Martensitic Alloys Research Articles

Influence of microscope settings on dislocation imaging in transmission forescattered electron imaging (t-FSEI)

This work analyzes the influence of several microscope settings, namely, sample-forescattered electron detector (FSD) distance, and tilting conditions on the characteristics of the dislocation contrast imaged in transmission forescattered electron imaging (t-FSEI). The dislocation contrast behaviors of characteristic dislocation configurations of two Fe-based alloys, namely an α’-martensitic (body-centered cubic, bcc) Fe33Ni alloy (wt%), and an austenitic (face-centered cubic, fcc) Fe-30Mn-6.5Al-0.3C alloy (wt%) were investigated on thin foil samples by using different on-axis transmission Kikuchi diffraction (TKD) configurations, namely t-FSEI, bright-field (BF) t-FSEI and electron channeling contrast imaging (ECCI). The set-ups use transmission Kikuchi electron patterns to orient the crystal into controlled diffraction conditions. Imaging parameters such as dislocation contrast intensity and information depth are analyzed and compared to those obtained in the ECCI mode under the same microscope conditions. These effects are associated with the attenuation of Bragg scattering by high-angle scattering processes and the electron channeling mechanism, respectively. The experimental analysis sets the microscope settings for optimum dislocation imaging in t-FSEI.

Chemical core-shell metastability-induced large ductility in medium-entropy maraging and reversion alloys

Maraging structural materials have been traditionally indicated as essential metallic alloys for hundreds of years. The highest strength of the aged martensite alloys requires the formation of high nanoprecipitate density; however, it often results in insufficient ductility (< ∼12%) which limits their application. Here, we describe how these alloys obtain enhanced ductility at high strength by injecting reversion-induced metastable austenite into the brittle microstructure, in which we develop a novel dual-phase medium-entropy Fe68Ni10Mn10Co10Ti1.5Si0.5 (at%) maraging alloys with a strength of 1.6 GPa and ductility of ∼25%. Generating the large fraction of austenite metastability with a chemical core-shell microstructure during a simple process of reversion drives profuse heterogeneities at chemical and structural states, including additional precipitation strengthening and transformation-induced plasticity effect. The combined metastability and heterogeneity, realized with heat-treatment techniques that are accessible processing routes in a wide range of academic and industrial applications, can provide a breakthrough to develop sustainable maraging materials with large ductility.

Lenticular twinning within α’ martensite in Ti-6Al-4V at early deformation stage

Twinning in α’ martensite in titanium alloys often occurs during rapid cooling or thermal cycling such as additive manufacturing process. This work reported α’ martensitic microstructure formed in Ti-6Al-4V alloy after room-temperature compression to ∼5% strain. Large-scale lenticular deformation twins were observed in martensite for the first time, which were evidently different in morphology and twinning system from nanotwins in quenched martensite. The emergence of such twins at only ∼5% strain resulted from variant selection and larger thickness of martensite and different stress state in martensite during deformation.

Asymmetric magnetic-field-induced phase transformation in Fe48Mn24Ga28 Heusler alloy

We report on the conceptual design and experimental realization of a ternary Heusler-based ferromagnetic martensitic phase transformation alloy Fe48Mn24Ga28 that exhibits asymmetric magnetostructural coupling between the forward austenite-to-martensite and the reverse martensite-to-austenite phase transformations. By tuning the Curie temperature of the high-temperature austenite to be within the thermal hysteresis of the first-order phase transformation, dissimilar change in magnetization can be associated with the forward and reverse phase transformations, respectively. We show that such an asymmetry leads to enlargement of the thermal hysteresis with an applied external magnetic field. Furthermore, partly due to the field-enhanced thermal hysteresis, the magnetic-field-induced martensitic phase transformation is irreversible in such a system. This work suggests the intricate correlation between magnetostructural coupling, thermal hysteresis, and magnetic-field-induced phase transformation, which can be exploited for designing magnetic phase transformation alloys that show extended functionalities.

The Effect of W Content on the Microstructure, Mechanics and Electrical Performance of an FeCrCo Alloy.

In this paper, FeCrCoW alloys with different W contents (0.4, 2.1 and 3.4 at%) are designed and studied in order to overcome the existing shortcomings of resistance materials. These resistance materials have high resistivity and a low temperature coefficient of resistivity. It is observed that the addition of W has a remarkable effect on the phase structure of the alloy. In particular, when the W content is 3.4 at%, the single BCC phase of the alloy can be transformed into the BCC and FCC phase. Meanwhile, when analyzed by transmission electron microscopy, there are stacking faults and martensite in FeCrCoW alloy with W content of 3.4 at%. These features are related to excessive W content. In addition, the strength of the alloy can be improved, and the ultimate tensile strength and yield strength are both very high, which are considered as grain-boundary strengthening and solid solution strengthening, caused by the addition of W. The electrical resistivity of the FeCrCoW alloys decreases when the content of W is more than 2.1 at%. The maximum resistivity of the alloy is 170 ± 1.5 μΩ·cm. Moreover, the unique properties of the transition metal allow the alloy to have a low temperature coefficient of resistivity in the temperature range of 298~393 K. The temperature coefficient of resistivity values of the W0.4, W2.1 and W3.4 alloys are -0.0073, -0.0052 and -0.0051 ppm/K. Therefore, this work provides a vision for resistance alloys, which can achieve highly stable resistivity and high strengths in a certain temperature range.

Evidence for austenite to non-modulated martensite transformation crystallography and variant organization in Ni-Mn-Ga-Co ferromagnetic shape memory alloys

Knowledge of transformation crystallography and variant organization of product phase was a prerequisite for tuning microstructure and enhancing functionalities in materials with displacive structure-transformation. However, the transformation orientation relationship (OR) between parent austenite and non-modulated (NM) martensite in traditional NiMnGa ferromagnetic shape memory alloy was not yet well-determined via direct experimental results, thus leading to ambiguity in understanding the self-accommodated configuration. In this work, by generating a microstructure with coexisting austenite and NM martensite through minor Co substitution and proper heat treatment, the transformation OR was unambiguously determined to be the N-W relation with {111}A//{101}NM and <2¯11>A//<101¯>NM on the basis of accurate EBSD orientation measurements. Analysis on deformation gradient matrix constructed from the N-W OR revealed that, the internal nano-twined structure allowed a maximum profit for eliminating the overall latticed deformation caused by transformation, overweighing the “sandwich” micro-variant pair structure. Such strain accommodation mechanisms consequently contributed to the resultant self-accommodated hierarchically twinned structure of martensite. Moreover, it was found that the prior austenite grain boundaries (PAGBs) provided dominated nucleation sites for NM martensite variants, where the preferential variant selection at PAGBs mainly depended on the inclination angle between the PAGB and matching close-packed direction (<2¯11>A//<101¯>NM) of variants. The present study provided comprehensive information on transformation crystallography and displacive characters of austenite to NM martensite transformation, which was useful for the efforts in property optimization and theoretical simulation.

Hysteretic behavior and recoverability research on the shape memory effect of SMA bars under cyclic loading

To utilize the special shape memory effect of shape memory alloy (SMA) to develop energy dissipation devices with a good energy dissipation effect and easy recovery of residual deformation after an earthquake, six SMA bars with a diameter of 14 mm were tested under multiple loading schemes in this paper. Hysteretic performance indicators, such as the hysteresis loop, energy dissipation capacity, secant stiffness, equivalent damping ratio and deformation recovery rate, were analyzed. The results illustrated that all specimens exhibited the desired full and stable hysteresis loops and a high residual deformation recovery rate achieved by water-bath heating and cooling after being subjected to 6.0% tensile strain. Due to the characteristics of SMA material, excessive compressive strain caused the stress value to soar, and hysteresis loops contributed to the phenomenon of tension–compression stress imbalance and affected the deformation recovery rate. No significant differences were observed in the bearing capacity and energy dissipation capacity of the same specimen with a strain amplitude of 6.0% after repeated loadings, indicating that martensitic shape memory alloys (SMAs) have stable mechanical characteristics and favorable recycling performance. The tensile cyclic constitutive model and tension–compression cyclic constitutive model of the martensite shape memory effect were proposed. Based on MATLAB/Simulink software, numerical simulations were carried out. The simulation results and fitted the experimental results, which proved the theoretical support for utilizing the shape memory effect of martensite SMA.

THE ROLE OF SELFACCOMMODATION COMPLEXES IN THE CRYSTALLOGRAPHIC REVERSIBILITY OF NONTHERMOELASTIC MARTENSITE TRANSFORMATIONS AND CONJUGATION OF ANISOTROPIC STRUCTURES

The possible structure of self-accommodation complexes of martensite crystals in alloys based on g-Mn and titanium nickelide has been analyzed. The analysis is based on the calculation of shape deformation, averaged over an ensemble of domains (equivalent versions of orientation relationships); for complete self-accommodation this deformation should be described by a unit matrix. In this case the compensation of the shape variation and minimization of elastic energy occur on the microscopic level of individual complexes of twinned martensite crystals. It is shown that complete self-accommodation of rhombohedral martensite R, as well as tetragonal and orthorhombic martensites in alloys based on y-Mn is implemented only in the complexes containing either all crystallographically equivalent versions of orientation relationships or their doubled number

Understanding Vogel-Fulcher–like equilibration times through the partial equilibration scenario

We test a post-quench Partial Equilibration Scenario (PES) of Ritort and colleagues, through Monte Carlo quench simulations, of a vector-spin model for a cubic-to-tetragonal, martensitic structural transition. We confirm the PES signature distribution of an exponential tail in heat releases, scaled in an effective search temperature that regulates energy-lowering passages between fixed-energy shells. Our simulations find that this effective temperature vanishes linearly in the deviation of the quench temperature from a divergence temperature, where passage bottlenecks in phase space segment their topology, and entropy barriers diverge. Equilibration delay times, exponential in the inverse effective temperature, are thus predicted to show singular Vogel-Fulcher behaviour, understood here as an arrest of PES heat releases to the bath, and confirmed by experimental data on martensitic alloys.

Anomalous hot deformation behavior and microstructure evolution of as-cast martensitic NiTi alloy during hot compression

Anomalous hot deformation behavior and microstructure evolution of as-cast martensitic NiTi alloy during hot compression

Reversible Diffusionless Phase Transitions in 3D Nanoparticle Superlattices

Nanocomposite tectons (NCTs), polymer brush-grafted nanoparticles that use supramolecular interactions to drive their assembly, form ordered nanoparticle superlattices (NPSLs) with well-defined unit cell symmetries when thermally annealed. In this work, we demonstrate that appropriate assembly and processing conditions can also enable control over the microstructure of NCT lattices by balancing the enthalpic and entropic factors associated with ligand packing and supramolecular bonding during crystallization. Unary systems of NCTs are assembled via the addition of a small molecule capable of binding to multiple nanoparticle ligands; these NCTs initially form face-centered-cubic (FCC) structures in solvents that are favorable for the particles' polymer brushes. However, the FCC lattices undergo a reversible, diffusionless phase transition to body-centered-cubic (BCC) lattices when transferred to a solvent that induces polymer brush collapse. The BCC superlattices maintain the same crystal habit as the parent FCC phase but exhibit significant transformation twinning similar to that seen in martensitic alloys. This previously unseen diffusionless phase transformation in NPSLs enables unique microstructural features in the resulting assemblies, suggesting that NPSLs could serve as models for the investigation of microstructural evolution in crystalline systems and extend our understanding of NPSLs as atomic material analogues.

Local Maxima in Martensite Start Temperatures in the Transition Region between Lath and Plate Martensite in Fe-Ni Alloys

In the binary Fe-rich Fe-Ni system, martensite start temperatures MS decrease from 500 to 200 K when Ni concentrations increase from 20 to 30 at.%. It is well known that alloys with Ni concentrations below 28.5 at.% exhibit lath martensite (LM) microstructures (athermal transformation, small crystals, accommodation by dislocations). Above this concentration, plate martensite (PM) forms (burst-like transformation, large crystals, accommodation by twins). The present work is based on a combination of (i) ingot metallurgy for the manufacturing of Fe-Ni alloys with varying Ni-concentrations, (ii) thermal analysis to measure phase transformation temperatures with a special focus on MS, and (iii) analytical orientation imaging scanning electron microscopy for a quantitative description of microstructures and crystallographic features. For Ni-concentrations close to 28.5 at.%, the descending MS-curve shows a local maximum, which has been overlooked in prior works. Beyond the local maximum, MS temperatures decrease again and follow the overall trend. The local maximum is associated with the formation of transition martensite (TM) microstructure, which exhibits LM and PM features. TM forms at higher MS temperatures, as it is accommodated by simultaneous twinning and dislocation slip. An adopted version of the Clausius-Clapeyron equation explains the correlation between simultaneous accommodation and increased transformation temperatures.

Twinning-induced Plasticity Mechanism of α″-martensitic Titanium Alloy

The deformation mechanisms of an α″-martensitic Ti–8Mo alloy were investigated by ex-situ electron microscopy tensile tests. Two deformation twins, {110}〈11¯0〉α″ and {130}〈3¯10〉α″ were simultaneously generated by two pathways: (1) direct from primary laths and (2) via reorientation twins. The generation of deformation twinning is strongly dependent on orientation and follows Schmid's law. The dominant twinning in α″ martensite was determined by comparing the shuffle magnitudes calculated from the lattice parameter and the magnitude of lattice modulation. {110}〈11¯0〉α″ and {130}〈3¯10〉α″ twins are activated simultaneously in Ti alloys with a low β-stabilizer content, such as Ti–8Mo alloy.

Antibacterial copper-bearing titanium alloy prepared by laser powder bed fusion for superior mechanical performance

• A boundary engineering strategy is proposed to improve the strength and ductility of materials simultaneously. • The addition of Cu successfully verified the feasibility of the boundary engineering strategy, and improved the comprehensive mechanical properties of LPBF martensitic titanium alloy. • The role of copper addition in microstructure development are clarified. • The strengthening mechanism of LPBF Ti, Ti-5Cu and Ti-6Al-4V is explained by calculation and fitting. Copper element was added in pure titanium to produce a new biomedical titanium-copper alloy by laser powder bed fusion (LPBF). Addition of copper can eliminate the mismatch of high strength but poor ductility problem caused by lath α' martensite, which is the usual microstructure of near α titanium alloy fabricated by LPBF. Instead of by the usual trade-off relationship between strength and ductility, which is a long-standing challenge for martensitic titanium alloys, in this study, we proposed a boundary engineering strategy and aim to synergistically enhance the strength and ductility of martensitic titanium alloy fabricated by LPBF. It is hypothesized that whilst both low-angle and high-angle grain boundaries are beneficial to the strength, high-angle grain boundary can simultaneously improve the ductility of materials. To test this strategy, a Ti-5Cu (wt.%) alloy is selected to compare against pure titanium and Ti-6Al-4V at the same laser processing conditions. EBSD, TEM and XRD analysis show that the as-fabricated LPBF Ti-5Cu alloy is comprised of partially tempered martensite with extraordinarily high number density of both high-angle and low-angle grain boundaries as well as low dislocation density. Such microstructure enables a high tensile strength of 940–1020 MPa, which is at a similar level as LPBF Ti-6Al-4V, and an excellent elongation of 13%–16%, twice as much as that of LPBF Ti-6Al-4V. The mechanism of microstructure refinement in LPBF Ti-5Cu at different levels from prior-β grains, martensitic packets, blocks to laths is also discussed.

Comparison of the Corrosion Behavior of TiNi Alloys with Martensitic and Austenitic Structures

The work is devoted to the study of the corrosion behavior of TiNi alloys with the original austenitic and martensitic structure in various states. In the course of the research, it was found that the rate of corrosion destruction for coarse-grained and ultrafine-grained Ti49.0Ni51.0 alloys is different: the coarse-grained alloy turned out to be less corrosion resistant and the destruction rate was higher than the corrosion rate in the ultrafine-grained state. X-ray phase analysis of TiNi alloys revealed that as a result of corrosion processes, a change in the phase composition of the alloy occurs, accompanied by the formation of TiNi hydride - TiNiH1.4. In the ultrafine-grained state, the volume fraction of the formed hydride is higher than in the coarse-grained state. This is typical for both alloys, however, in the martensitic alloy, the volume fraction is higher than in the austenitic one.

Hydrogen Embrittlement of Nitrogenating Layer on Martensitic Alloys

Hydrogen Embrittlement of Nitrogenating Layer on Martensitic Alloys

RELAXATION ROTATION DURING MARTENSITIC TRANSFORMATION AND SCATTERING OF MARTENSITE TEXTURE

In the our crystallographic theory of martensitic transformation, a new characteristic of martensitic transformation appears – a relaxation rotation, which was not in the generally accepted phenomenological theory of martensitic transformation. The relaxation rotation is necessary to obtain an invariant plane. The calculated angle of relaxation rotation for iron alloys with non-thermoelastic martensite is 1.75–1.85 degrees, for thermoelastic Geisler alloys with multilayer martensite 0.19–0.36 degrees. The relaxation rotation leads to the scattering of the texture of martensite. The scattering angles of single X-ray reflections of martensite in different alloys were measured and it was found that the scattering angle corresponds to the angle of relaxation rotation. This result shows that the our crystallographic theory of martensitic transformation describes the real mechanism of martensitic transformation.

State-of-the-Art Review on the Aspects of Martensitic Alloys Studied via Machine Learning

Though the martensitic transformation has been a commonly investigated topic in the field of experimental and computational materials science, the understanding of this mechanism in a variety of alloys is yet far from complete. In this era of Industry 4.0, there have been ongoing trends on employing machine learning (ML) techniques for the study of the martensitic alloys, and such data-driven approaches are expected to unravel a great amount of information about the process-structure-property behaviour relationship in this class of materials. However, with the availability of a large variety of datasets and with an option to use different ML models, a bulk amount of information has already been generated with regard to martensitic alloys. The discovery and design of shape memory alloys can be accelerated if the multi-principal element functional alloys and martensitic transformation phenomenon are studied extensively using machine learning techniques. Thus, it is necessary to highlight the major categories or aspects of these alloys that have been predicted with ML. The present work performs a state-of-the-art review on the machine learning models developed for the quantification of aspects such as martensitic start temperature (Ms), materials properties, microstructure, mechanisms etc., on the alloys.

Microstructural effects on the single crystal diamond tool wear in ultraprecision turning of Ti6Al4V alloys

Single crystal diamond tools are common cutters in the ultraprecision machining, but the tool wear is a great concern in machining of difficult-to-cut materials. In this paper, material microstructural effect on the diamond tool wear is firstly investigated in ultraprecision turning of difficult-to-cut Ti6Al4V alloys. Equiaxial, bimodal and lamellar microstructures are used in the machining. Results show that more adhesion of Ti6Al4V workpiece is observed at the flank face instead of the rake face in turning of all types of microstructures. The tool wear in turning of the lamellar alloy is less than that of the equiaxial and bimodal alloys, giving rise to low cutting forces and high surface quality. Furthermore, discontinuous serrated chips are observed for the equiaxial and bimodal alloys at the cutting distance of 3000 m, while the chips remain continuous in machining of the lamellar martensitic alloys. Besides, the diamond tool wear mechanism is discussed from perspectives of tool wear modes and chemical reactions by characterizations of the scanning electron microscope, atomic force microscope atom, X-ray photoelectron spectra and high-resolution transmission electron microscope. This study aims to clearly understand the diamond tool wear mechanism in ultraprecision machining of the Ti6Al4V alloys.

Solute rich cluster formation and Cr precipitation in irradiated Fe-Cr-(Ni,Si,P) alloys: Ion and neutron irradiation

Ion irradiation is a powerful and affordable tool to rapidly test a wide range of irradiation conditions and make the link with the corresponding microstructural evolution. However, several issues of transferability of results from ion to neutron irradiation have been evidenced. This paper presents an atom probe investigation of the microstructural evolution of FeCr-NiSiP alloys with different contents of Cr and minor solutes under both ion and neutron irradiation at 300°C. Minor solutes and Cr are known to form solute rich clusters (SRCs) and α' clusters in ferritic and martensitic FeCr alloys, which are one of the causes of hardening. The objective of this work is to highlight the differences and the commonalities between ion and neutron irradiations in these alloys. The use of two ion beam energies (8 MeV and 5 MeV) revealed that this parameter has an impact on the formation of SRCs. The SRCs present similar characteristics after 8 MeV Fe ion irradiation and neutron irradiation, despite the different dose rate, when Ni, Si and P are present. It is not the case for 5 MeV Fe ions, for which the SRCs were less developed. Influence of the concentration of minor elements has been evidenced, as well. The presence of Ni, Si and P has been shown to impact both the number density and the size of SRCs in Fe9Cr-NiSiP alloys and the onset of α' formation.