Parent Austenite Phase Research Articles

Origin of magnetoresistance across the martensitic transformation: formation of phase fraction

The influence of martensite and austenite phase volume fractions on the magnetoresistance have been studied across the first order martensite transformation of the Ni44Cu2Mn43In11 compound. The different phase volume fractions are calculated by measuring the resistivity as a function of temperature and magnetic field. Our experiment reveals that the field-induced austenite phase fraction ( f IA ) at any temperature depends on the availability and instability of the martensite phase fraction ( f M ) at that temperature. This f IA is found to contribute most significantly to the observed large magnetoresistance (MR), while the contributions from parent martensite and austenite phase fractions are negligible. It has been found that the MR does not depend linearly on the f IA . On application of a magnetic field, the ascending and descending branch of the f IA follows a different power law, giving rise to hysteresis in the MR.

Stochastic multiscale modeling of the thermomechanical behavior of polycrystalline shape memory alloys

A multi-scale model simulating hysteretic thermomechanical behavior of polycrystalline shape memory alloys (SMA) is presented. Using a kinetic Monte-Carlo approach, the energy-based model estimates the stochastic average in terms of volume fraction for a phase or phase variant deriving from the Gibbs free energy density as a selection inside a given population. Indeed pseudo-elastic behavior for example is well-known to be associated with the nucleation of martensite plates inside the austenite parent phase. Associated variants are similar to sub-domains inside a thermodynamic system following the statistical definition of Sommerfeld et al. (1964). The germination process of a variant is on the other hand dictated by a germination potential barrier identified from a differential scanning calorimetry (DSC) measurement. The stochastic average for each type of variants inside a grain leads then to the numerical simulation of hysteresis phenomena at the single crystal scale. Homogenization operations allow finally macroscopic quantities (phases fraction, deformation and temperature) to be calculated at the polycrystalline scale. This procedure is applied to model the whole thermomechanical behavior of an equiatomic Ni-Ti SMA polycrystalline alloy considering the phase transformation between austenite, R-phase and martensite.

The Role of Parent Phase Compliance on the Fatigue Lifetime of Ni\u2013Ti

It has been previously suggested that the fatigue lifetime of superelastic Ni–Ti might be improved if the R-phase were the parent to martensite rather than austenite. This body of work tests that hypothesis in two separate side-by-side fatigue tests both carefully constructed to match the superelastic properties in the two study arms. Both experiments show the R-phase parent to be more durable than the more commonly considered austenitic parent phase. The first experiment considers straight wire specimens fabricated from standard purity material, in a tension–tension fatigue test to 107 cycles, at mean strain ranging of 0.5–5.8% and strain amplitudes of 0.15–0.45%. The second experiment considers formed wire specimens in bending fatigue, more representative of realistic medical components, with a maximum mean strain of 1.2%, and maximum strain amplitudes ranging from 0.72 to 1.64%. Compared with the austenitic parent material, the R-phase material tolerated 0.1–0.3% higher strain amplitudes.

Characterization of Martensite Orientation Relationships in Steels and Ferrous Alloys from EBSD Data Using Bayesian Inference

Detailed analysis of the characteristics a martensite structure inherits from the parent austenite phase requires knowledge of the crystallographic orientation relationship between austenite and martensite, which varies with composition for steel alloys. The orientation relationship is typically observed to exhibit a significant degree of variability, such that measurements from each variant occupy a range of orientations within the transformed pole figure, complicating characterization of the orientation relationship. Here, we present a Bayesian methodology to measure the orientation relationship on martensite EBSD data from four different steels and a binary Fe-Ni alloy. The number of variants that must be exhibited for an accurate measurement as well as robustness to noisy data for this approach are investigated. The Bayesian approach is found to produce results which compare favorably to those from prior work while being more easily automatable.

Martensitic transformation at a crack under mode I and II loading

AbstractMetastable austenitic steels can undergo phase transformation. As an allotrope two crystal configurations are of interest: the softer austenitic parent phase and the martensitic phases. Here, the bain orientation relationship leads to distinct orientations for the martensitic variants with a different transformation strain [7]. A phase field approach is used to model the transformation, where a multi‐valued order parameter identifies the austenitic parent phase and the martensitic variants. This allows to define bulk and surface energies as regularized functions in terms of the order parameter and its gradient. The kinetics of the martensitic transformation are temperature dependent. Temperatures below an equilibrium temperature favour the growth of the martensitic phase, whereas temperatures above the equilibrium favour the austenitic phase. Approaching the equilibrium temperature slows down the transformation [5]. In this work we consider a static crack under mode I and mode II loading.

An Accurate Method for Crystallographic Reconstruction of Parent Austenite from Inherited Martensite in a Low-Alloy Steel

The microstructure of austenite at high temperatures, which cannot be reserved at room temperatures, determines the properties of its transformed phase in low-alloy steels. Consequently, an accurate method is herein developed to reconstruct local orientations of the parent austenite ( γ ) phase from electron backscatter diffraction maps of the martensite ( α ′ ) microstructure. The orientation map of α ′ is cropped into a grid of data squares as the reconstruction unit. The cropped square is then divided into the square inherited from only one γ grain and the square inherited from more than one γ grain. The local orientations around parent γ grain boundaries are more accurately determined using a newly proposed reconstruction criterion. Furthermore, the solution spaces for the orientation relationship and the parent γ orientation are refined, which simultaneously improves the calculation accuracy and efficiency of reconstruction procedure. The validated reconstruction method is applied to obtain local orientations of the deformed γ microstructure accurately.

Tnr Dependent Hot Rolling Microstructure and Texture Development in C-Mn Dual Phase and HSLA Steels

No recrystallization of austenite, Tnr, has an important influence on the transformed phase fractions and the final crystallographic texture after hot deformation. This paper investigates the evolution of microstructure and texture components during hot-rolling in two austenitic region based on Tnr along with three different cooling trajectory and coiling in dual-phase steels and high strength low alloys steel. The recrystallization of the austenite, the austenite deformation followed by the austenite-to-ferrite transformation influence the final microstructure and texture in dual phase steels, have been examined by means of optical microscopy, X-ray diffraction (XRD) measurements. Recrystallized and deformed austenite have clearly different texture components and, due to the specific lattice correspondence relations between the parent austenite phase and its transformation products, the resulting ferrite textures are different as well.

EBSD Characterization of Cryogenically Rolled Type 321 Austenitic Stainless Steel

Electron backscatter diffraction was applied to investigate microstructure evolution during cryogenic rolling of type 321 metastable austenitic stainless steel. As expected, rolling promoted deformation-induced martensitic transformation which developed preferentially in deformation bands. Because a large fraction of the imposed strain was accommodated by deformation banding, grain refinement in the parent austenite phase was minimal. The martensitic transformation was found to follow a general orientation relationship, {111}γ||{0001}ε||{110}α′ and 〈110〉γ||〈11-20〉ε||〈111〉α′, and was characterized by noticeable variant selection.

Correlating microstructure and superelasticity of directed energy deposition additive manufactured Ni-rich NiTi alloys

Laser-based directed energy deposition (LDED) additive manufacturing of Ni-rich NiTi shape memory alloys was shown to produce inhomogeneous precipitate morphologies and characteristic grain structures consisting of columnar grains coexisting with equiaxed and subgrain structures. Post-processing solutionizing and aging heat treatments impacted microstructure and martensitic phase transformation (MT) responses underpinning superelastic shape memory responses. A solution treatment of 950 °C for 24 h was found to produce a uniform composition of the B2 austenite parent phase without affecting the coexistence of columnar and equiaxed substructures. Aging the solution treated material brought about a spatially uniform Ni4Ti3 precipitate morphology. Due to the uniform morphology, an underlying austenite-martensite interface motion accompanies the compressive stress-induced MT (SIMT). Reversible interface motion underpinned the compressive superelastic response for the solutionized and aged condition. On the other hand, strain concentrations existed at different spatial locations in the as built condition as well as when the as built material was aged. The stark contrasts in the SIMT exposed precipitate morphology as a controlling factor in tailoring the superelastic response of Ni-rich NiTi SMAs fabricated by LDED.

Heat Conduction in a Phase Field Model for Martensitic Transformation

AbstractWe study the martensitic transformation with a phase field model, where we consider the Bain transformation path in a small strain setting. For the order parameter, interpolating between an austenitic parent phase and martensitic phases, we use a Ginzburg‐Landau evolution equation, assuming a constant mobility. In [1], a temperature dependent separation potential is introduced. We use this potential to extend the model in [2], by considering a transient temperature field, where the temperature is introduced as an additional degree of freedom. This leads to a coupling of both the evolution equation of the order parameter and the mechanical field equations (in terms of thermal expansion) with the heat equation. The model is implemented in FEAP as a 4‐node element with bi‐linear shape functions. Numerical examples are given to illustrate the influence of the temperature on the evolution of the martensitic phase. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of the kinetic arrest of martensitic transformation in Ni45Co5Mn36.8In13.2 melt-spun ribbons

The kinetic arrest (KA) of martensitic transformation (MT) observed in Ni45Co5Mn36.8In13.2 melt-spun ribbons has been studied. These alloy ribbons show an ordered columnar-like grain microstructure with the longer grain axis growing perpendicular to ribbon plane and transform martensitically from a single austenitic (AST) parent phase with the L21-type crystal structure to a monoclinic incommensurate 6M modulated martensite (MST). Results show that the volume fraction of austenite frozen into the martensitic matrix is proportional to the applied magnetic field. A fully arrest of the structural transition is found for a magnetic field of 7T. The metastable character of the non-equilibrium field-cooled glassy state was characterized by introducing thermal and magnetic field fluctuations or measuring the relaxation of magnetization. The relaxation of magnetization from a field-cooled kinetically arrested state at 5 and 7T follows the Kohlrausch–Williams–Watts (KWW) stretched exponential function with a β exponent around 0.95 indicating the weak metastable nature of the system under the strong magnetic fields. The relationship between the occurrence of exchange bias and the frozen fraction of AST into the MST matrix was studied.

Orientational Equidistance Method for Solving Orientation Relationship From Product Variants: Application to Steel

A three-step procedure has been evolved for finding the orientation relationship (OR) between parent austenite and product ferrite phase, from the room temperature microtexture data of ferritic/martensitic steel. In the first step, the frequently occurring misorientations between product ferrite variants are sought from the electron backscatter diffraction (EBSD) dataset. This is carried out by calculating the ‘misorientation angle-rotation axis’ values across all neighboring pair of ferrite pixels, and plotting them in a parametric space to detect the prominent misorientation peaks. Identification of ‘concurrent relative orientation’ of product variants, i.e., the second step, is resolved by comparing the observed misorientation peak positions with the inter-variant relation set of Kurdjumov–Sachs (KS) OR. The final crucial step of solving the OR based on the ‘Orientational Equidistance’ principle of parent phase from all its product phase variants is implemented in the Rodrigues–Frank parametric space. The procedure is illustrated using an large area EBSD dataset of normalized and tempered 9Cr–1Mo–0.1C steel, and the optimum orientation relation is determined to be ~44.0°[0.151 0.137 0.979], which is 2.4° deviated from KS OR.

Enhancement in magnetocaloric properties of NiMnGa alloy through stoichiometric tuned phase transformation and magneto-thermal transitions

The investigation is focussed on phase generation and magnetocaloric properties of a series of Ni77-XMnXGa23 (x=22, 23, 24, 25, 27) alloys prepared through arc melting furnace. With increase in Mn content, the alloys showed systematic transition from a non-modulated martensite (NM) to a fully austenitic parent phase through an appearance and coexistence of modulated (M) structure. Intermediate Mn containing alloy (#Mn24) not only displayed high magnetic entropy change (ΔSM) of −7.7J/kg−1K−1 but also large Refrigerant Capacity (RC) of 169J.Kg−1 at magnetic field change of 3 T compared to other alloys. The coexisting martensite (NM, M) and parent austenite as well as overlapping thermomagnetic and structural transformation was deliverable through tuning of alloy chemistry wherein Ni was systematically substituted by Mn. Transmission electron microscopy (TEM) supported the proposition with existence of martensite plates of different morphology in Mn24 alloy exhibiting superior magnetocaloric properties.

X-Ray and Neutron Diffraction Studies of Quenched and Partitioned (Q&P) Multiphase Steel Microstructures

Abstract Novel advanced steel developments in recent years have resulted in many steels with complex multiphase microstructures. Foremost currently are steels that contain significant fractions of untransformed parent austenite phase. “Quenched and partitioned” steels, commonly referred to as Q&P steels, important as potential advanced high-strength (AHS) automotive steels, are an example in which the austenite is chemically stabilized by carbon partitioning from martensite following an interrupted quench above the martensite finish temperature, Mf. It is evident that the progress of microstructural change during this novel quenching and partitioning treatment, the volume of untransformed austenite stabilized during partitioning and the behavior of this austenite according to its carbon content should be understood. In relation to these requirements, recent analysis of X-ray powder diffraction (XRD) and neutron diffraction experiments will be considered. Initially, the effect of sample preparation between the two measurement techniques is considered, followed by observation of the partitioning process in real time allowed by specially prepared Mn-containing experimental alloys, with and without a crucial silicon addition required to enable austenite retention during Q&P treatment.

In-situ TOF neutron diffraction studies of cyclic softening in superelasticity of a NiFeGaCo shape memory alloy

Real-time in-situ neutron diffraction was conducted during uniaxial cycling compression of a Ni49.3Fe18Ga27Co5.7 shape memory alloy to explore the mechanism on its superelasticity at room temperature, which was manifested by the almost recoverable large strains and the apparent cyclic softening. Based on the Rietveld refinements, the real-time evolution of volume fraction of martensite was in-situ monitored, indicating the incremental amount of residual martensite with increasing load cycles. Real-time changes in intensities and lattice strains of {hkl} reflections for individual phase were obtained through fitting individual peaks, which reveal the quantitative information on phase transformation kinetics as a function of grain orientation and stress/strain partitioning. Moreover, a large compressive residual stress was evidenced in the parent phase, which should be balanced by the residual martensite after the second unloading cycle. The large compressive residual stress found in the parent austenite phase may account for the cyclic effect on critical stress required for triggering the martensitic transformation in the subsequent loading.

Nature of gallium focused ion beam induced phase transformation in 316L austenitic stainless steel

The microstructural evolution and chemistry of the ferrite phase (α), which transforms from the parent austenite phase (γ) of 316L stainless steel during gallium (Ga) ion beam implantation in Focused Ion Beam (FIB) instrument was systematically studied as a function of Ga+ ion dose and γ grain orientations. The propensity for initiation of γ → α phase transformation was observed to be strongly dependent on the orientation of the γ grain with respect to the ion beam direction and correlates well with the ion channelling differences in the γ orientations studied. Several α variants formed within a single γ orientation and the sputtering rate of the material, after the γ → α transformation, is governed by the orientation of α variants. With increased ion dose, there is an evolution of orientation of the α variants towards a variant of higher Ga+ channelling. Unique topographical features were observed within each specific γ orientation that can be attributed to the orientation of defects formed during the ion implantation. In most cases, γ and α were related by either Kurdjumov-Sachs (KS) or Nishiyama-Wassermann (NW) orientation relationship (OR) while in few, no known OR's were identified. While our results are consistent with gallium enrichment being the cause for the γ → α phase transformation, some observations also suggest that the strain associated with the presence of gallium atoms in the lattice has a far field stress effect that promotes the phase transformation ahead of gallium penetration.

Anisotropic microstructure and superelasticity of additive manufactured NiTi alloy bulk builds using laser directed energy deposition

The microstructure and superelasticity in additive manufactured NiTi shape memory alloys (SMAs) were investigated. Using elementally blended Ni and Ti powder feedstock, Ni-rich build coupons were fabricated via the laser-based directed energy deposition (LDED) technique. The build volumes were large enough to extract tensile and compressive test specimens from selected locations for spatially resolving microconstituents and the underlying stress-induced martensitic phase transformation (SIMT) morphology. In the as-deposited condition, X-ray diffraction identified the B2 atomic crystal structure of the austenitic parent phase in NiTi SMAs, and Ni4Ti3 precipitates were the predominant microconstituent identified through scanning electron microscopy. The microstructure exhibited anisotropy, which was characterized by the Ni4Ti3 precipitate morphology being coarsest nearest the substrate, while a finer morphology was observed farthest from the substrate. In-situ full-field deformation measurements calculated using digital image correlation confirmed that the SIMT predominately occurred in the finer precipitate morphology. Heat treatment reduced the degree of anisotropy, and DIC analysis revealed localized SIMT strains increased compared to the as-deposited condition.

Magnetostructural transition and magnetocaloric effect in highly textured Ni-Mn-Sn alloy

Ni49.4Mn38.5Sn12.1 near single crystal was obtained by the Bridgman method. At room temperature, it consisted of a mixture of the parent austenite phase with the cubic L21 Heusler structure (ac = 5.984 Å) and modulated, tetragonal martensite phase 4M (at = 4.337 Å, ct = 5.655 Å). Under the application of a magnetic field, the specimen undergoes field induced reverse martensitic transformation, which combined with the Curie transition in austenite leads to the coexistence of direct and inverse magnetocaloric effects. The maximum entropy change at 280 K and under 5 T amounts to 3.4 J·kg−1·K−1 for the structural transition and at 316 K reaches −2.7 J·kg−1·K−1 for the magnetic transformation. The magnetic entropy change occurs over a wide temperature span leading to improved refrigerant capacity of 101 J·kg−1 (5 T). Hysteretic losses are considerably reduced, which is promising with respect to improved cyclic stability of such a material.

High-Resolution EBSD Study of Adiabatic Shear Band and Neighboring Grains After Dynamic Impact Loading of Mn-Steel Used in Vehicle Structure

We report the results of the microstructural characterizations and micro-texture analysis of a lightweight austenitic steel deformed at high strain rate (1200 s−1) using a split Hopkinson pressure bar. Formation of adiabatic shear bands (ASB) and plastic deformation mechanisms within neighboring grains are investigated by high-resolution electron backscatter diffraction (HR-EBSD). HR-EBSD reveals formation of athermal e-martensite and ά-martensite within the shear bands, resulting in the formation of a brittle intersection structure. Crack initiation and propagation is seen in intersection structure. The thermally induced e-martensite follows Shoji-Nishiyama crystallographic orientation relationship with parent austenite phase, while ά-martensite shows Burgers relationship with e-martensite. A detailed examination depicts the presence of deformation twins in grains adjacent to the ASB. Furthermore, strain-induced e and ά martensite are formed in the neighboring grains of ASB. The micro-texture of martensite variants is discussed in ASB and in the neighboring grains.

Variation in Transformation Temperature and Shape Memory Effect in Cu-Al-Be Shape Memory Alloys with the Effect of Quaternary Elements <sup></sup>

Cu-Al-Be Ternary alloys are prepared by ingot metallurgy route, which exhibits parent phase or Austenite phase at high temperature and Martensite phase at low temperature and also exhibits shape memory effect upon quenching to lower temperature. The Cu-Al-Be SMA was in the range of 10-12wt% of Al and 0.4-0.5 wt% of Be is chosen for present study and different amount of quaternary element is added to the ternary alloy. The variation in shape memory effect, transformation temperature and microstructure is studied by using bend test, differential scanning calorimeter and Optical microscope.