FCC → FCT martensitic transformation in two-phase Mn–Cu alloys

The fcc → fct martensitic transformation in Mn–Cu alloys, which contain two isomorphous fcc phases with different manganese contents, is considered. The influence of the ratio of contents of these phases, the phase dispersity, and the state of interphase boundaries on the development of the martensitic transformation in the volume of the alloys is described. As a result of these factors, the martensitic transformation can cover the entire volume of an alloy, including particles with the manganese content that is lower than the critical content required for the martensitic transformation to occur in single-phase solid solutions, and can take place only in manganese-rich particles. In the first case, a general tetragonal structure modulated with respect to lattice parameter c forms in the volume of a two-phase alloy.     

Development and characterization of high strength impact resistant Fe-Mn-(Al-, Si) TRIP/TWIP steels

Iron manganese steels with Mn mass contents of 15 to 30 % exhibit microstructural related superior ductility and extraordinary strengthening behaviour during plastic deformation, which strongly depends on the Mn content. This influences the austenite stability and stacking fault energy γ_fcc and shows a great impact on the microstructure to be developed under certain stress state or during severe plastic deformation. At medium Mn mass contents (15 to 20 %) the martensitic γ-ε-ά phase transformation plays an important role in the deformation mechanisms of the TRIP effect in addition to dislocation glide. With Increasing Mn mass content large elongation is favoured by intensive twinning formation. The mechanical properties of plain iron manganese alloys are strongly influenced by the alloying elements, Al and Si. Alloying with Al Increases the stacking fault energy and therefore strongly suppresses the martensitic γ-ε transformation, while Si sustains the γ-ε transformation by decreasing the stacking fault energy γ_fcc. The γ-ε phase transformation takes place in Fe-Mn-X alloys with γ_fcc ≤ 20 mJm⁻². The developed light weight high manganese TRIP and TWIP (twinning induced plasticity) steels exhibit high ultimate tensile strength (600 to 1100 MPa) and extremely large elongation of 60 to 95 % even at high strain rates of έ = 10³ s⁻¹. Particularly due to the advanced specific energy absorption of TRIP and TWIP steels compared to conventional deep drawing steels high dynamic tensile and compression tests were carried out in order to investigate the change in the microstructure under near crash conditions. Tensile and compression tests of iron manganese alloys with varying Mn content were performed at different temperatures and strain rates. The resulting formation of γ twins, ά- and ε martensite by plastic deformation was analysed by optical microscopy and X-ray diffraction. The deep drawing and stretch forming behaviour at varying deformation rates were determined by performing cupping tests and digitalised stress-strain-analysis.     

Transformation behavior of nearly stoichiometric Ni-Mn alloys

The transformation behavior of Ni-Mn alloys in the vicinity of the stoichiometric composition has been studied using transmission electron microscopy, X-ray diffraction, optical microscopy, and electrical resistivity measurements. The transformation behavior was found to be markedly different in Mn-rich alloys and Ni-rich alloys. In Mn-rich alloys a martensitic transformation between L2₀ (B2) and L1₀ structures takes place, which possesses many features common to alloys exhibiting a thermoelastic martensitic transformation. On the other hand, in Ni-rich alloys an order-disorder transformation between A2 and L1₀ structures occurs. The martensitic transformation features {111} transformation twins as the transformation substructure while the ordering reaction involves {101} order twins. In the Mn-rich alloys, the martensitic phase, if either slowly cooled or annealed at intermediate temperatures, becomes “tempered”, resulting in a noncrystallographic, essentially featureless microstructure apart from the presence of occasional {111} twins. Formerly with University of Illinois.     

High-resolution analytical electron microscopy study of isothermal plate-shaped products in some β-phase alloys

The isothermal plate-shaped products in β-phase Cu-Zn-Al, Cu-Zn-Au, and Ag-Cd alloys have been studied from the viewpoints of morphology, crystal structure, and composition using a high-resolution analytical electron microscope equipped with a field-emission electron gun. It is shown that thin plate-shaped products, at early stages of the isothermal formation, exhibit crystallographic properties characteristic of martensites formed at subzero temperatures but that their compositions were definitely different from those of the surrounding matrices. Therefore, atom diffusion is conceivable to be involved in the plate formation from the nucleation stage and to occur across the coherent plate/matrix interfaces. In order for nucleation of the isothermal transformation to occur, two kinds of reactions should be thermally activated at intermediate temperatures above T_o for the martensitic transformation: (1) atom diffusion to achieve the observed composition differences between plates and matrices and (2) martensitic transformation in hypothetical alloys with compositions less than those of the original ones by the observed composition differences. The incubation periods experimentally observed should be associated with these two thermal activation processes. This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18–22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

Potentiel pseudoelastique et plasticite de transformation martensitique dans les monoet polycristaux metalliques

The subject of this study is the behaviour of single and polycrystals undergoing a thermoelastic martensitic transformation which leads to creation of different variants of martensite. The overall behaviour is the superposition of contributions from the phase transformation and from other mechanisms of deformation—for example plasticity. Two classes of behaviour can then be distinguished: pure transformation plasticity exclusively due to phase transformation (the martensitic transformation); and coupled transformation plasticity in which plastic flow couples with the intrinsic (martensitic) contribution. For the first, a thermomechanical analysis based on the Gibbs-free energy gives the transformation criterion and the associated flow rule. The analysis, for the case of a single crystal, introduces the interaction matrix between the variants of the created martensite. This matrix determines the different classes of interaction between the variants. A phenomenological constitutive relation (transformation threshold for the rule of strain hardening) has been proposed for a polycrystal. It is similar to that proposed by Drücker and Prager. The results of theoretical analyses have been compared with the experimental data, for both single and polycrystals, obtained for pseudoelastic alloys of the Cu-Zn-Al type.     

Effects of Sm on Phase Transformation in Ni-Mn-Ga Alloys

The effects of small amount additions of Sm on the martensitic transition and magnetic phase transition of polycrystalline Ni-Mn-Ga alloys were investigated. The experimental results show that the Sm doped alloys also undergo a thermal-elastic martensitic transformation and reverse transformation during cooling and heating process and the addition of Sm decreases the martensitic transformation temperature and Curie temperature in different degree respectively. Ni-Mn-Ga alloys of adding Sm still possess Heusler structure, but their crystal lattice parameters are modified slightly. The addition of a proper amount of Sm does not basically decrease Tc of the alloy when avoiding the appearance of second phase. In addition, the doped alloys have favorable toughness because of grain refinement of Sm.     

基于Thermo--Calc的中锰中铝Fe--Mn--Al--C低密度钢类Schaeffler相图绘制与评估

含铝低密度钢由于其较好的综合力学性能和低密度特征引起结构钢领域研究人员的广泛关注.本文利用ThermoCalc热力学计算软件结合TCFE 7数据库,计算中锰中铝含量Fe-Mn-Al-C钢在不同温度的热力学平衡状态,总结其两相区相比例的变化规律,通过平移和修正等处理方法,绘制针对中锰中铝钢合金成分和相设计的类Schaeffler相图.结合马氏体转变温度的计算讨论对应不同合金成分条件下相种类存在可能,并通过已有材料的相比例和相形貌实验结果分析绘制的类Schaeffler相图的准确性和适用性.绘制的Fe-Mn-Al-C类Schaeffler相图可以直观地提供不同合金成分所对应的相比例、相种类等信息,可用于新型含铝低密度钢的合金设计.      

Homogeneous martensitic nucleation in FeCo precipitates formed in a Cu matrix

The martensitic transformation on subambient cooling has been monitored in defect-free nanocrystalline f.c.c. FeCo particles that have been coherently precipitated in a Cu matrix; such a CuFeCo system was chosen for study because the f.c.c. → b.c.c. transformational driving force in FeCo alloys is exceptionally large. Transformation is found to occur at a driving force of ∼ 10kJ/mol, a factor of 7 higher than the known critical driving forces for heterogeneous nucleation in bulk alloys. The observed transformation characteristics are entirely consistent with classical homogeneous coherent nucleation, whereas heterogeneous nucleation and homogeneous semicoherent nucleation can be ruled out in this case. An observed variation in transformation temperatures is explained by the experimentally-determined differences in Co content (and, hence, in transformational driving force) among the FeCo precipitates in relation to their distribution of particle sizes. The role of thermal activation in the homogeneous nucleation process is demonstrated by applying a high magnetic field to impose an increment of driving force at low temperatures.     

Ni55Mn25Ga18Ti2高温形状记忆合金的热循环稳定性

选择双相韧化的Ni?Mn?Ga?Ti高温形状记忆合金为研究对象。制备了淬火态Ni₅₅Mn₂₅Ga₁₈Ti₂高温形状记忆合金,并对其在室温至480 ℃之间进行高达500次的相变热循环,获得了5, 10, 50, 100和500次热循环态样品。采用X射线衍射、扫描电镜、能谱仪、同步热分析仪及室温压缩等实验方法,研究了淬火态和热循环态合金样品的微观组织、相变行为、力学及记忆性能,进而分析其热循环稳定性。研究结果表明:经500次循环后,Ni₅₅Mn₂₅Ga₁₈Ti₂合金相结构和显微组织未发生明显变化,均为由非调制四方结构的板条马氏体相和面心立方富Ni的γ相组成的双相结构;随着循环次数增加,马氏体相变温度几乎不变,逆马氏体相变温度和相变滞后在循环5次后趋于稳定;抗压强度及压缩变形率波动幅度较小;形状记忆性能下降,但形状记忆应变仍保持在1.4%以上;Ni₅₅Mn₂₅Ga₁₈Ti₂高温形状记忆合金显示出良好的热循环稳定性。      

Load and sliding velocity effect in dry sliding wear behavior of CuZnAl shape memory alloys

The wear behavior of shape memory alloys is linked to the thermoelastic martensitic transformation. Due to this transformation, these alloys have the ability to absorb a high amount of energy before undergoing plastic deformation and subsequent fractures caused by wear. In this study, the effect of sliding velocity and load on the dry wear behavior of CuZnAl alloys has been characterized. Weight loss as a function of the M_s transformation temperature at different sliding velocities and loads was studied for the different alloys. The weight loss and friction coefficient of the alloys as a function of load showed linear and exponential relationships, respectively; however, when considered versus applied sliding velocity, independently of which phase was present, they showed an exponential relation and no direct relation, respectively.     

Ti-V-Al轻质记忆合金的研究进展

Ti-V-Al合金基于热弹性马氏体相变而呈现出形状记忆效应。同时,Ti-V-Al合金不仅呈现出良好的冷热加工性能,还具有较低的密度,这可满足当今航空航天领域对轻量化制造的需求。文中主要综述国内外研究学者在Ti-V-Al轻质记忆合金研究方面的重要工作和进展,其中重点阐述了Ti-V-Al轻质记忆合金热循环稳定性、力学性能与功能特性方面的研究。最后,简单阐述了Ti-V-Al轻质记忆合金功能特性的演化规律与机制,并对后续Ti-V-Al轻质记忆合金的发展方向进行了展望。      

Kinetics of martensitic transformation induced by a tensile stress pulse

The kinetics of martensitic transformation induced by a tensile stress pulse (generated by the reflection of a compressive shock wave at a free surface) in time durations in the microsecond range, were studied in an Fe-32wt%Ni-0.035wt%C alloy. The tensile hydrostatic component of stress interacts with the dilatational strain (~0.04) of the martensitic transformation and increases the M_s temperature. Shock waves were produced by normal impact of a projectile on a target in a one-stage gas gun. Impact experiments were conducted by varying either the temperature (−10 to −50°C), or pulse duration (0.22−1.76 μs) at a constant pressure. The martensitic transformation, normally considered to be athermal in Fe-Ni-C alloys, exhibits an isothermal nature in the microsecond regime. The fraction transformed increases with decrease in temperature at a constant pulse duration, and increase in pulse duration at a constant temperature. The mean volume of the lenticular martensite was found to be constant throughout the progress of the transformation, consistent with the autocatalytic spreading of clusters. The activation energies for γ→α' transformation in the Fe-32wt%Ni-0.035wt%C alloy, calculated with a modified version of Pati and Cohen's kinetic equation [Acta metall. 17, 189 (1969)], range from 38,000 J/mol at −10°C to 25,000 J/mol at −60°C. The activation energies are linearly related to the total driving force (chemical free energy change + mechanical work due to the transformation). The activation volume for the transformation was calculated and found to be equal to approximately 60 atoms (0.7nm³). This indicates that the martensitic nucleation in this alloy, and under the imposed stress conditions, is interface-mobility controlled.     

Microstructural influence of Mn additions on thermoelastic and pseudoelastic properties of CuAlNi alloys

The thermoelastic and mechanical properties of CuAlNiBMn shape memory alloys have been studied as a function of manganese concentration and of heat treatment. Below a limiting value of manganese content, the loss of thermoelastic and pseudoelastic properties has been observed, in particular in the quenched specimens. The partial transformation and its degradation during thermal cycling observed in the low manganese content alloy has been attributed to the lower degree of B2 order achieved during the quench, leading to slower kinetics of DO₃ ordering. The accomodation of strains between martensite variants and between the martensite/austenite phases appear to need dislocation accumulation at their interfaces. The presence of dislocations observed during the reverse transformation seem responsible for the degradation of the transformation and the loss of pseudoelastic properties of this alloy.     

The effect of composition on the pressure-induced HCP (∈) transformation in iron

The pressure-induced phase transformations in iron-rich Fe?Mn and Fe?Ni?Cr alloys were studied using an opposed diamond anvil high-pressure X-ray diffraction unit and a liquidmedium hydrostatic pressure apparatus. Transformations occurring with both increasing and decreasing pressure were studied. It was found that alloy additions of manganese and of nickel plus chromium significantly reduce the formation pressure of the hcp phase and can in some cases stabilize the phase enough to prevent it from transforming into some other phase during pressure release. All of the transformations are shown to be martensitic. Pressurization of prepolished surfaces, a large transformation pressure hysteresis, and the “abaric” formation of the ∈ phase establish the transformation as martensitic.     

Stress-induced martensitic transformation and impact toughness of cast irons and high-carbon Fe-Ni-C steel

The relationship between the impact toughness and stress-induced martensitic transformation, which occurs during the impact process, has been studied in white cast irons and an Fe-Ni-C alloy at different temperatures. The experimental results have shown that in the brittle white cast irons, the stress-induced martensitic transformation makes a positive contribution to the impact toughness, and lowering the stability of austenite increases the toughness. In contrast, the transformation makes a negative contribution to the toughness of high-carbon austenitic steels, and lowering the stability of austenite decreases the toughness. The present work supports the early theory^[1] that the magnitude of the toughness change depends on the fracture properties of the new phase and the energy being dissipated during the transformation process. Using the crystallographic model for the stress-induced martensitic transformation, which was originally developed in ceramics and was then refined and extended to irons and steels, the effect of the stress-induced martensitic transformation on the impact toughness can be predicted.     

中国超超临界电站锅炉关键材料用钢及合金的研究现状

 综述了近几年国内用于超超临界火电机组锅炉的马氏体耐热钢、奥氏体耐热钢、镍基耐热合金的研究情况。国内学者们对马氏体和奥氏体耐热钢高温强化机理和生产工艺进行了系统研究,并实现了马氏体/铁素体耐热钢T/P91、T/P92与奥氏体耐热钢S31042、S30432、TP347HFG的国产化,对新型9%Cr马氏体耐热钢展开了研究。对镍基合金的投入和研究还不够,研究内容不够系统,但表现出较好的势头,学者们正对Inconel 740、Alloy 617及其改型合金和Alloy 263合金进行较广泛的研究。对用于超超临界火电机组关键材料的研究提出了建议,展望了国内耐热钢及合金的研究趋势。     

Phase Transformation Behaviors and Effects of Terbium in Polycrystalline Ni-Mn-Ga Magnetic Shape Memory Alloys

The phase transformation behaviors of two kinds of magnetic shape memory alloys NisoMn25 x Ga25-x and Ni50 Mn29Ga21-x Tbx were studied. When the composition of Ni in these alloys was constant, increasing Mn and reducing Ga contents make martensitic transformation temperatures rise obviously. Simultaneously, thermal hysteresis of phase transformation reduce but Curie temperature una|ters. When terbium was added, phase transformation temperature went up further and Curie temperature kept constant. The alloys still show strong ferromagnetism and properties of thermoelastic martensite phase transformation.     

Transformation relaxation and aging in a CuZnAl shape-memory alloy studied by modulated differential scanning calorimetry

The reverse martensitic transformation and aging processes in a polycrystalline Cu-23.52 at. pct Zn-9.65 at. pct Al shape-memory alloy have been studied using the recently developed modulated differential scanning calorimetry (MDSC) technique, and some new findings are obtained. By separating the nonreversing heat flow from the reversing heat flow, MDSC can better characterize the thermodynamic, kinetic, and hysteretic features of thermoelastic martensitic transformations. Two kinds of exothermal relaxation peaks have been identified and separated from the endothermal reverse martensitic transformation: one is associated with the movement of twin interfaces or martensite-parent interfaces, and another is due to the atomic reordering in the parent phase via a vacancy mechanism. The martensite aging processes have been examined, and two stages of the aging process been distinguished: the first stage of aging is characterized by the stabilization of martensite, as manifested in the increase in the reversing enthalpy of the reverse martensitic transformation and in the transformation temperatures, and the second stage is, in fact, the decomposition of the martensite on prolonged aging, accompanied by a decrease in the transformation enthalpy. The results suggest that the mechanisms of the relaxation in the martensite and in the parent phase may be quite different.     

Thermodynamics of the iron martensitic transformation and the ms temperature of iron

Based on the methodology put forward by Hsu on the thermodynamics of martensitic transformations of iron-carbon alloys, some aspects of thermodynamics of the iron martensitic transformation have been investigated by employing relatively reasonable experimental data. The M_s temperature of iron is 700 °C, as obtained in this paper. This is in good agreement with major experimental data in the literature. The driving force for the iron martensitic transformation is 438 J/mol, and the yield point of γ-iron at its M_s temperature is 46.3 MPa. Results presented in this paper can play as a modification to Hsu’s methodology. WEI QIUMING, formerly Ph.D. Candidate with the Department of Materials Science and Engineering, Northwestern Polytechnical University     

Development of Fe-based shape memory alloys associated with face-centered cubic-hexagonal close-packed martensitic transformations: Part II. transformation behavior

As part of a study on the newly developed Fe-based shape memory alloys associated with face-centered cubic-hexagonal close-packed (fcc-hcp) martensitic transformations, transfor-mation behavior is characterized utilizing a combination of electrical resistance, dilatometry, and magnetic susceptibility measurements. The characteristics of thermally induced and strain-induced ε martensitic transformations under the influence of antiferromagnetism are discussed based on the experimental results. The variations of shape memory properties with prestraining temperature are interpreted in terms of the transformation characteristics. It is shown that the ε martensite can be readily strain-induced under the stabilization effect of the antiferromagnetism which strongly suppresses the thermally induced transformation. The strain-induced transfor-mation of ε martensite is more preferred as a predominant deformation mechanism at low tem-peratures under a combined influence of the antiferromagnetism and other physical factors, whereas the irreversible deformation mode is more likely with prestrain at relatively high tem-peratures. The transformation characteristics can be significantly changed by alloying and mechanical /thermal treatments. This offers a possibility of developing new practical Fe-based shape memory alloys with a wide range of mechanical and physical properties.