Hcp Martensite Research Articles

Metastability-driven room temperature strain hardening in a nitrogen added FeMnCoCrN high-entropy alloy

This study deals with the strain hardening capability of a nitrogen added FeMnCoCr high-entropy alloy during room temperature tensile deformation with an emphasize on the mechanical stability of FCC phase. The heightened metastability of the FCC phase provides a proper condition for hierarchical evolution of dual-phase FCC-HCP structure which finally promotes the formation of 63% HCP martensite. Initially favoring slip mechanisms, the texture of the FCC phase transitions to geometrically hard orientations, thereby reducing its deformation accommodation capacity. This transition prompts the involvement of the HCP phase, initially evidenced by the emergence of new FCC phase and ε-twins at HCP martensite intersections. Subsequently, the formation of thickened ε-twins within the primary HCP lathes further contributes to deformation accommodation, explaining the observed excellent hardening behavior in the as-cast structure.

Pyramidal dislocation driven martensitic nucleation: A step toward consilience of deformation scenario in fcc materials (II)

Dislocation glides and their interactions with other defects are central to our understanding of deformation mechanism enhancing plasticity and damage tolerance. Martensitic transformation (MT) is a fascinating phenomenon overcoming strength-elongation trade-off and thus tailoring structure-property architecture. However, dislocation plasticity of Fe-based hexagonal close-packed (hcp) martensite is still challenging due to its metastability, further complicated by controversies surrounding pyramidal dislocation even in pure hcp metals. To resolve the uncertainties, we address the pyramidal-dislocation-driven plasticity in Fe-based hcp martensite by employing in-situ transmission electron microscopy (TEM) and high-resolution (HR) annular bright field (ABF) imaging together with dislocation-contrast analyses. We show that the activation and dissociation of pyramidal dislocation govern the initial stage of plastic deformation, while subsequent cross-slip by the cooperative motion of dissociated pyramidal dislocations plays a key role in nucleation of new hcp martensite. By incorporating current dislocation model into previous models coupled with stacking-fault-energy concept, we propose a synthesized deformation scenario that offers a comprehensive perspective on plasticity and transformability.

FCC twins and martensites responses of metastable dual-phase high entropy alloy to friction stir processing

Fe50Mn30Co10Cr10 dual-phase high entropy alloy (DP-HEA) has good application prospects in the nuclear and automotive industries due to its strength-ductility synergistic effect. The initial volume fractions of FCC twins and HCP martensites in Fe50Mn30Co10Cr10 HEA affect the relative contribution of transformation-induced plasticity and twinning-induced plasticity to the alloy's strength and ductility under loading conditions. Understanding the relationship between FCC twins and martensites is crucial during the friction stir process (FSP). In this study, the microstructure of Fe50Mn30Co10Cr10 HEA as FSP condition was studied. The HCP phase in the base metal almost completely transformed into the FCC phase during FSP. Both FCC twin and HCP martensite transformations serve to alleviate strain in the FCC matrix. The initiation of FCC twin and martensite formation depends on critical stress levels at different temperatures. Higher strain and coarser grains promote more martensite formation, while finer grains and lower strain impede it, causing FCC twins to occur before HCP martensites during the cooling process of materials in the processed zone. These findings offer insights into controlling the volume fractions of twins and martensites in FSPed DP-HEA, thereby potentially enhancing the mechanical properties of DP-HEA.

In-situ analysis of the effect of residual fcc phase and special grain boundaries on the deformation dynamics in pure cobalt

Polycrystalline hcp metals – an important class of engineering materials – typically exhibit complex plasticity because of a limited number of slip systems. Among these metals, deformation is even more complicated in cobalt as it commonly contains residual fcc phase due to the incomplete martensitic fcc→hcp transformation upon cooling. In this work, we employ a combination of in-situ (acoustic emission, AE) and ex-situ (scanning electron microscopy, SEM) techniques in order to examine deformation dynamics in pure polycrystalline cobalt varying in grain size and the content of residual fcc phase prepared using systematic thermal treatment and cycling. We reveal that the presence of the fcc phase and special ∼71∘ grain boundaries between different hcp martensite variants brings about higher deformability and strength. The fcc phase provides additional slip systems and also accommodates deformation via the stress-induced fcc→hcp transformation during loading. On the other hand, special boundaries enhance structural integrity and suppress the formation of critical defects. Both these non-trivial effects can dominate over the influence of grain size, being a traditional microstructural variable. The ex-situ SEM experiments further reveal that the stress-induced fcc→hcp transformation is sluggish and only partial even at high strains, and it does not give rise to detectable AE signals, unlike in other materials exhibiting martensitic transformation. In turn, these insights into cobalt plasticity provide new avenues for the microstructure and performance optimization towards the desired applications through the modern concept of grain boundary engineering.

Cryo-pre-straining contributes to achieving high yield strength in high-entropy alloys

In this study, we propose an effective and feasible strategy to significantly improve the room-temperature yield strength of the Co35Cr20Fe15Mn15Ni15 HEA by 338%, from 281 MPa (as-recrystallized, before CR) to 1231 MPa (30% CR) through cryo-pre-strain-inducing defects such as dislocations, stacking-faults (SFs), nano-twins, Lomer-Cottrell (L-C) locks, and HCP martensite, while maintaining good ductility (16.4% elongation in 30% CR). Additionally, the contribution of these defects to the yield strength enhancement was quantified. This study offers a new perspective for the preparation of advanced FCC high-entropy alloys.

Uncommon Cold-Rolling Faults in an Fe–Mn–Si–Cr Shape-Memory Alloy

The paper analyzes the occurrence of evenly spaced cracks on the surface of lamellar specimens of Fe-28Mn-6Si-5Cr (mass %) shape-memory alloy (SMA), during cold rolling. The specimens were hot rolled and normalized and developed cold rolling cracks with an approximate spacing of about 1.3 mm and a depth that increased with the thickness-reduction degree. At normalized specimens, X-ray diffraction patterns revealed the presence of multiple crystallographic variants of brittle α′ body-bcc martensite, which could be the cause of cold-rolling cracking. Both normalized and cold-rolled specimens were analyzed using scanning electron microscopy SEM. SEM micrographs revealed the presence of several crystallographic variants of α′-body-centered cubic (bcc) and ε hexagonal close-packed (hcp) martensite plates within a γ-face-centered cubic (fcc) austenite matrix in a normalized state. High-resolution SEM, recorded after 25% thickness reduction by cold-rolling, emphasized the ductile character of the cracks by means of an array of multiple dimples. After additional 33% cold-rolling thickness reduction, the surface of crack walls became acicular, thus revealing the fragile character of failure. It has been argued that the specimens cracked in the neutral point but preserved their integrity owing to the ductile character of γ-fcc austenite matrix.

Constructing a heterogeneous microstructure in the CoCrFeNi-based high entropy alloy to obtain a superior strength-ductility synergy

In the present work, the CoCrFeNi-based alloy was prepared, and a heterogeneous microstructure was constructed via thermo-mechanical processing. The heterogeneous microstructure consists of fine recrystallized grains and deformed matrix, containing high-density dislocations (HDDs), stacking faults (SFs), deformation/annealing twins (DTs/ATs) and precipitates of the γ′-L12, σ and (Ti, Nb, Mo)-rich phases. The precipitation particles are very effective in strengthening the alloy. In addition, dislocation strengthening and grain boundary strengthening play important roles. In tensile deformation, planar slip of the Shockley partial dislocations is active, generating Lomer-Cottrell locks which could promote twinning or HCP martensite transforming. The tensile yield strength of the alloy can reach as high as ∼2.2 GPa with a promising uniform elongation of ∼11.0%.

Deciphering the multiple deformation mechanisms responsible for sustained work hardening in a FeCrCoNi medium entropy alloy

Two important and desirable properties of materials for most structural applications are high tensile strength and ductility, which typically require high work hardening to delay necking. Here, we designed and tensile tested a face-centered cubic (fcc) Fe-Cr-Co-Ni medium-entropy alloy in which multiple deformation mechanisms are triggered during tensile loading at different temperatures to induce sustained work hardening. Our strategy involved control of the relative stabilities of the fcc, hcp (hexagonal close-packed), and bcc (body-centered cubic) phases in this quaternary system via high-throughput thermodynamic calculations. This alloy not only exhibits extensive deformation-induced nanotwinning at room temperature, but also displays a two-step sequential phase transformation [γ (fcc) → ε (hcp) martensite → α’ (bcc) martensite] at 77 K, which contrasts with the single-step phase transformation [γ → ε martensite] observed in many other fcc high/medium entropy alloys with a low stacking fault energy. The sequence of phase transformation at 77K was supported by first-principles density functional theory calculations. This work provides new templates for the design of alloys capable of multiple deformation mechanisms for sustained work hardening.

Twinning in Hexagonal Close-Packed Materials: The Role of Phase Transformation

Twinning is a major mechanism of plastic deformation in hexagonal close-packed (hcp) structures. However, a mechanistic understanding of twin nucleation and growth has yet to be established. This paper reviews the recent progress in the understanding of twinning in hcp materials—particularly the newly discovered phase transformation-mediated twinning mechanisms—in terms of crystallographical analysis, theoretical mechanics calculations, and numerical simulations. Moreover, the relationship between phase transformation-mediated twinning mechanisms and twinning dislocations are presented, forming a unified understanding of deformation twinning. Finally, this paper also reviews the recent studies on transformation twins that are formed in hcp martensite microstructures after various phase transformations, highlighting the critical role of the mechanical loading in engineering a transformation twin microstructure.

High corrosion resistance duplex fcc + hcp cobalt based entropic alloys: An experimental and theoretical investigation

A series of duplex fcc + hcp Co-based entropic alloys are being discovered as a new category of entropic alloys with outstanding mechanical properties, especially to overcome a typical mechanical trade-off between strength and ductility. In this work, CALPHAD-based (CALculation of PHAse Diagram) thermodynamic calculations were performed to facilitate alloy design and to understand corrosion behaviors. The kinetics of the electrochemical corrosion for designed alloys in typical aggressive anion Cl- was investigated by electrochemical tests, including open circuit potential (OCP), polarization and cyclic polarization curves, and electrochemical impedance spectroscopy (EIS). The valence state and the surface morphologies of the passive films were investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). High corrosion resistance materials with high strength and ductility performances were discovered in the present work. Except for Ni-oxides, various spinel compounds and many other oxides including Co2O3, Cr2O3, Fe2O3, MnO, MoO3, CoCr2O4, FeCr2O4, CoFe2O4, and CoMoO4 were observed in the passive films. The adsorbed and penetrated corrosive anion Cl- will be prone to breakdown the passive films with less Cr2O3, CoCr2O4 and MoO3 to form pitting corrosion (also include other localized corrosion, such as intergranular corrosion and crevice corrosion). The microstructure of the hcp martensite with the fcc matrix has played an important role in the propagation of the localized anodic dissolution in the form of cleavage and quasi-cleavage. The theoretical calculations are in good agreement with the experimental observations. This paper paves a way for the future development of high-performance Co-based entropic alloys served in some harsh environments.

Revealing the cryogenic-temperature toughness and deformation mechanisms in high manganese austenitic steels

High-Mn austenitic steels are promising structural metallic materials for cryogenic-temperature industries due to their good properties and economy. In this work, the cryogenic-temperature toughness and deformation mechanisms were revealed by Charpy impact testing and checking the microstructural characteristics in deformed Fe-(13– 30) Mn C steels. The results show that all studied steels showed good toughness at room temperature (i.e., 293 K), while an appropriate γ SFE level was essential for their low temperature toughness at 77 K. The plastic deformation mechanisms depended on stacking fault energy and accumulated strain or dislocation density levels simultaneously. Twinning-induced plasticity (TWIP) mechanism was found for the three studied steels at 293 K, which was beneficial for good Charpy impact toughness. Enhanced TWIP mechanism was found at 77 K owing to the decreased γ SFE values. Meanwhile, both strain-induced ε HCP martensite and α' BCC martensite were indicated in Fe-13Mn-0.9C steel, and few ε HCP martensite was examined in Fe-22Mn-0.9C steel. The α' BCC martensite was generally nucleated from prior strain-induced ε HCP martensite. The cryogenic-temperature toughness would deteriorate remarkably when strain induced α' BCC martensite was introduced or mechanical twins were delayed. The statistical dislocation densities in Fe-13Mn-0.9C steel were found to accumulate faster than Fe-22Mn-0.9C steel and Fe-30Mn-1.0C steel, especially at 77 K. And a full nonadditive strengthening mechanism between dislocations and varies obstacles were observed in the deformed steels. • The cryogenic-temperature toughness and deformation mechanisms in Fe-(13– 30) Mn C austenitic steels were revealed. • The temperature dependent Charpy impact energies showed special correlations with the stacking fault energies. • The plastic deformation mechanisms were the close functions of both stacking fault energies and accumulated dislocations. • The cryogenic-temperature toughness would deteriorate with strain induced α ' BCC martensite or delayed mechanical twins. • A full nonadditive strengthening mechanism between dislocations and varies obstacles were observed in the deformed steels.

Temperature-dependent universal dislocation structures and transition of plasticity enhancing mechanisms of the Fe40Mn40Co10Cr10 high entropy alloy

The FCC Fe40Mn40Co10Cr10 (at.%) high entropy alloy exhibits deformation twinning for room temperature deformation and deformation-induced HCP transformation for subzero deformation. Since the systematic investigation of temperature-dependent dislocation structures is not available, we present an in-depth characterization of the defects involved in deformation at room temperature (298 K) and subzero temperature (223 K) of the cold-rolled and annealed Fe40Mn40Co10Cr10 alloy. The material deformed at 223 K shows a higher strain hardening rate than the material deformed at room temperature while both materials show large ductility, 48% for the 223 K deformation and 55% for the 298 K deformation. The main deformation mechanisms of the investigated HEA include the development of inhomogeneous dislocation structures and interaction between dislocations and deformation twin/mechanically induced HCP martensite. The stacking fault energy measured using TEM weak-beam dark-field imaging of dissociated dislocations is 20±9 mJ/m2 at 298 K. The Fe40Mn40Co10Cr10 alloy exhibiting a positive temperature dependence of SFE leads to a decrease of SFE as deformation temperature decreases from 298 K to 223 K. The decrease of SFE results in the transition from deformation twinning to deformation-induced HCP transformation. Further, at higher strains at 223 K, kink banding of HCP and reverse transformation from HCP to FCC were observed, which could account for strain accommodation and stress relaxation, and the large ductility. The 298 K deformation leads to various types of dislocation structures: Extended dislocations, Taylor lattice of perfect dislocations, dislocation loops, highly dense dislocation walls, cell blocks, and cell structures. The observed dislocation structures at 298 K and 223 K are similar suggesting the minor effect of SFE on dislocation structures.

The Effect of the In-Situ Heat Treatment on the Martensitic Transformation and Specific Properties of the Fe-Mn-Si-Cr Shape Memory Alloys Processed by HSHPT Severe Plastic Deformation.

This work focuses on the temperature evolution of the martensitic phase ε (hexagonal close packed) induced by the severe plastic deformation via High Speed High Pressure Torsion method in Fe57Mn27Si11Cr5 (at %) alloy. The iron rich alloy crystalline structure, magnetic and transport properties were investigated on samples subjected to room temperature High Speed High Pressure Torsion incorporating 1.86 degree of deformation and also hot-compression. Thermo-resistivity as well as thermomagnetic measurements indicate an antiferromagnetic behavior with the Néel temperature (TN) around 244 K, directly related to the austenitic γ-phase. The sudden increase of the resistivity on cooling below the Néel temperature can be explained by an increased phonon-electron interaction. In-situ magnetic and electric transport measurements up to 900 K are equivalent to thermal treatments and lead to the appearance of the bcc-ferrite-like type phase, to the detriment of the ε(hcp) martensite and the γ (fcc) austenite phases.

Grain size dependent deformation behavior of a metastable Fe40Co20Cr20Mn10Ni10 high-entropy alloy

The effects of grain size on the deformation behavior in a metastable Fe40Co20Cr20Mn10Ni10 high-entropy alloy (HEA) were investigated. The dislocation substructures with two different grain sizes of 12 µm (FG) and 41 µm (CG) upon straining were revealed, of which the strong grain size dependent of martensite phase transformation was observed. The HCP martensite plates appear earlier in the CG sample than the FG sample, along with the formation of much thicker and higher fraction of the HCP plates. This is ascribed to the impediment of stacking faults (SFs) movement and further the growth of the HCP lamellae from grain boundaries. The differences between the strain hardening behavior of the FG and CG HEAs were clarified in terms of the deformation microstructural evolution. The FG HEA shows higher strain hardening rates at original stage of deformation resulted from pinning effects to dislocation movements by grain boundaries, yet values of strain hardening rates are lower for the FG HEA at later stage of deformation due to the lower level of martensite phase transformation.

Additive manufacturing of TRIP-assisted dual-phases Fe50Mn30Co10Cr10 high-entropy alloy: Microstructure evolution, mechanical properties and deformation mechanisms

Abstract The metastable dual-phase non-equiatomic Fe50Mn30Co10Cr10 high entropy alloy (HEA) was successfully prepared via laser melting deposition (LMD) technique. The phase composition, microstructure and mechanical performance were systematically characterized at different sections of LMDed HEA samples. The as-printed HEA specimens were dominated by FCC γ phase, plate and needle-like HCP e phase, and high density stacking faults and dislocations were observed at high magnification. The as-printed samples exhibited anisotropic mechanical properties, together with the maximum ultimate tensile strength of 760 MPa and maximum elongation of 28%. The fracture surface along X and Z directions exhibit smooth curve shape, while the fracture surface along Y direction shows saw tooth shape, indicating the boundary of melt pool is weakly bounded. After deformation, the deformation bands were observed in the FCC matrix, showing that the plastic strain was controlled by FCC γ phase. In addition, stacking faults, dislocation increase significantly owing to the formation of stacking fault is caused by sliding of Shockeley partial dislocation, and the stacking fault is the core of HCP martensite nucleation.

Structural Effects of Heat Treatment Holding-Time on Dynamic and Damping Behaviour of an Fe-28Mn-6Si-5Cr Shape Memory Alloy

The paper reports the structural effects of holding time period, during heat treatment, on the dynamic and damping behavior of a Fe-28Mn-6Si-5Cr (mass. %) shape memory alloy. After casting and hot rolling, solution treatment at 1050 oC was applied for five holding times, 2, 4, 6, 8 and 10 hours, followed by water quenching. The specimens were analyzed by scanning electron microscopy and X-ray diffraction which emphasized that only the 2-hours solution treated specimens contained ε-hexagonal close packed (hcp) martensite and experienced the highest internal friction value. These specimens were tested on a special device which transformed both tension and compression into tensile strain applied to the specimens and proved to be a promising solution for anti-seismic damper.

Effect of heating temperature and cooling rate on the microstructure and mechanical properties of a Mo-rich two phase α + β titanium alloy

In this work, influence of heating temperature and cooling rate on microstructure and mechanical properties of a Ti–6Al–1V–4Mo–Si alloy was investigated. The samples were heated to β (1020 °C) and α + β (950 °C and 850 °C) phase field followed by furnace cooling, air cooling and oil quenching. Basketweave morphology was witnessed for samples furnace cooled from β phase field, while Widmanstatten microstructure was observed for most air-cooled samples. For samples oil quenched from β phase field, HCP martensite (αʹ) was formed, while for samples heated in α + β phase field, lamellar α with α′ was observed. Oil quenching from the β phase field resulted in high strength and low elongation. The strength decreased and elongation increased with reduction in the heating temperature. Highest elongation (~ 16%) was obtained for samples heated to 850 °C followed by furnace cooling. The elastic modulus displayed a wide range of values (70–150 GPa) depending on the processing conditions.

Body-centered-cubic martensite and the role on room-temperature tensile properties in Si-added SiVCrMnFeCo high-entropy alloys

We present a new class of metastable high-entropy alloys (HEAs), triggering deformation-induced martensitic transformation (DIMT) from face-centered-cubic (FCC) to body-centered-cubic (BCC), i.e., BCC-DIMT. Through the ab-initio calculation based on 1st order axial interaction model and combined with the Gibbs free energy calculation, the addition of Si is considered as a critical element which enables to reduce the intrinsic stacking fault energy (ISFE) in SixV(9-x)Cr10Mn5Fe46Co30 (x = 2, 4, and 7 at.%) alloy system. The ISFE decreases from -30.4 to -35.5 mJ/m2 as the Si content increases from 2 to 7 at.%, which well corresponds to the reduced phase stability of FCC against HCP. The BCC-DIMT occurs in all the alloys via intermediate HCP martensite, and the HCP martensite provides nucleation sites of BCC martensite. Therefore, the transformation rate enhances as the Si content increases in an earlier deformation range. However, the BCC-DIMT is also affected by the phase stability of FCC against BCC, and the stability is the highest at the Si content of 7 at.%. Thus, the 7Si alloy presents the moderate transformation rate in the later deformation range. Due to the well-controlled transformation rate and consequent strain-hardening rate, the 7Si alloy possesses the superior combination of strength and ductility beyond 1 GPa of tensile strength at room temperature. Our results suggest that the Si addition can be a favorable candidate in various metastable HEAs for the further property improvement.

Strategies to increase austenite FCC relative phase stability in High-Mn steels

Several strategies to increase the FCC austenite stability compared to BCC and HCP martensites have been tested and are discussed. The relative stability of the different phases was analyzed by studying the effects of: a) grain size, b) antiferromagnetic ordering of the austenite, c) thermal cycling through the FCC-HCP transition, d) plastic deformation of the austenite and e) combined effects. As a first step, the effect of decreasing the grain size was analyzed in Fe–Mn alloys for Mn contents smaller than 18 wt.%, where BCC and HCP martensites compete in stability. Formation of the BCC phase is inhibited for 15 wt.% and 17 wt.% of Mn for grain sizes smaller than 2 μm. This enabled, for the first time at these compositions, the measurement of the Néel temperature of the austenite using specific heat and magnetic measurements. A comparison of the obtained transition temperatures with accepted models is discussed. The effect of modifying the grain size on the FCC-HCP transition temperatures was also analyzed for 15 wt.% and 17 wt.% Mn contents showing a complete HCP inhibition for grain sizes smaller than 200 nm. A nucleation model for the HCP martensite is considered which includes an additional resistance to the transformation term depending on the austenitic grain size. Additional combined effects on the FCC stabilization are discussed like the interaction between the antiferromagnetic ordering and the introduction of defects by thermal cycling through the martensitic transformation. The analysis can be easily applied to systems with a larger number of components. Results obtained in the Fe–Mn–Cr system are also presented.

Crystallographic Features of the Structure of Cast and Quenched Cobalt–Niobium Alloys

Specific features of structure formation at the β → α (fcc → hcp) polymorphic transformation in Co–Nb binary alloys have been investigated by metallography, scanning and transmission electron microscopies, and electron backscattering diffraction analysis. It is shown that gradual cooling of a crystallized ingot induces nucleation of α-phase crystals of several orientations (of the four ones that are possible in correspondence with the Wassermann orientation relationships) in each β grain of the alloy. Only α(hcp) martensite has been found in the structure of investigated alloys at room temperature. Misorientation of the substructure along the martensite-crystal length in the cast alloys does not exceed 1°. After homogenization and subsequent quenching in salty water, the structure of Co–Nb alloys does not undergo morphological and crystallographic changes but becomes significantly refined. In addition, misorientation of substructure elements along the martensite-crystal length increases several times, which is a consequence of the high level of quenching microstress in martensite. No phases with multilayer lattices of the NR-martensite type are revealed.