Dual-phase Alloy Research Articles

Effect of bath temperature on the growth kinetics and characteristics of permanganate conversion coating on LZ91 magnesium alloy

Permanganate conversion coating is one of the possible alternatives to chromate conversion coating for magnesium (Mg) alloys. Permanganate conversion coating enhances the corrosion resistance of Mg alloys, which is closely related to the thickness and defects of the coating. In this study, the growth kinetics of the permanganate conversion on the LZ91, a dual-phase magnesium alloy, as affected by the bath temperature ranging from 15 to 60 °C, was detailed. The permanganate conversion coating was mainly composed of MnO2/MnOOH and MgO/Mg(OH)2. Increasing the bath temperature reduced the growth rate and thickness of the coating although the bath temperature hardly influenced the composition and crystallinity of the coating. The permanganate conversion coating formed at 15 °C for 10 s was nonuniform with a thickness ranging from 100 to 300 nm, which was prone to cracking after post-coating drying at room temperature. The presence of cracks on the permanganate conversion coating was detrimental to the corrosion resistance. On the other hand, the permanganate conversion coating formed at 40 and 60 °C for 10 s was more uniform with a thickness of less than 100 nm, yet exhibited better corrosion resistance than the counterpart formed at low temperatures (15 and 25 °C).

Slip analysis and mechanical deformation behavior in dual-phase titanium alloy: Integrating crystal plasticity simulations with in-situ micro-compression

The strengthening effects of primary α grains in titanium alloys were systematically investigated by integrating micropillar compression tests and crystal plasticity simulations. This study focused on the deformation behavior of dual-phase micropillars consisting of a single primary α grain and a single-crystal β matrix. During compression, these micropillars exhibited two slip modes, i.e., single- and multi-slip features. Slip analysis was performed based on the compressed morphology of the pillars and the Schmid factors. It was found that the first slip step observed in multi-slip pillars was not attributed to the slip system with the largest Schmid factor. To validate the experimental findings, numerical simulations using crystal plasticity models were conducted. The simulation results accurately reproduced the engineering stress-strain responses and allowed for a comparison of the distribution of von Mises stress and cumulative shear strain between the dual-phase and single-β phase pillars. Also, the stress concentration induced by α precipitates affects the stress distribution and leads to heterogeneous deformation within the micropillars, correlating with the observed slip modes. The experimental and numerical results will provide a further understanding of meso-scale heterogeneous deformation behaviors and offer insights into the microstructure-properties relationship in titanium alloys.

Atomistic insights into the influence of Y element on the amorphization behavior of dual-phase Mg alloys

Molecular dynamics simulations are performed to investigate the effect of the interface structure, diffusion time and diffusion temperature on the diffusion behavior and amorphization mechanism of the dual-phase Mg alloys. The results indicate that there is a critical concentration of Y element in the amorphous/crystalline interface (ACI), which depends on the interface structure and temperature. And when the concentration of Y atoms in the ACI is higher than the critical concentration, the ACI will migrate towards the crystalline side, accompanied by the amorphization of the crystalline layer. In addition, it is found that the presence or absence of Y element in the amorphous layer has a significant impact on the amorphous layer thickness of the alloys. For the alloys containing Y atoms, the amorphous layer thickness increases with the increase of diffusion temperature and diffusion time.

Achieving prominent high-temperature mechanical properties in a dual-phase high-entropy alloy: A synergy of deformation-induced twinning and martensite transformation

Dual-phase high-entropy alloys are renowned for their favorable balance of strength and ductility at ambient conditions. However, their mechanical properties, especially the associated deformation mechanisms at elevated temperatures, remain less explored. In this study, the AlCoCrFeTi0.5Ni2.5 HEA consisting of FCC (i.e., L12) and BCC (i.e., B2 + A2) phases was subjected to compressive test at elevated temperatures ranging from 500 to 800 °C. The results demonstrate that this HEA exhibits excellent high-temperature mechanical properties up to 700 °C, superior to most refractory HEAs and Inconel 718 superalloy. Notably, its yield strength surpasses 1000 MPa, with pronounced strain-hardening evident at 600 °C. Upon reaching 700 °C, despite strain-softening occurring at the late deformation stage, the yield strength remains above 900 MPa. Microstructural analysis of the sample deformed at 600 °C indicates increased stacking faults and deformation twins in the FCC phase. Additionally, the BCC phase displays a high density of dislocation entanglements and nanoscale martensite lathes. It is revealed that the high-temperature twins in the FCC phase are triggered by the pre-existing local chemical ordered domains, while the martensitic laths in the BCC phase are activated via stress-induced phase transformation driven by stress concentration at the B2/A2 interface. The synergistic effect of multiple deformation mechanisms operating in both the FCC and BCC phases significantly enhances the strain-hardening capability, contributing to the remarkable mechanical properties at elevated temperatures. These findings provide valuable insights for developing dual-phase HEAs with outstanding mechanical properties over an extensive range of temperatures.

Microstructure, properties and natural ageing behavior of friction stir welded dual-phase Mg-Li alloy

In this study, the super-light α + β dual-phase Mg-9Li-3Al-3Zn alloy was successfully jointed by friction stir welding (FSW). The microstructure, mechanical properties as well as corrosion resistance of the welds were specially explored. The natural age-softening of the welds was found and the underlying mechanism for the properties durability was revealed. FSW resulted in significant grain refinement as well as dissolution of α-Mg phases into β-Li phases in the stirred zone (SZ). Meanwhile, Mg17Al12 (λ) and AlLi (γ) precipitates were dissolved into the matrix while orderly-arranged MgLi2Al (θ) precipitates were formed, which contributed to a precipitation-strengthening effect. Therefore, the welds exhibited extensive hardening and demonstrated superior tensile strength than base metal (BM). Under the optimal welding parameters, the ultimate tensile strength (UTS) of the weld reached 165.8% of that of the BM. On the other hand, the welds obtained at the various rotation speeds also exhibited lower corrosion rates than that of the BM, and higher rotation speeds led to superior corrosion resistance. Furthermore, owing to increase in γ precipitate content during natural ageing, the precipitation-strengthening effect in the weld was weakened, resulting in reduction of the strength of the welds. Meanwhile, the increasing γ precipitate content aggravated the local micro-galvanic coupling corrosion, causing the decrease in corrosion resistance of the welds during natural ageing. This welding method might be useful for obtaining the dual-phase Mg-Li alloys with high mechanical properties and corrosion resistance.

Achieving ultra-high mechanical properties in metastable Co-free medium entropy alloy via hierarchically heterogeneous microstructure

A new metastable dual-phase Fe59Cr13Ni18Al10 medium entropy alloy (MEA) with hierarchically heterogeneous microstructure from micro- to nano-scale was designed in this work. Partially recrystallized FCC phase and lots of NiAl-rich B2 precipitates are obtained by annealing and aging treatment. The yield strength of the MEA at room temperature (298 K) and liquid nitrogen temperature (77 K) increased from ∼910 MPa and ∼1250 MPa in the annealed state, respectively, to ∼1145 MPa and ∼1520 MPa in the aged state, while the uniform elongation maintained more than 15%. The excellent mechanical properties of the MEA both at 298 and 77 K are attributed to the co-activation of multiple strengthening mechanisms, including fine grain, dislocation, precipitation, transformation-induced plasticity, stacking faults, and nano-twins.

Introduction of rare-earth element Sc in alloy design to modify wear features of dual-phase high-entropy alloy

Introduction of rare-earth element Sc in alloy design to modify wear features of dual-phase high-entropy alloy

Tailoring Mechanical and Magnetic Properties in Dual-Phase FeCoNi(CuAl)0.8 High-Entropy Alloy.

For tailoring the mechanical and magnetic properties of dual-phase high-entropy alloys (HEAs), it is crucial to understand the effect of each phase on the overall properties. In this paper, the effects of individual FCC and BCC phases on the mechanical and magnetic properties of the FeCoNi(CuAl)0.8 HEA are investigated by nanoindentation and first-principles calculations. The nano-hardness of the BCC phase is 8.73 GPa, which is nearly double the 4.60 GPa of the FCC phase, which ascribes to spherical nanoprecipitates that are only observed in the BCC phase leading to precipitation hardening. First-principles calculations on the electronic structure show that calculated saturation magnetization (Ms) of the BCC phase is 0.81 T, higher than 0.77 T of the FCC phase. An approximate yield strength and Ms can be estimated by summing the volume-fraction-weighted contributions from each phase, and are in good agreement with experimental values. It indicates that the overall mechanical and magnetic properties of the dual-phase HEAs can be tailored by tuning the volume fraction of the individual phase. Our findings are helpful to design prospective dual-phase HEAs with both good mechanical properties and soft magnetic performance by adjusting the content of each phase.

Data-driven mapping-relationship mining between hardness and mechanical properties of dual-phase titanium alloys via random forest and statistical analysis

Data-driven mapping-relationship mining between hardness and mechanical properties of dual-phase titanium alloys via random forest and statistical analysis

An ultrasound-based evaluation of cubic symmetry preservation and homogeneity in elastic behavior of β + ω and β + α Ti-alloys

Elastic constants of dual-phase β-titanium alloys with different phase compositions and different volume fractions of individual phases were investigated using resonant ultrasound spectroscopy and transient grating spectroscopy. It was shown that even for high volume fractions of the secondary phases (ω and α), the crystals can retain cubic symmetry and homogeneity in their elastic behavior, both at the macro-scale and at the micro-scale, unless the secondary phases precipitate under external stress violating this symmetry. In contrast, a material heat-treated under uniaxial compression deviates from cubic symmetry in its macroscopic behavior, and its micro-scale elastic behavior is heterogeneous. The results also reveal that the ω and α particles have different impacts on the elastic constants of the β−matrix, the former affecting only the shear part of the elastic response, while the latter also the bulk modulus.

A Hf-doped dual-phase high-entropy alloy: phase evolution and wear features

A Hf-doped dual-phase high-entropy alloy: phase evolution and wear features

Achieving excellent mechanical properties in a dual-phase FeCrNi medium entropy alloy through athermal transformations and dislocation structures

In this study, a face-centered cubic (FCC) + body-centered cubic (BCC) dual-phase Fe40Cr40Ni20 (at.%) medium-entropy alloy (MEA) with outstanding mechanical properties was developed. Deformation and strain-hardening mechanisms of the MEA were investigated using a transmission electron microscope. In the FCC phase, dislocation slip was the main deformation mechanism, and twinning appeared at the high-stress stage. Dislocation tangle induced by multi-slip positively contributed to the improvement of strain hardening ability. In the BCC phase, deformation mainly depended on dislocation slip, stress-induced martensitic transformation, and twinning. ω particles resulting from martensitic transformation strongly inhibited dislocation motion. Deformation twins and dislocation structures enhanced the strain-hardening ability by reducing the dislocation mean free path. The BCC/FCC interface also contributed to strain hardening ability by hindering the dislocation movement. The combined action of multiple mechanisms led to the high strain-hardening rate of the MEA.

Effect of texture on the mechanical and micromechanical properties of a dual-phase titanium alloy

The room temperature deformation behaviors of Ti–6Al–4V were investigated using crystal plasticity (CP) simulations to analyze the influence of texture on the mechanical and micromechanical properties of the alloy. The equiaxed microstructure with four different textures, namely random, basal, basal/transverse, and transverse, were generated using Dream 3D software. The alloy with random texture was used to study the influence of basal and prism critical resolved shear stress (CRSS) on the alloy properties. The findings revealed that the strength of the alloy is more sensitive to an increase in basal CRSS, whereas an increase in prism CRSS significantly affects plasticity. The analysis of microstructural stress and strain showed that the hard α grains accommodate the majority of the stress in the microstructure, while the soft α and β grains accommodate plastic strain. The simulations indicated that the basal texture exhibits the lowest strength and twin fractions, the basal/transverse texture displays intermediate strength, and the transverse texture has the highest strength along both the rolling direction (RD) and transverse direction (TD), accompanied by the highest twin fractions along TD. The alloy showed the highest strength when loaded along TD, parallel to the c-axes of the most frequently occurring grains, while the lowest strength was observed when loaded along RD, perpendicular to the c-axes of the most frequent grains. Under both tension and compression, the highest twin fractions were observed along TD, followed by RD, and then the through-thickness (TT). This research provides valuable insights into the mechanical and deformation mechanisms of titanium alloys under common textures.

Unveiling mechanisms of self-healing in CoCrFeMnNi/HfNbTaTiZr dual-phase high-entropy alloys: A molecular dynamics simulation study

This study investigates the mechanical properties and self-healing behavior of FCC CoCrFeMnNi/BCC HfNbTaTiZr dual-phase high-entropy alloy (DP-HEA) multilayers using molecular dynamics simulations. The relaxed DP-HEA multilayers have low hexagonal close-packed (HCP) percentages and decreasing "Other" fractions with increasing thickness. DP-HEA multilayers exhibit ductile behavior under uniaxial tensile loading, with stress increasing linearly until the yield strain and then gradually decreasing. Dislocation motion, twinning in the FCC phase, and BCC to HCP local structural transformation contribute to this behavior. Single crystal FCC CoCrFeMnNi and BCC HfNbTaTiZr alloys show different stress-strain responses, while DP-HEAs exhibit no significant stress drop beyond the ultimate strain. Self-healing is observed in DP-HEAs with 5 and 10 nm thickness, with a transformation from HCP to BCC structures. The DP-HEA with 15 nm thickness shows less pronounced self-healing. The interlayer distance between BCC/FCC interfaces influences the self-healing ability of the BCC phase. These findings enhance our understanding of the mechanical behavior and self-healing mechanisms in DP-HEA multilayers, facilitating the design of advanced materials with improved properties.

Deformation mechanisms of β-Li in Mg–Li dual-phase alloy and effects of surface oxide layer

This study investigates the deformation mechanisms of β-Li micropillar prepared from a Mg–7Li dual-phase alloy with or without surface oxide layer. The β-Li micropillar with three cross-sectional sizes, 2.5 × 2.5 μm2, 4.0 × 4.0 μm2 and 5.5 × 5.5 μm2, are studied. The results show that the oxide layer improves the strength of micropillar due to the dislocation trapping effect. When increasing the micropillar size, the oxide layer having nanoporous structure becomes relatively thinner, and the strengthening effect resulting from the oxide layer decreases. Regardless of the existence of the surface oxide layer, all β-Li micropillars show continuous deformation characteristics. This is ascribed to the interactions between pre-existing dislocations and non-planar dislocations activated during compression, which induces low-angle grain boundary and effectively traps the dislocations at the central area.

Creep mechanism in ductile dual-phase aluminum alloys with a fully continuous fiber state

To understand the creep behavior of a practical alloy that has a multiphase consisting of a matrix and several types of precipitate phases, the role of individual phases needs to be studied. However, it is difficult to elucidate the contribution of each phase to creep owing to their complex microstructural morphologies and very complex creep behavior. To understand the role of individual phases and the creep behavior, fundamental creep theories for ductile dual-phase (DDP) alloys must be established initially for specimens with simple microstructural morphologies. To apply these theories practically, they must also be experimentally verified. In this study, DDP alloys composed of 99.99 % Al (matrix phase) and an Al–Mg solid-solution alloy (reinforced phase) were fabricated as a fully continuous fiber state by accumulative roll bonding. Tensile and creep tests at elevated temperatures were performed on alloys composed of systematically varying volume fractions of the reinforced phase. Tensile data at 546 K indicated a linear relationship between the maximum stress and volume fraction of the reinforced phase; the lines were also connected to the data from single-phase materials of the matrix and reinforced phase. Thus, the amount of strain in the matrix and reinforced phases was the same as that during the tensile test, and the relationship between the maximum stress and volume fraction of a DDP alloy followed the classical linear law of mixtures (CLLM). The creep data for Al + Al–2Mg (DDP alloy) at 546 K were nearly equal to those calculated using the CLLM. Thus, the matrix and reinforced phases deformed at the same creep rate. Creep tests were also conducted for Al + Al-1M(5ARB), which had a fine-grained reinforced phase. The creep data for Al + Al-1M(5ARB) at 523 K were also a good approximation to the lines representing the data calculated using the CLLM. Hence, although the grain sizes of the two phases were very different, corresponding to different rate-controlling mechanisms, the creep data for a DDP alloy could be analyzed using the CLLM. Consequently, the creep behavior of the DDP alloys was a superposition of the creep behavior in both phases, and the interpretation of the creep data for DDP alloys using the creep theory of single-phase materials would lead to incorrect conclusions. These findings will contribute to a better understanding of the creep behaviors of practical alloys and will aid in the correct estimation of the creep lifetime.

In-situ study of damage mechanisms in Mg–6Li dual-phase alloy

Interfaces play a crucial role in influencing the mechanical properties of Mg alloys. For Mg–Li dual-phase alloy, the type of interfaces is complex, which includes both grain boundary and phase boundary, and the influence of such interfaces on the damage nucleation is yet to be explored. In this paper, in-situ scanning electron microscopy (SEM) based measurements were carried out to investigate the meso‑scale damage nucleation mechanisms of the Mg–6Li dual-phase alloy. Results show that 94.8% of cracks are nucleated at the α-Mg grain boundary in the post-uniform elongation stage, while 5.2% are at phase boundary and almost no crack at the β-Li grain boundary. The initiation of α-Mg grain boundary cracks is attributed to strain incompatibility, which induces micro-strain localization, and then causes grain boundary sliding (GBS) and crack nucleation. Deformation compatibility analysis reveals that the geometric compatibility factor (Mk) can be used to predict the nucleation of α-Mg grain boundary crack. When Mk is lower than 0.075, α-Mg grain boundary cracks tend to form. Few cracks are generated at the phase boundary is due to the mild strain partitioning between α-Mg phase and β-Li phase and may also be partly attributed to multiple slip systems in body-centered cubic (BCC)-structured β-Li phase, which can accommodate well with the deformation of adjacent α-Mg phase.

Formation process, microstructure, and mechanical properties of an ultrafine dual-phase alloy formed through phase transition of 18R-type long-period stacking ordered in Mg85Zn6Y9 under high pressure

The 18R-type long-period stacking ordered (LPSO) structure in Mg85Zn6Y9 exhibits phase transition into D03 and α-Mg (D03/α-Mg) before it transforms into a liquid phase with the increase in temperature at 7.8 GPa. First-principles calculations suggest that the energy barrier between 18R-type LPSO and D03/α-Mg decreases with pressure. A further increase in the temperature decreases the energy difference between LPSO and D03/α-Mg. Therefore, the free energy variations due to the effects of both pressure and temperature cause phase transition. The D03/α-Mg phases formed at high pressure can be recovered at 0.1 MPa and room temperature. The phase transition from 18R-type LPSO structure to D03/α-Mg causes inner-grain diffusion and generates a lamellar structure. The thickness of the lamellar structure decreases with the increasing pressure until it reaches 65 nm at 15 GPa. Moreover, the alloy solidified above 5 GPa also comprises D03/α-Mg phases. The alloy solidified at 5 GPa exhibits a typical eutectic lamellar structure. However, spherulite-like polycrystalline microstructures containing ultrafine grains are obtained when the alloys are solidified at 10 and 15 GPa. The microstructure variations are caused by an increase in the degree of supercooling due to atomic diffusion suppression with the increasing pressure. The alloys containing D03/α-Mg exhibit both low modulus and ultrahigh strength. The Young’s modulus is related to the presence of the D03 and α-Mg phases in the alloy structure. In contrast, the increase in σy can be attributed to the grain refinement strengthening.

Improving the microstructure and mechanical properties of ultralight Mg-Li-Zn alloy by multidirectional asymmetric rolling

In this work, a multi-pass asymmetric rolling of Mg-8Li-1Zn alloy with four different directional routes were carried out with a diameter ratio of 1:1.08 for the upper and lower rolls, and the deformation of the rolling processes were analyzed by finite element simulation. The effects of asymmetric rolling routes on microstructure, mechanical properties, and stress-strain of the Mg-Li alloy were investigated in detail. Results reveal that the thickness of α-Mg phase (≈47 µm) in the annealed alloy is significantly refined (≈12 µm) and distributed in the matrix as lamellar. The ultimate tensile strength (UTS) reaches up to 336.7 MPa by the forward rolling (FR), while the alloy still possesses an ultra-high elongation (18.38%) by the turned-reverse rolling (TRR) and a good combination of UTS (≈310 MPa) and elongation (≈14.51%) in the turned rolling (TR). This is attributed to the multidirectional asymmetric rolling which results in different cross-shear areas. Additionally, the magnitude and direction of the shear force produced during rolling have different effects on the activation of the slip system within the dual-phase alloy.

Hydrogen induced cracking behavior of the dual-phase Co30Cr10Fe10Al18Ni30Mo2 eutectic high entropy alloy

In this paper, the hydrogen induced cracking behavior of the Co30Cr10Fe10Al18Ni30Mo2 eutectic high entropy alloy was studied. The results showed that the strength and ductility of the alloy decreased after hydrogen pre-charging, and quasi-cleavage fracture features and hydrogen-induced cracks were found. Moreover, the cracks induced by hydrogen pre-charging were found to propagate along the two-phase interface and inside the BCC phase.