R-phase Research Articles

Evolution of microstructure and properties of nanocrystalline NiCo alloy under high-energy laser shock

In this study, nanocrystalline NiCo alloy was prepared through electrodeposition and subjected to high-energy laser shock (HELS) treatment to investigate the influence of laser shock on the microstructures and properties. An analysis of phase composition, microstructural evolution, texture evolution, and hardness was conducted. The results indicated that under HELS, there was no change in phase composition, while there was a slight reduction in grain size, as well as in the quantities of Low-angle grain boundaries (LAGBs) and twin boundaries (TBs). The texture evolution revealed that HELS led to a more uniform overall texture due to grain boundary (GB) mediate deformation mechanisms, along with the generation of a significant amount of 9R phases, causing a shift in texture orientation from (110) to (100). The internal structure of nanocrystalline NiCo showed an accumulation of numerous dislocations post-laser shock, with randomly oriented dislocations interacting to form Lomer-Cottrell Locks (L-C Locks) and 9R phases. Additionally, interactions with large stacking faults (SFs) also resulted in their dissociation into multiple 9R phases. The hardness enhancement was mainly attributed to the accumulation of dislocations within the grain and the generation of a significant quantity of 9R phases.

Dynamic plastic deformation at liquid nitrogen temperature and aging effects on microstructure and properties of Cu-0.3Zr-0.15Cr alloys

The 9R phase is rarely formed in copper alloys because of its high stacking fault energy. In this study, we generated a large amount of 9R phase in Cu-0.3Zr-0.15Cr alloys by dynamic plastic deformation (DPD) at liquid nitrogen temperature for the first time, and revealed the transformation mechanism of 9R to nanotwins. The process of DPD + aging treatment enables the alloy to obtain excellent comprehensive properties (strength of 657 MPa, electrical conductivity of 75.2 % IACS, and elongation of 14.6 %), which are derived from the heterogeneous microstructure composed of ultrafine structures, coarse grains, and nano precipitated phases. In addition, we have evaluated the phase transformation kinetics after DPD at liquid nitrogen temperature. This work provides a new way to better understanding the microstructure transition and precipitation kinetics under dynamic plastic deformation with aging treatment at liquid nitrogen temperature.

Self-Stacked 1T-1H Layers in 6R-NbSeTe and the Emergence of Charge and Magnetic Correlations Due to Ligand Disorder.

The emergence of correlated phenomena arising from the combination of 1T and 1H van der Waals layers is the focus of intense research. Here, we synthesize a self-stacked 6R phase in NbSeTe, showing perfect alternating 1T and 1H layers that grow coherently along the c-direction, as revealed by scanning transmission electron microscopy. Angle-resolved photoemission spectroscopy shows a mixed contribution of the trigonal and octahedral Nb bands to the Fermi level. Diffuse scattering reveals temperature-independent short-range charge fluctuations with propagation vector qCO = (0.25 0), derived from the condensation of a longitudinal mode in the 1T layer, while the long-range charge density wave is quenched by ligand disorder. Magnetization measurements suggest the presence of an inhomogeneous, short-range magnetic order, further supported by the absence of a clear phase transition in the specific heat. These experimental analyses in combination with ab initio calculations indicate that the ground state of 6R-NbSeTe is described by a statistical distribution of short-range charge-modulated and spin-correlated regions driven by ligand disorder. Our results demonstrate how natural 1T-1H self-stacked bulk heterostructures can be used to engineer emergent phases of matter.

Deformation mechanisms of the Fe40Mn20Cr20Ni20 high entropy alloy upon dynamic tension

In this study, the Fe40Mn20Cr20Ni20 HEA is successfully prepared, and the deformation mechanism and microstructure evolution upon dynamic tension are investigated. The alloy achieves excellent strength-ductility synergies and extraordinary work-hardening capabilities under conditions of decreasing the temperature or increasing the strain rate. After cold rolling and annealing, the Fe40Mn20Cr20Ni20 HEA exhibited apparent secondary work hardening during dynamic tension (the temperature is 298 K and the strain rate is ∼1400 s−1). The yield strength and ultimate tensile strength increased from ∼416 to ∼555 MPa and from ∼742 to higher than 950 MPa, respectively. At the same time, the plasticity is increased. This improvement is similar to that at cryogenic temperature and quasi-static tension (the temperature is 77 K and the strain rate is ∼10−3 s−1). The above two cases have a different deformation mechanism. The activation and evolution of the deformation mechanism under dynamic loading at room temperature with increasing strain are determined through dynamic interruption experiments. Deformation mechanisms such as dislocations, nano-twins, stacking faults & deformation twin networks, precipitated phases, and possibly 9R phases undergo evolutions with increasing plastic deformation. Their extensive interactions are the dominant reasons for the improved mechanical performance, which is rarely identified under quasi-static deformation. The σ phase plays a crucial role during the plastic deformation of the alloy at high strain rates, delaying the onset of necking together with deformation twins.

Synergy effect of multi-strengthening mechanisms in CoNiCr-based high-entropy superalloy at cryogenic temperature

The evolution and formation mechanism of microstructures in a high-entropy CoNiCr-based superalloy induced by cold-drawing deformation at cryogenic temperature were investigated. The improvement of strength while maintaining the ductility during cryogenic-drawing deformation in the CoNiCr-based high-entropy superalloy originated from the activation of multiple deformation mechanisms including deformation twins, dislocations, 9R phase and hexagonal close-packed (HCP) phase transformation. Two dominant contributions have been suggested for the increase in strength, namely, nanotwins strengthening and the existence of 9R and HCP phase. In this paper, cryogenic-deformation-induced phase transformation from face-centered cubic (FCC) to HCP in the CoNiCr-based high-entropy superalloy was found, and the atomic-scale mechanism of FCC-9R-Twin/HCP transformation was revealed. A new crystallographic model was established to explain the FCC→9R→Twin/HCP transformation. The phase transformation is significant for the comprehensive understanding of the deformation behavior of this CoNiCr-based high-entropy superalloy at cryogenic temperature, and for guiding the design of high-entropy superalloy with advanced mechanical properties.

Atomic-scale observation and characterization of deformation twins in uniaxial tensile-deformed 120Mn13 steel

High-resolution transmission electron microscope was used to observe and characterize the different formation stages of zero-strain twins in uniaxial tensile-deformed 120Mn13 steel at the atomic scale, which revealed the growth process of zero-strain twins in coarse-grained austenitic manganese steels. Here, plenty of zero-strain twins are observed in deformed 120Mn13 steel and they can be formed by three types of precursors. Their processes of expansion and connection, thickening and thinning can be carried out through the transformation of 9R phase with the matrix and the twin. A new 9R phase with different stacking sequences is firstly discovered and defined as 9R-Ⅱ to distinguish it from the classical 9R phase (9R-Ⅰ). 9R-Ⅰ and 9R-Ⅱ have the same lattice structure but different internal atomic arrangements and satisfy a twin-like relationship. In addition, a new formation mechanism of zero-strain twins based on the 9R phase transformation is proposed.

The effect of boron on the structure and lattice parameters of diamond single crystals

Boron doped diamonds (BDD) with boron concentration above 1019 at./cm3, grown under conditions of high isostatic pressure by the temperature gradient method, were studied by Transmission Electron Microscopy methods, EELS analysis and X-ray diffraction analysis. It was shown that boron is distributed unevenly in the grown crystal. It was found that in places of maximum boron content, interplanar distances d111 increase from 0.206 to approximately 0.2065 nm. Parallel bands were discovered, the boundaries of which are fragments of the intermediate carbon phase (ICP-phase). Numerous twins along the {111} plane, as well as fragments of rhombohedral 9R phase, were discovered.

Enhancing strength and ductility of CrCoNi medium-entropy alloy through the 9R phase and doping

The 9R phase model based on experimental orientation relationship are constructed, the mechanical moduli of the face-centered cubic (FCC) and 9R phases in CrCoNi medium-entropy alloy (MEA) are investigated by first-principles calculations with the meta-generalized gradient approximation (meta-GGA) functional. This functional accurately describes CrCoNi MEA, captures localized electronic states, and overcomes the challenge of DFT + U parameters in special quasi-random structure (SQS) based transition metal atom models. Results indicate that the 9R phase exhibits higher mechanical moduli than the FCC phase, with a 12% increase in bulk modulus and a 7% increase in Poisson's ratio. Consequently, the strength and toughness of CrCoNi MEA are significantly enhanced. Doping Mo, Ta, and W in the MEA structures improves strength by promoting the formation of the 9R phase compared to the doped single FCC phase. Calculations of the work of adhesion and fracture toughness for the 9R/FCC interface reveal improved interface properties with Mo and W doping. These enhancements are attributed to the hybridization of electronic orbitals in the doped CrCoNi MEA, resulting in changes in bond types between neighboring atoms and subsequent modifications of mechanical properties in the 9R phase. Comparative computational values strongly support an alloy design strategy involving Mo doping and the construction of the 9R phase to achieve superior mechanical properties in MEA.

Thermodynamic modelling for the long-period stacking ordered phase in Mg–Y–Al system: Construction and application

An adequate thermodynamic model is required for describing the long-period stacking ordered (LPSO) phase because previously developed models cannot account for the accurate atomic occupancy of the LPSO structure. In this study, atomic structures of the equilibrated 18R and 10H phases in Mg–Y–Al alloys were systematically investigated via atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-TEM) and first-principles calculations. Three structures (α-type, β-type, and γ-type) with different numbers of Mg layers (one, two, and three, respectively) sandwiched between L12 structures were present in the 18R phase. Meanwhile, δ-type (without Mg layers), ε-type (with one Mg layer), and ζ-type (with two Mg layers) structures were detected in the 10H phase. The presence of the interstitial atoms in the L12 structure was confirmed by TEM and theoretical calculations. Based on the obtained results, a new thermodynamic model for the LPSO phase was constructed and successfully applied to establish the thermodynamic description of the Mg–Y–Al system. The CALPHAD-type calculation data obtained using the proposed thermodynamic description were in good agreement with the experimental results. The findings of this study can help elucidate the formation mechanism of LPSO phases and guide Mg–Y–Al alloy design.

Refining 18R-LPSO phase into sub-micron range by pre-kinking design and its prominent strengthening effect on Mg97Y2Zn1 alloy

In this study, a composite deformation strategy of pre-kinking (equal channel angular pressing (ECAP)) followed by large-ratio hot extrusion (HE) was designed to refine the 18R long period stacking ordered (LPSO) phase into sub-micron range in a Mg97Y2Zn1 (at.%) alloy. After the composite processing, the mechanical properties of the alloy are significantly enhanced, superior to the majority of reported Mg97Y2Zn1 and other LPSO-containing Mg alloys. Among the composite deformed alloys, the 16P-HE alloy exhibits the best mechanical properties with tensile yield strength (TYS) of 475 MPa, ultimate tensile strength (UTS) of 526 MPa, and fracture elongation (FE) of 14.5%. Quantitative analysis of 18R phase indicates that increasing ECAP pass from 1 to 16 gradually decreases the average size of 18R phase from 5.1 µm to 2.3 µm. After HE, the 18R phase is further refined with a corresponding decrease in the average size in the descending order of 1P-HE (4.3 µm), 4P-HE (3.2 µm), and 16P-HE (1.4 µm) alloys. Calculation of the strengthening contributions confirms that the superior mechanical properties of 16P-HE alloy are mainly due to its strongest interface strengthening (145 MPa) and grain boundary strengthening (189 MPa) from the sub-micron 18R phase and α-Mg grains. Moreover, the strengthening effect of 18R phase decreases gradually with their morphology changing from particles to fibers, and to blocks. The obtained results further deepen and broaden the strengthening-toughening theory of 18R phase.

Strong and ductile CoCrFeNi high-entropy alloy microfibers at ambient and cryogenic temperatures

Exploring metallic microfibers with an excellent combination of high strength and ductility is a challenge for structural applications under extreme conditions. In this work, a heterogeneous gradient structure was introduced into CoCrFeNi high-entropy alloy (HEA) microfibers via thermomechanical processing. The annealed CoCrFeNi microfiber displays an ultrahigh yield strength of ∼ 1 GPa, an ultimate tensile strength of 1.45 GPa, and an outstanding uniform elongation of 75% at 150 K. These excellent properties originate from the ultrafine grains and heterogeneous gradient structure, as well as the activation of multiple deformation mechanisms including deformation twins, dense dislocations, stacking faults, Lomer-Cottrell locks, phase transformation and 9R phase. Our work not only suggests that HEA microfibers have great potential for structural applications in cryogenic environments, but also sheds light on the design of advanced multi-component metallic microfibers with superior mechanical properties.

Twin boundary defects-assisted dual-nanoprecipitation in a selective-laser-melted Al alloy

A novel phenomenon of twin boundary-assisted dual-nanoprecipitation in a selective-laser-melted (SLM) Al alloy is systematically investigated. The dual-nanoprecipitation of Mg-Zn-rich η’ and Al-Sc-Zr-rich L12 phases is observed at a twin boundary. This twin boundary can be decomposed into incoherent twin boundaries with 9R phases, whereas provide nucleation sites for the dual-nanoprecipitation during in-situ heating at 120 °C. Such dual-nanoprecipitation is primarily driven by the Gibbs-free energy difference between the incoherent twin boundaries with 9R phases and the adjacent matrix. The results offer primary guidance to design a novel SLM Al alloy with dense twins and precipitations.

Elucidating friction properties and strengthening mechanisms of Ni-based surfacing layer containing 9R phase

This study analyzed the strengthening mechanisms and friction properties of Inconel 625 surfacing layers fabricated by plasma transfer arc surfacing (PTA-surfacing) in ball-on-disc friction tests at room temperature and 650 °C. Results showed that Inconel 625 exhibited poor tribological performance at room temperature, but excellent at 650 °C. The average Vickers hardness of the high-temperature wear area (1.6826 ± 0.4059 GPa) was higher than that of the room temperature wear area (1.3900 ± 0.1875 GPa). The coefficient of friction for Inconel 625 decreased from 0.5362 at room temperature to 0.2846 at 650 °C. The wear rates of Inconel 625 at 650 °C and room temperature were 1.2063 × 10−5 mm3/N·m and 20.9740 × 10−5 mm3/N·m, respectively. The wear mechanisms at 650 °C were mainly abrasive, adhesion, oxidation wear, and some degree of delamination wear. The microstructural characterizations revealed that the wear subsurface at 650 °C comprised polycrystalline-mixed oxide layers (Ni(Cr,Fe)2O4, (Cr,Fe)2O3, and NiO), gradient grain refinement, nanoscale precipitations (M23C6, M6C, and laves), 9R structural phases, I-type twinning, and high-density dislocations, which synergistically enhance the outstanding high-temperature friction properties and strength.

Consolidation and properties of porous Cu–Al–Ni shape memory alloys manufactured by powder metallurgy

ABSTRACT A method to manufacture a porous Cu–Al–Ni shape memory alloy by powder metallurgy using space holders is presented. Two aluminium powders with different particle morphologies were employed to investigate their influence on phase formation, microstructure and mechanical properties. The variation of the relative amount of space holders in the mixture allows to obtain different porosities. Samples prepared with irregular-shaped aluminium powder include both 18R and 2H martensitic phases and exhibit the shape memory effect and pseudoelastic behaviour under uniaxial compression tests. In contrast, the samples made with aluminium flakes present the α phase accompanying the 18R martensitic phase, and do not exhibit the shape memory effect. Both the aluminium flakes flat shape and the higher proportion of aluminium oxide associated with its larger surface area to volume ratio hindered the interdiffusion of the metals, resulting in an aluminium-depleted martensitic phase surrounded by an aluminium oxide-rich layered structure.

Precipitation kinetics and mechanical properties of Mg–Y–Zn and Mg–Y–Ni alloys containing long-period stacking ordered (LPSO) structures

Microstructures, mechanical properties by tensile deformation, and precipitation kinetics during elevated-temperature homogenization/aging heat treatment of Mg95.5Y3Zn1.5 and Mg95.5Y3Ni1.5 (at.%) magnesium alloys were systematically compared in the present work. The microstructure of both alloys in the as-cast state consisted of α-Mg and 18R long-period stacking ordered (LPSO) phase. For peak aging at 473 K (maximum hardness), the 18R phase in Mg95.5Y3Zn1.5 alloy partly transformed to lamellar 14H-LPSO phase, and hence, the ultimate tensile strength (UTS) and total elongation increased from 160.6 MPa to 2.6% to 210.5 MPa and 8.5%, respectively. After similar aging treatment of the Mg95.5Y3Ni1.5 alloy, the βʹ-Mg7Y particles precipitated in the structure, and as a result of precipitation hardening effect, the UTS and total elongation increased from 143.4 MPa to 1.9% to 201.6 MPa and 9.2%, respectively. It was observed that the formation of LPSO and precipitating phases during thermomechanical processing might lead to an improvement of strength-ductility synergy, for which overaging at hot temperatures might also be considered for improving plasticity and work-hardening behavior. The investigation of the formation kinetics of 14H-LPSO in Mg95.5Y3Zn1.5 alloy and βʹ-Mg7Y in Mg95.5Y3Ni1.5 alloy based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis led to the activation energy (Q) of 104 kJ/mol and 89 kJ/mol, respectively. The former is similar to that of the diffusion of zinc in the matrix and the latter is close to that for the diffusion of yttrium in the matrix. Accordingly, the present work can shed light on the phase transformations and strengthening mechanisms of high-strength LPSO-containing Mg alloys.

Enhanced strength and crack resistance in CoCrNi-based medium entropy alloy with nano-precipitates, 9R structures and nanotwins produced by hot isostatic pressing

In the present study, hot isostatic pressing (HIP) subjected to high intermediate cooling rate is exploited to seal keyholes and tune the microstructure of equiatomic CoCrNi and (CoCrNi)94Al3Ti3 medium entropy alloys (MEAs) prepared by spark plasma sintering (SPS). For equiatomic CoCrNi, fracture toughness (KIC) is significantly increased but the yield strength (σ0.2) is almost constant when the relative density is increased from 98.9% to 99.7% by HIP. By contrast, σ0.2 and KIC of (CoCrNi)94Al3Ti3 are considerably advanced by 27.3% and 38.6%, respectively. Meanwhile, the change of grain size introduced by HIP treatment is very inconspicuous in present study. Obvious plastic deformation traces are observed near the crack in the samples subjected to HIP treatment. Systematic microstructure observations indicate that the nano-precipitates and the concurrent of the long split 9R structure and nanotwins in (CoCrNi)94Al3Ti3 formed during HIP treatment are responsible for the enhancement of σ0.2. The formation mechanisms of 9R phase in terms of the hierarchical architecture twins and dislocations activity are explored. Our findings offer new insights on design and processing of MEAs parts by the powder metallurgy.

Multi-principal element alloys with High-density nanotwinned 9R phase

Nanostructure design is an effective strategy for strength enhancement for metals and alloys. Nevertheless, stacking faults and nanotwins are not thermodynamically favoured. Here, it is demonstrated that high-density 9R phase with a columnar nanotwinned structure can be obtained through rational compositional design in a novel NiMnAlFeCr multi-principal element alloy. A combined density functional theory (DFT) study and molecular dynamics simulation (MD) was also performed to study the impact of alloy composition on the stability of 9R phase and its consequences on strength. The results showed that the energy barrier of formation of stacking fault can be obviously reduced by introduction of Fe or Cr in NiMn solid solution and its stability can be enhanced by simultaneously addition a certain range of Fe and Cr, which in favoring formation of high-density 9R phase.The experimental and simulation results demonstrate the substantial impact of 9R phase on the mechanical properties of the (Ni50Mn30Al20)100-x(FeCr)x alloys. This study provides a novel insight into nanotwinned structure as well as a promising strategy to design materials with high-strength.

Investigation on microstructure, superior tensile property and its mechanism in Al0.3CoCrFeNi high-entropy alloy via thermo-mechanical processing

In this study, the fully annealed Al0.3CoCrFeNi high entropy alloy (HEA) with single-phase face-centered cubic (FCC) was cryo-rolled and isochronal annealed at different temperatures from 1123 K to 1473 K for 1 h. The effects of annealing temperature on microstructure, grain boundary evolution, and mechanical properties, especially the deformation mechanism of cryo-rolled Al0.3CoCrFeNi alloys were systematically investigated using XRD, EBSD, TEM, microhardness, and tensile tests. A partial recrystallization was achieved after annealing in the 1123 K and 1223 K temperature ranges, which was mainly attributed to that the precipitation of B2 phase can effectively improve the nucleation rate of phase boundary and play a barrier role in growth of grain. Additionally, The remarkable combination of strength (yield strength of ∼911 MPa and ultimate tensile strength of ∼1146 MPa) and ductility (fracture elongation of ∼23%) achieved in cryo-rolled Al0.3CoCrFeNi alloy annealed at 1123 K with heterogeneous microstructure was mainly due to the contributions of back stress and grain size. It was noticeable that the FCC → 9R phase → twin transformation was found in cryo-rolled Al0.3CoCrFeNi HEA annealed at 1473 K during the tensile deformation, which indicated that twin-induced plasticity and phase transformation induced plasticity were reason for the outstanding ductility and strain hardening ability of this sample. This research are important not only for understanding the strengthening mechanisms of HEAs with various microstructure in general, but also for guiding thermo-mechanical processing of single-phase FCC HEAs.

Atomic scale simulations of [formula omitted] symmetric incoherent twin boundaries in gold

The crystalline defects may play an important role in the improvement of the properties of materials. This is the case of twin boundaries which improve the mechanical properties. Σ3{111} coherent twin boundaries (CTBs) and Σ3{112} symmetric incoherent twin boundaries (ITBs) are two defects studied in this work. Atomistic simulations with four interatomic potentials commonly used in the literature and density functional theory have been used to characterize in detail these two defects in gold. The results show that the excess volumes introduced by twin boundaries strongly depend on the potential. After the ITB relaxation, a portion of 9R phase is formed. This phase is rotated from the face centred cubic (fcc) phase through an angle α which has been determined. The 9R phase formation energy γform has been quantified through atomic scale simulations. We put forward a model to calculate this formation energy γform as a function of the intrinsic stacking fault energy γISF and the excess volume. This model shows that the interaction energy between ISFs is much larger for the Ackland and Baskes potentials than for the Foiles and Grochola potentials. Furthermore, the elastic deformation ɛxx produced in the system to ensure consistency between the 9R phase and the fcc phase is computed. It allows us to discuss the width of the 9R phase in terms of γform and elastic energy and not uniquely in terms of γISF, as previously done in the literature.

In-situ and ex-situ investigation of deformation behaviors of a dual-phase Mg−Ni−Y alloy

The relationship between the deformation behaviors of the bulk α-Mg and Long Period Stacking Ordered (LPSO) phases is important for the design of high-strength alloys. In this work, the deformation behaviors of dual-phase Mg alloy which contains α-Mg and 18R-LPSO phases are investigated by in-situ tensile scanning electron microscopy and ex-situ transmission electron microscope. The results show that the 18R phase accounting for 11.3 vol.% provides 23.0% deformation of the overall alloy. At the early stage of deformation, the stacking faults are bent to coordinate twinning deformation through the slip of basal dislocations in the α-Mg matrix, while the kinking deformation 18R induces the kinking deformation of Mg slices. At the end of deformation, large numbers of T2-type stacking faults are generated. These deformation behaviors improve the ductility of the alloy. This work provides a novel strategy to design the high strength and good ductility dual-phase Mg alloys.