Martensite Nucleation Research Articles

Martensite nucleation site and grain refinement in duplex titanium alloy weldment by active flux with nanoparticle addition

The effects of activating flux containing nanoparticles on the welding microstructure and hardness distribution of titanium alloy were investigated. The fluxes, consisting of NaF with 1 wt-% of either nano-TiO2 or nanodiamond, effectively yielded refined grain size and deep weld penetration. The hardness of the weldment was close to that of the base metal for 99 wt-% NaF plus 1 wt-% diamond, leading to a desirable uniform mechanical property in the weld design. In the meantime, the transmission electron microscopy images illustrate that the nanoparticles promote β phase coarsening and martensitic α′ phase formation at the β veins.

Acoustic emission in bearing steel during isothermal formation of midrib

The first attempt of an acoustic emission (AE) application for the control of isothermal transformation, at the temperature near the martensite start (M S ) in 100CrMnSi6-4 bearing steel, is made herein. A unique method for recording the signals of the AE was developed. The Short Time Fourier Transform algorithm was used for displaying the signal values on the spectrogram graphs. Acoustic emission showed the nucleation and growth of the isothermal martensite in the residual austenite. The maximum power spectral density of signals was determined using the relative intensity of the RMS values. The observed energy of separated signals during the second stage of residual austenite decomposition is comparable to the energy of the initial stage at lower temperatures. It was proposed as a new process concept under paraequilibrium that involves the midrib formation and swing back in the kinetics of lower bainite.

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

AbstractUltra‐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.

Quantitative analysis of stress-induced martensites by in situ transmission electron microscopy superelastic tests in Cu–Al–Ni shape memory alloys

Stress-induced martensite nucleation and further growing, in Cu–Al–Ni shape memory alloys, have been studied during in situ superelastic tests in the transmission electron microscope. Two kinds of martensite, β 3 ′ and γ 3 ′ , are induced and can coexist under stress, both exhibiting in a high density of stacking faults. The interface plane and the orientation relationships between the different variants of such martensites have been determined, and the atomic configurations of the lattices across the interface have been described. Finally, in light of the results, selection rules for the stress-induced promoted martensites at the nano-scale have been established, being determined by the shear direction and the basal plane of the martensite lattice.

Probing deformation processes in near-defect free volume in high strength–high ductility nanograined/ultrafine-grained (NG/UFG) metastable austenitic stainless steels

The deformation behavior of a nanograined/ultrafine-grained (NG/UFG) austenitic stainless steel characterized by high strength–high ductility combination is investigated via nanoindentation and electron microscopy , and the behavior compared with its coarse-grained (CG) counterpart. In NG/UFG steel, mechanical twinning was an active deformation mechanism , while in CG steel, nucleation of strain-induced martensite at the shear bands occurred. The differences in deformation mechanisms of NG/UFG and CG steels are reflected in the discrete burst in the force–displacement plots and attributed to austenite stability.

Quantifying transient states in materials with the dynamic transmission electron microscope

The dynamic transmission electron microscope (DTEM) offers a means of capturing rapid evolution in a specimen through in situ microscopy experiments by allowing 15-ns electron micrograph exposure times. The rapid exposure time is enabled by creating a burst of electrons at the emitter by ultraviolet pulsed laser illumination. This burst arrives at a specified time after a second laser initiates the specimen reaction. The timing of the two Q-switched lasers is controlled by high-speed pulse generators with a timing error much less than the pulse duration. Both diffraction and imaging experiments can be performed, just as in a conventional TEM. The brightness of the emitter and the total current control the spatial and temporal resolutions. We have demonstrated 7-nm spatial resolution in single 15-ns pulsed images. These single-pulse imaging experiments have been used to study martensitic transformations, nucleation and crystallization of an amorphous metal and rapid chemical reactions. Measurements have been performed on these systems that are possible by no other experimental approaches currently available.

Electro-thermomechanical properties of superelasticity in single crystals shape memory alloys

In this work, superelastic behavior of single crystals shape memory alloys has been investigated by the electric resistivity (ER) measure technique coupled simultaneously during traction tests. Samples in austenitic phase were submitted to stress induced martensite (SIM) under several conditions such as: different temperatures, different deformation rate, throughout successive martensitic transformations and different crystallographic orientation axes. ER vs. deformation curves presented a linear relation without hysteresis and independence on temperature. In the case on the increasing of deformation rate, these curves show a little hysteresis due to adiabatic transformations. For samples submitted to successive transformations, ER curves showed changes with induced martensite types. In samples of the same alloy with different crystallographic orientations was observed that single variant martensite nucleation presented distinct values, this characterizes martensite electric resistivity anisotropy.

Real-time martensitic transformation kinetics in maraging steel under high magnetic fields

We have monitored the isothermal transformation kinetics of the austenite phase into the martensite phase in a metastable austenitic maraging steel by time-dependent magnetization measurements for temperatures from 4 to 298 K and continuous applied magnetic fields up to 30 T. The transformation kinetics is shown to be accelerated by several orders of magnitude when high magnetic fields are applied. Analyzing the transformation rate as a function of magnetic field and temperature provides direct insight into the martensite nucleation process.

Critical grain size in the γ→α martensite transformation. Thermodynamic analysis with regard to spatial scales characteristic of martensite nucleation

In the work, it is shown that taking into account the ratio of spatial scales characteristic of martensite crystal nucleation in the elastic field of an individual dislocation localized in a grain of diameter D makes it possible (i) to estimate the critical grain size D c (∼1 μm) characteristic of the γ→α transformation at a martensite starting temperature M s ∼ 100 K from the requirements that threshold strain arises in the elastic region, (ii) to consistently describe the D dependence of M s with M s(D c) = 0 from analysis of the phase free energy difference, (iii) to express D c through macroparameters and interpret the limiting case M s∞→0, D c→∞ (D c ∼ (M s∞) −1 with M s∞ ≡ M s (∞)), and (iv) to more exactly specify the qualitative dynamic model of the effect of dislocation grain boundaries on the onset of initial excitation.

Phase transformation in δ Pu alloys at low temperature: In situ dilatometric study

The purpose of this work is to precisely study the martensitic transformation in a plutonium-gallium alloy. Thus, the thermodynamics and kinetics of the δ→α'+δ phase transformation in a Pu-Ga alloy were studied under isochronal and isothermal conditions. The activation energy of the δ→α'+δ phase transformation at a constant cooling rate (0.5 K.min−1) was determined by using Kissinger and Ozawa models. The average value of the activation energy was found to be at −56 kJ.mol−1. Dilatometry measurement was also used to trace 'in situ' the entire transformation for several temperatures. The kinetics of the δ→α'+δ transformation were modelled under isothermal conditions in the theoretical frame of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. It is proposed that the transformation consists of three stages. The α' transformation begins with a nucleation of pre-existing embryos. Then, both nucleation and rapid growth of α' occurs simultaneously and finally, the plates width expend. Apparent activation energies for nucleation and growth transformation were determined from the temperature dependence of the constant K at respectively −34 kJ.mol−1 and −60 kJ.mol−1. Adler et al. [1] investigated also the thermodynamics and the kinetics of the martensitic transformation in Pu alloys. These nucleation energies were found by modelling of heterogeneous martensitic nucleation via strain interaction with observed superdislocation-like nucleation sites in PuGa alloys. The values obtain by this model was very close to those we find. Investigations in steels alloys indicate that these energies are of the same order for nucleation near dislocation. Then, it could be indicating a strong relationship between these dislocations and martensitic nucleation sites.

Martensite start temperature and the austenite grain-size

The dependence of the martensite start temperature on the austenite grain-size is modeled based upon aspects of martensite nucleation and validated by data reported in the literature. The results obtained support the view that grain boundaries provide an environment for the formation of martensite embryos in annealed austenite. The model can account for a two-stage dependence of martensite start temperature on the austenite grain-size, considering that grain boundaries provide a suitable environment for martensite nucleation, and grain-size also has a stabilizing effect by limiting the volume of individual martensite units.

A crystallographic model of fcc/bcc martensitic nucleation and growth

A crystallographic model of face-centered cubic/body-centered cubic martensitic nucleation and growth has been developed in which the glissile martensitic interface is described in terms of discrete transformation and misfit dislocations within the framework of the Frank–Bilby equation. In the present model heterogeneous nucleation of a martensitic embryo occurs spontaneously by dissociation of an existing perfect dislocation under certain conditions, such as the catalysis on grain boundaries. Autocatalytic dislocation is proposed to be responsible for embryo growth, then nucleation and subsequent growth of a single embryo can be regarded as an autocatalytic process. As a natural result the crystallographic features of grown martensite are consistent with the phenomenological theory of martensite crystallography. Martensitic nucleation in this model agrees with many experimental observations.

Displacive Phase Transitions at Large Strains: Phase-Field Theory and Simulations

The Landau potential for multivariant displacive phase transformations (PTs) is derived for the most general case of large rotations, elastic and transformational strains, as well as nonlinear and different elastic properties of phases. The method of repetitive superposition of large strains is extended for PTs and is utilized in the finite-element method approach for solution of corresponding coupled phase-field and mechanical problems. Problems of martensitic variants nucleation and evolution in a nanosize sample, including a sample with two nanovoids, are solved. A similar approach can be applied for twinning, dislocations, and reconstructive PTs.

Molecular dynamics study of surface effect on martensitic cubic-to-tetragonal transformation in Ni–Al alloy

Molecular dynamics (MD) study of martensitic transformation (MT) in nickel and aluminum alloy is performed. The behavior focused on is transformation between crystalline structures from B2 cubic cell to body-centered tetragonal cell, which is simply realized by uniaxial tensile loading. The potential function used is Finnis–Sinclair type having only single energy minimum where B2 structure exists. The availability of this specific many-body potential for stress-induced MT phenomena under uniaxial loading is fully discussed. In MD simulations, martensite phase is induced by tensile stress or strain in the atomic system, as predicted by a potential energy map. It is understood that the characteristic of the potential energy function with regard to deformation is crucial for MT studies and investigating energy-strain or stress–strain map is worthwhile. The MT behavior in the atomic system occurs during a plateau region of stress–strain (S–S) curve of the whole specimen, that is typical for experimental superelastic or shape-memory alloys under uniaxial loading. It is found that, during each MT event, large jump of atomic strain is observed. Owing to single energy minimum, the atomic system shows almost perfect recovery in S–S curve, where the graph comes completely back to initial state after unloaded. Besides, the present paper focuses on surface effect for MT behavior. Since the surface effect is dominant in MT phenomena especially in microscopic specimens, a novel computational scheme for stabilizing condition in which uniaxial loading is always applied together with arbitrary periodic boundary condition(s) is devised. By comparing one-, two-, and three-dimensional models under uniaxial loading, it is recognized that the nucleation behavior depends strongly on the existence of free surface region (including corner edge). When there is no surface, a chaotic nucleation of martensite is observed. On the other hand, the free surface induces first martensite because of less constraint in tensile deformation of unit cells. It is confirmed that the tendency toward MT nucleation corresponds to yield stress or strain of the specimen. In order to define and detect martensite structure as for each atom, an atomic strain measure (ASM) with our own formation is introduced. It is shown that the ASM is very effective to distinguish martensite bct unit structure from others.

Multiscale experimental investigations about the cyclic behavior of the 304L SS

This work follows a series of experiments carried out earlier at INSA of Rouen (Hassan, T., Taleb, L., Krishna, S., 2008. Influence of non-proportional loading paths on ratcheting responses and simulations by two recent cyclic plasticity models. Int. J. Plast. 24, 1863–1889). It investigates the elastoplastic cyclic behavior of a 304L stainless steel at room temperature. In a first step the cross path effect on ratcheting is confirmed, as well as the crucial role of the loading path non-proportionality. Strain controlled tests are also conducted for different strain amplitudes and loading paths. Cross-hardening effect appears more important when the shearing sequence is followed by the axial one. Moreover for alternating axial and shearing cycles, this phenomenon occurs after each crossing sequence leading to a very significant strain hardening, at least of the same order as the one obtained after a circular strain path. Yet, the magnitude of the observed over hardening does not necessarily seem a function of the cumulated plastic strain. The relative contributions of the isotropic and kinematic parts of the cyclic strain hardening are also very different in the axial direction. The material generally exhibits a transient period of cyclic hardening followed by cyclic softening. Under tension-compression, the importance of the cyclic strain hardening period seems to increase with the strain amplitude. Microstructural investigations have been performed in order to understand the main physical phenomena responsible for this macroscopic behavior. The additional cyclic hardening observed in cross paths may be explained by the high defect density generated: multiple slip systems, intersecting stacking faults and twins, formation of dislocation heterogeneous structures. Furthermore, it is shown that martensite nucleation takes place at the intersections between micro-shear bands or twin faults; the quantity of martensite being estimated through magnetic measurements. After the application of a magnetic field, the material anisotropy induced by the loading path is evaluated; the martensitic needles and platelets seem to develop in the axial direction.

Electron irradiation induced rock salt type structure from ZnO/Zn intergrowth

Electron irradiation of nano-size wurtzite (W)-type ZnO condensates with intimate mixture of parallel epitaxial Zn caused { 1 0 1 ¯ 1 } W slip to form a single domain of rock salt (R)-type core and W-type shell. The two polymorphs follow ( 1 1 ¯ 1 ) R / / ( 0 1 ¯ 1 1 ) W ; [ 0 1 1 ] R / / [ 1 ¯ 2 1 ¯ 3 ] W , i.e. chair type Peierls distortion with additional 38° tilting (0 0 1) R along the ( 1 ¯ 2 1 ¯ 0 ) W plane for a fair match of ( 1 0 1 ¯ 1 ) W / ( 1 1 ¯ 1 ) R , the same as one of the two paths for the back-transformation of R-ZnO into a lower crystal symmetry. The martensitic nucleation of R-type ZnO can be attributed to dynamic migration of interstitials/vacancies, lattice mismatch stress, and capillarity effect.

Modeling of isothermal bainite formation based on the nucleation kinetics

Abstract Using a new approach to modeling bainite nucleation, the kinetics of isothermal bainite formation have been calculated under the assumption of displacive growth. The nucleation rate is assumed to depend on the number density of potential nucleation sites N i, a factor λ accounting for autocatalytic nucleation, and an activation energy Q*. Compatible with the theory for athermal martensite nucleation, N i is assumed to be proportional to the driving pressure. Analogous to the Koistinen – Marburger model for martensite formation, the average volume of bainitic sub-units is assumed to be constant over the extent of the transformation and the growth of sub-units is very fast, and thus the change in fraction is directly related to the nucleation rate of bainite. The model results in an analytical expression for the fraction bainite as a function of time that contains only two adjustable parameters: a (temperature independent) autocatalytic parameter λ and a rate parameter κ, which has a temperature dependence that is mainly governed by Q*. The calculations are compared with experimental fraction curves measured isothermally with dilatometry for the carbon steels C45, C50 and C60 at a range of temperatures. From the best agreement between the calculations and the experimental data it follows that Q* decreases linearly with temperature, which is consistent with other bainite nucleation models. By austenitizing steel C60 at different temperatures it is found that λ depends on the austenite grain size: when the austenite grain size is increased, λ becomes larger.

Sensitivity of the magnetization curves of different austenitic stainless tube and pipe steels to mechanical fatigue

In meta-stable austenitic stainless steels, fatigue is accompanied by a partial strain-induced transformation of paramagnetic austenite to ferromagnetic martensite [G.B. Olsen, M. Cohen, Kinetics of strain induced martensite nucleation, Metall. Trans. 6 (1975) 791–795]. The associated changes of magnetic properties as the eddy current impedance, magnetic permeability or the remanence field may serve as an indication for the degree of fatigue and therefore the remaining lifetime of a component, even though the exact causal relationship between martensite formation and fatigue is not fully understood. However, measuring these properties by magnetic methods may be limited by the low affinity for strain-induced martensite formation. Thus other methods have to be found which are able to detect very small changes of ferromagnetic contents. With this aim the influence of cyclic strain loading on the magnetization curves of the austenitic stainless tube and pipe steels TP 321, 347, 304L and 316L is analysed in the present paper. The measured characteristic magnetic properties, which are the saturation magnetization, residual magnetization, coercive field and the field dependent permeability (AC-magnetization), are sensitive to fatigue and the corresponding material changes (martensitic transformation). In particular, the AC-magnetization was found to be very sensitive to small changes of the amount of strain induced martensite and therefore also to the degree of fatigue. Hence we conclude that applying magnetic minor loops are promising for the non-destructive evaluation of fatigue in austenitic stainless steel, even if a very small amount of strain induced martensite is formed.

Isothermal martensitic transformation in a 12Cr–9Ni–4Mo–2Cu stainless steel in applied magnetic fields

This work concerns an in situ study of the isothermal formation of martensite in a stainless steel under the influence of magnetic fields up to 9 T at three different temperatures (213, 233 and 253 K). It is shown that the presence of a constant applied magnetic field promotes the formation of martensite significantly. The activation energy for the nucleation of martensite has been derived using a semi-empirical kinetic model. The experimental results have been analyzed using the Ghosh and Olson model. While this model describes the time and field dependences of the experimental data well, the thermal frictional energy and the defect size values are much lower than those expected from earlier work.

Magnetization easy axis in martensitic Heusler alloys estimated by strain measurements under magnetic field

We study the temperature dependence of strain under constant magnetic fields in Ni–Mn based ferromagnetic Heusler alloys in the form Ni–Mn–X (X: Ga, In, Sn, and Sb) which undergo a martensitic transformation. We discuss the influence of the applied magnetic field on the nucleation of ferromagnetic martensite and extract information on the easy axis of magnetization in the martensitic state.