Martensite Lattice Research Articles

Нейтронографическое исследование кинетики низкотемпературного распада мартенсита среднеуглеродистой стали

It is found that the low-temperature decomposition of martensite in quenched medium-carbon steel occurs in two stages. In the first stage, the rate of decomposition is higher than that in the subsequent stage. Application of the neutron diffraction method allows the identification of two stages of transformation in the first stage of martensite decomposition. It is shown that the first stage is associated predominantly with carbon segregation at dislocations, and the second, with the outdiffusion of carbon from the supersaturated solid solution with the formation of dispersed particles of metastable carbides. It is shown that the change in the concentration of carbon and, accordingly, the degree of tetragonal lattice of martensite at aging and low tempering occurs to a certain limit, independent of the cooling rate during quenching and tempering temperature. This is due to the establishment of a relative equilibrium between a supersaturated solid solution and fine particles of metastable iron carbide. It is found that the determining process, which leads to a change in the microhardness the low-temperature decomposition, is the out diffusion of carbon from the supersaturated solid solution.

Peritectoid carbide transformation based on ε-carbide Fe2C in Fe-C-system alloys. Part 1. Basics of theory

In the present work low-temperature carbide phase transformations in the system of Fe-C alloys based on ε-сarbide Fe2C with consideration of identification of θ-Fe3C cement as a solid solution were studied. It has been proved that the θ-Fe3C cement slurry is colourfastonide, and the ε-Fe2C carbide slurry is bertollide. When tempering hardened steels, ε-сarbide Fe2C is emitted in the structure of hardened martensite, which is absent in the phase diagram of iron-carbon system alloys. It is believed that ε-сarbide Fe2C is not a stable phase, and since it is metastable, it is formed only in quenched steels under non-equilibrium conditions. The isolation and dissolution of ε-сarbide Fe2C is a classic phase transformation and the absence of this transformation on the diagram is not caused by the metastable phase of ε-сarbide Fe2C, but by the incomplete iron-carbon diagram. The martensite decomposition phase transformation is based on the formation of carbon enriched zones. The processes of carbon segregation on dislocation structures and grid planes of martensite create zones with excess energy. Beginning approximately with temperature 100 °С in structure of martensite begins to allocate ε-сarbide Fe2C, finishing a stage of two-phase segregational disintegration of martensite. At rather small concentrations of carbon in cluster zones the fastest and most effective way of relaxation of redundant energy in these zones, as well as in the tetragonal lattice of martensite is the formation of phases with low value of work of nucleation, first of all ε-сarbide Fe2C and α-Fe(C) or ferrite. The main stages of phase transformations in the peritectoid reaction of martensite decomposition are considered. It is proposed to introduce the peritectoid transformation horizontal at 382 °C and the peritectic transformation horizontal of cement at 1650 °C into the Fe-C alloy state diagram.

Phase transformation and magnetocaloric effect of Co-doped Mn–Ni–In melt-spun ribbons

Ribbon-shaped magnetocaloric materials are favorable to achieve high heat-transfer efficiencies due to their large specific surface area. In this work, Mn50Ni41−xIn9Cox (0 ≤ x ≤ 4) ribbons were prepared using a melt-spinning technique, and the corresponding phase transformation and magnetocaloric properties were studied. The large temperature gradient during melt-spinning caused the initial austenite in the ribbons to form typical columnar-shaped grains with a strong ⟨001⟩A preferred orientation perpendicular to the ribbon plane. After cooling, the ribbons undergo martensitic transformation from cubic austenite to monoclinic eight-layered modulated (8 M) martensite. High angle annular dark field-scanning transmission electron microscopy observations indicate that martensite lattice modulation is inhomogeneous at atomic scales. Co substitution for Ni not only strongly influences the phase transformation temperatures but also greatly enhances ferromagnetic coupling. As a result, an enlarged magnetization difference across the martensitic transformation under a field change of 5 T in the Mn50Ni38In9Co3 ribbon induces a large magnetic entropy change up to 12.1 J kg−1 K–1 and a refrigeration capacity of 197 J kg–1 around room temperature. In addition, a wide operational temperature region up to 31 K is obtained in the Mn50Ni37In9Co4 ribbon due to the enhanced sensitivity of the transformation temperature shift under a magnetic field.

Prediction of the Bain spin memory materials (BSMM) revealed by Kaneyoshi theory

ABSTRACT In this work, we investigate the spin transformation from ferromagnetic austenite BCT lattice to antiferromagnetic martensite BCC lattice (ji = +J → ji = -J or ↑↑→↑↓) in Bain transformation (or Bain strain) by using effective field theory which is developed by Kaneyoshi. We find that the spin orientation changes the magnetic properties in the Bain BCT→BCC transformation. Therefore, we predict that material can memory their spin orientations in the austenite-martensite transformation similar to the shape memory effect in the shape memory materials. Since the austenite-martensite transformation mechanism is first modelled by Bain, we call this spin memory effect as 'Bain spin memory materials (BSMM)’.

ВПЛИВ ВУГЛИЦЮ НА КОРОЗІЙНУ ПОВЕДІНКУ ТЕРМІЧНО ЗМІЦНЕННОГО ПРОКАТУ

In this work, we studied the corrosion behavior of hot-rolled and heat-hardened rolled products made of carbon steel with different carbon content in order to select the optimal conditions for its heat treatment to obtain a satisfactory combination of strength and corrosion resistance during the development of processes of general corrosion with hydrogen depolarization. The studies were carried out on samples of carbon steel (0.14-0.8% C) in hot-rolled and heat-hardened states. It has been established that the corrosion rate (K) in specimens made of steels in the quenched state is lower than in the hot-rolled state; however, even in this state, with an increase in the carbon content within the studied limits, it increases 40 times. An increase in the corrosion rate with an increase in the carbon content in hot-rolled steel is explained by an increase in the amount of pearlite and an increase in the length of the ferite-cementite interphase boundaries. It was determined that in quenched hardened steel in the carbon content range from 0.14 to 0.80 % C, an increase in the corrosion rate is apparently due to an increase in microstresses (accumulation of excess internal energy) associated with an increase in the tetragonality of the martensite lattice and the appearance of concentration inhomogeneity in the solid solution. It was found that during the course of corrosion with hydrogen depolarization, the dependences of the general corrosion rate of thermally hardened structural steels on the tempering temperatures are described by extreme dependences with a maximum corrosivity in the temperature range of 350-500 oC, while, with an increase in the carbon content in steel, rental increases. It was determined that at the tempering temperature corresponding to the maximum K, the corrosion rate is 1,5-3,6 times higher than in the quenched state, and 1,4-2,6 times higher than in hot-rolled, and with an increase in the carbon content this difference is growing. An increase in the rate of corrosion with hydrogen depolarization in the tempering temperature range of 350-450 oC is apparently associated with the process of carbide formation and the appearance of coherent stresses during the release of dispersed particles. An increase in corrosion losses in this interval with an increase in carbon content is due to an increase in the volume fraction of fine particles of the carbide phase and an increase in the level of elastic microdistortions of the crystal lattice (Δa/a).

Macroscopic modeling of strain-rate dependent energy dissipation of superelastic SMA dampers considering destabilization of martensitic lattice

Superelastic shape-memory alloys (SMAs) are unique smart materials with a considerable energy dissipation potential for dynamic loadings with varying strain-rates. The energy dissipation arises from a hysteretic phase transformation of the polycrystalline atomic grid structure. In fact, the nucleation from austenite to martensite phase and vice versa exhibits a strong thermomechanical coupling. The hysteresis depends on the latent heat generated by the austenitic-martensitic transformation and the convection of that heat. Due to the thermomechanical coupling, the martensitic nucleation stress level and thus the propagation of martensitic phase fronts strongly depends on the material temperature. High strain-rate interferes with the release of the latent heat to the environment and determines quantity, position and propagation of martensitic transformation bands. Lastly, high strain-rate reduces the hysteresis surface. The propagation velocity and quantity of martensitic bands have an impact on martensitic phase stability. The degree of atomic disorder and accordingly the change in entropy influences the reverse phase transformation from martensite to austenite. In other words, the stability of the martensitic state affects the stress level of the reverse transformation. However, in phenomenological superelastic SMA models, which are used to simulate energy dissipation behavior, the effects of the rate-dependent entropy change are not considered in particular. To incorporate the rate-dependent entropy change, we improved a one-dimensional numerical model by introducing an additional control variable in the free energy formulation for solid-solid phase transformation of superelastic SMAs. In this model, the observed effects of the strain-rate on the reverse transformation are taken into account by calculating the rate-dependent entropy change. A comparison of the numerical results with the experimental data shows that the model calculates the dynamic superelastic hysteresis of SMAs more accurately.

The Effect of Local Arrangement of Excess Mn on Phase Stability in Ni–Mn–Ga Martensite: An Ab Initio Study

The effect of excess Mn on the stability of modulation and elastic properties was investigated using ab initio electronic structure calculations in Ni–Mn–Ga magnetic shape memory alloy. We used the structure of four layered modulated martensite known as 4O to describe modulation of the martensitic lattice. We found that elastic properties of stoichiometric 4O martensite are very similar to elastic properties of 10M martensite reported in previous calculations. The modulated structure becomes less stable than nonmodulated martensite above 3 at.% of excess Mn which corresponds very well to experimental observation at low temperature. Elastic properties of NM martensite are not significantly affected by Mn content nor local arrangements of Mn-excess atoms. We also found that Mn-excess atoms prefer occupation of distant positions for low Mn-excess composition. The occupation of closest position is preferred for alloys with higher content of Mn.

The reinforcement role of deep cryogenic treatment on the strength and toughness of alloy structural steel

Deep cryogenic treatment (DCT) is believed to have superior performance in coordination with traditional heat treatment. In present work, the effect of DCT combined with inter-critical quenching treatment on the mechanical properties and microstructure of 30CrMnSi alloy structural steel was investigated. Before DCT employed, quenching treatment was conducted at different austenitizing temperatures (780 °C, 820 °C and 900 °C) to obtain diverse initial states of microstructure. The influence of different sequence between DCT and tempering was also studied. The results showed that compared to quenching-tempering (QT) treatment, the impact toughness of 30CrMnSi alloy austenitized at all the three temperatures was improved by conducting DCT after quenching and tempering (QTC). The strength and toughness were concurrently improved by quenching-tempering-cryogenic (QTC) treatment at the austenitizing temperature of 820 °C. Microstructural characterizations were carried out by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron backscattered diffraction (EBSD). Strip ferrite and lath martensite were refined, as well as high-angle grain boundaries increased after 820-QTC. Deep cryogenic treatment has also promoted the dispersed precipitation of stable carbides in the boundaries of ferrite and martensite, through constricting the martensite lattice and increasing the nucleation energy under ultra-low temperature.

Combinatorial synthesis of Ni–Mn–Ga-(Fe,Co,Cu) high temperature ferromagnetic shape memory alloys thin films

High temperature ferromagnetic shape memory alloys (HT-FSMAs) in form of a thin film materials library of Ni–Mn–Ga, alloyed with Fe, Co and Cu has been fabricated by co-sputtering and characterized by high-throughput screening techniques. The weak dependence of martensitic transformation temperature and tetragonal ratio of the martensitic lattice on the doping composition and their irregular variation are ascribed to the competitive influence of the alloying elements Fe, Co and Cu.

Highly mobile twin boundaries in seven-layer modulated Ni–Mn–Ga–Fe martensite

We report on low twinning stress of 0.2 MPa for type 1 twin boundaries with rational (101) twinning plane and 0.1 MPa for type 2 twin boundaries with rational [1¯01] twinning direction, in Ni2MnGa0.8Fe0.2 seven-layer modulated martensite exhibiting about 12% stress-induced strain. The studied samples were free of compound twins with (100) and (110) twinning planes. The martensite lattice is monoclinic and shows a long-period commensurate modulation with wave vector q=(2/7)g110, where g110 is the reciprocal lattice vector. The investigated alloy has high potential for practical applications due to the low twinning stress and the elevated Curie point TC=400 K.

Evaluation of structure and magnetic properties of maraging steel powder feedstock for additive manufacturing of magnetic components

We investigated the structure as well as the magnetic properties of a commercial maraging steel powder feedstock—EOS MS1—and elucidated the stability of supersaturated martensite phase upon thermal-treatment. The investigation unfolds the potential reusability/recyclability of the powder feedstock, which is essential from an economic as well as an environmental perspective in the manufacturing of magnetic components. The as-supplied powder feedstock predominantly constituted martensite (lattice parameter ∼0.2881 nm). The coercivity (HCI), remanence (MR), and magnetic saturation (MS) of the powder feedstock were estimated as 3.03 ± 0.12 kA/m, 1.4 ± 0.1 A m2/kg, and 176.0 ± 2.0 Am2/kg, respectively. The supersaturated martensite phase was stable during thermal treatment. The MS, MR, and HCI of the thermally-treated powders, on an average, were ∼1% more, ∼0.8 times less, and ∼0.9 times less, respectively, than the as-supplied powder feedstock: thermal-treatment marginally enhanced the soft-magnetic behavior. The similarities in the structure and the magnetic properties between the as-supplied and thermally-treated powders suggest that the powder feedstock is reusable/recyclable in the manufacturing of magnetic components.

The Influence of Molybdenum Equivalent on the Anisotropy of Thermal Expansion of Titanium Martensite Lattice

.

Parameters Evolution of Orthorhombic Martensite Lattice in VT16 Titanium Alloy During Heating

.

Modeling the martensite reorientation and resulting zero/negative thermal expansion of shape memory alloys

Recent experimental investigation shows that macroscopic overall zero/negative thermal expansion (TE) can be obtained through the martensite reorientation of NiTiPd shape memory alloy (SMA). In this work, a microstructure-based theoretical model is established to quantify such phenomenon. Based on the crystallographic symmetry, the microstructures and the coefficients of thermal expansion of the austenite and martensite lattices of NiTiPd SMA are analyzed. A crystal plasticity-type constitutive model at the individual grain level is constructed to describe the martensite transformation (MT) and martensite reorientation (MR) processes, and the evolution of thermal expansion coefficient. The nonlinear constitutive model is linearized by adopting the implicit backward Euler time discretization scheme and an incremental affine stress–strain–temperature relationship is obtained. An incremental self-consistent homogenization scheme is proposed to describe the interaction among the grains and calculate the elastic, inelastic (MT and MR) and thermal deformations of the polycrystalline aggregates. The established model is used to simulate the MR and the resulting zero/negative TE of polycrystalline NiTiPd SMA. It is shown that the observed quantitative features in experiments can be captured reasonably by the model.

On Tetragonality of the Martensite Crystal Lattice in Steels

This article addresses the pseudo-tetragonal nature of the crystal lattice of martensite in carbon-containing steels. It is proposed that the periodic distortion of c-cube edges by the presence of carbon atoms is better reflected in the root-mean-square distortions of martensite lattice than by the value of doublet splitting of the corresponding X-ray reflections of the martensite crystal.

Theory of Mn-Based High-Magnetization Alloys

The realization of the magnetic moment of Mn atoms in ferromagnetic alloys is investigated theoretically. This paper is partly motivated by the recent discovery of high-magnetization Fe–Co–Mn thin films by Snow et al. , who reported moments of up to 3.25 $\mu _{\text {B}}$ per atom. Structural and strain effects are discussed with particular emphasis on the distinction between tetragonal martensitic lattice distortions and Bain transitions in body-centered cubic, CsCl, and Heusler alloys. It is outlined how these lattice distortions affect the electronic structure of the alloys, and ab initio calculations are used to determine the net moment of cubic Fe6Co63Mn31. The calculated moment, $2.90~\mu _{\text {B}}$ per atom, is reasonably close to the experimental value.

Deep Cryogenic Treatment of AISI M2 Tool Steel and Optimisation of Its Wear Characteristics Using Taguchi‘s Approach

The present research paper focuses on optimisation of wear test parameters for different deep cryogenic treated AISI M2 tool steel material. Commercially available, AISI M2 tool steel material is machined as per ASTM G99-05 standards and was subjected to deep cryogenic treatment for the different holding time of 12, 24 and 36 h, followed by tempering for 2 h at $$150\,^{\circ }\hbox {C}$$ . The specimens were tested for their wear resistance using a pin-on-disc wear testing setup adopting Taguchi‘s design of experiments approach. The control variables selected were deep cryogenic treatment holding time, the speed of the rotating disc and load applied to the specimen pin. The wear test was conducted as per trials generated by Taguchi‘s $$\hbox {L}_{27}$$ orthogonal array, and the results are analysed using signal-to-noise ratio and analysis of variance. The results show that the cryogenic holding time has majorly affected the wear resistance followed by the load on the pin and disc speed. It is also observed that 24 h holding time for deep cryogenic treatment yields better wear resistance compared to 12 and 36 h. Microstructure and X-ray diffraction analysis of the cryogenically treated specimens reveals the conversion of retained austenite into martensite and the formation of fine carbides in the martensite lattice after deep cryogenic treatment process which are responsible for the increase in wear resistance. Also, the worn surface analysis shows that the adhesion wear, oxidation wear and abrasion wear are the predominant wear mechanisms observed under different testing conditions.

Carbon Ordering in Martensite Lattice Under External Stress: Thermodynamic Theory and Molecular Dynamics Simulation

Rapid cooling of the Fe–C fcc solution leads to the formation of a phase called martensite, which has high mechanical properties. It is well known that under stress a martensite has the ability to plastically deform within a small range, despite the large value of the macroscopic yield strength. This effect is explained in different ways, mainly on the basis of dislocation theory. We study the impact of external stress on ordering of interstitial atoms in Fe–C solid solutions and formation of tetrahedral distortion of martensite lattice. Molecular dynamics with embedded atom (EAM) interatomic potential is used. According to the simulations, interstitial carbon atoms migrate under external stress applied along the tetragonality direction from z‐sublattice, and tetrahedral distortion of lattice changes its orientation to other ones when the stress exceeds some critical value. This result enables us to justify a new look at the nature of the martensite plastic deformation. The influence of temperature and carbon content on the critical stress value is investigated.

Hydrogen trapping in carbon supersaturated α‑iron and its decohesion effect in martensitic steel

Our first-principles calculations demonstrate that hydrogen is more stable in carbon supersaturated martensite than in α‑iron, due to the carbon-induced tetragonality in martensite lattice. The trapped hydrogen leads to remarkable decohesion between (110) planes both inside the martensite and along the martensite/ferrite interface, with the former being more significant than the latter. This decohesion can explain recent precise observations that in martensite/ferrite dual-phase steels the hydrogen-promoted crack was initiated in the martensite region and that in lath martensite steel it propagated not on lath boundaries but showed quasi-cleavage feature along (110) planes at very high hydrogen concentration.

The Application of Scanning Contact Potentiometry Method and Diffraction of Thermal Neutrons at Physico-Mechanical Tests of Materials

The results of physical and mechanical tests on the tensile strength of austenitic steel 12X18H10T, in the stress range from 100 MPa to the maximum value of 700 MPa, at which the sample was destroyed, are presented. Structural changes were registered synchronously by two methods: the method of scanning contact potentiometry and the method of diffraction of thermal neutrons. At loads above 650 MPa, the α-martensite phase was found in the austenite matrix, as well as the appearance of diffraction peaks characteristic of a cubic martensitic BCC lattice was observed in neutron spectra. On the potentiograms, this process corresponds to the appearance of local regions in which high values of electric potential gradients were observed. This is the case of discovery of the fatigue nucleus cracks in the tensile testing of steel ЭИ847 by the method of scanning contact potentiometry.