Nano-scale Precipitates Research Articles

Unravelling the impact of Al-Ti content on the microstructure-toughness relationship in the coarse-grained heated-affected zone of super-high-strength pipeline steels

This study aims to investigate the role of Al-Ti addition on particle modification in super-high-strength pipeline steels, and its following effect on microstructural evolution and impact toughness in the coarse-grained heated-affected zone (CGHAZ) is also explored. The steel plates with low and high Al-Ti content (0.009 wt-% Al + 0.006 wt-% Ti vs. 0.033 wt-% Al + 0.025 wt-% Ti), respectively, are subjected to gas metal arc welding with 6–7 kJ/cm heat input. The results reveal that the second-phase particles including micron-scale inclusion of Al-Mg oxide covered with CaS and nano-scale precipitate of titanium nitride are obtained in low Al-Ti steel, which are modified to the Al-Mg-Ti oxide core surrounded by CaS and TiN precipitate in high Al-Ti steel. The nano-scale TiN precipitate and micron-scale inclusion can restrict the austenite grain growth at high temperatures and induce acicular ferrite nucleation at medium temperatures, respectively, leading to the fine-grained mixed microstructure of bainite and acicular ferrite in CGHAZ of both steels. Because of the variant selection during the decomposition of austenite to bainite and acicular ferrite, those fine-grain structures with amounts of high-angle grain boundaries exhibit excellent CGHAZ toughness in both specimens. Compared with low Al-Ti steel, more precipitates induce smaller austenite grains, and the inclusions containing titanium increase the ability of inclusion to promote acicular ferrite nucleation in CGHAZ of high Al-Ti steel. Both of them resulted in smaller crystallographic grain and toughness improvement in CGHAZ of high Al-Ti steel than that in low Al-Ti steel.

Revealing grain refinement and hydrogen trapping mechanism for anti-hydrogen susceptibility of Nb-alloyed 34MnB5 press hardened steel

Hydrogen embrittlement (HE) extant a substantial concern to press-hardened steel (PHS) owing to superior strength. The high strength to light-weight automobile structures necessitates the advancement of superior HE resistance PHS. This study investigated the HE susceptibility of Nb-microalloyed PHS by slow strain rate tensile testing, u-shaped constant bending load test, and thermal desorption spectroscopy. Nb enhances microstructure and HE resistance by introducing retained austenite, refining prior austenite grains (21.14–13.73 μm), forming low-angle grain boundaries, and nano-scale precipitates. Nb-alloyed steel exhibits no-cracking over 300 h under high pre-bending stress and decreases elongation loss up to 48% in hydrogen environment as compared to Nb-free steel. Diffusible H-content in 0.12 wt% Nb-steel reduces to 14.9% of that in Nb-free steel owing to enhanced hydrogen traps, the Fcc/Bcc matrix, and carbide precipitation. The multi-phase microstructure with nano-scale NbC precipitation impeded the localized H-dispersion, enhancing the HE resistance in PHS despite its high strength.

Quantification of nanoscale precipitation in AA7050 using X-ray scattering, electron microscopy and automated particle counting techniques

Material strength is dependent on several factors, including precipitation strengthening, which is the main strengthening mechanism in AA7050 aluminum alloys. These alloys contain numerous nanometer-scale precipitates that occur throughout grains and along grain boundaries when subjected to a range of heat treatment conditions. In this study, three different characterization methods were used to characterize these nanoscale precipitates: conventional scanning transmission electron microscopy (STEM); laboratory-based small-angle X-ray scattering (SAXS); and a new software analysis tool developed by Thermo Fisher Scientific: automated particle workflow (APW). Each method was used to determine average precipitate size and volume fraction of precipitation in AA7050-T7451 specimens with variable post-T7 heat treatment procedures. STEM techniques measured the average particle size as ranging from 7.6 to 17.6 nm, compared to 6.3–10.7 nm for SAXS, and 7.4–10.8 nm for APW. Volume fraction determinations varied from 0.23 to 16 %, depending on method, and were the most difficult to quantify. Significant outcomes of the current study are that SAXS and APW are the most accurate methods for determining average particle size and that future work is needed to effectively analyze precipitate volume fraction.

Microstructure and strength-conductivity synergy of Al-Mg-Si-Cu alloy sheets prepared via vacuum melting and thermo-mechanical treatment

This work has investigated the response of microstructural evolution on the strength and electrical conductivity (EC) of vacuum melted Al-Mg-Si-Cu alloys during T6 heat treatment, cold rolling (CR) and re-aging (RA). The precipitation behavior of nano-scaled precipitates during thermo-mechanical treatment was revealed. The strength- conductivity improvement mechanisms of the alloy were discussed. The results showed that compared with the non-vacuum melting alloy, there were superior enhancements in the strength and EC of the alloy prepared by vacuum melting due to the high compactness. After T6 heat treatment, nanoscale β", Q' and L phases were precipitated in the alloy sheet. Owing to the increased dislocation density and precipitate dissolution during CR, the strength of the alloy sheet was improved, followed by a decrease in EC. After RA, a good combination of strength and EC of the alloy sheet was obtained, i.e., ultimate tensile strength and EC were 350MPa and 57.48% IACS, respectively, which was associated with the formation of nano-sized precipitates and the decrease in solute atom concentration. The precipitates volume, dislocation density, grain refinement and solid solubility during thermo-mechanical treatment were the main factors determining the strength and EC of the alloy. This work provides a strategy for the preparation of high strength-conductivity Al alloys.

Breaking strength-plasticity trade-off in heterostructure CrFeNiAl0.28Si0.09Ti0.02Cu0.01 high entropy alloy by heat-treatment-induced nano-precipitation

In this study, heat treatment was employed to enhance the mechanical properties of an as-cast CrFeNiAl0.28Si0.09Ti0.02Cu0.01 high entropy alloy (HEA) with the heterostructure consisting of face-centered cubic (FCC), body-centered cubic (BCC) and B2 phases. It was observed that, after heat treatment at 873 K for 1 h followed by water-quenching, well-dispersed L12 and L21 nano-precipitates could be formed in FCC phase and BCC phase of this as-cast HEA, respectively. Furthermore, it is worth noting that despite a noticeable reduction in dislocation density, this heat treatment process significantly enhances yield strength, ultimate tensile strength, plasticity and hardness simultaneously, which could be attributed to the formation of nano-scale precipitates mentioned above. These findings provide a novel approach for fabricating high-performance HEAs by introducing complex multi-level heterogeneity.

Effects of heat treatment and geometry on mechanical properties and precipitate development in martensitic additively manufactured 17-4 PH

Reliable mechanical properties of precipitation strengthened alloys like 17-4 PH stainless steel are conventionally obtained through tightly controlled thermomechanical processing to optimize the size and distribution of strengthening precipitates. In additive manufacturing, however, the thermal history (e.g. heating and cooling rates) can vary significantly across a part or individual layer causing the precipitate distribution and resulting mechanical properties to be non-uniform. This is especially true for precipitation strengthened alloys like 17-4 PH which require a specific thermal history to achieve the expected phases and properties. To explore this material and the effect that standard heat treatments have on the processing-structure-properties relationship for AM 17-4 PH, a series of thin-wall (0.8 mm) and ASTM E8 size dogbones were printed via SLM and tensile tested in the as-built, solution annealed, and H900 condition. A Vic-2D DIC system was used during tensile testing to obtain full-field strain data. SEM BSE imaging and TEM EDS were used to characterize the larger (60–120 nm) precipitate phases (SiO and NbC) along with the overall microstructure. GISAXS and TEM EDS were used to quantify the size (5–15 nm), shape, distribution, and composition of nanoscale copper-rich precipitates. Overall, the mechanical properties between the E8 and thin-wall samples were similar with yield strengths of 677 and 734 MPa and ultimate strengths of 1049 and 941 MPa in the H900 condition, respectively. The other sample conditions follow similar trends with the only significant difference being a drop in average ductility (0.27 vs 0.17) for the thin-wall geometry. These similarities were hypothesized to stem from several factors which work to lessen the cooling rate dependence of 17-4 PH.

Synchronously upgrading of hydrogen storage thermodynamic, kinetics and cycling properties of MgH2 via VTiMn catalyst

To effectively enhance the hydrogen storage performance of MgH2, the multimetallic catalyst VTiMn was triumphantly synthesized by mechanical alloying, and the MgH2-10 wt% VTiMn composite was further fabricated. The results revealed that VTiMn significantly improved the thermodynamic, kinetics and cycle properties of MgH2. Compared with pure MgH2, VTiMn promotes its plateau pressure to increase by 0.16 MPa and the initial desorption temperature to decrease by 47 K, showing obvious thermodynamic instability. Meanwhile, the MgH2-VTiMn composite can adsorb 5.5 wt% H2 within 72 s and release within 1200 s at 523 K. The activation energies of hydrogen absorption and desorption are 30.1 and 90.5 kJ/mol, respectively. After 130 cycles at 598 K, 83 % hydrogen capacity retention can still be achieved. Characterisation indicated that the composite formed a core–shell structure with MgH2 coated on the VTiMn surface. Part of the Mn element in VTiMn diffused into the Mg matrix, accompanied by the precipitation of nanoscale α-Mn particles at high temperature. The improved absorption kinetics can be attributed to the accelerated H atoms diffusion by the microcracks in VTiMn and the phase boundaries provided by the nano-precipitated phase, while the desorption kinetics benefits from the promotion of Mg nucleation and growth by α-Mn. Due to the effect of temperature, the rate-determining steps of the hydrogen absorption and desorption process of the composite at 598 K change from H atoms diffusion and Mg nucleation growth to H atoms surface penetration, respectively. These findings will facilitate a more comprehensive understanding of multimetallic catalysts.

The thermodynamic effects of solute on void nucleation in Mg alloys.

Replica exchange transition interface sampling simulations in Mg-Al alloys with high vacancy concentrations indicate that the presence of a solute reduces thermodynamic barriers to the clustering of vacancies and the formation of voids. The emergence of local minima in the free energy along the reaction coordinate suggests that void formation may become a multi-step process in the presence of a solute. In this scenario, vacancies agglomerate with solute before they coalesce into a stable void with well-defined internal surfaces. The emergence of vacancy-solute clusters as intermediate states would imply that classical nucleation theory is unlikely to adequately describe void formation in alloys at high vacancy concentrations, a likely precursor for alloy strengthening through nanoscale precipitation.

Oxide precipitation strengthening in Cu alloy modified with trace amounts of Nd during dynamic impact

In this study, three kinds of Cu-Nd alloys were prepared. The effect of Nd content on the dynamic impact behavior of the Cu-Nd alloys and its strengthening mechanism were studied. The results show that during the dynamic impact process, nano-scale oxide particles are precipitated in the alloy matrix, and a large number of cracks are formed along the grain boundaries, and the formation of shear bands and thermal grooves are also observed in local areas. With the increase of Nd content, the compressive strength of the alloys increases significantly, the grain size is refined, the number of twins increases, the grooves are much deeper and the number of the precipitated nano-oxide particles also increase significantly. In addition, due to the refinement of the grain size, the crack length is reduced. The increase of the alloy strength is mainly attributed to the nano-scale oxide precipitation strengthening and grain refinement strengthening.

Microstructures and compositions of nano-precipitates and reverted austenite in custom 455 stainless steel in over-aged condition studied by transmission electron microscopy and atom probe tomography

The microstructures and nano-scale precipitate phases of Custom 455 stainless steel quenched from 850 °C and over-aged at 480 °C for 256 h have been studied in a quantitative way by using transmission electron microscopy and atom probe tomography. The results show that plate reverted austenite is formed between the martensite laths. In the reverted austenite, a rod-shaped Ni3Ti phase particle of about 40 nm long is precipitated at the interphase interface between martensite and reverted austenite, with an adjacent Cu-rich precipitate with a size of ∼20 nm. In the martensite, spherical G-phase particles with a diameter of about 8.0 ± 1.6 nm are precipitated adjacent to the Cu-rich precipitates with a size of about 10.0 ± 2.0 nm. These Cu-rich precipitates in the martensite are much smaller than those in the reverted austenite, but have a ten times higher number density than that in the latter. The orientation relationships between the martensitic matrix, reverted austenite, Ni3Ti phase, G-phase and Cu-rich precipitates are: (01−1)M//(1−11)γ//(0004)η//(02−2)G//(111)Cu,[111] M //[011]γ//[2−1−10]η//[111]G//[11−2]Cu.

Mechanisms of phase decomposition in a non-equimolar CrFeMnNi alloy during thermal aging

While high entropy alloys and multi principal element alloys have been largely studied and developed as single solid solutions, fewer investigations have focused on phase transformations in these multicomponent alloys. This study reports on the aging behavior at 500 °C of a non-equimolar CrFeMnNi alloy. The results unveil a complex evolution, leading to complete decomposition into three main phases, namely an L10 NiMn phase, an FCC Fe-rich phase, and a BCC Cr-rich phase. The intricate microstructure is achieved in stages through continuous precipitation of L10 Ni-Mn nanoscale precipitates, which are progressively overtaken by a sweeping discontinuous front involving the co-precipitation of the L10 NiMn and BCC Cr-rich phases in addition to the parent FCC Fe-rich phase. Furthermore, the Cr-rich phase initially forms with metastable compositions leading to further precipitation of Fe-rich precipitates within the Cr-rich region behind the transformation front. Understanding these transformation pathways through metastable states emphasizes the potential for expanding the range of achievable properties through deliberate microstructure tailoring of multi principal element alloys.

Effect of Hf addition on structural transformation and aging stability of Cu–15Ni–8Sn alloy

The Cu–15Ni–8Sn-xHf (wt.%, x = 0–0.6) alloys were prepared by vacuum melting, and the effect of Hf on the micro-structure of as-cast, as-solution-treated and as-aged alloys was systematically investigated, as well as the alloy aging stability. The Hf addition provided a significant role on refining the solidification structure and reducing the micro-segregation degree, and it also made γ-D03 phases more likely to form in needlelike-shape rather than lamellar-shape. With the presence of more Hf atoms, their existence form was accordingly changed from solid solute atoms to nano-scale precipitation phases and then to micron-scale segregation phase. A small content of Hf (0.1 wt%) could effectively improve the aging stability by inhibiting the growth of γ-D03. Once its content reached or exceeded 0.3 wt%, the harmful discontinuous precipitation was completely inhibited, and the common lamellar-shape γ-D03 locating around the grain boundary was no longer formed. Even the alloy failed at higher temperatures, it could only be carried out through the continuous precipitation method, namely the generation of needlelike-shape γ-D03 uniformly in the matrix. The appropriate Hf addition was capable of extending the alloy aging window, and the hardness of Cu–15Ni–8Sn-0.3Hf alloy could still exceed 360 HV even after 48 h aging.

Achieving well-balanced strength and ductility via dual nanoscale precipitate structures in Co-free high-entropy alloys

Recent studies confirmed that introducing nanoprecipitates as strengthening phase into high-entropy alloys (HEAs) is an extremely effective means to achieve the excellent strength-ductility synergy. In this work, we successfully created a dual nanoscale precipitate structure in a Co-free Ni2.5CrFeAl0.3Ti0.2 HEA by controlling thermo-mechanical process. Compared to the recrystallized alloy, homogeneous L12 precipitates in FCC matrix and heterogeneous L12/BCC precipitates near grain boundaries were formed in the aged alloys. The dual-nanoprecipitate structure induces a long-range back stress during deformation process, effectively contributing to a balance of high strength and good ductility at 298K. Especially, the mechanical properties of the recrystallized and aged alloys are dramatically improved without strength-ductility tradeoff, showing a multiple-stage strain-hardening behavior at 77 K. In the aged alloys, the nanoprecipitates shorten the mean free path of dislocation movement to effectively induce the precipitation strengthening appreciably depending on the precipitation size. Different from planar dislocations slip dominated by stacking faults at 298K, multiple stacking fault networks and deformation twins were activated sequentially with increasing the tensile strain, thus forming a dynamic Hall-Petch effect at 77K. The current work could help to understand the correlation between mechanical properties and deformation mechanisms in precipitation-strengthening HEAs, and the transformation of these deformation mechanisms provide an insight for developing outstanding-performance HEAs through trigging various hardening modes.

Interplay between high-temperature creep deformation behavior and nanoscale precipitate evolution in a newly designed Fe–Ni-based superalloy

In an endeavor to achieve the critical goals of carbon neutrality, an explorative study was conducted to scrutinize the precipitate evolution and high-temperature creep behavior of an innovative and cost-effective Fe–Ni-based superalloy that was meticulously engineered for the next-generation ultra-supercritical (USC) power plants. The creep experiments conducted at temperatures and stresses of 650 °C/280 MPa (17,179 h), 675 °C/250 MPa (10,271 h), and 700 °C/250 MPa (2378 h), coupled with proficient data fitting were employed to accurately assess the long-term service performance of the superalloy under real-world operational environments. Though it can withstand theoretically an intense stress of 130 MPa at 700 °C for approximately 30 years as calculated by the Larson-Miller equation, its performance was not without a degree of perceived vulnerability. This was primarily traced to the conspicuous coarsening behavior of the precipitate-free zones (PFZs)/discontinuous coarsening zones (DCZs) formed in the grain boundaries (GBs) on a longer time scale. It had a significant diminishing impact on the overall strength of the GBs, resulting in a recorded creep life that was considerably shorter than initially projected. Moreover, a synergistic mechanism between PFZs/DCZs with TiC carbides was advanced to interpret the intergranular fracturing dynamic, which was postulated due to the evidence gathered from electron backscatter diffraction (EBSD), highlighting that the banded distribution of TiC carbides circumambient to GBs engendered considerable stress aggregation. Conversely, the plentiful supply of coincidence site lattice (CSL) boundaries disrupted the continuous network structure of random high-angle grain boundaries (HAGBs), thereby creating an effective barricade against the proliferation of cracks. These empirical findings paved the way for the proposition of an enhanced blueprint and the industrial utilization of such.

Effect of compositional variations on the heat treatment response in 17-4 PH stainless steel fabricated by laser powder bed fusion

17–4 precipitate hardening (PH) stainless steel is used in various applications including in the aerospace, marine, and chemical industries, largely due to its unique combination of corrosion resistance and high strength, which is achieved by the formation of nanoscale Cu-rich precipitates during aging. 17–4 PH has been widely researched for its applicability for laser powder bed fusion (LPBF). However, there are discrepancies in the literature on its heat treatment response, which seem to be linked to compositional variations. Systematic studies of the interplay between these variations and nanoscale precipitation are currently missing. Using atom probe tomography, we present a systematic study of the heat treatment responses of two variants of LPBF 17–4 PH builds fabricated from different powder feedstocks, with significant differences in N contents (High vs Low N 17–4). Both variants formed predominantly δ-ferritic as-built microstructures. The as-built High N 17–4 variant showed a higher volume fraction of austenite which further increased upon solution annealing and quenching. The consequence was no appreciable hardening effect due to the absence of Cu precipitation in either austenite or martensite after aging, degrading the alloy's desirable property profile. Conversely, Low N 17–4 showed no austenite in the as-built condition and a fully martensitic matrix after solution annealing. This variant had the desired aging response; a ∼ 140 HV 5 increase in hardness due to nanoscale Cu precipitation. Our findings describe the deleterious effects of compositional variations incurred during the LPBF process flow and how they can be overcome in 17–4 PH and similar steels.

Developing high strength/high toughness grades steels by dual-precipitates co-configuration during aging process

Development of new ultrahigh strength steels with high strength and high toughness requires thorough comprehension of nanoscale precipitation mechanisms. In this investigation, a comprehensive array of techniques including atom probe tomography, transmission electron microscopy, scanning electron microscopy, electron backscatter diffraction, small angle x-ray Scattering and thermodynamic calculations were employed. These methods unveiled an intriguing co-precipitation mechanism involving NiAl and carbide nanoparticles in a 2.2 GPa grade strength steel. The results demonstrate that the precipitation mechanisms of NiAl and carbides at different aging temperatures have a significant impact on the strength and toughness of the steel. Sub-micron ε-carbide at the interface of martensite lath and uniformly distributed NiAl cluster in the matrix are independent nucleation during the low-temperature aging process. The high distribution density of NiAl cluster in the matrix results in an increase in the impact absorbed energy of the steel to 136 J, albeit with a slight decrease in strength. On the other hand, the NiAl with minimal lattice misfit and low interfacial energy preferentially nucleates uniformly in the matrix, inducing the nucleation of M2C and increasing the number density of M2C precipitates at the secondary hardening temperature. The homogeneous precipitation of spherical NiAl, along with needle-shaped M2C in the martensite, significantly enhances the strength of the steel, with a tensile strength reaching up to 2216 MPa and an impact absorbed energy of 21 J.

Research progress of coincidence Doppler broadening of positron annihilation measurement technology in materials

Positron annihilation technique is an atomic-scale characterization method used to analyze the defects and microstructure of materials, which is extremely sensitive to open volume defects. By examining the annihilation behaviour of positrons and electrons in open volume defects, local electron density and atomic structure information around the annihilation site can be obtained, such as the size and concentration of vacancies, and vacancy clusters. In recent years, positron annihilation spectroscopy has evolved into a superior tool for characterizing features of material compared with conventional methods. The coincident Doppler broadening technique provides unique advantages for examining the local electronic structure and chemical environment (elemental composition) information about defects due to its effectiveness describing high momentum electronic information. The low momentum portion of the quotient spectrum indicates the Doppler shift generated by the annihilation of valence electrons near the vacancy defect. Changes in the peak amplitudes and positions of the characteristic peaks in the high momentum region can reveal elemental information about the positron annihilation point. The physical mechanism of element segregation, the structural features of open volume defects and the interaction between interstitial atoms and vacancy defects are well investigated by using the coincidence Doppler broadening technology. In recent years, based on the development of Doppler broadening technology, the sensitivity of slow positron beam coincidence Doppler broadening technology with adjustable energy has been significantly enhanced at a certain depth. It is notable that slow positron beam techniques can offer surface, defect, and interface microstructural information as a function of material depth. It compensates for the fact that the traditional coincidence Doppler broadening technique can only determine the overall defect information. Positron annihilation technology has been applied to the fields of second phase evolution in irradiated materials, hydrogen/helium effect, and free volume in thin films, as a result of the continuous development of slow positron beam and the improvement of various experimental test methods based on slow positron beam. In this paper, the basic principles of the coincidence Doppler broadening technique are briefly discussed, and the application research progress of the coincidence Doppler broadening technique in various materials is reviewed by combining the reported developments: 1) the evolution behaviour of nanoscale precipitation in alloys; 2) the interaction between lattice vacancies and impurity atoms in semiconductors; 3) the changes of oxygen vacancy and metal cation concentration in oxide material. In addition, coincident Doppler broadening technology has been steadily used to estimate and quantify the sizes, quantities, and distributions of free volume holes in polymers.

Depth-dependent microstructure and mechanical properties of hot rolled AA7075

In this study, the through-thickness mechanical properties of a 32 mm thick hot rolled 7075 aluminum plate were measured with microscale uniaxial tensile tests. The mechanical strength across the thickness of the plate exhibited a complex M-shaped profile, with the maximum yield strength and ultimate tensile strength of 440.45 MPa and 493.81 MPa respectively attained at 11 mm depth from the surface of the plate. Representative samples from 0, 4, 11 and 16 mm depths were investigated to understand the influence of the precipitate distribution on the evolution of through-thickness mechanical properties in this alloy. The size and distribution of the microscale (>50 nm) and nanoscale (<20 nm) precipitate phases were quantified with a combination of large area scanning electron microscopy (SEM) and small angle x-ray scattering (SAXS) techniques and correlated with through-thickness mechanical properties obtained through microscale tensile testing. The nanoscale precipitate density as investigated with SAXS showed that the 11 mm depth had the highest volume fraction (0.0086) in comparison to the other representative depths. Moreover, the quench and temper processes were modeled using the precipitation module of Thermo-Calc with various assumptions about cooling rate and microstructure and they were found to be in good agreement with the experimental findings.

Study on the Microstructure of Mg-4Zn-4Sn-1Mn-xAl As-Cast Alloys.

In this study, the microstructure of the Mg-4Zn-4Sn-1Mn-xAl (x = 0, 0.3 wt.%, denoted as ZTM441 and ZTM441-0.3Al) as-cast alloys was investigated using scanning electron microscopy (SEM), focused-ion/electron-beam (FIB) micromachining, transmission electron microscopy (TEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The analysis results revealed that the microstructure of the ZTM441 and ZTM441-0.3Al as-cast alloys both mainly consist of the α-Mg matrix, skeleton-shaped MgZn2 eutectic texture, block-shaped Mg2Sn, and Zn/Sn-rich nanoscale precipitate bands along the grain boundary and the interdendrite. Nanoscale α-Mn dispersoids formed in the grain in the ZTM441 alloy, while no α-Mn formed in the ZTM441-0.3Al alloy instead of nanoscale Al3Mn2 particles. In the ZTM441 as-cast alloy, part of the Zn element is dissolved into the α-Mn phase, and part of the Mn element is dissolved into the MgZn2 phase, but in the ZTM441-0.3Al alloy, there are no such characteristics of mutual solubility. Zn and Mn elements are easy to combine in ZTM441 as-cast alloy, while Al and Mn are easy to combine in ZTM441-0.3Al as-cast alloy. The Mg-Zn phases have not only MgZn2-type crystal structure but also Mg4Zn7- and Mg149Zn-type crystal structure in the ZTM441-0.3Al as-cast alloy. The addition of Al changes the combination of Mn and Zn, promotes the formation of Al3Mn2, and the growth of the grain.

Coercivity mechanism of high-performance anisotropic heterostructure SmCo5 magnets

Due to its large magnetocrystalline anisotropy field and high Curie temperature, SmCo-based magnets are irreplaceable in high-temperature applications. The hot-deformed SmCo-based magnets are especially attractive for their excellent corrosion resistance, making them more suitable for harsh environments. However, it is still challenging for hot-deformed SmCo-based magnets to obtain high magnetization and high coercivity simultaneously. Therefore, it is necessary to analyze the relationship between its microstructure and magnetic properties and explore its magnetic hardening mechanism to guide the further improvement of magnetic properties. In this paper, the anisotropic heterostructure SmCo5 magnet was prepared by combining large height-reduction and introducing a nano-grained Sm-rich phase. Strong c-axis texture, (BH)m of 16.1 MGOe, and Hci of 9.6 kOe were obtained via the heterostructure with SmCo5 micron-grain and Sm-rich nano-scale precipitates. The magnetic domain observation and technical magnetization analysis demonstrate that the coercivity mechanism is dominated by nucleation. The results also show that the smooth and Sm-rich precipitated grain boundary provides weak pinning effects. It provides a reference for the development of high-performance nanocrystalline SmCo5 magnets.