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。     

Crystallographic texture evolution and martensite transformation in the strain hardening process of a ferrite-based low density steel

The strain-induced martensite transformation is of great importance in the strain hardening process of ferrite based low-density steel.Based on the microstructure analysis,the texture evolution and martensite transformation behavior in the strain hardening process were studied.The results show that martensite transformation accompanied by TWIP effect and high density dislocations maintains the con-tinuous hardening stage.As the strain increases,the texture of retained austenite evolves towards the F orientation{111}〈112〉,which is not conducive to martensite transformation.After the strain of 5％,the number of austenite grains with high Schmid factor orientations is gradually increased,and then signif-icantly reduced when the strain is over 10％due to the occurrence of martensitic transformation,which results in a high martensitic transformation rate.However,the unfavorable orientation and the reduced grain size of austenite slow down the martensite transformation at the final hardening stage.Moreover,because of the coordination deformation of austenite grains,strain preferentially spreads between adja-cent austenite grains.Consequently,the martensite transformation rate in strain hardening process is dependent on the orientation and grain size evolution of austenite,leading to a differential contribution to each strain hardening stage.     

Two-stage recovery strain of prestrained TiNi shape memory alloy after phase transformations under constraint

A two-stage strain recovery was observed in a TiNi alloy fiber, which had been prestrained and experienced a constraint heating and cooling cycle with a fixed strain constraint. The results of differential scanning calorimeter (DSC) measurements showed that after the thermomechanical process, the self-accommodation martensite in the TiNi alloy was divided into two sorts of martensite, the redeformed martensite and the inheriting deformation martensite. The first stage of the recovery strain vs. temperature curve corresponds to the transformation from the inheriting deformation martensite to parent, and the second one corresponds to the transformation from the redeformed martensite to parent.     

Very high strain-rate response of a NiTi shape-memory alloy

The compressive response of a NiTi shape-memory alloy is investigated at high strain rates, using UCSD’s modified split Hopkinson pressure bar and a mini-Hopkinson bar with specially designed striker bars. To obtain a constant strain rate during the formation of the stress-induced martensite phase in a Hopkinson test, a copper-tube pulse shaper of suitable dimensions or a stepped striker bar is employed, since without a pulse shaper or with a uniform striker bar, the strain rate of the sample will vary significantly as the material’s microstructure changes from austenite to martensite, whereas with proper pulse shaping techniques a nearly constant strain rate can be achieved over a certain deformation range. At a very high strain rate, the yield stress and the stress-induced martensite formation process are significantly different from those at moderately high strain rates, suggesting that, correspondingly, different microstructural changes may be involved in the phase transition regime. The material’s yield stress appears lower when measured in a mini-Hopkinson bar (with very small samples) as compared with that measured by a 1/2-in. Hopkinson bar (with relatively large samples), possibly due to the sample size that may produce different deformation mechanisms within the superelastic strain range. The transition stress from the austenite to the martensite phase shows strain-rate sensitivity. This may be explained by considering the interfacial motion of the formed martensite phase, based on the thermally activated and dislocation-drag models. There exists a certain critical strain-rate level, at which the transition stress for the stress-induced martensite formation equals the yield stress of the austenite phase. Therefore, the shape-memory alloy deforms by the formation of stress-induced martensites, accompanied by the yielding of the martensite phase at this critical strain rate, while the material deforms plastically by the dislocation-induced plastic slip at strain rates above this critical level.     

Influence of Cobalt and Molybdenum Additives on the Structure and Shape Memory Parameters of Reaction-Sintered Porous Nickel Titanium Alloys

We have studied the structure and properties of porous nickel titanium (TiNi) alloys obtained upon reaction sintering of Ti and Ni powders with Co and Mo additives. It is established that Co and Mo doping additives retain the compaction of Ni powder achieved at the initial stage of sintering. The maximum deformation of porous samples loaded in the austenite state was observed upon adding Co, while the addition of Mo resulted in minimum deformation. The addition of Co leads to single-stage martensitic transformation in TiNi phase, while the addition of Mo leads to the two-stage transformation that is more homogeneous over the volume. Both Co and Mo additives lead to increase in the maximum accumulated strain due to the formation of favorably oriented stress-induced martensite and reoriented quench-induced martensite.

     

承受各种循环加载的TiNi形状记忆合金的超弹性变形行为

TiNi形状记忆合金由于其优良的机械性能、抗腐蚀能力和生物适应性得到广泛的使用。超弹性是TiNi形状记忆合金重要的力学性能之一。本文通过实验研究了不同加载速率和不同实验温度下承受完全循环加载以及部分加载卸载的TiNi形状记忆合金超弹性变形行为。分析了循环变形期间马氏体相变应力和弹性模量变化的特性。研究表明在完全循环加载过程中，由于残余应变的存在，马氏体相变应力随循环增加而减小。马氏体相变应力的变化量（即残余应力）与残余应变成线形关系。对于受过循环变形的机械训练的TiNi形状记忆合金，研究了部分加载和卸载情况下其超弹性变形，分析了相变开始与结束的应力特性。     

The effects of austenite phase deformation on microstructure and magnetic properties in Fe–13.4%Mn–5.2%Mo alloy

The effects of austenite phase deformation on martensitic transformations and magnetic properties in Fe–13.4%Mn–5.2%Mo have been investigated by scanning electron microscopy, transmission electron microscopy, and M?ssbauer Spectroscopy. The increase of plastic deformation rates on austenite phase created considerable changes in amounts of ε (h.c.p.) and α′(b.c.c.) martensite, and austenite grains size decreased. Analysis of microstructure and M?ssbauer spectra show that the amount of ε martensite increased at low deformation rates whereas it decreased at high deformation rate. Besides, M?ssbauer spectra of the alloy reveal a ferromagnetic character with a broad sextet for α′ martensite phase and a paramagnetic character with a singlet for the γ (f.c.c.) austenite and ε martensite phases. In the other hand, the magnetic character of the alloy exhibits a different magnetic order depending on strain rates.     

A physical mechanism based constitutive model for temperature-dependent transformation ratchetting of NiTi shape memory alloy: One-dimensional model

In this paper, the effect of test temperature on the transformation ratchetting of super-elastic NiTi shape memory alloy was first investigated in the cyclic tension-unloading tests. It is shown that all the residual strain, dissipation energy, the start stress of martensite transformation and their evolutions during the cyclic loading depend greatly upon the test temperature. Based on the experimental observations, a new one-dimensional constitutive model is constructed by considering two different inelastic deformation mechanisms (i.e., martensite transformation and transformation-induced plasticity). The proposed model employs a new evolution rule of transformation-induced plasticity which considers the physical mechanism of the plastic deformation, i.e., the dislocation slipping in the austenite phase near the austenite–martensite interfaces. Furthermore, the interaction between dislocation and martensite transformation is also taken into account in the proposed model. The capability of the proposed model to predict the uniaxial temperature-dependent transformation ratchetting of NiTi shape memory alloy is verified by comparing the predictions with the experimental data.     

Tensile deformation of a duplex Fe-20Mn-9Al-0.6C steel having the reduced specific weight

The room temperature deformation characteristics of a duplex Fe-20Mn-9Al-0.6C steel with the reduced specific weight of 6.84 g/cm³ in the fully solutionized state were described in conjunction with the deformation mechanisms of its constituent phases. The phase fraction was insensitive to annealing temperature in the range of 800-1100 °C. The ferrite grain size was also nearly unaltered but the austenite grain size slightly increased with increasing annealing temperature. This revealed that there is little window to control the microstructure of the steel by annealing. The steel exhibited a good combination of strength over 800 MPa and ductility over 45% in the present annealing conditions. Ferrite was harder than austenite in this steel. Strain hardening of both phases was monotonic during tensile deformation, but the strain hardening exponent of austenite was higher than that of ferrite, indicating the better strain hardenability of austenite. In addition, the strain hardening exponent of austenite increased but that of ferrite remained unchanged with increasing annealing temperature. The overall strain hardening of the steel followed that of austenite. Considering element partitioning by annealing, the stacking fault energy of austenite of the steel was estimated as ∼70 mJ/m². Even with the relatively high stacking fault energy, planar glide dominantly occurred in austenite. Neither strain induced martensite nor mechanical twins formed in austenite during tensile deformation. Ferrite exhibited the deformed microstructures typically observed in the wavy glide materials, i.e. dislocation cells. The mechanical properties of the present duplex steel were compared to those of advance high strength automotive steels recently developed.     

基于TiNi合金的超弹性对其相关磨损机制的FEM研究

本文在重点考虑TiNi合金高弹性变形量的前提下，采用等向强化模型，对不锈钢和超弹TiNi合金在法向接触载荷作用下的六种模型进行了有限元(FEM)分析。结果表明：在相同载荷条件下，超弹TiNi合金产生的von Mises弹性应变要高于不锈钢，但其von Mises应力和塑性应变却恰恰相反，在同一载荷下该合金发生塑性变形的区域要小于不锈钢；此外，超弹TiNi合金发生塑性变形要比不锈钢困难，所需的临界载荷值随其最大弹性变形量(屈服点处的应变值)的增加而增加。最后，基于本文的有限元计算结果对超弹TiNi合金的蘑粒磨损和疲劳磨损机制进行了讨论。     

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.     

Dynamic precipitation during cyclic deformation of an underaged Al-Cu alloy

The cyclic deformation behavior of Al-4Cu alloy containing shear-resistant particles was investigated systematically as a function of precipitate state. Pronounced cyclic hardening was observed in the under aged Al-4Cu-0.05Sn (wt.%) alloy strained under various imposed plastic strain amplitudes at room temperature. Such cyclic hardening is absent from the longer aging treatments. Microstructural characterization reveals that the pronounced cyclic hardening of the under aged alloy is due to the dynamic precipitation of GP zones. The dynamic precipitation occurs during all the cyclic loading process and only at the peak stress, where the hardening increment from dynamic precipitation saturates, does strain localization occur which is soon followed by failure of the material. The dynamic precipitation of GP zones has a positive effect on the low cycle fatigue performance of this alloy, and can significantly elevate the strength of this alloy without loss in ductility. Experiments performed to test the dependence of the cyclic hardening on plastic strain amplitude and strain-rate illustrate a relatively strain-rate independent and strain amplitude dependent behavior. Such kinetic behavior is approximately consistent with that expected if the GP zone formation is controlled by the vacancies production process during plastic deformation.     

Modelling Residual Strains During Cycling of Ti–Ni and Ti–Ni–Cu Shape Memory Alloys in a Pseudoelastic Range of Behaviour Conditions

Abstract: Pseudoelastic behaviour of three types of Ti–Ni shape memory alloys in a pseudoelastic state has been studied under conditions of maximum strain‐ and maximum stress‐controlled cycling. Experimental results proved that residual deformation after unloading increases with the number of cycles; however, critical stress for the induction of martensite and the energy dissipated in one cycle decline during cycling. A higher critical stress for slip, and more intense cyclic dislocation hardening promoted by greater maximum deformation and greater maximum applied stresses, generally reduce the rate at which residual elongation grows with the number of cycles, and tend to stabilise the cyclic stress‐elongation diagrams. The small magnitude of critical stress for slip in low‐nickel alloys, and also cyclic strain hardening, induce greater internal stresses and a more marked decrease in critical stress for the induction of martensite as cycling progresses. Detailed analysis of plastic deformation propagation in cyclically loaded specimen helped develop a model of dependence of residual elongation on the number of cycles. This model enables identification of three main factors that govern the magnitude of residual elongation: one residual plastic elongation caused by dislocation hardening after the alloy is heat treated, and two cyclic strain hardening parameters describing how residual elongation grows with number of cycles, and how this residual elongation is reduced, as cycles increase, by the rising critical stress level for slip. The model has proved to yield very close agreement with experimental findings.     

Analysis of deformation induced martensitic transformation in stainless steels

Abstract

Many studies monitoring the formation of martensite during the tensile deformation of austenite report data which are, in principle, affected by both the applied stress and the resulting plastic strain. It is not clear in these circumstances whether the transformation is stress induced (i.e. the stress provides a mechanical driving force) or whether the generation of defects during deformation helps nucleate martensite in a scenario better described as strain induced transformation. The authors demonstrate in the present work that a large amount of published data relating the fraction of martensite to plastic strain can in fact be described in terms of the pure thermodynamic effect of applied stress.     

Micro-defect effect on gigacycle fatigue S-N property and very slow crack growth of high strength low alloy steel

Abstract

Transformation induced plasticity (TRIP) assisted steels contain a small quantity of carbon enriched retained austenite, which transforms into martensite during the course of plastic deformation. Transformation of this kind can be induced by both stress and plastic strain. The detailed mechanism by which the martensite is induced is different for these two scenarios. An attempt is made here to discover the relative importance of these mechanisms and it is found that stress affected transformation can explain much of the variation in retained austenite content as a function of plastic strain.     

中锰Q&P钢残余奥氏体的稳定性及其TRIP效应

通过单向拉伸及平面应变实验研究了Mn含量为7%的中锰淬火-配分(QP)钢残余奥氏体的机械稳定性,利用X射线衍射仪(XRD)测定试验钢残余奥氏体的含量,通过观察试验钢的拉伸曲线及扫描电镜(SEM)、透射电镜(TEM)照片,分析变形前后的微观组织,研究中锰QP钢的变形机制。结果表明:应力状态对残余奥氏体稳定性有较大的影响,平面应变更有利于相变诱导塑性(TRIP)效应的发挥;中锰QP钢的拉伸变形特征是由超细晶硬化机制和TRIP效应相互作用产生的,通过微观组织观察发现中锰QP钢的塑性变形主要是残余奥氏体的TRIP效应,其中薄膜状的残余奥氏体的稳定性最高。     

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.     

Transformation start lines in TiNi and Fe-based shape memory alloys after incomplete transformations induced by mechanical and/or thermal loads

The transformation start condition in the stress-strain or strain-temperature planes is examined in a TiNi polycrystalline shape memory alloy and an Fe-based polycrystalline shape memory alloy under mechanical and/or thermal loads. The martensite start and austenite start stresses in the TiNi alloy are revealed to be almost constant during isothermal loading at the pseudoelastic temperature range, being independent of the extent of prior transformations. In the Fe-based alloy, however, both the martensite start stress and the austenite start temperature are strongly dependent on the extent of prior transformations. The alloy performance is fully contrary to the prediction of the thermodynamic theory of 1. Miller and co-workers (I. Müller, 1989, Cont. Mech. Thermodyn. 1, 125; I. Müller and H.B. Xu, 1991, Acta Metall. Mater. 36, 263) and B. Raniecki et al. (1992, Arch. Mech. 44, 261)     

High strain rate behavior, transformation-induced plasticity and fracture toughness characterization of cast and additionally tempered Fe85Cr4Mo8V2C1 alloy manufactured using a rapid solidification technique

This study focuses on the characterization of the microstructures of an FeCrMoVC alloy in two states (an as-cast and a heat-treated state) as well as the compressive strain rate-dependent material and fracture toughness behavior. Both microstructures consist of martensite, retained austenite and complex carbides. Tempering results in a transformation of retained austenite into martensite, the precipitation of fine alloy carbides, and diffusion processes. High yield stresses, flow and ultimate compressive strength values at a relatively good deformability were measured. The yield and flow stresses at the onset of deformation are higher for the heat-treated state due to higher martensitic phase fractions and fine precipitations of alloy carbides respectively. Compressive deformation causes a strain-induced transformation of retained austenite to α′-martensite. Hence, both high-strength alloys are TRIP-assisted steels (TRansformation-Induced Plasticity). However, the martensitic transformation is more pronounced in the as-cast state due to higher phase fractions of retained austenite already in the initial state. Examinations of strained microstructures showed decreased crystallite sizes with increasing deformation. It is assumed that, during plastic deformation, the amount of low angle grain boundaries increases while the incremental formation of α′-martensite leads to decreased crystallite size. In general, lower microstrains were determined in the heat-treated state as a consequence of stress relaxation during tempering. In comparison to commercially available tool steels, the determined fracture toughness K _Ic of both variants revealed relatively high fracture toughness values. It was found that the lower shelf of K _Ic is already reached at room temperature. Higher loading rates $ \dot{K} $ resulted in lower dynamic fracture toughness K _Id values. Notch fracture toughness K _A measurements indicate that the critical notch tip radii of the examined materials are slightly smaller than 0.09?mm.     

The strain induced martensite transformation in austenitic stainless steels: Part 1 – Influence of temperature and strain history

Abstract

The strain induced martensite transformation in austenitic stainless steels is of considerable interest, because it results in materials with attractive combinations of strength and ductility. The present work examines the mechanical response for a variety of strain and temperature paths, and relates these to microstructural observations. New evidence of the detailed transformation sequence is presented, along with direct evidence of codeformation of the austenite and martensite. Using different deformation temperature sequences enables the transformation to be changed from one that is heterogeneous to one that propagates axially along the sample. The strain hardening that occurs due to combined plasticity and martensitic transformation results in high kinematic hardening that is revealed by microstructural observations here, and which are linked directly to the mechanical response of these materials described in Part II of the present work.