Features of austenitic steels' microstructure following plastic deformation

The carried out transmission electron microscopic (TEM) analysis of steels 316L and TWIP, has shown that important mechanisms of plastic deformation are mechanical twinning, shear band formation and deformation martensitic transformations. Mechanical twinning is a characteristic feature of a plastic deformation at ε ≈ (20–50) % deformation degrees. At ε = (20–70) % the basic contribution in plastic deformation of a material brings shear band formation. The contribution deformation martensitic transformations into plastic deformation is defined by level of phase instability of steels.     

TiNi记忆合金的马氏体相变与形状记忆效应

对近些年来有关ＴｉＮｉ记忆合金中马氏相变和形状记忆效应的最新研究成果进行综合评述。主要内容包括马氏体的形成，相变晶体学和变体自协作以及变形行为和形状记忆效应。     

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.     

Perspective in Development of Shape Memory Materials Associated with Martensitic Transformation

By consideration of the characteristics of martensitic transformation and the derivation from the application of the group theory to martensitic transformation, it may be concluded that the shape memory effect (SME) can be attained in materials through a martensitic transformation and its reverse transformation. only when there forms single or nearly single variant of martensite, with an absence of the factors causing the generation of the resistance against SME. on this principle, various shape memory materials including nonferrous alloys. iron-based alloys and ceramics containjng zirconia are expected to be further developed. A criterion for thermoelastic martensitic transformation is presented, Factors which may act as the resistance against SME in various materials are briefly described     

铁基形状记忆合金马氏体相变研究进展

铁基形状记忆合金的形状记忆效应和超弹性取决于合金的马氏体相变特征,掌握铁基合金的马氏体相变规律是开发和优化铁基形状记忆合金的前提。根据马氏体相变类型将目前发现的铁基形状记忆合金分成3类：Fe-Mn-Si系,Fe-Ni-Co系和Fe-Pt／Fe-Pd系,分别阐述了3类铁基形状记忆合金马氏体相变的研究进展,总结了铁基合金形状记忆效应的不同机理和影响马氏体相变特征的各种因素,探讨了开发新型铁基形状记忆合金的需要关注的方向。     

磁性形状记忆合金研究进展

磁性形状记忆合金是上世纪90年代开始出现的一类新型金属功能材料。这类合金兼具热弹性马氏体相变和磁性转变,其形状记忆效应可以由磁场控制。此外这类合金还具有磁阻、磁热等丰富的物理效应,因而一直是近期研究热点。首先介绍了磁性形状记忆合金的3个基本特征,即马氏体相变与磁性转变、磁场驱动孪晶再取向和磁场诱发相变。然后分别对Ni基、Co基和Fe基磁性形状记忆合金的研究现状进行了评述。最后展望了磁性形状记忆合金的发展趋势。     

Ti-Ni-Hf高温形状记忆合金的研究进展

Ti-Ni-Hf记忆合金因具有高相变温度、相对低廉的价格和高输出功等诸多优点而成为最具潜力的高温形状记忆合金之一。然而,Ti-Ni-Hf记忆合金基体强度低,变形过程中易优先发生塑性变形,从而使其可实现的可恢复应变远低于理论值。目前改善应变恢复特性的措施主要包括:热机械处理(冷轧+退火)、合金化、时效处理、制备单晶合金等。研究表明,Ti-Ni-Hf合金的应变恢复特性与微观组织结构密切相关。本文主要阐述了Ti-Ni-Hf记忆合金在近年来的最新研究进展,包含微观组织结构的演化规律、马氏体相变行为以及力学性能和应变恢复特性,并基于前期研究成果建立了微观组织结构-马氏体相变-力学与应变恢复特性的关联特性。当前,Ti-Ni-Hf高温形状记忆合金冷、热加工性能差是其广泛应用的瓶颈。因此,Ti-Ni-Hf高温记忆合金的粉末冶金和增材制造可能是未来研究的热点与重点。     

Phenomenological analysis on subloops and cyclic behavior in shape memory alloys under mechanical and/or thermal loads

A theoretical framework is presented, from the phenomenological point of view, for the cyclic uniaxial deformation in shape memory alloys subjected to the thermal and/or mechanical loads by introducing three internal variables; the local residual stress and strain and the volume fraction of the martensic phase accumulated during cyclic forward and reverse martensitic transformations. The cyclic effect on the stress-strain and strain-temperature hysteresis loops is discussed. The subloops due to incomplete transformations are also analyzed by assuming the transformation starting stress or temperature which depends on the preloading. Numerical results explain qualitatively well the observations on the thermomechanical behaviors of shape memory alloys.     

Electroplastic effect in nanocrystal and amorphous alloys

The influence of the structural and phase state in nanocrystalline and amorphous alloys, as well as the pulsed current modes, on the electroplastic effect is studied under their tension. The decreasing grain size to nanoscale, the emergence of secondary phases, and amorphization in alloys cause the attenuation or disappearance of the electroplastic effect. The application of current pulses upon tension of alloys with a reversible thermoelastic martensitic transformation suppresses the step down of stress on the tensile diagrams caused by the electroplastic effects, but activates the jump up of stress attributed to the shape memory effect.     

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.     

Effect of previous plastic deformation on the resistance of chromium-manganese steels to cavitation erosion

The investigation described in this paper was concerned with austenitic chromium-manganese steels characterized by various resistances to the martensitic transformation during plastic deformation. It was established that the influence of previous plastic deformation on the resistance of these steels to cavitation erosion varies. In each case it is necessary to consider both the susceptibility of austenite to decomposition (accompanied by the formation of martensite and -phase) under the influence of cavitation and various phase and structural transformations taking place in the steel as a result of previous plastic deformation.     

An investigation of the mechanical behaviour of fine tubes fabricated from a Ti–25Nb–3Mo–3Zr–2Sn alloy

This study investigates the mechanical properties and the deformation mechanisms active in Ti–25Nb–3Mo–3Zr–2Sn fine tubes. Ti–25Nb–3Mo–3Zr–2Sn alloy is a recently developed metastable β titanium alloy intended for biomedical applications. Tensile tests were carried out on the fine tubes. The modulus of the Ti–25Nb–3Mo–3Zr–2Sn fine tubes increased with reductions in the diameter for tubes in the cold rolled and annealed conditions. In comparison with cold rolled tubes, the annealed tubes exhibit increased strain hardening behaviour and superior ductility. Mechanical twins, stress-induced martensitic transformation and the textures of the β and α″ phases were investigated. The results show that the fine tubes exhibit different moduli which are related to the evolution of β and α″ phase textures during processing. Cold rolling facilitates the transformation from β to the α″ phase and mechanical {332}〈113〉 twinning. For the annealed tubes, mechanical twinning as well as primary and secondary martensitic transformations was activated at specific levels of tensile strain. Twins developed with increasing levels of strain, and secondary martensitic transformations occurred within the twinned β regions. Annealed fine tubes exhibit multistage strain hardening behaviour and superior ductility due to the synergetic effects of twinning induced plasticity and transformation induced plasticity during tensile deformation.     

Physical metallurgy of Ti-Ni-based shape memory alloys

Ti-Ni-based alloys are quite attractive functional materials not only as practical shape memory alloys with high strength and ductility but also as those exhibiting unique physical properties such as pre-transformation behaviors, which are enriched by various martensitic transformations. The paper starts from phase diagram, structures of martensites, mechanisms of martensitic transformations, premartensitic behavior, mechanism of shape memory and superelastic effects etc., and covers most of the fundamental issues related with the alloys, which include not only martensitic transformations but also diffusional transformations, since the latter greatly affect the former, and are useful to improve shape memory characteristics. Thus the alloy system will serve as an excellent case study of physical metallurgy, as is the case for steels where all kinds of phase transformations are utilized to improve the physical properties. In short this review is intended to give a self-consistent and logical account of key issues on Ti-Ni based alloys from physical metallurgy viewpoint on an up-to-date basis.     

Ti-Ni-Cu形状记忆合金快速凝固条带的研究进展

回顾了Ti-Ni-Cu形状记忆合金快速凝固条带的研究进展.主要介绍了Ti-Ni-Cu合金条带的组织结构、晶化行为、织构和析出物、马氏体相变温度及其形状记忆特性.合金条带的铸态组织随工艺参数不同可在非晶-晶态之间变化;独特的析出物和织构影响其相变行为和形状记忆性能;条带具有良好的形状记忆效应和超弹性性能,与体材相比,其相变应变大而相变应力滞后小.总结了研究中存在的问题并展望了未来的研究方向.     

Displacive processes in systems with bcc parent lattice

The changes of sample shape are caused by plastic deformation or by martensitic phase transformations. In both cases the mechanisms of atomic rearrangements are based on collective displacements of atomic aggregates. The internal structure of dislocations, carriers of plastic deformation, can be examined using the energies of generalized stacking faults displayed by so called γ-surfaces calculated for bcc metals by Vasek Vitek already more than 40 years ago. This approach can be extended to the shuffling of atomic planes that plays a crucial role in martensitic phase transformations. Similarities and differences between displacive processes of lattice shearing and atomic plane alternate shuffling are discussed.     

On the effect of cobalt doping on thermoelastic martensitic transformations in ferromagnetic Heusler Ni50 − x Co x Mn29Ga21 magnetically controlled shape memory alloys

The phase composition, structure, magnetic and thermoelastic martensitic transformations, and physical properties (magnetic susceptibility, magnetization, electric resistivity) in multicomponent magnetic Ni_{50 ? x} Co _x Mn₂₉Ga₂₁ alloys (with a Co content of x = 0, 1, 2, 3, and 10 at %) exhibiting a magnetically controlled shape memory effect are studied. The critical temperatures of the magnetic transitions and thermoelastic martensitic transformations are determined. It is shown that the studied alloys in the austenitic state are substitutional solid solutions based on a quasi-ternary L2₁ superstructure (Ni, Co)₅₀Mn₂₉Ga₂₁.     

Martensitic transformation and shape memory properties of Ti-Ta-Sn high temperature shape memory alloys

The effects of Ta and Sn contents on the martensitic transformation temperature, crystal structure and thermal stability of Ti-Ta-Sn alloys are investigated in order to develop novel high temperature shape memory alloys. The martensitic transformation temperature significantly decreases by aging or thermal cycling due to the formation of ω phase in the Ti-Ta binary alloys. The addition of Sn is effective for suppressing the formation of ω phase and improves stability of shape memory effect during thermal cycling. The amount of Sn content necessary for suppressing aging effect increases with decreasing Ta content. High martensitic transformation temperature with good thermal stability can be achieved by adjustment of the Ta and Sn contents. Furthermore, the addition of Sn as a substitute of Ta with keeping the transformation temperature same increases the transformation strain in the Ti-Ta-Sn alloys. A Ti-20Ta-3.5Sn alloy reveals stable shape memory effect with a martensitic transformation start temperature about 440 K and a larger recovery strain when compared with a Ti-Ta binary alloy showing similar martensitic transformation temperature.     

Effect of loading prehistory on phase transformations in chromium-nickel steel with plastic deformation

Results are given for a study of the kinetics of phase transformations in chromium-nickel austenitic steel with repeated plastic deformation including under conditions when prior and repeated loading differ in temperature and form of stressed state. It is established that prior low-temperature deformation intensifies the process of martensite formation at room temperature whereas deformation under room temperature conditions has almost no effect on martensitic transformation kinetics at low temperature.Translated from Problemy Prochnosti, No. 10, pp. 46–50, October, 1991.     

Improvement of NiTi Shape Memory Actuator Performance Through Ultra‐Fine Grained and Nanocrystalline Microstructures

Ultra‐fine grain sizes have been shown to enhance some key mechanical and functional properties of engineering materials, including shape memory alloys. While the effect of ultra‐fine and nanocrystalline grain sizes on pseudoelastic shape memory materials is well‐appreciated in medical device engineering, the effect of such microstructures on actuators has not been sufficiently characterized. In the present work, it is demonstrated that NiTi spring actuators with ultra‐fine grained microstructures can be obtained by conventional wire drawing in combination with heat treatments and that the final grain size can be controlled by varying the final annealing temperature. Annealing at 400 °C for 600 s allows for the evolution of microstructures with median grain sizes of about 34 nm, while annealing at 600 °C for the same length of time results in median grain sizes of about 5 µm. It is observed that the grain size strongly affects the elementary processes of the martensitic phase transformation. Small austenite grain sizes inhibit twinning accommodation of transformation strains, such that a higher driving force is required to nucleate martensite. This increase in the martensite nucleation barrier decreases the martensite transformation temperatures such that only partial transformation to martensite is possible upon cooling to room temperature. The incomplete martensitic transformation reduces the exploitable actuator stroke; however, a reduction in grain size is shown to improve the functional stability of the material during thermal and thermomechanical cycling by reducing the irreversible effects of dislocation plasticity.