Ratcheting Behavior of a Non-conventional Stainless Steel and Associated Microstructural Variations

Ratcheting fatigue behavior of a non-conventional stainless steel X12CrMnNiN17-7-5 has been investigated with varying combinations of mean stress (σ_m) and stress amplitude (σ_a) at room temperature using a servo-hydraulic universal testing machine. X-ray diffraction profile analysis has been carried out for assessing possible martensitic phase transformation in the steel subjected to ratcheting deformation. The results indicate that ratcheting strain as well as volume fraction of martensite increases with increasing σ_m and/or σ_a; the phenomenon of strain accumulation is considered to be governed by the associated mechanics of cyclic loading, increased plastic damage as well as martensitic transformation. A correlation between strain produced by ratcheting deformation and martensitic transformation has been established.     

冷变形对FeMnSiCrNi合金形状恢复率和转变温度的影响

通过Fe16Mn6Si7Cr7Ni合金变形量的大小和形变的类型来研究马氏体的转变特征.形变类型及变形量为:冷轧13%～28%;拉伸4%～17%.并通过热膨胀法测量合金相转变起始温度和转变终了温度,合金的相组成和微观结构特征通过XRD、SEM获得.研究结果表明:冷变形过程均会导致合金的形状恢复率降低,导致应力诱发ε、α/马氏体的形成,并使热马氏体的帆低于室温.     

Detection of shock-wave-induced internal stresses in Cu-Al-Ni shape memory alloy by means of acoustic technique

1. The present results indicate that the stress-induced β₁→γ₁′ martensitic transformation occurs for an impact duration of 2 × 10⁻⁶ s. This time interval appears to be sufficient also for the subsequent deformation of the γ₁′ martensitic phase to occur.2. A structure memory effect has been found: Cu-Al-Ni austenitic crystals, shock-loaded at room temperature to induce γ₁′–martensite, recall during subsequent temperature-induced martensitic transformation the martensitic variant structure (elastic properties) formed under the shock loading.3. Elastic properties of quenched β₁′ and γ₁′ crystals of the Cu-Al-Ni system are extremely sensitive to the shock-wave loading. Mechanisms of these effects, as well as of the structure memory effect, include the generation of internal stresses due to the high elastic anisotropy of the martensitic phases. These internal stresses either change the distribution of martensitic variants or govern the formation of the martensitic variant structure during the temperature-induced martensitic transformation. The generation of high internal stresses by impact loading of the β₁′ martensitic phase is also detected by several anelastic phenomena.4. In contrast to elastic and anelastic properties, transformation temperatures are insensitive to the impact loading, pointing to the difference of structural elements responsible for the anelastic effects and for the interval and hysteresis of the thermoelastic martensitic transformation.     

Effect of superrapid crystallization on the structure and properties of maraging steels

Conclusion Super rapid crystallization of melts for maraging steels has a considerable effect on the nature of their phase transformation, structures, and properties. Depending on solidification rate and subsequent cooling it is possible to form in steel 01N17K12M5T structures of the following types: 1) entirely martensitic; 2) entirely ferritic; 3) a mixture of martensitic and austenitic components. The maximum microhardness is achieved with formation in the test steel of a highly dispersed entirely martensitic structure. An increase in the microhardness of martensitic structure is governed mainly by the small width of martensitic platelets. Results of experimental data are in good agreement with the Hall-Petch relationship, which points to the decisive role of the interface in metal strengthening.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 32–39, January, 1988.     

Time effect of martensitic transformation in Ni43Co7Mn41Sn9

Effect of thermal arrest on the L2₁-tetragonal martensitic transformation in a NiCoMnSn shape memory alloy was investigated. The phenomenon was studied by interrupted heating/cooling in differential scanning calorimetry analysis. The experimental evidence indicates that the forward martensitic transformation continued to completion during cooling arrest between M_s and M_f. The same behavior was also observed for the reverse transformation on heating. These observations demonstrate that the L2₁-tetragonal martensitic transformation in the Ni₄₃Co₇Mn₄₁Sn₉ alloy is time dependent at the finite cooling rate.     

Cavitation failure of austenitic and martensitic steels in liquid oxygen

Conclusion Austenitic steels with unstable austenite at low temperatures have the highest cavitation resistance in liquid oxygen. The cavitation resistance of martensitic (N9, G7) and transition steels (SN2AL) is some-what lower but fairly high, and the cavitation resistance can be increased substantially by tempering in the temperature range at which the reverse martensitic transformation begins. Stabilized austenitic steels (10Kh11N23T3MR, N36, G38) have low cavitation resistance in liquid oxygen.S. M. Kirov Polytechnical Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 9–14, March, 1976.     

马氏体相变切变机制的实验和理论研究——评刘宗昌等的马氏体相变非切变机制

刘宗昌等学者近几年相继在国内杂志发表了有关马氏体相变非切变机制的论文,并将这些论文作为他们2012年出版的《马氏体相变》一书中的主要创新内容予以强调。他们否定已被国际公认的马氏体相变切变机制的主要依据有两条:1)马氏体相变切变机制缺乏实验基础,即缺乏现代实验技术,如透射电镜(TEM),原子力显微镜(AFM)的实验证明;2)根据他们的计算,马氏体切变能太大,相变驱动力无法克服切变能。本文作者首先结合马氏体晶体学表象理论(PTMC)给出论证马氏体相变切变机制的TEM和AFM实验;随后介绍徐祖耀提出的计算马氏体相变切变能的方法;最后评论刘宗昌等的论断和切变能的计算方法。     

Algebra and geometry of martensitic transformations in the iron alloys

The martensitic transformation is a special form of phase transformation that is not associated with the conventional temperature-dependent parameters: the number of centers and the crystal growth rate. The internal stresses are important for this transformation. How can we define the martensitic transformation? Possibly as follows: “The martensitic transformation is a diffusionless phase transformation that is induced by the stresses.” The stream of studies devoted to the martensitic transformation is not abating, and each author often has his own point of view. But they all contribute very little to clarification of the nature of this phenomenon and, unfortunately, even less to their use in practice; for example, for regulation of the kinetics of the transformation. We shall present two articles that are devoted to the profound causes and subtle characteristics of the martensitic transformation.     

A direct method for the crystallography of martensitic transformation and its application to TiNi and AuCd

A simple and straightforward method to obtain a complete set of twining planes and habit planes of martensitic crystals by using the crystallographic data is proposed under the bulk strain energy minimization hypothesis. This method can also be used to obtain the diffraction pattern corresponding to martensitic transformation forming the invariant planes. Applications to the cubic→trigonal and cubic→monoclinic martensitic transformation are presented. The results well explain the morphological differences between R-phase in TiNi and ζ′₂ martensite in AuCd.     

Computational thermodynamics and the kinetics of martensitic transformation

To assist the science-based design of alloys with martensitic microstructure, a multicomponent database kMART (kinetics of MARtensitic Transformation) encompassing the components Al, C, Co, Cr, Cu, Fe, Mn, Mo, N, Nb, Ni, Pd, Re, Si, Ti, V, and W has been developed to calculate the driving force for martensitic transformation. Built upon the SSOL database of the Thermo-Calc software system, a large number of interaction parameters of the SSOL database have been modified, and many new interaction parameters, both binary and ternary, have been introduced to account for the heat of transformation, T ₀ temperatures, and the composition dependence of magnetic properties. The critical driving force for face-centered cubic (fcc) → body-centered cubic (bcc) heterogeneous martensitic nucleation in multicomponent alloys is modeled as the sum of a strain energy term, a defect-size-dependent interfacial energy term, and a composition-dependent interfacial work term. Using our multicomponent thermodynamic database, a model for barrierless heterogeneous martensitic nucleation, a model for the composition and temperature dependence of the shear modulus, and a set of unique interfacial kinetic parameters, we have demonstrated the efficacy of predicting the fcc → bcc martensitic start temperature (M _s ) in multicomponent alloys with an accuracy of ± 40 K over a very wide composition range.     

Composition dependence on the martensitic structures of the Mn-rich NiMnGa alloys

The Mn-rich Ni₅₀Mn_25+xGa_25−x (x=0–5) alloys were developed to investigate the structural transitions and magnetic properties. Structural transitions from austenite to 5M, 7M, and non-modulated martensite were observed with the increase of Mn content. The lattice parameter a elongates, as where b and c contract, and the unit cell volume reduces with increasing Mn content. The martensitic transformation start temperatures M_s increase monotonically from 10.7 °C for x=2 to 102.7 °C for x=5. The saturation magnetization was measured at 5 K, where all the samples exhibit a martensitic structure. The average magnetic moments per Mn atom vary from 4.38 μ_B to 2.93 μ_B for x=0 to x=5. The negative effect of excess Mn atoms changes from −3.00 μ_B for x=2 to −7.25 μ_B for x=5. The excess Mn atoms modify the electronic structures of the unsubstituted Mn atoms, resulting in the sharp decrease of the magnetic moments of the unsubstituted Mn atoms with increasing Mn content. Structural incommensurability was observed with 7M for powder and non-modulated for bulk samper in a specific range of compositions and proved to be reversible when performing martensitic transformation. The 7M and non-modulated martensites Ni₅₀Mn₃₀Ga₂₀ possess similar saturation magnetizations and Curie temperatures. The non-modulated martensite was estimated to have a lower free energy than 7M, and should be more stable for a reverse martensitic transformation, leading to a higher austenite start temperature A_s, which is consistent with the experimental result.     

Microstructure and martensitic transformation of Ni–Fe–Ga–Si ferromagnetic shape memory alloys

The effects of the fourth element Si on the martensitic transformation and magnetic properties of Ni–Fe–Ga magnetic shape memory alloys were investigated. A complete thermoelastic martensitic transformation in Ni–Fe–Ga–Si alloys was observed in the temperature range of 218–285 K. The martensitic transformation temperatures of Ni–Fe–Ga alloys are obviously decreased by the substitution of Si for Ga element, that is, the substitution of 1 at.% Si for Ga leads to a decrease of martensitic transformation temperature of about 39.6 K. Moreover, the substitution of Si for Ga leads to a decrease of the saturation magnetic field and the magnetic anisotropy constant K₁ obviously.     

In situ TEM observation of two-step martensitic transformation in aged NiTi shape memory alloy

Martensitic transformation was investigated in an aged NiTi alloy with DSC and a temperature controllable TEM specimen stage to observe the influence of Ti₁₁Ni₁₄ precipitates and R-phase on martensitic transformation in situ. The R-phase, conventional martensitic twins, and a new morphology of interwoven austenite/martensitic structure were observed.     

Effect of Co addition on martensitic phase transformation and magnetic properties of Mn50Ni40-xIn10Cox polycrystalline alloys

This study investigated the use of Co to enhance the magnetic driving force for inducing the martensitic transformation of Mn₅₀Ni_40-xIn₁₀Co_x alloys. These alloys present a martensitic transformation from a Hg₂CuTi-type austenite to a body centered tetragonal martensite, with a large lattice distortion of 15.7% elongation along the c direction and 8.2% contraction along a and b directions. The martensitic transformation temperatures, transformation enthalpy and entropy changes decreased with increasing the Co content in these alloys. The maximum magnetization of the austenite increased significantly, whereas that of the martensite changed much less prominently with increasing the Co substitution for Ni, leading to increase of the magnetic driving force for the transformation. The magnetization increase of the austenite is found to be due to (i) formation of ferromagnetically coupled Mn–Mn due to new atomic configuration in off-stoichiometric composition, (ii) magnetic moment contribution of Co and (iii) widening of the temperature window for magnetization of the austenite by lowering the temperature of the martensitic transformation. These findings clarify the effect of Co addition on martensitic transformation and magnetic properties in Mn-rich ferromagnetic shape memory alloys, and provide useful understanding for alloy design for magnetoactuation applications.     

Surface saturation of low-carbon martensite steels with boron and copper

Conclusions  

EFFECT OF γ-IRRADIATION ON CuZnAl SHAPE MEMORY ALLOY

Effect of γ-irradiation on the shape memory alloy CuZnAl has been studied by the techniquesof TEM,positron annihilation etc..The martensitic transformation temperature of the alloyincreases obviously ofter γ-irradiation at a dose of 2×10~7 Gy but not for a dose of 1×10~7 Gy.The shape memory effect in both irradiated alloys remains unaffected.     

Microstructure and properties of Ni-Mn-Ga alloys produced by rapid solidification and pulsed electric current sintering

Ni-Mn-Ga alloys were compacted using pulsed electric current sintering (PECS) at 850-875 °C (50 MPa, 8 min) of flake-like powders made from the rapidly quenched melt-spun ribbons. Two kinds of ribbons were used: one made with a relatively slow wheel speed (6 m/s; average grain size ∼14 μm), and another with a faster wheel speed (23 m/s; average grain size ∼5 μm). Both sets of flake-like powders consisted of a mixture of non-modulated martensite (NM) and seven-layered modulated martensite (7M) structure. The amount of NM was greater in the slower speed material, while the other one exhibited mostly the 7M structure. These crystal structures were inherited by the sintered samples. In the compacts having the NM structure the multi-step martensitic reaction overlapped with the magnetic transition, and the Curie temperatures during heating and cooling differed from each other. In the compacts having mainly 7M structure the Curie point was about 100 °C and the martensitic transition took place in the paramagnetic state, while the intermartensitic one occurred in the region of 60-85 °C. This material demonstrated good magnetic properties and saturation magnetization, at best ∼50 emu/g. Mechanical properties of the compacts were good, and comparable to those of the polycrystalline Ni-Mn-Ga samples in compression.     

Suppression of γ phase in Ni38Co12Mn41Sn9 alloy by melt spinning and its effect on martensitic transformation and magnetic properties

Microstructure, martensitic transformation and magnetic properties of melt-spun Ni₃₈Co₁₂Mn₄₁Sn₉ ribbon were investigated and compared with its bulk counterpart. The formation of second phase (γ phase) was prevented by melt spinning. Ni₃₈Co₁₂Mn₄₁Sn₉ ribbon underwent a thermoelastic martensitic transformation at a little higher temperature than the master alloy. Due to the disappearance of γ phase, the magnetic properties of as-spun ribbon in the plane was close to its bulk counterpart. A magnetic-field-induced reverse martensitic transformation was verified in this ribbon under a relatively low magnetic field of 1.5 T. In addition, the kinetic arrest behavior was also observed even though there still existed a forward martensitic transformation when applying a magnetic field of 8 T.     

Computational thermodynamics and the kinetics of martensitic transformation

To assist the science-based design of alloys with martensitic microstructure, a multicomponent database kMART (kinetics of MARtensitic Transformation) encompassing the components Al, C, Co, Cr, Cu, Fe, Mn, Mo, N, Nb, Ni, Pd, Re, Si, Ti, V, and W has been developed to calculate the driving force for martensitic transformation. Built upon the SSOL database of the Thermo-Calc software system, a large number of interaction parameters of the SSOL database have been modified, and many new interaction parameters, both binary and ternary, have been introduced to account for the heat of transformation, T ₀ temperatures, and the composition dependence of magnetic properties. The critical driving force for face-centered cubic (fcc) → body-centered cubic (bcc) heterogeneous martensitic nucleation in multicomponent alloys is modeled as the sum of a strain energy term, a defect-size-dependent interfacial energy term, and a composition-dependent interfacial work term. Using our multicomponent thermodynamic database, a model for barrierless heterogeneous martensitic nucleation, a model for the composition and temperature dependence of the shear modulus, and a set of unique interfacial kinetic parameters, we have demonstrated the efficacy of predicting the fcc → bcc martensitic start temperature (M _s ) in multicomponent alloys with an accuracy of ± 40 K over a very wide composition range.     

Evidence for a monoclinic incommensurate superstructure in modulated martensite

Structural modulation of martensitic phases is regarded as a prerequisite for magnetically or electrically induced reversible strains in shape-memory alloys. Controversy surrounds the crystal structure of modulated martensite. Here, we explore this critical issue through combined spatially resolved microstructural and crystallographic characterizations of a polycrystalline Ni₅₃Mn₂₂Ga₂₅ alloy, by comparing the high-resolution Kikuchi patterns with those predicted according to various hypotheses—7M(IC) or nanotwin combination structure—on the modulated martensite. Detailed analysis has demonstrated that the modulated martensite plates may possess a monoclinic incommensurate superstructure, other than the nanotwinned tetragonal non-modulated structure. Such an incommensurate superstructure can generate a more favorable plate interface configuration for field-driven twinning/detwinning, capable of producing large reversible actuation strain. Moreover, the thermodynamically metastable modulated martensite may transform into the non-modulated martensite by further local lattice distortion, which would degrade the magnetic shape-memory effect. By taking into account the microstructure–property correlation, the ambiguity concerning the modulated martensitic structure could be clarified, which is essential to the understanding of the stability of modulated martensitic phases and their functionality as shape-memory materials.