Effect of Different Shape-Memory Processing Methods on the Thermomechanical Cyclic Properties of a Shape-Memory Polyurethane

Shape-memory polymers are materials that are capable of changing their shape when an external stimulus is applied. This effect is called the shape-memory effect (SME) and takes place by means of a thermomechanical cycle called programming. The SME depends on the thermomechanical conditions at which programming is performed, and the influence of these conditions differs depending on whether the programming is performed with a strain- or stress-controlled protocol. This study focuses on finding the thermomechanical cycling conditions in stress-controlled programming (T _prog and σ_m) that stabilize the material in the fewest cycles while obtaining the best mechanical and shape-memory properties over the highest number of cycles. Using a T _prog above or below, the glass transition temperature makes a big difference in terms of shape recovery and the maximum stress is a key factor in the stabilization of shape-memory properties.     

Thermomechanical Properties of Porous NiTi Alloy Produced by SHS

Samples obtained from relatively large powders (<150 µm), with total porosity in the range 30-68%, were characterized mainly from a morphological point of view. Total porosity, as well as pore size, shape and distribution, was analyzed. Sample microstructure was also investigated, indicating that the main phase produced during the self-propagating high-temperature synthesis (SHS) reaction is Ti reach NiTi phase, as confirmed by calorimetric analyses. Moreover, the presence of secondary phases, suggested by the low transformation enthalpy, was confirmed by SEM observations. In fact, EDS microanalyses and EBSD mapping helped in the identification of such secondary phases, such as Ni₃Ti, Ti₂Ni and Ti₄Ni₂O _x . Other samples were successively produced starting from the same powders but introducing a different powder compression methodology and operating conditions. In this way, the obtained samples showed higher porosity featured by more uniform size, shape and distribution while, from a micro-structural point of view, no significant differences were observed. Mechanical compression tests were carried out at room temperature and, on selected samples, also above A _f in order to highlight the influence of pore shape and distribution. Results obtained at room temperature show that the mechanical properties decrease with the porosity augmentation. For higher temperatures, the samples presented a pseudoelastic behavior. Dilatometric tests were also performed and the results well indicated the martensite to austenite transformation at the same temperature showed by the DSC analyses. Thermal analysis was completed by evaluating the thermal diffusivity temperature and porosity dependence using an experimental-numerical approach especially developed.     

Effect of Impact-Induced Strain on the Stress-Induced Martensitic Transformation of Superelastic NiTi Shape-Memory Alloy Wires

The stress-induced martensitic (SIM) transformation of NiTi shape-memory alloy wires has been studied as a function of the maximum strain induced during tensile deformation at impact and quasi-static strain rates. The SIM transformation stresses are higher at impact than at quasi-static strain rates. Only the lower plateau strength is sensitive to the maximum strain achieved during transformation when this is higher than necessary to complete the SIM transformation. For the same maximum strain achieved, the deformation energy (E _d) and recoverable strain energy (E _r) are greater at impact than at quasi-static strain rates, and the dissipated energy (W _d) is slightly lower at impact, reaching values close to those obtained at quasi-static strain rates.     

Cr离子注入镍钛合金的表面组织结构

针对镍钛合金(NiTi)进行3种不同浓度Cr离子注入,采用扫描电子显微镜、三维白光形貌干涉仪、小角度掠射X射线衍射仪、X射线光电子能谱仪分析了离子注入前后镍钛合金表面形貌、粗糙度、组织结构及化合态。结果表明,随着注入Cr离子浓度的增大,镍钛合金表面更加平整,粗糙度变小。所有注入试样表面形成了近60nm厚的注入膜层,主要成分为三氧化二铬和铬碳化合物,并且随Cr离子注入剂量的增加,三氧化二铬和铬碳化合物的含量增加。     

Influence of Quenching Rates on Equiatomic NiTi Ribbons Fabricated by Melt-Spinning

Quenching rates, in terms of circumferential wheel speed in melt-spinning, play an important role in the resultant characteristics of the ribbons. In this study, the influence of various wheel speeds on ribbon dimensions, cross-sectional microstructures, and crystallographic phases within samples were investigated. The Ni-50.3at.%Ti melt-spun ribbons were produced under 200 mbar He atmosphere at wheel speeds of 5-30 ms^?1 using a quartz crucible coated internally with Y₂O₃. The different wheel speeds led to different sample dimensions and cooling rates. The microstructures of the samples were observed using optical microscopy and TEM, while the shape memory effect of samples and the existing phases were studied by differential scanning calorimetry, x-ray diffraction, and tensile tests.     

Thermomechanical cyclic response of an ultrafine-grained NiTi shape memory alloy

We focus on the effects of ultrafine grains on the thermomechanical cyclic stability of martensitic phase transformation in Ni_49.7Ti_50.3 shape memory alloy fabricated using equal-channel angular extrusion (ECAE). The samples were ECAE-processed between 400 and 450 °C resulting in average grain sizes of 100–300 nm. Tensile failure experiments demonstrated that the strength differential between the onset of transformation and the macroscopic plastic yielding increases after ECAE. Such increase led to a notable improvement in the thermal cyclic stability under relatively high stresses. The experimental observations are attributed to the increase in critical stress level for dislocation slip due to grain refinement, change in transformation twinning mode in submicron grains, the presence of R-phase, and multi-martensite variants or a small fraction of untransforming grains due to grain boundary constraints. The effects of these microstructural factors on the transformation behavior are discussed in the light of transformation thermodynamics.     

Characterization of the High-Strength Mg–3Nd–0.5Zn Alloy Prepared by Thermomechanical Processing

Magnesium alloys based on Nd and Zn are promising materials for both aviation industry and medical applications.Superior mechanical properties of these materials can be achieved by thermomechanical processing such as extrusion or rolling and by aging treatment, which can significantly strengthen the alloy. The question remains especially about the connection of texture strength created in the alloys based on the specific conditions of preparation. This work focuses on the Mg–3 Nd–0.5 Zn magnesium alloy prepared by hot extrusion of the as-cast state at two different temperatures combined with heat pre-treatment. Extrusion ratio of 16 and rate of 0.2 mm/s at 350 and 400 °C were selected for material preparation. The structures of prepared materials were studied by scanning electron microscopy and transmission electron microscopy. The effect of microstructure on mechanical properties was evaluated. Obtained results revealed the strong effect of thermal pre-treatment on final microstructure and mechanical properties of extruded materials. The Hall–Petch relation between grain size and tensile yield strength has been suggested in this paper based on the literature review and presented data. The observed behavior strongly supports the fact that the Hall–Petch of extruded Mg–3Nd–0.5 Zn alloys with different texture intensities cannot be clearly estimated and predicted. In addition, Hall–Petch relations presented in literature can be sufficiently obtained only for fraction of the Mg–3Nd–0.5 Zn alloys.     

V含量对等原子比NiTi形状记忆合金微观组织、相变行为和显微硬度的影响

采用光学显微镜、扫描电子显微镜、X射线衍射仪、差示扫描量热仪和显微硬度计等测试手段,研究了V含量对等原子比NiTi形状记忆合金微观组织、相变行为和显微硬度的影响规律。结果表明：当V含量为0.5at%时,具有等轴晶组织的NiTiV形状记忆合金包含B19'' 和 Ti₂Ni相;当V 含量大于0.5at%时,NiTiV形状记忆合金形成B19''相、Ti₂Ni相和V的富集相,并且随着V含量增加,V的富集相越来越多聚集于晶界。进一步分析表明,Ni_49.75Ti_49.75V_0.5和Ni_49.25Ti_49.25V_1.5 形状记忆合金发生了B2?B19''的一级相变,而Ni_48.75Ti_48.75V_2.5和Ni_48.25Ti_48.25V_3.5形状记忆合金发生了B2?R?B19''的二级相变,尽管降温过程中同时发生了部分的R相变与B19''马氏体相变。随着V含量增加,相变温度随着V含量增加逐渐降低,其主要原因是Ni/Ti比例的增加。此外,随着V含量增加,合金的显微硬度值呈现先降低然后几乎保持不变的变化规律。     

Influence of Fe Addition on Phase Transformation,Microstructure and Mechanical Property of Equiatomic NiTi Shape Memory Alloy

Based on equiatomic nickel and titanium, three kinds of NiTiFe alloys with a nominal chemical composition of Ni_(49)Ti_(49)Fe_2, Ni_(48)Ti_(48)Fe_4 and Ni_(47)Ti_(47)Fe_6(at.%), respectively, have been designed to investigate the influence of the addition of Fe element on phase transformation, microstructure and mechanical property of equiatomic NiTi shape memory alloy. The microstructures of three kinds of NiTiFe alloys are characterized by the equiaxed grains instead of the dendrites.Consequently, some Ti_2Ni precipitates are found to distribute in the grains interior and at the grain boundaries. The content of Fe element has an important influence on mechanical property of NiTiFe alloy. With increasing content of Fe element,the strength of NiTiFe alloy increases substantially, but the plasticity decreases sharply. It can be concluded that precipitation strengthening and solution strengthening play a significant role in enhancing the strength of NiTiFe alloy. In the case of three NiTiFe alloys, neither martensitic transformation nor reverse transformation can be observed in the range from-150 to 150 ℃. On the one hand, the phase transformation temperature is probably out of the scope of the present experimental temperature. On the other hand, the addition of Fe element probably suppresses first-order martensitic transformation or reverse transformation, and consequently the second-order-like phase transformation from an incommensurate stage to a commensurate stage can probably take place.     

Preparation of an Nanocrystalline Ta-Cu Alloy by Mechanical Alloying

Nanocrystalline alloy wlth graln size of about 10～20nm was prepared by mechanlcal alloying of elemen-tal powders in an imnliscible Ta-Cu system, The structure changes of Ta_70Cu_30 during mechanical alloyingwere monitored by X-ray diffraction. scanning electron microscopy and transmission electron microscopy.High-energy ball milling can efficiently reduce the grain size and considerably increase the Cu solubility in Ta.The significant enhancement of hardness of alloyed powders was also observed.     

Microstructure Modification of Powder Compact of Al-Zn-Mg Nanostructured Alloy by a Semisolid Thermomechanical Processing

A thermomechanical process(TMP) consisting of three cycles of cold pressing at 154 MPa and liquid-phase sintering at 600 ℃ for 30 min in each cycle was applied to modify the microstructure of nanostructured Al-Zn-Mg alloy.The alloy powders were produced by mechanical alloying.Also,solid-state sintering at 550 ℃ for 90 min was done to compare the results with those obtained from the TMP.The powders and the thermomechanically(TM) processed samples were analyzed by XRD to reveal the present phases in addition to calculating the crystallite size changes by the Williamson-Hall method.Moreover,scanning electron microscope was employed to observe the morphology of the powder and the microstructures of the sintered and the TM processed samples.The results revealed that the TMP affected the microstructure noticeably as well as the microhardness by removing the continuous grain boundary porosities and uniform distribution of the intermetallic phase particles as well as obtaining a near globular microstructure after the second cycle.Also,the average grain sizes in the first and the second cycles of the TMP were lower than those of the sintered sample.Furthermore,nanocrystalline grains were stable up to the second cycle of the TMP.     

镍钛形状记忆合金丝材加工工艺及其影响因素

综述了TiNi形状记忆合金医用丝材加工工艺,包括热加工、冷加工和热处理工艺,对影响产品质量的因素进行评价.TiNi合金由于具有优良的超弹性、形状记忆性能、耐蚀性和生物相容性,而被广泛用于医疗器械领域,如牙弓丝、血管内支架等.     

Structure and Properties of Ti-19.7Nb-5.8Ta Shape Memory Alloy Subjected to Thermomechanical Processing Including Aging

In this work, the ternary Ti-19.7Nb-5.8Ta (at.%) alloy for biomedical applications was studied. The ingot was manufactured by vacuum arc melting with a consumable electrode and then subjected to hot forging. Specimens were cut from the ingot and processed by cold rolling with e = 0.37 of logarithmic thickness reduction and post-deformation annealing (PDA) between 400 and 750 °C (1 h). Selected samples were subjected to aging at 300 °C (10 min to 3 h). The influence of the thermomechanical processing on the alloy’s structure, phase composition, and mechanical and functional properties was studied. It was shown that thermomechanical processing leads to the formation of a nanosubgrained structure (polygonized with subgrains below 100 nm) in the 500-600 °C PDA range, which transforms to a recrystallized structure of β-phase when PDA temperature increases. Simultaneously, the phase composition and the β → α″ transformation kinetics vary. It was found that after conventional cold rolling and PDA, Ti-Nb-Ta alloy manifests superelastic and shape memory behaviors. During aging at 300 °C (1 h), an important quantity of randomly scattered equiaxed ω-precipitates forms, which results in improved superelastic cyclic properties. On the other hand, aging at 300 °C (3 h) changes the ω-precipitates’ particle morphology from equiaxed to elongated and leads to their coarsening, which negatively affects the superelastic and shape memory functional properties of Ti-Nb-Ta alloy.     

电化学沉积Ni-W合金纳米晶镀层的组织与硬度研究

采用电沉积制备Ni-W合金纳米晶镀层，研究了纳米晶的形成与镀层组织结构和硬度。随着镀液组分的变化，镀层的表面形貌发生变化；X射线衍射结果表明，Ni-W合金的晶粒尺寸在17-30m之间；镀液组成、pH值、电流密度、温度等因素对Ni-W合金纳米晶沉积层的硬度都有影响，最主要的影响因素是pH值及电流密度。     

溅射态原位加热NiTi形状记忆合金薄膜的结构性能研究

研究了原位加热溅射的NiTi形状记忆合金薄膜的结构，讨论了制备工艺及织构对薄膜相变特征的影响，采用电阻法及X射线衍射分析了薄膜的结构及相变过程，确定了最适合于微器件的溅射工艺。结果发现：溅射时采用原位加热可直接获得具有织构的晶化薄膜，提高溅射功率将使薄膜的相变温度升高。     

半固态合金球状组织形成过程的相场法模拟

采用耦合溶质场和流场的相场模型,模拟了半固态金属材料(Al-2.0mol%Si合金)成形过程中微观组织的演变,分析和讨论了扰动强度对微观组织演变的影响。结果表明:随着浓度扰动的增加,二次枝晶臂越来越发达,枝晶形貌由树枝状逐渐变成蔷薇状,并向近似的球状组织发展;在扰动强度达到0.4时,枝晶基本上趋于球形生长,枝晶生长前沿溶质扩散层的厚度也趋于一致。模拟结果与半固态组织的成形机理基本一致。     

Processing of NiTi Foams by Transient Liquid Phase Sintering

Porous NiTi was produced by sintering pre-alloyed NiTi powders (with small Ni addition to form Ni-rich composition) with NaCl powders which are removed to create 40-60 vol.% macropores which are open to the surface, blocky in shape, and 100-400 ??m in size. The microporosity present between the NiTi powders is infiltrated by an in situ created NiTi-Nb eutectic liquid which, after solidification, densifies the NiTi powders into dense struts. This processing technique allows for separate control of the macroporous structure, and the densification and composition of the NiTi struts.