Shape memory behavior and tension-compression asymmetry of a FeNiCoAlTa single-crystalline shape memory alloy

A 〈1 0 0〉 textured polycrystalline FeNiCoAlTa shape memory alloy was recently shown to possess large superelastic strain and stress levels. In this study, the shape memory behavior of a Fe-28Ni-17Co-11.5Al-2.5Ta (at.%) single-crystalline material oriented along the 〈1 0 0〉 direction was studied, for the first time, by thermal cycling under constant stress levels in both tension and compression. When γ′ precipitates with an average size of 5 nm are introduced by an aging heat treatment, the single crystals show fully recoverable transformation strains up to 3.75% in tension and 2% in compression. The change in transformation temperatures for a unit change in applied stress level was higher in compression than in tension, in accord with the lower transformation strains in compression obtained both from theoretical calculations and experimental observations. However, in all specimens, the observed transformation strain levels were lower than theoretically predicted, possibly owing to significant volume fraction of non-transforming precipitates, incomplete martensite reorientation due to martensite variant interactions, and a slightly higher-than-expected martensite c/a ratio in the samples used in this study. The ramifications of relevant structural parameters and microstructural features on reaching theoretical transformation strain and high strength levels are also discussed.     

TiNi形状记忆合金与不锈钢激光钎焊接头界面组织特征

通过扫描电子显微镜、能谱仪及X射线衍射技术分析TiNi形状记忆合金与1Cr18Ni9Ti不锈钢激光钎焊接头界面反应层的组织结构特征。结果表明：TiNi形状记忆合金与不锈钢激光钎焊接头钎缝主要由α-Ag固溶体、α′-Cu固溶体和Ag-Cu共晶相组成;不锈钢/银基钎料界面反应区由3层连续的反应层构成,分别为：奥氏体（A）,马氏体（M）/A和M/α-Ag＋α′-Cu＋M;TiNi形状记忆合金/银基钎料界面反应层主要由Ti（Ni,Cu）＋（Ni,Cu）Ti2化合物组成。     

Microstructures and properties of welded joint of TiNi shape memory alloy and stainless steel

The fracture characteristics of the joint were analyzed by means of scanning electron microscope(SEM).Microstructures of the joint were examined by means of optical microscope, SEM and an image analyzer. The results show that the tensile strength of the inhomogeneous joint of TiNi shape memory alloy and stainless steel is lower than that of the homogeneous joint and a plastic field appears in the heat affected zone on the side of TiNi shape memory alloy. Because TiNi shape memory alloy and stainless steel melted, a brittle as-cast structure was formed in the weld. The tensile strength and the shape memory effect of the inhomogeneous joint are strongly influenced by the changes of composition and structure of the joint. Measures should be taken to reduce the base metal melting and prevent the weld metal from the invasion by O for improving the properties of the TiNi shape memory alloy and stainless steel inhomogeneous joint.     

TiNi形状记忆合金与不锈钢激光焊接头裂纹分析及防止措施

采用激光焊对TiNi形状记忆合金与不锈钢异种材料进行焊接,用扫描电镜和激光共聚焦显微镜研究了TiNi合金/不锈钢接头裂纹及断口特征,分析了焊缝裂纹的形成机理,并提出了防止裂纹的措施.结果表明,裂纹多以微裂纹的形式出现于焊缝中心和TiNi合金侧熔合区.焊缝中存在大量的脆性化合物是产生裂纹的内在原因,接头受到拉伸应力是产生裂纹的必要条件,焊缝裂纹是二者共同作用的结果.通过焊接区加镍和钴中间层材料、改变激光光斑位置、焊接区施加轴向力及优化激光焊接参数的方法均能在一定程度上改善焊缝金属的裂纹敏感性,其中加金属中间层效果更为明显,加镍和钴中间层后,接头抗拉强度分别达到372和347 MPa,比未加中间层的接头的抗拉强度分别提高98.9%和85.6%.     

Microstructures and mechanical properties of laser-welded TiNi shape memory alloy and stainless steel wires

The Nd:YAG laser welding was used to join the TiNi shape memory alloy and AISI304 stainless steel wires. The microstructural features of the dissimilar material joint were analyzed. The tensile and hardness tests were carried out to examine the mechanical properties and microhardness distribution of the welded joint. The results show that the joint has the non-homogeneous microstructure and element distribution. The brittle phases such as Fe2Ti, FeTi, Cr2Ti, Ti3Ni4, Fe0.2Ni4.8Ti5 and TiN mainly segregate in rich Ti region of fusion zone. The laser-welded joint has the tensile strength of 298 MPa with the elongation of 3.72% and exhibits the brittle fracture features on the fracture surfaces. The reasons for low joint strength were discussed in this investigation.     

医用封堵器中形状记忆合金与不锈钢的焊接

研究医用封堵器制造过程中的焊接工艺,首先利用钨极氩弧焊(TIG)将一束NiTi形状记忆合金丝焊接在一起,然后采用激光点焊将NiTi形状记忆合金丝接头与不锈钢管焊接起来.采用光学显微镜观察接头的显微结构,并利用电子探针(EPMA)研究接头中合金元素的分布.结果表明,在形状记忆合金TIG焊接头中弥散分布有TiC相,但TiC的含量在形状记忆合金与不锈钢的激光点焊焊缝处较少.在熔合线附近存在元素间的相互扩散,并且在熔合线附近产生金属间化合物Ni3Ti+(Fe,Ni)Ti.     

The corrosion behavior of Fe-based shape memory alloys

Fe-30Mn-6Si, Fe-30Mn-6Si-5Cr and Fe-13Mn-5Si-12Cr-5Ni shape memory alloys were prepared by a VIM technique. The various corrosion tests were conducted to investigate the corrosion behaviors of these alloys. Experimental results show that in 3.5% NaCl solution, the Fe-13Mn-5Si-12Cr-5Ni alloy had the best chemical corrosion resistance, whereas the Fe-30Mn-6Si-5Cr alloy was locally attacked, forming many corrosion pits after immersion test. In addition, the detachment of the corrosion product covering the Fe-30Mn-6Si alloy caused an abrupt increase in the weight loss. After 2 h of heat treatment at 1000 °C, the corrosion potential of the Fe-30Mn-6Si alloy increased due to the formation of α-ferrite, while the Fe-30Mn-6Si-5Cr alloy became more active. In the stress-corrosion cracking test, the Fe-13Mn-5Si-12Cr-5Ni alloy, having the highest fracture stress in the atmosphere among these alloys, exhibited the largest decrease in fracture stress in the saturated H₂S solution due to the existence of α-martensite.     

Microstructures and properties of capacitor discharge welded joint of TiNi shape memory alloy and stainless steel

0IntroductionTiNi SMAis a newkind of functional material used inmany fields,such as aviation and spaceflight,atomic ener-gy,ocean development,instruments and medical purpo-ses,etc.due to its special SME and superelasticity,aswell as excellent erosion resi…     

经形变热处理铜基形状记忆合金的晶粒生长、形状记忆和腐蚀行为

比较研究了Cu-11．8％Al-3．7％Ni-1％Mn和Cu-11％A1—5．6％Mn形状记忆合金（SMAS）的形状记忆、腐蚀性能。采用光学显微镜（0M）、扫描电子显微镜（SEM）、差示扫描量热法（DSC）、动电位极化、弯曲和拉伸试验,研究了晶粒细化对这些性能的影响。在800。C退火时,在首先的15S内静态再结晶和动态晶粒长达显示出一个快速的再结晶过程,随后才是晶粒生长。退火15S后得到的Cu—A1—Ni-Mn和Cu-Al—Mn合金的最小晶粒尺寸分别为90gm和260pm。拉伸试验表明2种合金呈现典型的三阶段曲线,由此可以看出,晶粒细化后合金具有高的断裂应力和应变。显微组织表明,Cu—Al-Ni—Mn合金中存在锯齿状的所马氏体形态,通过差示扫描量热法也证实了所和rf共存于Cu—A1-Mn合金中。评估了形变热处理前、后及800℃退火15min,随后进行水淬的合金的形状记忆性能。另外,采用动电位极化法分析了晶粒细化后合金的腐蚀行为。结果表明,铜溶解过程中主要为阳极反应,Cu—Al—Ni—Mn合金比Cu—A1-Mn合金具有更好的耐腐蚀性。     

TiNi形状记忆合金与不锈钢激光钎焊AgCuZnSn钎料的研制

成功研制一种适合于TiNi形状记忆合金与不锈钢异质材料钎焊，并可安全应用于医学领域的新型AgCuZnSn银基钎料。对AgCuZnSn系钎料的熔化特性、微观组织、钎焊工艺特性及接头力学性能进行了研究。结果表明，所研制AgCuZnSn银基钎料成分为Ag51～53Cu21～23Zn17～19Sn7～9，固相线温度为590．0℃，液相线温度为635．3℃。该钎料主要由α—Ag固溶体、α-(Cu，Zn)固溶体和Ag-Cu共晶相组成，并含有少量Cu41Sn11、AgZn、Ag3Sn、Cu5Zn8等化合物。采用该钎料钎焊TiNi形状记忆合金与不锈钢，接头强度达320～360MPa，同时TiNi形状记忆合金性能损失较小。新型钎料熔点低、钎焊冶金特性优异，钎焊接头界面冶金结合平直、致密。     

Electrochemical and microstructural study of Cu-Al-Ni shape memory alloy

The corrosive behaviour of Cu-Al-Ni shape memory alloy in deaerated 0.5 M NaCl solution at 20 °C was studied by means of open circuit potential measurements, linear polarization, potentiodynamic polarization measurements, cyclic voltammetry and electrochemical impedance spectroscopy measurements. The electrode surface was examined by light microscope, SEM, XRD and EDX methods. The polarization measurements have shown that corrosive behaviour of Cu-Al-Ni alloy in NaCl solution was dominated by the Cu component. The results of impedance measurements at open circuit potential have shown that the overall impedance of the system increases with immersion time due to continuous growth of the passive film on the alloy surface. The XRD and EDX analysis showed the presence of copper, aluminium and nickel compounds, Cu-oxides and Cu-chlorides on alloy surface.     

Corrosion behavior of martensitic and precipitation hardening stainless steels treated by plasma nitriding

Plasma nitriding is a well established technology to improve wear and corrosion properties of austenitic stainless steels. Nevertheless, in the case of martensitic stainless steels, it continues being a problem mainly from the corrosion resistance viewpoint.In this work, three high chromium stainless steels (M340, N695 and Corrax) were hardened by ion nitriding at low temperature, intending to preserve their corrosion resistance.Corrosion behavior was evaluated by CuSO₄ spot, salt spray fog and potentiodynamic polarization in NaCl solution. Microstructure was analyzed by optical microscopy, SEM (EDS) and glancing angle X-ray diffraction. All the samples showed an acceptable corrosion resistance in experiments with CuSO₄, but in salt spray fog and electrochemical tests, only Corrax showed good behavior. The poor corrosion performance could be explained by chromium carbides formed in thermal treatment stage in martensitic steels and chromium nitrides formed during nitriding, even though the process was carried out at low temperature.     

Compressive training of the shape memory alloy washer

This paper evaluates the performance and engineering aspects of 44Ti-47Ni-9Nb alloy in a pretensioning washer application. Previously the behavior of this alloy has mainly been studied for tensile predeformation with wires and strips up to 1 mm thickness. The present work investigated the effect of compression deformation and different homogenization temperatures on ring-shaped washers with larger dimensions. The best results with the compression-trained washer were achieved after homogenization at 1073 K where the compression training deformation was -10.4%, the lowest applied. The deformation training was carried out at 213 K near the martensitic reaction start temperature of the material. The expansion method, applied to the commercial reference ring, seemed to be a fairly efficient training method for the studied application, even though the parallel course of the radial surfaces was poor.     

Corrosion properties of laser surface melted NiTi shape memory alloy

A Nd-YAG laser was used to melt the surface of NiTi shape memory alloy in air and argon environment respectively. The laser surface melted (LSM) layer is free of porosity and crack. The corrosion resistance of the LSM specimen in 3%NaCl solution has been improved significantly and is attributed to the increased amount of TiO₂ and Ti/Ni ratio on the surface.     

TiNi形状记忆合金与不锈钢TLP-DB接头界面组织及力学性能

采用AgCu金属箔作中间层,对TiNi形状记忆合金与不锈钢进行了瞬间液相扩散焊.分析了接头的显微组织、元素分布、物相组成等,研究了接头的显微硬度和不同工艺参数下的抗剪强度.结果表明,接头界面区由TiNi侧过渡区、中间区和不锈钢侧过渡区组成,主要相分别为Ti(Cu,Ni,Fe),AgCu,TiFe等.过渡区的显微硬度值高达500~650 HV,但中间区的硬度值只有大约120 HV.随加热温度的升高和保温时间的延长,接头抗剪强度均呈先增大后减小的趋势,最大抗剪强度为239.4 MPa.断裂发生在TiNi母材和AgCu中间层扩散界面上,断口为混合断裂形貌.     

一种具有高形状回复率的双相 Ni-Mn-Ga-Gd 高温形状记忆合金

本文通过弯曲试验研究了双相Ni58Mn25Ga16.9Gd0.1合金的形状记忆效应和回复率。结果表明,该合金具有优异的形状回复能力。在预变形4.6%加热后,可以得到86%的形状回复率,高于已报道的双相Ni-Mn-Ga合金。这是因为Gd掺杂提高了γ 相的临界滑移应力,使γ 相不易发生变形。     

Corrosion behavior of wire-arc-sprayed stainless steel coating on mild steel

The corrosion characteristics of a wire-are-sprayed stainless steel coating on mild steel have been investigated in regards to atomizing gases and sealing treatment. Salt spray test was performed. The corrosion behavior of the coating was observed under a microscope succesively through a cycling test of wetting and drying in a salt solution. The sealing-treated coating was found to rust faster compared with the non-sealing-treated coating; it protected the mild steel substrate against corrosion, but even it deteriorated the coating itself due to the interruption of the substrate as an anode. The air-atomized coating ruste more heavily than the nitrogen-atomized one. Four different phases were observed in the coating in regards to corrosion behavior; namely, chromium-based oxide, iron-based oxide, chromium-depleted metallic phase, and stainless steel matrix phase. It was found that the chromium-depleted metallic phase and the iron-based oxide are non-corrosion-resistant, whereas the chromium-based oxide and the stainless steel matrix phase are corrosion-resistant.     

Effects of nanoprecipitation on the shape memory and material properties of an Ni-rich NiTiHf high temperature shape memory alloy

Shape memory properties of a Ni_50.3Ti_29.7Hf₂₀ (at.%) polycrystalline alloy were characterized after selected heat treatments. The effects of heat treatment temperature and time on the transformation temperatures (TTs) and temperature hysteresis were determined by differential scanning calorimetry. Thermal cycling under constant compressive stress was carried out to reveal the changes in transformation strain, temperature hysteresis, and TT as a function of stress. Isothermal stress cycling experiments were conducted to reveal the critical stresses, transformation strain, and stress hysteresis as a function of temperature. The crystal structure and lattice parameters of the transforming phases were determined by X-ray diffraction at selected temperatures. Precipitate characteristics and martensite morphology were revealed by transmission electron microscopy. Precipitation was found to alter the martensite morphology and significantly improve the shape memory properties of the Ni-rich NiTiHf alloy. For the peak aged condition shape memory strains of up to 3.6%, the lowest hysteresis, and a fully reversible superelastic response were observed at temperatures up to 240 °C. In general, the nickel-rich NiTiHf polycrystalline alloy exhibited a higher work output (≈16.5 J cm^?3) than other NiTi-based high temperature alloys.     

Gamma-phase influence on shape memory properties in Ni-Mn-Co-Ga-Gd high-temperature shape memory alloys

The effect of γ-phase on two-way shape memory effect(TWSME) of polycrystalline Ni_(56)Mn_(25-x)Co_xGa_(18.9)Gd_(0.1) alloys was investigated. The results show that an appropriate amount of ductile γ-phase significantly enhances the TWSME. The largest TWSME of 1.4% without training is observed in Ni_(56)Mn_(21)Co_4Ga_(18.9)Gd_(0.1) alloy, and this value is increased to 2.0% after thermomechanical training. The as-trained TWSME decays over the first five thermal cycles and then reaches a stable value as the number of cycles further increasing. Only the degradation of 0.2% is observed after 100 thermal cycles. The better TWSME and thermal stability are ascribed to the stable extra stress field formed by the plastically deformed γ-phase.     

Corrosion properties of active screen plasma nitrided 316 austenitic stainless steel

AISI 316 austenitic stainless steel has been plasma nitrided using the active screen plasma nitriding (ASPN) technique. Corrosion properties of the untreated and AS plasma nitrided 316 steel have been evaluated using various techniques, including qualitative evaluation after etching in 50%HCl + 25%HNO₃ + 25%H₂O, weight loss measurement after immersion in 10% HCl, and anodic polarisation tests in 3.5% NaCl solution. The results showed that the untreated 316 stainless steel suffered severe localised pitting and crevice corrosion under the testing conditions. AS plasma nitriding at low temperature (420 °C) produced a single phase nitrided layer of nitrogen expanded austenite (S-phase), which considerably improved the corrosion properties of the 316 austenitic stainless steel. In contrast, AS plasma nitriding at a high temperature (500 °C) resulted in chromium nitride precipitation so that the bulk of the nitrided case had very poor corrosion resistance. However, a thin deposition layer on top of the nitrided case, which seems to be unique to AS plasma nitriding, could have alleviated the corrosion attack of the higher temperature nitrided 316 steel.