can occur both during quenching or plastic deformation. Martensite formed during plastic deformation, depending on the thermo-mechanical treatment applied, exhibits a different morphology from that achieved during quenching and forms the so-called composite-like structure. The morphology and volume fraction of martensite depends both on strain and temperature. In the present studies Fe–30Ni alloy was deformed by monotonic rolling in one path and perpendicular rolling in the temperature range M_D–M_S. The aim of the investigations was a determination of martensite volume fraction depending on the strain and temperature. To examine the influence of strain, the alloy was deformed by rolling in one path or perpendicular rolling at a temperature of − 30 °C, in the strain range of 10–30%. The dependence of temperature was investigated by rolling with 30% strain in a temperature range from − 30 °C to − 80 °C. The variants of thermo-mechanical treatment performed enabled us to achieve different martensite morphologies and volume fractions. Microstructural analysis was performed by means of light microscopy and transmission electron microscopy. The results of quantitative microstructural analysis of martensite and retained austenite volume fractions formed in different thermo-mechanical treatments were compared with those obtained by magnetic measurements. The fraction of deformation-induced martensite determined varied from 2% to 86%. The partial volume fractions V_V^MF of martensite formed in different deformation directions were also determined. It was found that the influence of the temperature on the martensite volume fraction is more pronounced than the influence of strain.     

Effects of Sn content on the microstructure, phase constitution and shape memory effect of Ti–Nb–Sn alloys

The effect of Sn content on the microstructure, phase constitution and shape memory effect of Ti–16Nb–xSn (x = 4.0, 4.5, 5.0 at%) alloys were investigated by means of optical microscopy, X-ray diffraction, transmission electron microscopy and bending test. With the increase of Sn content, the β phase becomes stable. The solution-treated Ti–16Nb–4Sn alloy is composed of ″ and β phases at room temperature, whereas the solution-treated Ti–16Nb–5Sn alloy is only composed of β phase at room temperature. TEM observation shows that there is parallel lamellar ″ martensite with the substructure of () type I twin in the Ti–16Nb–4Sn alloy. There exists the dislocation wall inside the single β phase in the Ti–16Nb–5Sn alloy. The shape recovery ratio decreases with increasing the bending strain and the bending temperature, which is in correspondence with the different deformation mechanisms at different temperature ranges. The shape recovery ratio shows a decreasing trend with the increase of Sn content at the same bending strain and temperature. The maximum completely recovery strain is around 4%.     

Internal friction associated with phase transformation of nanograined bulk Fe–25 at.% Ni alloy

The internal friction and modulus of a nanograined bulk Fe–25 at.% Ni prepared by an inert gas condensation and in situ warm consolidation technique were measured in temperature range −100 to 400 °C by means of a dynamic mechanical analyzer (DMA). An internal friction peak at around −75 °C associated with martensitic transformation was observed. During heating, an internal friction peak at about 200 °C accompanied with the decrease of modulus was also observed, which was proved by XRD that this may mainly be attributed to the reverse phase transformation of stress-induced martensite (SIM). Some abnormal features of modulus versus temperature were observed and discussed.     

Martensitic transformation and shape memory effect in Fe–Mn–Si based alloys

The research status of the Fe–Mn–Si based alloys is reviewed with emphasis on the recent progress in the martensitic transformation and the associated shape memory effect (SME). Particular interest is given to the fcc(γ)–hcp(ε) transformation mechanism in the alloys featured by low stacking fault energy and the approaches aiming to the enhancement of SME through alloy design including microalloying and microstructure control by introducing texture and precipitates into the parent γ matrix. Potential topics of oncoming focus are briefly highlighted.     

Internal friction during martensitic transformation in high manganese Mn–Cu alloys

Low-frequency internal friction and elastic modulus were studied for manganese-rich Mn–Cu alloys in the temperature range of martensitic transformation (20–300 °C). It is shown that the some special features of the transformation peak and its temperature are caused by the degree of the spinodal decomposition. The phenomenological model connecting an-elastic effects with the stages of evolution of the structure during martensitic transformation in manganese-rich Mn–Cu alloys (tweed structure–“parquet” structure–classical twinning martensite) is presented.     

Study of transformation behavior in a Ti–4.4 Ta–1.9 Nb alloy

An alloy of composition Ti–4.4 wt.% Ta–1.9 wt.% Nb is being developed as a structural material for corrosion applications, as titanium and its alloys possess excellent corrosion resistance in many oxidizing media. The primary physical metallurgy database for the Ti–4.4 wt.% Ta–1.9 wt.% Nb alloy is being presented for the first time. Determination of the β transus, M_s temperature and classification of the alloy have been carried out, employing a variety of microscopy techniques, X-ray diffraction (XRD), micro-hardness and differential scanning calorimetry (DSC). The β transition temperature or β transus determined using different experimental techniques was found to agree very well with evaluations based on empirical calculations. Based on chemistry and observed room temperature microstructure, the alloy has been classified as an + β titanium alloy. The high temperature β transforms to either ′ or + β by a martensitic or Widmanstatten transformation. The mechanisms of transformation of β under different conditions and characteristics of different types of have been studied and discussed in this paper.     

TiNi合金在恒应力约束下的不完全相变

利用DSC对预应变TiNi形状记忆合金丝在恒应力约束下的马氏体不完全逆相变进行了研究,发现不完全相变热循环样品在第二次自由态加热过程中出现两步马氏体逆转变和两段应变回复现象.分析认为:经过恒应力约束下的不完全逆相变后,TiNi样品中存在不同的马氏体,在随后的加热过程中先后逆转变,产生两段回复应变.     

The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys undergoing stress-induced martensitic transformation

The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys in β solution-treated condition was investigated. The fracture toughness of the alloys was found to increase with an increase in grain size initially, reach a maximum and subsequently decrease with further increase in grain size. This trend was attributed primarily to the effect of grain size on the enhancement of fracture toughness due to stress-induced martensitic transformation (SIMT) at the crack tip, which in turn can be related to the effect of grain size on trigger stress for SIMT. Alloys containing higher Al and Nb showed a higher toughness for the same grain size, which was also explained in terms of effect of composition on the trigger stress.     

CuZnAl合金在应力下的马氏体相变

采用附加有加载装置及电视系统的光学金相显微镜,对CuZnAl形状记忆合金的应力诱发马氏体进行了动态观察,发现该合金的应力诱发马氏体具有热弹性马氏体的相变特征,马氏体的形貌及转变量随应力的变化而变化;此外,还发现该合金有些晶粒的马氏体在应力诱发过程中消隐,代之出现取向、大小不同的新的马氏体。     

Crystal plasticity analysis of stress–assisted martensitic transformation in Ti–10V–2Fe–3Al (wt.%)

A new model based on crystal–plasticity, crystallography, thermodynamics, kinetics and statistics is developed for stress–assisted martensitic transformation. The model includes the essential features of the stress–assisted martensitic transformation, such as: nuclei of progressively lower potency are activated in the course of transformation, the martensite phase appears in the form of thin plates, the parent phase exerts a higher resistance toward the growth of a plate in the thickness than in the radial direction, the average plate size decreases while the average plate aspect ratio increases with the extent of transformation, etc. The model is implemented in the commercial finite element code ABAQUS/Standard to analyze the evolution of martensite, materials texture and the resulting equivalent stress–equivalent strain curve during the stress–assisted martensitic transformation under different stress and strain states in a polycrystalline Ti–10V–2Fe–3Al (wt.%) alloy. The equivalent stress–equivalent strain curves and the volume fraction of martensite–equivalent strain curves are found to be mainly controlled by the applied stress state. Conversely, the texture observed in the transformed Ti–10V–2Fe–3Al is found to be primarily controlled by the imposed macroscopic strain state. The validity of the proposed materials constitutive model has been established by demonstrating a reasonable agreement between the model predictions and the available experimental data.     

Peculiarities of strain and resistivity variations in TiNi

Simultaneous measurements of electrical resistivity and inelastic strain accumulation in Ti–49.5 at.% Ni undergoing a two-step B2→R→B19^′ martensitic transformation were performed. To exclude the influence of specimen size changes during the deformation all experiments were made in torsion. It was detected that noticeable part of strain recovery after heating under a stress and active deformation in martensitic state takes place before the start of the reverse transformation and is obviously connected with twinning. At the same time the two-way shape memory effects are caused by the martensitic transformation. The obtained data may be useful for adequate modelling of mechanical properties of the shape memory alloys because allow to connect the macroscopic parameters with microscopic physical processes.     

Influence of subgrain orientation on martensitic transformation in β1–Cu–Zn–Al-single crystals

In β₁–Cu–Zn–Al single crystals the course of cyclic martensititic transformation ‘β₁ parent phase↔γ′₁ martensite’ induced by tensile stress were studied with use of X-ray topography, light microscopy and etch pits. Two groups of single crystals were studied. The first one (OR) contained single crystals of subgrain boundaries parallel to the direction of elongation [001], the second one (RA) consisted of single crystals of random subgrain boundaries orientations. Single crystals from the RA group cracked after about 300 cycles of martensitic transformation; single crystals from the OR group did not crack even after 1200 cycles. In OR single crystals changes of dislocation density inside the subgrains caused by cycling occurred much more slowly than in the RA single crystals. This has been related to the dislocation movement from inside the subgrains to their boundaries.     

Martensitic Transformation and Magnetic-Field-Induced Strain in Magnetic Shape Memory Alloy NiMnGa Melt-Spun Ribbon

A magnetic shape memory alloy with nonstoichiometric Ni50Mn27Ga23 was prepared by using melt-spinning technology. The martensitic transformation and the magnetic-field-induced strain (MFIS) of the polycrystalline melt-spun ribbon were investigated. The experimental results showed that the melt-spun ribbons underwent thermal-elastic martensitic transformation and reverse transformation in cooling and heating process and exhibited typical thermo-elastic shape memory effect. However the start temperature for martensitic transformation decreased from 286 K for as-cast alloy to 254 K for as-quenched ribbon and Curie temperature remains approximately constant. A particular internal stress induced by melt-spinning resulted in the formation of a texture structure in the ribbons, which made the ribbons obtain larger martensitic transformation strain and MFIS. The internal stress was released substantially after annealing, which resulted in a decrease of MFIS of the ribbons.     

Molecular dynamics study of surface effect on martensitic cubic-to-tetragonal transformation in Ni–Al alloy

Molecular dynamics (MD) study of martensitic transformation (MT) in nickel and aluminum alloy is performed. The behavior focused on is transformation between crystalline structures from B2 cubic cell to body-centered tetragonal cell, which is simply realized by uniaxial tensile loading. The potential function used is Finnis–Sinclair type having only single energy minimum where B2 structure exists. The availability of this specific many-body potential for stress-induced MT phenomena under uniaxial loading is fully discussed. In MD simulations, martensite phase is induced by tensile stress or strain in the atomic system, as predicted by a potential energy map. It is understood that the characteristic of the potential energy function with regard to deformation is crucial for MT studies and investigating energy-strain or stress–strain map is worthwhile. The MT behavior in the atomic system occurs during a plateau region of stress–strain (S–S) curve of the whole specimen, that is typical for experimental superelastic or shape-memory alloys under uniaxial loading. It is found that, during each MT event, large jump of atomic strain is observed. Owing to single energy minimum, the atomic system shows almost perfect recovery in S–S curve, where the graph comes completely back to initial state after unloaded. Besides, the present paper focuses on surface effect for MT behavior. Since the surface effect is dominant in MT phenomena especially in microscopic specimens, a novel computational scheme for stabilizing condition in which uniaxial loading is always applied together with arbitrary periodic boundary condition(s) is devised. By comparing one-, two-, and three-dimensional models under uniaxial loading, it is recognized that the nucleation behavior depends strongly on the existence of free surface region (including corner edge). When there is no surface, a chaotic nucleation of martensite is observed. On the other hand, the free surface induces first martensite because of less constraint in tensile deformation of unit cells. It is confirmed that the tendency toward MT nucleation corresponds to yield stress or strain of the specimen. In order to define and detect martensite structure as for each atom, an atomic strain measure (ASM) with our own formation is introduced. It is shown that the ASM is very effective to distinguish martensite bct unit structure from others.     

TiNi形状记忆合金在定应变约束下的马氏体相变

通过DSC及回复应变的测量,研究了定应变约束态加热-冷却过程对预应变TiNi形状记忆合金相变行为的影响。结果表明,在约束态相变中,逆相变温度区间拓宽;取向马氏体除向母相转变外,应变还要进一步增大;在正相变过程中,从母相生成的马氏体也具有变形结构。约束态不完全相变后,样品中存在两种马氏体;再变形马氏体和继承变形马氏体,在随后的无约束逆相变过程中,前者的相变温度高于后者,并且输出两段回复应变。