Martensitic Mechanism Research Articles

Acoustic shock wave-induced sp2-to-sp3-type phase transition-part II: Evidence of the presence of diamond from valance band spectra and electronic diffraction pattern

Here, we introduce a novel technique that utilizes repeated exposure to low-pressure (2.0 MPa) millisecond acoustic shock waves on a graphite sample to facilitate the successful transformation of graphite into diamond. This transformation is based on a martensitic nucleation mechanism verified through valance-band X-ray photoelectron spectroscopic and electron diffraction observations. Typically, diamond formation occurs only under pressures of 50–60 GPa or more in nanosecond dynamic compression experiments. The present work offers a new platform to make synthetic diamonds and a new scientific path for diamond formation.

A deep insight of re-entrant martensitic transformation mechanism in Co2Cr(Ga,Si) shape memory alloys

Co2Cr(Ga,Si) shape memory alloys have a wide application temperature range due to the unique re-entrant martensite phase transformation (RMT) behavior. Nevertheless, the microscopic mechanism of RMT remains elusive and unsystematic. The heart of this investigation lies the comprehensive exploration of phase stability, phase transformation path, and martensite slip direction during RMT from martensite (D022-PM) to austenite (L21-FM). In this work, we systematically investigate the re-entrant martensitic transformation of Co2Cr(Ga,Si) SMAs using first-principles methods for the first time. Firstly, the density of states (DOS) calculation indicates that the stability of L21-FM is higher than D022-PM. The charge density calculation further indicates that the bonding strength between Co atoms and Cr (Si) atoms in the L21-FM phase is the highest. In addition, Fermi surface calculation further reveals that phase instability is due to the Fermi surface nesting caused by electron-phonon coupling. Moreover, the minimum energy path was calculated by the G-SSNEB method for the first time, which indicates the RMT can occur. Finally, the calculation of the elastic constants indicates that RMT is caused by the crystal plane slip of the D022-PM phase along the <110> direction. Our calculations provide the electronic-level and thermodynamic mechanism for RMT of Co2Cr(Ga,Si) alloy and shed some light on the revelation of the phase transformation mechanism.

Martensitic phase mechanism of ternary FeCrMn thin films influenced by the substrate rotation speeds: structural and magnetic properties

In order to investigate the martensitic phase mechanism of the ternary FeCrMn thin films sputtered under the effect of substrate rotation speeds, the structural and related magnetic properties were studied. A range of thin films were deposited at varying rotational speeds of 0, 15, 30, and 45 rpm on flexible amorphous polymer substrates through the use of DC magnetron sputtering. The films were 50 nm thick and were produced at 0.09 nm s−1. The crystal structures showed that all films have a mixture of the body-centred tetragonal (bct) and tetragonal structure. The peak intensity of bct (110) martensitic α’phase increased with the increase of the rotation speeds whereas the tetragonal (430) and (333) peaks stayed almost stable. And, the morphologic surface analysis displayed that the smooth surface turned into a rough surface with the increase of the rotation speeds. After the measurements of hysteresis loops, the films obtained by sputtering of austenitic target have ferromagnetic character with increasing saturation magnetization, MS and coercivity, HC as the substrate rotation speeds increase. With increasing rotation speeds, the increase of the MS from 148 to 242 emu cm−3 and the rise of the HC of the films from 21 to 185 Oe might be explained by the increase of the grain sizes with the increase of % martensitic α’phase caused by increasing rotation speeds. The ternary FeCrMn thin films exhibit increasing % martensitic α’phase and corresponding ferromagnetic properties with increasing substrate rotation speeds. It is concluded that the nanostructured films of FeCrMn have different properties from those of their bulk counterparts under the influence of substrate rotation speeds. Therefore, the martensitic mechanism of the films can easily be controlled by changing rotation speed for potentially flexible new device applications such as spintronics, magnetic hetero-structures, magnetic separators, etc.

Temperature-dependent lattice dynamics study on phase stability, martensitic transformation mechanism, and vibrational entropy properties of B2-type Ni–Mn–Ti Heusler alloy

The all-d-metal Ni–Mn–Ti Heusler alloy with its good mechanical properties and colossal caloric effects has attracted significant attention for mechanocaloric refrigeration. However, the phase stability of the B2-type disordered structure at finite temperatures and the physical origin of large phase transformation entropy change in theoretical calculation remain unclear. In view of thermal and strongly anharmonic effects, we combined the first-principles and temperature dependent effective potential calculations to systematically study the phase stability, martensitic transformation mechanism, and vibrational entropy properties of B2 partial disordered structures for Ni8Mn5Ti3 and Ni8Mn6Ti2. Our results showed the austenitic structures of Ni8Mn5Ti3 and Ni8Mn6Ti2 both exhibit antiferromagnetic interaction behaviors at targeted temperatures, resolving the contradiction between previous calculations and experiments. By the analysis of phonon dispersion relations and Helmholtz free energies at low and high temperatures, we provided a deeper explanation for the underlying mechanism of martensitic transformation. Meanwhile, the phase transformation temperatures and vibrational entropy changes were accurately predicted compared with the experiment results. Specifically, we demonstrated that the large vibrational entropy change of Ni–Mn–Ti material originates from the contributions of both the tetragonal distortion ratio and volume change, which provides important theoretical guidance and has great practical significance for designing the solid-state material with large vibrational entropy change in the application of solid-state refrigeration.

Optimizing crack initiation energy in austenitic steel via controlled martensitic transformation

Although the seemingly negative effect of deformation-induced martensite (DIM) volume fraction on the impact toughness of austenitic steels has been well documented, it relies mostly on analyzing crack propagation without explicitly considering the crack initiation process which, however, plays a crucial role in these ductile alloys. The dependence of crack initiation energy (Ei) on martensitic transformation mechanisms is still ambiguous, inhibiting the precise design of damage-tolerant and ductile alloys. Here, we explore the temperature-dependent crack initiation energy of a SUS321 stainless steel at various temperatures (25, –50, and –196 °C). Contrary to the crack propagation energy (Ep), the Ei has a weak correlation with the volume fraction of α′-martensite but a strong correlation with the martensitic transformation rate. Also contrary to the traditional viewpoint of Ep considering ε-martensite as a detrimental phase, a high volume fraction of ε-martensite turns out to be beneficial to the increase of Ei, thereby enhancing impact toughness. As such, an optimal value (15 mJ/m2) for the stacking fault energy (SFE), which dictates the γ→ε→α′ transformation sequence, is given as a new design guideline for enhancing the Ei and consequently the impact toughness of ductile steels. The generality of this guideline is further validated in multiple austenitic steels with different compositions and grain sizes.

Abnormal superelastic cyclic behavior and stress-induced martensitic transformation mechanism of NiTi alloy with R phase

In this work, an abnormal phenomenon of the superelastic cycle in NiTi alloy with R phase is reported. The phase transformation of the 500°C-annealed-sample is B2-R-B19'. During the first superelastic cycle, the critical stress of stress-induced martensitic transformation (SIMT) is reduced to 305.5 MPa and the phase transition plateau is shortened. With the loading process, the transformation of R–B19′ and the elastic deformation of the B19′ occur. With the unloading process, martensite reversal transformation occurs and the high-density dislocation zone and residual martensite are formed. During the superelastic cycle, the R phase is consumed. The increase of irreversible residual martensite and crystal defects leads to the deterioration of superelasticity.

A Review of Deformation Mechanisms, Compositional Design, and Development of Titanium Alloys with Transformation-Induced Plasticity and Twinning-Induced Plasticity Effects

Metastable β-type Ti alloys that undergo stress-induced martensitic transformation and/or deformation twinning mechanisms have the potential to simultaneously enhance strength and ductility through the transformation-induced plasticity effect (TRIP) and twinning-induced plasticity (TWIP) effect. These TRIP/TWIP Ti alloys represent a new generation of strain hardenable Ti alloys, holding great promise for structural applications. Nonetheless, the relatively low yield strength is the main factor limiting the practical applications of TRIP/TWIP Ti alloys. The intricate interplay among chemical compositions, deformation mechanisms, and mechanical properties in TRIP/TWIP Ti alloys poses a challenge for the development of new TRIP/TWIP Ti alloys. This review delves into the understanding of deformation mechanisms and strain hardening behavior of TRIP/TWIP Ti alloys and summarizes the role of β phase stability, α″ martensite, α′ martensite, and ω phase on the TRIP/TWIP effects. This is followed by the introduction of compositional design strategies that empower the precise design of new TRIP/TWIP Ti alloys through multi-element alloying. Then, the recent development of TRIP/TWIP Ti alloys and the strengthening strategies to enhance their yield strength while preserving high-strain hardening capability are summarized. Finally, future prospects and suggestions for the continued design and development of high-performance TRIP/TWIP Ti alloys are highlighted.

Structure formation of ultradispersed diamond by detonation synthesis

In the present work, we report on structure formation of ultradispersed diamond through detonation synthesis. In particular, based on the present data, we can state that ultradispersed diamond of detonation (UDD) synthesis is characterized by a rail substructure formation, which is typical for diamonds of dynamic and static synthesis formed from graphite by the martensitic mechanism. The formation of UDD is characterized by a two-stage process: the first stage is the formation of graphite from detonation products, and the second stage is the transformation of graphite into diamond by the martensitic mechanism. A microlamellar substructure in the rails of diamonds formed from the original graphite is due to the presence of basic packing defects that is characteristic for covalent crystals with a wurtzite lattice deformed at high pressures and temperatures typical for lonsdaleite. The presence of lonsdaleite in UDD also indicates its formation in the process of synthesis of diamond from graphite through the martensitic transformation.

Strain rate-dependent tensile deformation behavior and fracture mechanism of Mn–N bearing lean duplex stainless steel

The tensile deformation behavior of an Mn–N bearing lean duplex stainless steel (LDSS) with metastable austenite was investigated at the strain rate range of 10−4 - 100 s−1. The mechanical response and microstructural evolution at various strain rate was characterized and in particular, the influence of strain rate on the work hardening, strain-induced martensitic transformation (SIMT) and fracture mechanism was focused on. The results show that the deformation of austenite dominates the overall work hardening and damage mode of the test LDSS. At the strain rate of 10−4 s−1, the test LDSS exhibits excellent strength-ductility combinations with ultimate tensile strength (UTS) of 928 MPa and total elongation (TEL) of 59 %, which are attributed to the occurrence of the SIMT with a typical sequence of γ→ε→α′ and resultant transformation induced plasticity (TRIP) effect. However, the increasing strain rate suppressed the TRIP effect, causing a negative correlation between UTS/TEL and strain rate, because the elevated temperature rise (ΔT) resulting from the adiabatic heating effect promoted increased austenite stability and decelerated SIMT kinetic. Especially at the case of 100 s−1, the higher Ta (average temperature ∼ 23 + 38 = 61 °C), which is close to the Md temperature (the critical temperature up to which martensite formation can be forced by mechanical stress, ∼ 60 °C), leads to a significant increase in stacking fault energy (SFE) within the austenite. Consequently, the microstructural evolution during austenite deformation does not exhibit pronounced SIMT but rather maintains limited twinning. Although the samples deformed at various strain rates exhibit a dimple-type fracture mechanism, the secondary cracking on the fracture faces becomes more pronounced (i.e., reduced fracture resistance) with the decreasing strain rate. Crack initiation frequently appears at the interface of γ/α (or α′/α) for all strain rate cases because of the strain partitioning among constituents, moreover, at lower strain rates (≤10−2 s−1), the cracks inside of the α′-martensite are another damage site, which is responsible for the degradation of fracture resistance.

Molecular dynamic simulations of the martensitic transformation for the dual-phase structure and dislocation activities in Ti80 alloys

Titanium alloy (Ti alloy) has been widely used in various industry areas and attracted extensive attentions due to their excellent properties. In this work, the martensitic transformation (MT) and complex dislocation activities are thoroughly simulated and analyzed for the commercial Ti80 alloy in molecular dynamic (MD) simulations. One specific interatomic potential is developed for Ti80 alloy within the framework of embedded atom method (EAM), which is capable of describing the related physical properties and the plastic deformation mechanism for this alloy. The stabilities of hexagonal close-packed (hcp) and body-centered cubic (bcc) phase boundary and the martensitic transformation mechanism is fully investigated in MD simulations. The orientation relationship between hcp and bcc phases obeys the Burgers relationship. Complex dislocation activities are reproduced in the simulation of the compression of the polycrystalline configuration, including the formation of dislocation wall, (112‾2) compression twin boundaries and dislocation cross slip mechanisms. The MD simulation results are well agreement with our experimental observations and provides an in-depth understanding of the deformation mechanisms of the Ti80 alloy and related Ti alloys. The revealed deformation mechanism and the newly-developed interatomic potential should be a useful guide to the development of the new series of high-performance Ti alloys in the future.

Formation mechanism of faulted bands and its effect on α′-martensitic transformation

Understanding the martensitic transformation mechanism is of critical importance to control martensitic transformations and hence retained austenite. Linear defects reported in a previous study can provide nucleation sites for martensitic transformation, but their nature, formation mechanism and effect on martensitic transformation were not revealed. In this study, detailed evidence was provided that linear defects were formed by the motion of Shockley partial dislocation on a close-packed plane and usually could be in the form of faulted austenite, twins and ɛ-martensite, defined as faulted bands. Faulted bands were formed prior to martensitic transformation at a starting transition temperature higher than Ms. They can provide nucleation sites for α′-martensitic transformation but can act as an effective barrier to limit the growth of α′-martensite laths. In addition, the same plane was found between the formation plane of the faulted band and the habit plane of α′-martensite, which indicates that the variant types of α′-martensite were decided by the variant types of faulted bands.

Martensitic twinning transformation mechanism in a metastable IVB element-based body-centered cubic high-entropy alloy with high strength and high work hardening rate

Realizing high work hardening and thus elevated strength–ductility synergy are prerequisites for the practical usage of body-centered-cubic high entropy alloys (BCC-HEAs). In this study, we report a novel dynamic strengthening mechanism, martensitic twinning transformation mechanism in a metastable refractory element-based BCC-HEA (TiZrHf)87Ta13 (at.%) that can profoundly enhance the work hardening capability, leading to a large uniform ductility and high strength simultaneously. Different from conventional transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) strengthening mechanisms, the martensitic twinning transformation strengthening mechanism combines the best characteristics of both TRIP and TWIP strengthening mechanisms, which greatly alleviates the strength-ductility trade-off that ubiquitously observed in BCC structural alloys. Microstructure characterization, carried out using X-ray diffraction (XRD) and electron back-scatter diffraction (EBSD) shows that, upon straining, α” (orthorhombic) martensite transformation, self-accommodation (SA) α” twinning and mechanical α” twinning were activated sequentially. Transmission electron microscopy (TEM) analyses reveal that continuous twinning activation is inherited from nucleating mechanical {351}α” type I twins within SA ‘‘{351}’’<2¯11>α” type II twinned α” variants on {351}α” twinning plane by twinning transformation through simple shear, thereby accommodating the excessive plastic strain through the twinning shear while concurrently refining the grain structure. Consequently, consistent high work hardening rates of 2–12.5 GPa were achieved during the entire plastic deformation, leading to a high tensile strength of 1.3 GPa and uniform elongation of 24%. Alloy development guidelines for activating such martensitic twinning transformation strengthening mechanism were proposed, which could be important in developing new BCC-HEAs with optimal mechanical performance.

Unusual martensitic strain accommodation mechanism in an electron beam processed metastable β-Ti alloy

This study reports an unusual martensitic strain accommodation mechanism, in an electron beam processed metastable β-Ti alloy, exhibiting a fine-grained microstructure with good yield strength and elastic modulus matching. Microstructural analysis reveals that ultra-fine laths of martensites were embedded within the fine-grained β structure. Detailed TEM and orientation relationship analyses further unravel that the observed microstructure consists of α’’ martensites associated with different habit plane and transformation twins, one with {7100}α’’//{755}β type habit plane and forming {111}α’’ type I twin while another with {471}α’’//{443}β type habit plane, forming a {351}<2¯11>α’’ type II twin. This structure can be interpreted as the product of rapid solidification and thermal stress relaxation which manipulated the formation of distinct martensites that originates from the fluctuated lattice instability, lattice distortion, and transformation strain. This finding provides an alternative route for utilizing the martensitic strain accommodation mechanism, benefiting additive manufacturing β-Ti alloy components with desired mechanical performance.

The Formation Mechanism of Nanocrystals after Martensitic Transformation.

Understanding the ultrafine substructure in freshly formed Fe-C martensite is the key point to reveal the real martensitic transformation mechanism. As-quenched martensite, whose transformation temperature is close to room temperature, has been investigated in detail by means of transmission electron microscopy (TEM) in this study. The observation results revealed that the freshly formed martensite after quenching is actually composed of ultrafine crystallites with a grain size of 1–2 nm. The present observation result matches well with the suggestion based on X-ray studies carried out one hundred years ago. Such nanocrystals are distributed throughout the entire martensite. The whole martensite shows a uniform contrast under both bright and dark field observation modes, irrespective of what observation directions are chosen. No defect contrast can be observed inside each nanocrystal. However, a body-centered cubic {112}<111>-type twinning relationship exists among the ultrafine α-Fe grains. Such ultrafine α-Fe grains or crystallites are the root cause of the fine microstructure formed in martensitic steels and high hardness after martensitic transformation. The formation mechanism of the ultrafine α-Fe grains in the freshly formed martensite will be discussed based on a new γ → α phase transformation mechanism.

Studying the strengthening mechanisms and mechanical properties of dissimilar laser-welded butt joints of medium-Mn stainless steel and automotive high-strength carbon steel

In this study, a post-weld heat treatment (PWHT) at a high temperature of 700 °C for 10 min was proposed to significantly improve the mechanical performance of dissimilar laser-welded butt joints between medium-manganese stainless steel (MMn-SS) and high-strength carbon steels (HS-CS) as base metals (BMs). Electron backscatter diffraction and transmission electron microscopy were employed to characterise the microstructures and study the deformation mechanisms in the paired metals of the dissimilar joints during PWHT and tensile straining. Micro-indentation hardness tests were performed to analyse the mechanical responses in the weld zones after PWHT. The results revealed that PWHT at 700 °C significantly affected the microstructure and mechanical strength of the BMs and their weldments. A fully tempered martensitic microstructure with recrystallised grain regions was formed in the fusion zones (FZs) of the weldments, which were processed at low and high specific point energies (SPEs) of 19, 25, and 30 J. BM HS-CS was significantly strengthened owing to the interphase precipitation hardening of vanadium carbide precipitates during the proposed PWHT. The strength-ductility balance of BM MMn-SS was improved by combining the martensitic transformation and mechanical twinning mechanisms. Although, the tensile strength of BM HS-CS increased from 500 to 630 MPa and that of the metal in the FZ decreased from 1100 to 800 MPa owing to PWHT. Hence, HS-CS is a weak metal for dissimilar joints, and joint failure was promoted in it.

Damping performance and martensitic transformation of rapidly solidified Fe-17%Mn alloy

The rapid solidification of Fe-17%Mn alloy was performed to investigate the influence of cooling rate on its damping performance and martensitic transformation mechanism. A proper heat treatment was also carried out to clarify its coupled effects with rapid solidification. The stacking fault probability and martensitic transformation temperature were determined to demonstrate their relationship with the cooling rate and the heat treatment process. With the increase of cooling rate, the volume fraction of ε-martensite increased and the stacking fault probability of ε-martensite was enhanced. The formation of ε-martensite phase was remarkable for the increase of damping capacity and microhardness. It was found that rapid solidification was beneficial for the formation of ε-martensite and the improvement of damping capacity. This effect can be facilitated by the incorporation of the heat treatment process.

Highly ordered graphite (HOPG) to hexagonal diamond (lonsdaleite) phase transition observed on picosecond time scales using ultrafast x-ray diffraction

The response of rapidly compressed highly oriented pyrolytic graphite (HOPG) normal to its basal plane was investigated at a pressure of ∼80 GPa. Ultrafast x-ray diffraction using ∼100 fs pulses at the Materials Under Extreme Conditions sector of the Linac Coherent Light Source was used to probe the changes in crystal structure resulting from picosecond timescale compression at laser drive energies ranging from 2.5 to 250 mJ. A phase transformation from HOPG to a highly textured hexagonal diamond structure is observed at the highest energy, followed by relaxation to a still highly oriented, but distorted graphite structure following release. We observe the formation of a highly oriented lonsdaleite within 20 ps, subsequent to compression. This suggests that a diffusionless martensitic mechanism may play a fundamental role in phase transition, as speculated in an early work on this system, and more recent static studies of diamonds formed in impact events.

On the computational treatment of fully coupled crystal plasticity slip and martensitic transformation constitutive models at finite strains

AbstractIn this article, four numerical techniques are formulated to improve the reliability and numerical efficiency of stress update algorithms of crystal‐plasticity‐like phenomena at finite strains. The resulting algorithmic setting is especially relevant for structural analyses of polycrystals and the multiscale evaluation of anisotropic microstructures in metallic materials. These techniques are exemplified with a constitutive model, which couples crystallographic slip and martensitic transformation deformation mechanisms with a viscous regularization. The model is characterized by a set of highly coupled equations expressed in a single system and solved by a monolithic solution procedure. The first technique employs a sub‐stepping procedure to generate a better initial guess for the Newton–Raphson scheme used in the iterative solution of the equilibrium problem at the Gauss quadrature point. Then, a logarithmic discretization of the exponential parameter of the viscoplastic law is proposed to approximate the target value of the exponential parameter incrementally in the return mapping algorithm. A strategy to remove the strain‐rate dependence and reduce the necessary viscoplastic parameters is also presented to help with the stiff equations that arise in the rate‐independent limit. Finally, an efficient strategy is suggested to complete the transformation process when the full martensitic transformation is approached. These techniques can be easily used in any combination and dramatically improve the model's efficiency by enabling more significant incremental steps to be used within the monolithic solution procedure. A thorough assessment of their impact is shown in a series of ablation studies.

Computational Simulation by Phase Field: Martensite Transformation Kinetics and Variant Selection under External Fields

Tailoring martensite transformation is critical for improving the mechanical properties of advanced steels. To provide preliminary guidance for the control of martensite transformation behaviour using external fields by computational simulation method, the phase-field method was used to calculate the morphology evolution, kinetics, and variant selection of the martensite transformation under different loading modes and magnetic field intensities. The incubation, transformation, and stable stages of the three variants based on the Bain strain group were investigated using different kinetic curves. These results clearly indicate that both uniaxial tension and compression can greatly promote the formation of martensite during the transformation stage and cause an obvious preferred variant selection. In contrast, the different variants have relatively balanced forms under shearing conditions. In addition, the magnetic field is a gentler way to form a state with balanced variants than other techniques such as shearing. Additionally, all these simulation results are consistent with classical martensitic transformation theory and thermodynamic mechanism, which proves the rationality of this research. The aim of the present study was to provide qualitative guidance for the selection of external fields for microstructural improvement in advanced steels.

Multiple-mechanism and microstructure-based crystal plasticity modeling for cyclic shear deformation of TRIP steel

The transformation-induced plasticity (TRIP) steel has an excellent synergy between strength and ductility and is widely used in industry. Cyclic loading is often seen in its industrial application. Therefore, it is of great significance to study cyclic plastic behavior of the TRIP steel. The TRIP steel's cyclic shear behavior and microstructure evolutions were investigated by mechanical testing, electron backscatter diffraction (EBSD) characterization, and in-situ electron channeling contrast imaging (in-situ ECCI) observation. Then, a multiple-mechanism crystal plasticity constitutive model was developed, considering both dislocation slip and martensitic transformation mechanisms. Furthermore, an isotropic hardening law and a modified kinematic hardening rule were taken into account. The crystal plasticity model was implemented into DAMASK with the spectral method. A polycrystalline RVE with realistic grain morphology was constructed from the EBSD data. The simulations of the TRIP steel under cyclic shear loading showed that the samples under higher strain amplitude exhibit stronger cyclic shear hardening. The activation of the martensitic transformation mechanism promotes cyclic hardening. In grain level, the grains with Taylor factor (related to orientation) between 3.3 and 3.9 can activate more slip systems and have a better cyclic hardening ability. So, the Taylor factor is defined as an indicator describing the cyclic response of individual grains in polycrystalline materials. Considering the capability of the established modeling framework, it is of great significance to guide the TRIP steel to serve safely and help the microstructural design in grain scale.