Stability in Phase Transformation After Multiple Steps of Marforming in Ti-Rich Ni-Ti Shape Memory Alloy

Nickel-titanium (Ni-Ti) alloys are the most attractive among shape memory alloys (SMA) due to their good functionality properties coupled with high strength and ductility. The transformation temperatures in Ti-rich Ni-Ti SMA can be altered by subjecting them to suitable thermal and/or mechanical treatments to obtain martensitic transformation in one or more steps above 0 °C. The goal of the present work is to investigate the stability of phase transformation characteristics, such as, type of sequence (one, two, and multiple steps) and transformation temperatures in Ti-Rich Ni-Ti SMA (Ni-51 at.%Ti), after being subjected to an initial heat treatment at 500 °C for 30 min in air followed by multiple steps of marforming (cold rolling, 30% thickness reduction) intercalated with heat treatments at 500 °C for 30 min in air and a final heat treatment at four different temperatures (400, 450, 500, and 600 °C) for 30 min in air atmosphere. Differential scanning calorimetry (DSC) and electrical resistivity (ER) were used to identify the phase transformation sequences and the stability of transformation temperatures during initial 10 thermal cycles for each sample with distinct thermo-mechanical treatment.     

Thermoelastic transition kinetics of a gamma irradiated CuZnAl shape memory alloy

Many types of shape memory alloys have been used in nuclear reactors and aerospace applications where they are exposed to high levels of various kinds of radiation. The effect of gamma irradiation on the transformation kinetics of thermoelastic transformations in a shape memory CuZnAl alloy was investigated by differential scanning calorimetry (DSC) with heating/cooling rates of 10, 20, 30 and 40 °C/min. Irradiation doses of 10, 20, 30 and 40 kGy were applied to the samples of the alloy. Changes in Gibbs free energies, entropies and elastic energies were calculated. Reverse transition temperatures A _s and A _f systematically decreased with increasing doses, although forward transition temperatures M _s and M _f underwent a minimum value at a dose of 20 kGy. Hysteresis in the transition temperatures changed as an inverse parabolic function of the irradiation dose. The activation energies of transformations were calculated by using Kissinger and Osawa methods.     

Thermal Memory Degradation in a Cu-Zn-Al Shape Memory Alloy During Thermal Cycling with Free Air Cooling

A fragment of a Cu-15Zn-6Al (mass%) shape memory alloy (SMA), in hot rolled-water quenched condition, was subjected to thermal cycling performed by means of a differential scanning calorimetry (DSC) device. Each cycle comprised controlled heating, isothermal maintaining, and free air-cooling, repeated three times, up to maximum temperatures increased by every 10 K, between 450 and 490 K, aiming to reproduce actual functioning conditions of a SM electrical actuator with uncontrolled cooling. Both the endothermic peaks, associated with thermally induced reversion of martensite to austenite, and their derivatives were analyzed in terms of critical transformation temperatures and specific enthalpy absorptions and in terms of transformation rates, respectively, with the aim of revealing their variational tendencies with increasing heating temperature to the maximum. The progressive decay of reverse martensitic transformation was associated with the reciprocal blocking of differentially oriented populations of stabilized lath-martensite needles, observed by scanning electron microscopy.     

The Mechanical and Thermal Behaviors of Heat-Treated Ni-Rich NiTi Orthodontic Archwires

The near equiatomic nickel-titanium alloy is an outstanding intermetallic compound exhibiting distinctive properties associated with characteristic thermal and stress-induced martensitic transformations. The process of producing orthodontic wires has been modified to obtain the optimal shape memory behaviors. Phase transformation temperatures and load-deflection characteristics of this binary alloy are very significant variables in the performance of this alloy and can be manipulated by different thermomechanical treatments via inducing precipitation or dislocation networks in the matrix. In this study, one brand of commercial heat-activated nickel-titanium archwire (3 M Unitek) was selected and solution treated. Then, the wires annealed at 400 °C for 10, 30, and 60 min. Thermal transformation temperatures were determined using differential scanning calorimeter. It was showed that these temperatures increased with increasing the time of heat treatment and multistage transformation occurred as the result of inhomogeneities. In order to evaluate mechanical parameters of heat-treated archwires, they were placed on an arch-form fixture simulating maxillary dentition and load-deflection curves were obtained by three-point bending test at 37 °C. The results compared to as-received archwires and the best superelasticity was observed after 30 min aging.     

Influence of carbon on martensitic phase transformations in NiTi shape memory alloys

In the present paper we show how carbon affects martensitic transformations in Ni-rich NiTi shape memory alloys. During vacuum induction melting in graphite crucibles, NiTi melts dissolve carbon and TiC particles form during solidification. Differential scanning calorimetry (DSC) shows that this is associated with a decrease in the phase transition temperatures. We provide new experimental evidence for increasing temperature intervals between the start and the end of the martensitic transformations (from B2 to B19′) with increasing C content in as-cast and solution-annealed (850 °C) microstructures. The nucleation and growth of TiC particles in intercellular/interdendritic regions causes variations in the local Ni/Ti ratios. This results in wider transformation temperature intervals (DSC peak broadening) in as-cast and solution-annealed microstructures. Subsequent intense heat treatments (1000 °C) homogenize the alloy and re-establish sharp DSC peaks during martensitic transformations.     

Effect of the High-Temperature Deformation on the M s Temperature in a Low C Martensitic Stainless Steel

The effects of cooling rate and plastic deformation at 1100 °C on the start of the austenite-to-martensite transformation in a low carbon Fe-12Cr stainless steel were investigated by dilatometry and thermal analysis of pre-strained samples. The experimental M _s was found constant (397.2 °C) for cooling rates between 15 and 30 °C/min. Outside this range, the M _s apparently decreases as the cooling rate decreases below 15 °C/min or increases above 30 °C/min. An overall average value of 380.9 ± 11.8 °C was observed within the complete range of cooling rates investigated (8-40 °C/min). This value is 50 °C higher than that predicted from the chemical composition of the steel and Andrews’ equation for M _s. Prior plastic deformation of austenite at 1100 °C increases the M _s temperature and broadens the martensitic domain (M _f-M _s). This effect increases as the magnitude of the applied pre-strain increases.     

Study of the Evolution of the Structure and Kinetics of Martensitic Transformations in a Titanium Nickelide upon Isothermal Annealing after Hot Helical Rolling

Differential scanning calorimetry has been used to study the effect of the temperature and holding time upon isothermal annealing on the calorimetric effects of the martensitic transformations (MTs) of an aging Ti–50.7 at % Ni alloy produced by hot helical rolling at a temperature of 850–900°С with subsequent cooling in air. The sequence of the phase transitions upon reverse MT has been determined using X-ray diffractometry. The regularities of the evolution of the characteristic temperatures of martensitic transformations after annealing at temperatures of 400, 430, and 450°С for 1–10 h have been analyzed. The results of electron-microscopic studies of the structure of the alloy in the initial state and after annealing have been presented; the features of the morphology of the Ti₃Ni₄ phase precipitated upon aging have been analyzed. Based on the results of calorimetric and structural studies, the temperature of the highest intensity of the aging processes has been determined.     

高温合金差示扫描量热分析(DSC)的影响因素研究III：合金状态和升降温速率

摘 要：对固溶强化型镍基高温合金625进行升、降温差示扫描量热分析（DSC）试验,研究了同一合金不同状态（粉末态、粉末+热等静压态和铸态）以及升/降温速率（5-10℃/min）对相变温度的影响。采用场发射扫描电镜（FESEM）、电子探针（EPMA）对不同状态625合金的微观组织和元素分布进行表征。结果表明：（1）铸态比粉末态合金的枝晶间距大2个数量级,而热等静压态合金为无枝晶偏析的细等轴晶结构。（2）升、降温速率对DSC曲线中加热时基体开始熔化（等于固溶强化型合金的初熔温度）和冷却时开始凝固温度（偏离基线的拐点）无影响,但对合金加热熔化结束、冷却时大量凝固析出温度（峰位）和终凝温度（拐点）有明显影响。采用加热、冷却曲线相应相变温度平均值的方法可减少DSC试验和样品条件的影响,获得相对固定且更具可比性的合金相变温度。（3）合金状态对初熔温度和DSC升温曲线固相线温度附近的圆弧段有明显影响。根据DSC加热曲线固相线温度附近的圆弧大小可以判断合金的偏析倾向,弱偏析倾向的粉末态和热等静压态PM625合金DSC加热曲线固相线附近区域拐点尖锐,表现为合金开始熔化温度（偏离基线的拐点）与名义固相线温度（切线交点）差异很小,分别仅为5℃和6℃;偏析倾向较大的铸态IN625合金的DSC加热曲线中固相线温度附近区域为较大圆弧,开始熔化温度与名义固相线温度差异可达52℃。铸态625合金的初熔温度比热等静压态和粉末态分别低45℃和40℃,在实际热处理和热等静压等热工艺参数选择时应注意圆弧段较大的合金降低初熔温度的影响。在所有DSC冷却曲线中,由于完全熔化重新凝固消除了合金原始显微组织特征,不同状态625合金固相线温度区域附近曲线形态相似,均为较大的圆弧。     

Effect of thermocycling on the temperatures of phase transformations,structure, and properties of the equiatomic alloy Ti<Subscript>50.0</Subscript>Ni<Subscript>50.0</Subscript>

This article is devoted to studying the influence of thermocycling in the range of temperatures of the thermoelastic martensitic transformation B2–B19' on the microstructure, the temperatures of the martensitic transformations, and the mechanical properties of the equiatomic alloy Ti₅₀Ni₅₀ in the coarse-grained (CG) and ultrafine-grained (UFG) states, the latter obtained by equal-channel angular pressing (ECAP). One hundred cycles of thermocycling and the related increase in the dislocation density in the CG alloy led to a decrease in the temperatures of martensitic transformations. In the UFG alloy, the temperatures of the forward transformation (M_s, M_f) decrease by 2–3 K, and the temperatures of the reverse transformation (A_s, A_f) increase by 6 K. The ultimate strength remains almost unaltered upon the thermocycling, but the yield stress increases substantially from 430 to 550 MPa and from 935 to 1120 MPa for the CG and UFG states, respectively.     

热循环对 Cu Zn Al Mn Ni记忆合金中双可逆相变的影响

通过差示扫描量热技术 ,研究了热循环对 Cu- 2 3.6 Zn- 4 .4 7Al- 0 .2 3Mn- 0 .17Ni形状记忆合金中双可逆相变的影响。结果表明 ,X→ M转变温度随热循环的进行而降低 ,而马氏体逆转变的温度几乎不发生变化。     

Microstructure and martensitic transformation behavior of Ni56-xMn25FexGa19 shape memory alloys简

Microstructure, martensitic transformation behavior, mechanical and shape memory properties of Ni56-x Mn25 Fex Ga19(x = 0, 2, 4, 6, 8) shape memory alloys were investigated using optical microscopy(OM), X-ray diffraction analysis(XRD), differential scanning calorimeter(DSC), and compressive test. It is found that these alloys are composed of single non-modulated martensite phase with tetragonal structure at room temperature, which means substituting Fe for Ni in Ni56 Mn25 Ga19 alloy has no effect on phase structure. These alloys all exhibit a thermoelastic martensitic transformation between the cubic parent phase and the tetragonal martensite phase. With the increase of Fe content, the martensitic transformation peak temperature(Mp) decreases from 356 °C for x = 0 to 20 °C for x = 8, which is contributed to the depressed electron concentration and tetragonality of martensite. Fe addition remarkably reduces the transformation hysteresis of Ni–Mn–Ga alloys. Substituting Fe for Ni in Ni56 Mn25 Ga19 alloy can decrease the strength of the alloys and almost has no influence on the ductility and shape memory property.     

Ti-Ni-Nb宽滞后形状记忆合金的应力诱发马氏体相变行为

用在（Ms＋30℃）温度下的拉伸实验和差示扫描量热仪（DSC）较系统地研究了Ti44Ni47Nb9宽滞后形状记忆合金应力诱发马氏体的相变行为。研究结果表明：当形变量达到14％左右时，应力诱发马氏体相变过程基本完成。应力诱发马氏体的逆相变温度间隔要比热诱发马氏体约小一个数量级。形变对该合金应力诱发马氏体的逆转变开始温度、逆转变温度间隔以及相变潜热均有明显影响，随着拉伸变形量的增加而增加。而在随后的冷却循环中，相变潜热和马氏体相变开始温度均随着形变的增加缓慢降低。     

加热速率对CuAlBe形状记忆合金相变的影响

通过差示扫描量热技术研究了不同的加热速率对CuAlBe形状记忆合金的相变特征的影响。结果表明：在不同加热速率下的相变点在一个区域内变化（Ms=120-128℃,Mf=99-105℃,As=142-130℃,Af=151-141℃）。快速加热使两个相变峰靠近,随着加热速率的降低,两个相变峰逐渐分离。用超低降温速率法分离后的DSC曲线,右瑞的大峰变为了一连串由许多微区依次发生马氏体逆相变所产生的许多小的放热峰。     

Preparation of multi-walled carbon nanotube-reinforced TiNi matrix composites from elemental powders by spark plasma sintering

Carbon nanotube (CNT)-reinforced TiNi matrix composites were synthesized by spark plasma sintering (SPS) employing elemental powders.The phase structure,morphology and transformation behaviors were studied.It was found that thermoelastic martensitic transformation behaviors could be observed from the samples sintered above 800 ℃ even with a short sintering time (5 min),and the transformation temperatures gradually increased with increasing sintering temperature because of more Ti-rich TiNi phase formation.Although decreasing the sintering temperature and time to 700 ℃ and 5 min could not protect defective MWCNTs from reacting with Ti,still-perfect MWCNTs remained in the specimens sintered at 900 ℃.This method is expected to supply a basis for preparing CNT-reinforced TiNi composites.     

PHASE TRANSFORMATION BEHAVIOR IN THE SPUTTERDEPOSITED Ti-Pd-Ni THIN FILMS

1. IntroductionIn comparing with the conventional shape memory thin films, Ti-Pd-Ni films exhibithigher phase transformation temperatures, respond quickly to the efficient thermal ex-change, and thus have been received much attention for fabricating microactuators.It is important to study the phase transformation behavior, because the shape memoryeffect is generally realized through the transformation process. In Ti5oPd.Ni5o--. allOys,phase transformation has been studied by many authors[1-…     

Structure and Thermoelastic Martensitic Transformations in Ternary Ni–Ti–Zr Alloys with High-Temperature Shape Memory Effects

X-ray diffraction analysis and transmission electron and scanning electron microscopies were used to study the effect of alloying with zirconium (0–20 at %) on the phase composition and structural and morphological features of thermoelastic martensitic transformations in ternary Ni–Ti–Zr alloys. The electrical resistivity of alloys has been measured in a wide range of temperatures, the critical temperatures have been determined, and a complete diagram of the high-temperature thermoelastic forward and reverse martensitic transformations B2 ? B19' occurring with a hysteresis in the range of 32–50 K has been constructed based on XRD data. The intercritical range of the temperatures of transformations increases as the zirconium content increases within the indicated limits. The lattice parameters of the monoclinic crystal lattice of the B19' martensite have been measured at room temperature. The twinning types most frequently observed in B19' martensite have been found as follows: I-(011), \(\left( {11\;\bar 1} \right)\) and (001), II-(011).     

Study of martensitic transformation in the Fe-22 Mn-3 Si alloy by mechanical spectroscopy

Methods of mechanical spectroscopy (determination of the internal-energy dissipation and the elasticity modulus), X-Ray diffraction, and calorimetry have been used to study the specific features of the reversible martensitic transformation in the Fe—22Mn—3Si alloy (wt %). A detailed study has been given to the specific features of the internal-energy dissipatio n during the direct and reverse martensitic transformations in a wide range of heating rates, frequencies, and deformation amplitudes. Particular emphasis has been placed on multiple thermal cycling of the sample through the martensitic-transformation range and its effect on the martensitic-transformation parameters, such as the transformation start and finish temperatures, the value of the thermal hysteresis, and the transformation kinetics.     

Fabrication of shape memory alloys using the plasma skull push–pull process

In this work, the fabrication of shape memory alloys (SMAs) through the use of the plasma skull push–pull (PSPP) process was studied. In this process, small quantities of metallic elements can be quickly melted on a copper crucible using a rotating plasma torch and injected into a metallic mold. In order to validate the utilization of the PSPP process to produce SMA, several Ni–Ti-based and Cu–Al-based binary, ternary and quaternary alloys were tested. It was verified that five melts and re-melts of the SMA before injection are enough to obtain a homogeneous product. The obtained SMAs were characterized by optical microscopy and microhardness measurements. The thermoelastic martensitic transformation at the origin of the shape memory effect (SME) was studied via electrical resistance as a function of temperature and differential scanning calorimetry (DSC). All studied SMAs were shown to undergo a thermoelastic transformation, thus demonstrating the viability of the PSPP process to fabricate a great variety of SMAs.     

Time effect of martensitic transformation in Ni43Co7Mn41Sn9

Effect of thermal arrest on the L2₁-tetragonal martensitic transformation in a NiCoMnSn shape memory alloy was investigated. The phenomenon was studied by interrupted heating/cooling in differential scanning calorimetry analysis. The experimental evidence indicates that the forward martensitic transformation continued to completion during cooling arrest between M_s and M_f. The same behavior was also observed for the reverse transformation on heating. These observations demonstrate that the L2₁-tetragonal martensitic transformation in the Ni₄₃Co₇Mn₄₁Sn₉ alloy is time dependent at the finite cooling rate.     

PHASE TRANSFORMATIONS IN RAPIDLY SOLIDIFIED TiNi SHAPE MEMORY ALLOYS

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