Evolution Of Martensite Research Articles

Simulation of martensitic transformations in TRIP-steel and Fe-based shape memory alloy

Several important effects in the mechanical behavior of materials are due to martensitic transformations. In case of TRIP-steels, the γ→ α ′ martensitic transformation during plastic deformation is responsible for high strength combined with an outstanding ductility. Another martensitic transformation ( γ→ ε) is the reason for the perfect shape recovery in Fe-based shape memory alloys (SMAs). By means of a finite element analysis the evolution of martensite both in a TRIP-steel and in an Fe-based SMA is investigated. The model is able to explain the experimentally observed dependence of the austenite start temperature in an Fe-based SMA on the extent of prior martensitic transformation. Dilatometric experiments with an high-alloyed TRIP-steel are used to validate the predictions of the TRIP-simulations.

Modeling of Plastic Strain-Induced Martensitic Transformation for Cryogenic Applications

A simplified model of martensitic transformation in stainless steels at cryogenic temperatures is proposed. The constitutive modeling of plastic flow under cryogenic conditions is based on the assumption of small strains (⩽0.2). The hardening law for the biphase material (α′ martensite platelets embedded in the γ austenite matrix) has been obtained from the Mori-Tanaka homogenization. A mixed hardening with combined isotropic and kinematic contributions is proposed. The constitutive model, containing a reasonable number of parameters, has been numerically implemented and checked with respect to experimental data. Finally, the model is applied to compute the martensite evolution in thin-walled corrugated shells designed for cryogenic temperatures (mechanical compensation system of the Large Hadron Collider at CERN).

NONLINEAR FE ANALYSIS OF THE HYSTERESIS BEHAVIOR OF SMA

This paper presents a nonlinear finite element procedure for the simulation of the hysteresis behavior and the shape memory effect of shape memory alloy (SMA) materials. The consitutive equation is based on the consistent mathematical description of martensite fraction evolution during the thermoelastic phase transformation in shape memory alloys induced by general thermomechanical loading. The kinematics law is based on an experimentally defined stress-temperature phase diagram, transformation functions for one-dimensional SMA body, and a novel vector hysteresis temperature phase diagram for an initial value of martensite fraction. The solution of the geometrically and physically nonlinear problem is achieved by application of Newton's iteration method. Numerical results which consider the temperature and pseudoelastic effects show that the proposed procedure is an effective computational tool for the simulation of a broad range of shape memory material based applications.

Optimal design of a flexural actuator

A minimum-weight flexural actuator is designed. The actuator comprises a triangular corrugated core with shape memory alloy (SMA) faces. It is clamped at one end and free at the other. For design and optimization, the temperature history of the face sheets upon heating and subsequent cooling is first obtained as a function of the cooling efficiency (Biot number) and the operational frequency deduced. Based upon this response, a phenomenological model is employed to represent the martensite evolution. Thereafter, the end deflection is calculated as a function of temperature. The minimum weight is calculated subject to the provisos that: (i) the end deflection attains a specified value; (ii) the power consumed is less than the upper limit of the supply; and failure is averted by (iii) face/core yielding and (iv) face/core buckling; (v) the operational frequency of the panel achieves a specified limit.

Micromechanical study of formation and evolution of martensite for Cu–Al–Ni single crystal by moiré interferometry

The basic properties of pseudoelasticity of Cu–Al–Ni single crystal are studied to analyze the morphology associated with the formation and evolution of martensite and the shape memory recovery process at different temperatures. Use is made of a multifunctional macro–micro moiré interferometry measurement system. The β 1 to β 1 ′ phase changes are identified with the stress-induced transformation of a Cu–13.7%Al–4.18%Ni (wt%) alloy. The invariant plane between the martensite and the parent phase is shown directly by fringe patterns. It is found that martensite appears in the shape of bands or thin plates on the surface of the specimen. The formation of martensite is a very rapid process and martensite `jumps' out until the specimen is completely transformed into a single variant. The results reveal the mechanism and process of stress-type and temperature-induced martensitic transformation.

Crystal plasticity analysis of stress–assisted martensitic transformation in Ti–10V–2Fe–3Al (wt.%)

A new model based on crystal–plasticity, crystallography, thermodynamics, kinetics and statistics is developed for stress–assisted martensitic transformation. The model includes the essential features of the stress–assisted martensitic transformation, such as: nuclei of progressively lower potency are activated in the course of transformation, the martensite phase appears in the form of thin plates, the parent phase exerts a higher resistance toward the growth of a plate in the thickness than in the radial direction, the average plate size decreases while the average plate aspect ratio increases with the extent of transformation, etc. The model is implemented in the commercial finite element code ABAQUS/Standard to analyze the evolution of martensite, materials texture and the resulting equivalent stress–equivalent strain curve during the stress–assisted martensitic transformation under different stress and strain states in a polycrystalline Ti–10V–2Fe–3Al (wt.%) alloy. The equivalent stress–equivalent strain curves and the volume fraction of martensite–equivalent strain curves are found to be mainly controlled by the applied stress state. Conversely, the texture observed in the transformed Ti–10V–2Fe–3Al is found to be primarily controlled by the imposed macroscopic strain state. The validity of the proposed materials constitutive model has been established by demonstrating a reasonable agreement between the model predictions and the available experimental data.

Pseudoelastic behavior of a CuAlNi single crystal under uniaxial loading

In order to study the basic properties of pseudoelasticity of a CuAlNi single crystal, an investigation was carried out to observe and analyze the orientation dependence of the stress-induced martensitic transformation. The transformation is the β1 to β′1 stress-induced transformation in a Cu-13.7 pct Al-4.18 pct Ni (wt pct) alloy. From the uniaxial tension of three groups of differently oriented flat specimens, we obtained a series of stress-strain curves. In addition, the micrograph of martensitic evolution was observed by utilizing a long-focus microscope. It is found that martensite appears in the shape of bands or thin plates on the surface of the specimen. The formation of martensite is a very quick process, and martensite “jumps” out until the specimen is completely transformed into a single variant. The experimental results are analyzed and compared to a constitutive model proposed recently. It is found that the constitutive model cannot describe transformation hardening, since the model ignores the surface-energy change. Nevertheless, the proposed constitutive model cannot only precisely predict the forward and reverse transformation, but can also characterize the stress-strain hysteresis behavior during pseudoelastic deformation under uniaxial tension loading.

The role of texture in tension–compression asymmetry in polycrystalline NiTi

The purpose of the present study is to thoroughly understand the stress–strain behavior of polycrystalline NiTi deformed under tension versus compression. To do this, a micro-mechanical model is used which incorporates single crystal constitutive relationships and experimentally measured polycrystalline texture into the self-consistent formulation. For the first time it is quantitatively demonstrated that texture measurements coupled with a micro-mechanical model can accurately predict tension/compression asymmetry in NiTi shape memory alloys. The predicted critical transformation stress levels and transformation stress–strain slopes under both tensile and compressive loading are consistent with experimental results. For textured polycrystalline NiTi deformed under tension it is demonstrated that the martensite evolution is very abrupt, consistent with the Luders type deformation experimentally observed. The abrupt transformation under tension is attributed to the fact that the majority of the grains are oriented along the [111] crystallographic direction, which is soft under tensile loading. Since single crystals of the [111] orientation are hard under compression it is also demonstrated that under compression the martensite in textured polycrystalline NiTi evolves relatively slower.

A role of δ-ferrite in edge-crack formation during hot-rolling of austenitic stainless steels

Non-destructive characterization methods to observe phase transformations and thus, gain insights to transformation mechanisms in representative volumes are key for the development of advanced materials and manufacturing. Conventional methods are constrained to the surface and small sizes, thus, access to the bulk often implies tedious destructive approaches and hinders in-situ observations of phase evolution. In this work, we introduce a non-destructive technique that overcomes key limitations prevailing today for mapping the spatial distribution of magnetic phases in bulk materials. The use of polarized neutrons, being sensitive to sub-percent fractions of ferromagnetic phases and able to penetrate centimeter sized samples, enables micrometer-scale spatial and second-scale time resolutions. We demonstrate ex-situ and in-situ quantitative mapping of magnetic phases, in particular the evolution of martensite induced by uniaxial- and biaxial deformation in metastable 304 steel. The quantitative results obtained during in-situ deformation testing prove polarization contrast neutron imaging to be particularly effective in detecting small fractions of martensite. Especially during early stages of deformation where neutron diffraction, which in contrast does not provide full field spatial resolution, fails. The short exposure times and high sensitivity renders the method well suited for rapid 3D tomographic mapping and/or operando investigations of phase distributions.

Compositional evolution in aging duplex Fe-Mn-Al-C alloy

Compositional evolution of 18R martensite and α-Mn particles in aging a duplex Fe-Mn-Al-C alloy was studied by a Field Emission Gun Transmission Electron Microscope (FEG TEM) JOEL JEM-2010F equipped with EDS and Gatan imaging filter. A model was proposed to explain the compositional and structural evolution of the 18R martensite during aging treatment.

Structural evolution of the aging martensite in duplex steel

In the past, the structure and crystallography of needle-like martensite in the as-quenched Fe-Al-Mn-C alloy were studied. The structure of the needle-like martensite was identified to be a structure between 18R(5)3 and 18R(4)3. The orientation relationship between the BCC matrix and martensite phase was determined to be and . The shape strain, habit plane and other crystallography of martensite were also determined. Comparison with the asquenched Fe-Mn-Al-C martensite, there is only little information about the aging behavior of the martensite phase. The purpose of this work is to study the structural evolution of the martensite phase after aged at high temperature.The wrought Fe-29.4-Mn-8.03Al-0.3C alloy was heated at 1300°C for 1 hour in argon atmosphere, and followed by a quenched into water. Aging process were performed at 400 °C, 500 °C and 600 °C, respectively, for 1 hour in salt bath, and the specimens were then quenched into water. TEM specimens were examined with a JOEL-200CX STEM.

Microstructural evolution and creep behaviour of bainitic, martensitic, and martensite–ferrite dual phase Cr–2W steels

The microstructural evolution and creep behaviour of Cr–2W–0·lC (wt-%) steels containing from 2 to 15%Cr have been investigated after creep rupture testing at 873 K for up to about 72×106 s (20 000 h). The creep curves consisted mainly of a transition creep region, where creep rate decreased with time, and an accelerated creep region, where creep rate increased with time. For the bainitic or martensitic steels, which contain from 2 to 9%Cr, the minimum creep rate decreased and the time to rupture increased with increasing Cr concentration. However, for the dual phase martensite and δ ferrite steels (12–15%Cr), the minimum creep rate increased and the time to rupture decreased with increasing Cr concentration, i.e. with increasing δ ferrite volume fraction. The microstructural evolution during creep was greater in the bainitic than in the martensitic steels. Evolution was not observed in the δ ferrite. The resistance to creep deformation is considered to depend on pinning by carbides preferentially arranged along the lath subgrain boundaries in the bainite and martensite. The effect of microstructural evolution during creep on the creep rate and time to rupture of the steels is comprehensively discussed.MST/1617

Microstructural evolution and creep behaviour of bainitic, martensitic, and martensite–ferrite dual phase Cr–2W steels

The microstructural evolution and creep behaviour of Cr–2W–0·lC (wt-%) steels containing from 2 to 15%Cr have been investigated after creep rupture testing at 873 K for up to about 72×106 s (20 000 h). The creep curves consisted mainly of a transition creep region, where creep rate decreased with time, and an accelerated creep region, where creep rate increased with time. For the bainitic or martensitic steels, which contain from 2 to 9%Cr, the minimum creep rate decreased and the time to rupture increased with increasing Cr concentration. However, for the dual phase martensite and δ ferrite steels (12–15%Cr), the minimum creep rate increased and the time to rupture decreased with increasing Cr concentration, i.e. with increasing δ ferrite volume fraction. The microstructural evolution during creep was greater in the bainitic than in the martensitic steels. Evolution was not observed in the δ ferrite. The resistance to creep deformation is considered to depend on pinning by carbides preferentially arranged along the lath subgrain boundaries in the bainite and martensite. The effect of microstructural evolution during creep on the creep rate and time to rupture of the steels is comprehensively discussed.MST/1617

The Mössbauer kinetics of the evolution of martensite during the ageing and first stage of tempering in high carbon steel

57Fe Mossbauer spectroscopy is, with13C NMR, the only technique enabling to follow the redistribution of interstitials occuring during low temperature aging in ferrous martensite and responsible for the formation on the carbides obtained by tempering. We report the isochronal aging of martensite from as-quenched samples of high carbon content in order to follow the kinetics, all over the temperature range from 78°K to 400°K, thus covering the aging and the first stage of tempering as well. X-ray diffraction analysis is done in parallel for all temperatures above ambient. The results sustain the fact that one unique process is involved, leading to the precipitation of ∈ or ν carbide by stress-induced mechanism.

A TEM Study of the Evolution of Strain-Induced Martensite

We have demonstrated previously the effects of strain, strain state, strain rate, and temperature on the morphology of strain-induced martensite in type 304 stainless steel. In all cases α' martensite nucleated at shear band intersections as suggested by Olson and Cohen, but the final morphologies were different. As the amount of martensite increased (with increasing strain, biaxial tension, low strain rates, and low temperatures), α' appeared more blocky and less lath-like. We have demonstrated that α' embryos grow by coalescence of new embryos, rather than actual growth.In the present study, we concentrated on the early stages of transformation to determine if α' embryos are identical for test conditions including uniaxial tension, biaxial tension, and shock loading, at room and cryogenic temperatures (as detailed in the caption of Fig. 1). Strain levels were controlled to produce only small amounts of α'. Sheet specimens, 0.18 mm thick, were used for all experiments, thinned to electron transparency, and examined in a JEOL 200B microscope operated at 200 kV.