Grain Boundary Phase Research Articles

The influence of Gallium doping on the magnetic performance and microstructure of Nd-Fe-B sintered magnets

The influence of Gallium (Ga) doping on the microstructure, thermal stability and the magnetic properties of Nd-Fe-B based sintered magnets was investigated in this paper. It is shown that the intrinsic coercivity increases significantly from 12.4 kOe to 19.4 kOe while the remanence decreases slightly from 14.03 kGs to 13.70 kGs when Ga content increases from 0.0 wt% to 0.5 wt%. The thermal stability of the Ga-doped sample is much better than that of Ga-free sample. Temperature coefficient of intrinsic coercivity β @ 20-120 ℃ by doping 0.5 wt% Ga is −0.585 %/K, which is better than that of Ga-free sample (-0.622%/K). It was observed by the microstructures that the addition of Ga can promote the diffusion of Nd and Cu from triple junction phase (TJP) into grain boundary phase while Fe atoms are enriched around TJPs, resulting in continuous grain boundary (GB) phase and reducing the Fe concentration in GB phase. The Curie temperature increases from 583 K to 585 K after doping 0.5 wt% Ga.

Phase field study on the microscopic mechanism of the cyclic degradation of shape memory effect in nano-polycrystalline NiTi shape memory alloys

A three-dimensional non-isothermal and crystal plasticity based phase field model was proposed to simulate the cyclic deformation of nano-polycrystalline NiTi shape memory alloys (SMAs) reflecting their one-way shape memory effect (OWSME) and stress-assisted two-way effect (SATWSME), and then the microscopic mechanism of the cyclic degradation of shape memory effect (SME) was systematically revealed. The proposed model was numerically implemented in a two-dimensional plane strain nano-polycrystalline NiTi SMA system, where each grain was divided into a grain-interior (GI) phase and grain-boundary (GB) phase, and the GI phase was set to be embedded in the GB phase. For the GI phase, a crystal plasticity based phase field model was newly established by considering four inelastic deformation mechanisms, i.e., martensitic transformation (MT), martensitic reorientation (MR), austenitic plasticity caused by dislocation slipping and martensitic plasticity caused by dislocation slipping and deformation twinning, simultaneously; for the GB phase, a visco-plastic model was used to describe its plastic deformation. The simulated results show that: during the cyclic deformation of nano-polycrystalline NiTi SMAs involving the OWSME or SATWSME, the local internal stress caused by the MR or MT can lead to the accumulation of plastic deformation, which further increases the fraction of the martensite variants with favorable orientations by inducing a specific internal stress field after cooling and then results in the accumulation of residual martensite phase after heating; the accumulation of the irrecoverable strain mainly from the martensitic twinning and the decrease of recoverable strain cause the cyclic degradation of OWSME, which is further dependent on the loading rate and loading level; the SATWSME of the SMAs can exhibit a cyclic strengthening during the cyclic loading with a relatively low constant stress, while a cyclic degradation with a relatively high stress, which results from the competition between the transformation strain and the irrecoverable strain mainly caused by the austenitic plasticity that increases gradually during the cyclic deformation involving the SATWSME. These new findings are very helpful to clarify the physical nature of the cyclic degradation of SME and then alleviate the functional fatigue of NiTi SMAs by modulating the material design.

Microstructure of die-cast alloys Mg–Zn–Al(–Ca): a study by electron microscopy and small-angle neutron scattering

AbstractThe microstructure of a magnesium die-cast alloy with 8 wt.% Zn and 5 wt.% Al (ZA85) and of two alloy modifications with 0.3 and 0.9 wt.% Ca (ZACa8503 and ZACa8509) are compared. Optical, scanning electron and transmission electron microscopy (TEM) were used to study the microstructure in as-cast and annealed conditions. Small-angle neutron scattering (SANS) measurements were carried out to characterize the early stages of the precipitation process. The die-cast microstructure of ZA85, ZACa8503 and ZACa8509 consists of dendritic α-Mg grains and coarse intermetallic grain-boundary phases. In the Ca-free ZA85 alloy, grain boundaries are decorated with a quasi-crystalline phase with icosahedral point-group symmetry (I phase), a metastable modification of the equilibriumτphase (Mg32(Al, Zn)49). At the grain boundaries of the Ca-containing ZA85 alloys, small amounts of the stable cubicτphase Mg32(Al, Zn, Ca)49as well as Al2(Ca, Zn) phase, were detected in addition to the I phase. In the as-cast condition, regions supersaturated with Al and Zn were found to surround the α-Mg grains and to becomeτphase precipitation sites during high-temperature annealing in all alloys investigated. TEM and SANS measurements revealed no influence of Ca on the size distribution and the coarsening rate of the precipitates, which are formed by a continuous precipitation process.

Nucleation of Grain Boundary Phases.

We derive a theory that describes homogeneous nucleation of grain boundary (GB) phases. Our analysis takes account of the energy resulting from the GB phase junction, the line defect separating two different GB structures, which is necessarily a dislocation as well as an elastic line force due to the jump in GB stresses. The theory provides analytic forms for the elastic interactions and the core energy of the GB phase junction that, along with the change in GB energy, determines the nucleation barrier. We apply the resulting nucleation model to simulations of GB phase transformations in tungsten. Our theory explains why under certain conditions GBs cannot spontaneously change their structure even to a lower energy state.

Microstructural stability of a two-phase (O + B2) alloy of the Ti-25Al-25Nb system (at.%) during thermal cycling in a hydrogen atmosphere

<abstract> <p>In this work, the stability of the microstructure of the experimentally obtained two-phase (O + B2) alloy of the Ti–25Al–25Nb (at.%) system were studied during thermal cycling in a hydrogen atmosphere. It was found that the two-phase structure (O + B2) of the alloy of the Ti–Al–Nb system shows high thermodynamic stability. In this case, phase transformations of secondary phases (α2, AlNb<sub>2</sub>) are observed in the microstructure of the alloy, the volumetric content of which at all stages of testing does not exceed 2%. Thus, after the first cycle of high-temperature exposure, single inclusions of the α2 phase precipitate, while in the areas enriched in Ti and Al, due to the redistribution of Nb, a new colony of the α2 phase is observed. After five test cycles, it was found that large accumulations of the α2 colony, due to the α2 → B2 phase transformations, form new micron-sized grains of the B2 phase. A volumetric accumulation of nanosized precipitates of the AlNb<sub>2</sub> phase was found near the triple joints of the grain boundaries of the B2 phase after 10 cycles of thermal exposure, which is caused by the supersaturation of B2 grains with niobium.</p> </abstract>

Structure and magnetic properties of Ga doped Nd13.65Fe74.15B5.6Co6.6 ribbons

The addition of low melting point Ga changed the crystallization process and microstructure of the Nd13.65Fe74.15B5.6Co6.6 ribbons. The Ga addition improves the glass-forming ability of the ribbons, and the ribbons melt-spun at 25 m/s are composed of Nd2(Fe, M)14B phases with different Nd contents and Nd-rich amorphous grain-boundary phase. Nd2(Fe, M)14B grains have a gradient size of 50–300 nm, also, the large-sized grains are slightly rich in Fe-Co, and the small-sized ones are rich in Nd. Compared to the Ga-free alloy, 0.6 at% Ga addition narrows the grain boundary width to 5.5 ± 4.5 nm, promotes the enrichment of Ga atoms at grain boundaries and form Nd(Fe, Co)1.62Ga0.04 composition, improved the intergranular exchange coupling and magnetic properties of the ribbons most significantly, increasing the coercivity and remanence by 52.6% and 36.6% respectively. Besides, the microstructure models changed by Ga addition are established, and the corresponding solidification processes and magnetic mechanisms are discussed.

Pr80Al20 surface-coated DyF3 modified sintered Nd-Fe-B magnets for large coercivity increment via grain boundary diffusion

A large coercivity increment of 11.63 kOe is obtained by grain boundary diffusion with composite powders of Pr80Al20 surface-coated DyF3. The coercivity increases from the original 12.76 kOe to 24.39 kOe and 23.28 kOe in PA-0.7 and PA-0.9 diffused magnets (Pr80Al20:DyF3 = 7:3, 9:1 in wt%), which shows an increase of 91.15% and 82.45%, respectively. The temperature coefficient of coercivity of diffused magnets is improved due to the high magnetic anisotropy field (HA) of (Nd, Pr, Dy)2Fe14B and the optimized microstructure. Excellent diffusion efficiency and Dy diffusion depth are obtained owing to the betterment of Pr-Al-rich grain boundary (GB) phases. Microstructural features show that the combined strengthening effect of magnetic isolation by the continuous Pr-Al-rich GB phases and magnetic hardening by Dy-Pr-rich shells significantly improve the coercivity of the PA-0.7 and PA-0.9 diffused magnets. Especially, no accumulation of Dy element occurs in the PA-0.9 diffused magnet. The composite diffusion sources provide a new method to improve diffusion efficiency of Dy.

Early stages of liquid-metal embrittlement in an advanced high-strength steel

Grain-boundary degradation via liquid-metal embrittlement (LME) is a prominent and long-standing failure process in next generation advanced high-strength steels. Here we reveal, well ahead of the crack tip, the presences of nano-scale grains of intermetallic phases in Zn-infiltrated but uncracked grain boundaries with scanning- and 4D transmission electron microscopy. Instead of the often-reported Zn-rich Fe-Zn intermetallics, the nano-scale phase in the uncracked infiltrated grain boundaries is identified as the Γ-phase, and its presence reveals the local enhancement of strain heterogeneities in the grain boundary network. Based on these observations, we argue that intermetallic phase formation is not occurring after cracking and subsequent liquid Zn infiltration but is instead one of the primary nanoscopic drivers for grain-boundary weakening and crack initiation. These findings shift the focus of LME from micro- and meso-scale crack investigations to the very early stages immediately following Zn diffusion, after which secondary phase nucleation and growth emerge as the root-cause for failure.

Enhanced High‐Temperature Strength of a Low‐Density Dispersion‐Strengthened Fe–Mn–Al–C Steel

A novel low‐density dispersion‐strengthened Fe–18%Mn–8%Al–0.8%C (wt%) steel exhibiting superior high‐temperature strength at 700–900 °C, is developed. The finely dispersed ferritic grains that form near the grain boundaries of the austenite phase provide significant strengthening by pinning the dislocations and enable high microstructural stability by restricting the dynamic recovery/recrystallization. The high Mn content provides remarkable solid solution strengthening to this steel, while the high aluminum content prominently lowers the density to enhance the specific strength. This newly developed steel with low alloying and manufacturing costs has great potentials for structural applications at elevated temperatures.

Geometry of Triple Junctions during Grain Boundary Premelting.

Grain boundaries (GBs) whose energy is larger than twice the energy of the solid-liquid interface exhibit the premelting phenomenon, for which an atomically thin liquid layer develops at temperatures slightly below the bulk melting temperature. Premelting can have a severe impact on the structural integrity of a polycrystalline material and on the mechanical high-temperature properties, also in the context of crack formation during the very last stages of solidification. The triple junction between a dry GB and the two solid-liquid interfaces of a liquid layer propagating along the GB cannot be defined from macroscopic continuum properties and surface tension equilibria in terms of Young's law. We show how incorporating atomistic scale physics using a disjoining potential regularizes the state of the triple junction and yields an equilibrium with a well-defined microscopic contact angle. We support this finding by dynamical simulations using a multiphase field model with obstacle potential for both purely kinetic and diffusive conditions. Generally, our results should provide insights on the dynamics of GB phase transitions, of which the complex phenomena associated with liquid metal embrittlement are an example.

The Effects of Forming CeFe2 on Phase Structure and Magnetic Properties in Ce-Rich Nd-Ce-Fe-B Permanent Magnetic Materials.

The decomposition of the Nd-Ce-Fe-B phase to form CeFe2 has been usually believed to have an important positive effect on the magnetic properties of Nd-Ce-Fe-B permanent magnetic materials. In this work, a new decomposition process of the Nd-Ce-Fe-B phase on the formation of the CeFe2 phase was observed to play a negative role in its magnetic properties. It is demonstrated that the Nd-Ce-Fe-B phase decomposes into non-magnetic CeFe2, accompanied by the precipitation of Fe soft-phase. The kinks usually occurring in the demagnetization curves of Ce-rich Nd-Ce-Fe-B magnets have been determined to be related to the Fe soft-phase. Instead of using CeFe2 as a grain-boundary phase, another Ce-Cu boundary phase has been explored to efficiently improve the coercivity of Ce-rich Nd-Ce-Fe-B magnets, provided that the Ce-Cu boundary phase has an appropriate Ce to Cu ratio. The present results contribute to the mechanism comprehension and high-performance design of Nd-Ce-Fe-B permanent magnetic materials.

Study and selection of most appropriate filler rod for GTAW of S32750 super duplex steel joints: A comprehensive study on microstructural, mechanical and corrosion properties

In the present study, the effect of using different filler metals on the microstructure, mechanical and corrosion properties of gas tungsten arc welding (GTAW) of S32750 super duplex steel joints has been investigated. Three different filler rods of 316 L, S32202, and S32750 containing different concentrations of ferrite and austenite stabilizer elements were used to fabricate GTAW S32750 joints. From the microstructural studies, it was found that elongated structure of austenitic-ferritic transforms into a dendritic γ-austenite structure within an α-ferrite matrix, when S32202 and S32750 were used. While, the microstructure of sample welded by 316 L was consisted of dendritic δ-ferrite phase in the austenite grain boundaries. Owning the higher percentage of α-ferrite phase, the sample welded by S32202 filler rod showed the highest microhardness and tensile strength. The best combination of tensile strength (826 MPa), ductility (27%) and absorbed energy prior to fracture (51 J) was observed in the sample welded by S32750 filler rod due to the higher percentage of γ-austenite phase or a decrease in the percentage of the α and δ-ferrite phase in the weld microstructure. The Tafel polarization test showed that the highest and lowest corrosion resistance belonged to the samples welded by S32750 and 316 L, respectively. The presence of higher percentage of γ-austenite phase caused an increase in the corrosion resistance of the weld metal, while the presence of higher percentage of α and δ-ferrite caused a significant decrease in the corrosion resistance.

Microstructure and mechanical properties of AlCoCrNiTi–C High Entropy Alloy processed through Spark Plasma Sintering

AlCoCrNiTi–C High Entropy Alloy (HEA) fabricated through mechanical alloying (10 h) and Spark Plasma Sintering (SPS) at 1000 °C develops a mixture of NiAl B2 phase, Cr rich phase and fine TiCs. Transmission Electron Microscope (TEM) images before and after milling confirms the presence of BCC phases in the alloy. Carbon Hydrogen and Nitrogen (CHN) analysis reveal the presence of carbon content in milled powders after 10 h. Back Scattered Electron (BSE) images and elemental mapping indicates the presence of TiCs along grain boundaries of NiAl B2 phase. Microstructural characterization before and after heat treatments reveals the fact that there is an increase in the volume fraction of Cr rich phase. Moreover the alloy develops a good room temperature compressive strength of 2750 ± 4.3 MPa, high temperature strength of 871 ± 3.1 MPa at 800 °C along with micro hardness of 860 ± 7.8 HV 0.5 . Nano indentation results shows that the major contribution to hardness and strength comes from NiAl phase which has a value of 10.2 ± 2.1 GPa. Enhanced properties of the alloy can be attributed to the presence of NiAl B2, disordered Cr rich BCC phase along with fine TiCs. • Composite microstructure resulting from in situ reactions. • Enhanced strength due to grain boundary strengthening and precipitation hardening. • Limited grain growth at temperatures of 800 °C, 900 °C and 1000 °C. • Heat treatment studies reveals increase in volume fraction of Cr rich phase.

Phase Transformations in the AlMg Alloys Driven by High‐Pressure Torsion

As‐cast aluminum–magnesium alloys with 3, 5, and 10 wt% Mg are subjected to high‐pressure torsion (HPT) at room temperature: 5 GPa, five turns with rotation speed of 1 rpm. HPT leads to the strong grain refinement for both (Al) solid solution and intermetallic β‐phase. Transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and X‐ray diffraction (XRD) are used to characterize the phase transitions under heating of the HPT‐treated alloys. The decomposition of the solid solution in the ultrafine‐grained Al–10 wt% Mg alloy obtained by HPT does not follow the equilibrium diagram. The results of DSC, XRD, and TEM show that the Mg‐rich intermetallic phases appear during heating in the following sequence: GP zones → β → ε → β → γ → β. The (Al)/(Al) grain boundaries (GBs) in the fine‐grained Al–10 wt% Mg alloy after HPT and annealing up to 400 °C (i.e., in the solid solution area, far from solvus and solidus lines) contain the Mg‐rich areas. This can be the result of possible GB phase transitions with the formation of Mg‐rich GB phases. The GBs in the binary AlMg can contain the thin layers of a GB phase far away from the solvus line.

A micromechanics-based constitutive model for nanocrystalline shape memory alloys incorporating grain size effects

A micromechanics-based model is developed to capture the grain-size dependent superelasticity of nanocrystalline shape memory alloys (SMAs). Grain-size effects are incorporated in the proposed model through definition of dissipative length scale and energetic length scale parameters. In this paper, nanocrystalline SMAs are considered as two-phase composites consisting of the grain-core phase and the grain-boundary phase. Based on the Gibbs free energy including the spatial gradient of the martensite volume fraction, a new transformation function determining the evolution law for transformation strain is derived. Using micromechanical averaging techniques, the grain-size-dependent superelastic behavior of nanocrystalline SMAs can be described. The internal length scales are calibrated using experimental results from published literature. In addition, model validation is performed by comparing the model predictions with the corresponding experimental data on nanostructured NiTi polycrystalline SMA. Finally, effects of the internal length scales on the critical stresses for forward and reverse transformations, the hysteresis loop area (transformation dissipation energy), and the strain hardening are investigated.

Incorporating elasticity into CALPHAD-informed density-based grain boundary phase diagrams reveals segregation transition in Al-Cu and Al-Cu-Mg alloys

The phase-like behavior of grain boundaries (GBs), recently evidenced in several materials, is opening up new possibilities in the design of alloy microstructures. In this context, GB phase diagrams are contributing to a predictive description of GB segregation and (interfacial) phase changes. The influence of chemo-mechanical solute-GB interactions on the GB phase diagram remains elusive so far. This is particularly important for multi-component alloys where the elastic interactions among solute atoms, of various sizes and bonding energies, can prevail, governing a complex co-segregation phenomenon. Recently, we developed a density-based model for GB thermodynamics that intrinsically accounts for GB elasticity in pure elements. In this work, we incorporate the homogeneous and heterogeneous elastic energies associated with the solutes into the density-based framework. We derive the multi-component homogeneous elastic energy by generalizing the continuum misfitting sphere model and extend it for GBs. The density-based free energy functional directly uses bulk CALPHAD thermodynamic data. The model is applied to binary and ternary Al alloys. We reveal that the elastic energy can profoundly affect the GB solubility and segregation behavior, leading to Cu segregation in otherwise Cu-depleted Al GBs. Consequently, GB segregation transition, i.e., a jump in the GB segregation as a function of alloy composition, is revealed in Al-Cu and Al-Cu-Mg alloy systems with implications for subsequent GB precipitation in these alloys. CALPHAD-informed elasticity-incorporated GB phase diagrams enable addressing a broader range of GB phenomena in engineering multi-component alloys.

Magnetic properties evolution with grain boundary phase transformation and their growth in Nd-Fe-Cu-Ga-B sintered magnet during post-sinter annealing process

The formation of Nd6Fe13Ga phase promoted the continuity and non-ferromagnetism of grain boundary (GB) phases of Nd-Fe-Cu-Ga-B magnets during proper post-sintering annealing process. The coercivity increased dramatically with a decrease of remanence. After further annealing at the sintering temperature, the remanence recovered with a drop of coercivity. It indicated that Nd2Fe14B phase decomposed with the formation of Nd6Fe13Ga phase. Overextending the annealing time led to the gradual decrease of coercivity as the growth of Nd6Fe13Ga phase weakened the continuity of GB phases. Increasing the annealing temperature to the optimum temperature for Nd6Fe13Ga phase can aggravate the decrease of coercivity and remanence. Owing to the intensifying segregation of Ga and Fe in GB phases with the formation of Nd6Fe13Ga phase by dehydrogenation (HD) at 560 °C, high coercivity can be obtained by annealing at a lower temperature or for shorter time compared with HD at 480 °C. The enrichment of Cu in Nd-rich phase ensured its wettability and non-ferromagnetism when the diffusion of Fe and Ga became more local with the formation of Nd6Fe13Ga phase. Therefore, the combined effect of Cu doping and reasonable high temperature HD was conducive to obtain high coercivity in a larger range of annealing time and temperature for Nd-Fe-Ga-B magnet, which was a potential method for its mass production.

Correlation Between Local Chemical Composition and Formation of Different Types of Ordered Phases in the Polycrystalline Nickel‐Base Superalloy A718Plus

Allvac 718Plus is a derivative of alloy 718 for applications at higher temperatures in rotating and static components of gas turbines. According to literature, various phases as γ, γ′, η, δ, and γ″ can form with so far contradicting results. Herein, the precipitation behavior in different alloy conditions is investigated using transmission electron microscopy and atom probe tomography. The aim is to clarify how local variations in chemical composition alter the sequence of precipitation and type of precipitates that form. Material conditions with a minor volume fraction of η/δ phase and a high fraction of primary γ′ precipitates form γ″ phase due to excess of available Nb. Spherical γ′ precipitates with disc‐shaped γ″ precipitates at γ′ interfaces are observed. During discontinuous coarsening of η/δ plates mainly Nb is consumed which leads to an enrichment of γ′ forming elements and hence the heterogeneous nucleation of γ′ at the η/γ interface. The coexistence of thin η plates embedded in δ phase in grain boundary pinning precipitates could additionally be confirmed. These results show under which conditions certain primary phases form and how the resulting consumption and redistribution of phase promoting elements trigger the formation of other secondary phases.

Suppression mechanism of abnormal grain growth by Zr addition in pressless processed Nd-Fe-B sintered magnets

Microstructures of pressless processed (PLP) Nd-Fe-B sintered magnets with and without Zr addition were investigated by scanning electron microscopy and scanning/transmission electron microscopy. In the Zr-free magnet, huge polygonal abnormal grains (AGs) about 100 times larger than the normal grain size were generated by PLP sintering at 975 °C. Electron backscattered diffraction analysis revealed that the PLP sintered magnet had a biaxial oriented texture of fine Nd2Fe14B grains with the orientation of the AGs significantly deviated from the biaxial oriented grains. By adding a small amount of Zr, fine plate-like Zr-B precipitates with a long period stacking ordered structure derived from the ZrB2 phase formed on the growth surfaces of Nd2Fe14B grains with a crystallographic orientation relationship. There were two Nd-rich grain boundary (GB) phases, namely, orthorhombic Nd3(Co, Cu) and metastable CaF2-NdO2, which became liquid during sintering. The boron consumption associated with the formation of Zr-B precipitates increased the Nd-rich GB phases, resulting in a homogeneous distribution of the liquid phase around the Nd2Fe14B grains. Therefore, the addition of Zr effectively suppressed the abnormal grain growth during PLP sintering owing to the pinning effect of the Zr-B precipitates and the formation of a homogeneous liquid phase distribution.

On the role of molybdenum on the microstructural, mechanical and corrosion properties of the GTAW AISI 316 stainless steel welds

A comparative study was carried out on GTAW AISI 316 welds fabricated by three different filler metals including ER-317M, ER-316 and ER-308. Filler metals were selected in a way to induce varying Molybdenum (Mo) concentrations in the weld metal, so that the effect of Mo on the microstructural, mechanical and corrosion behavior of the welds was investigated. Microstructural studies revealed that as Mo concentration increased in the weld metal, the dendritic morphology of this zone gradually was changed from cellular dendritic to columnar dendritic and a noticeable amount of austenitic phase and precipitations were formed. Mo-containing ER-316 and ER-317M welds showed appreciable amount of Mo- and Cr-rich carbides such as (Fe,Cr) 23 C 6 which formed at the grain boundaries of δ-ferrite phase resulting in increased the hardness and strength of welded samples. The highest microhardness value of 188 HV and yield strength of 330 MPa belonged to ER-317M welds containing the highest Mo concentration. Also, corrosion resistance of the welds significantly improved by increasing Mo concentration and a minimum corrosion rate of 4.178 mm/year was accounted for ER-317M welds in which, the formation of passive film was more facilitated due to the higher concentration of Mo.