Homogeneous Grain Research Articles

Crystallographic studies of the intergranular phases in sintered PrNd23Ce7Gd1.5FebalB0.93Cu0.35M1 magnet with commercial N40M magnetic properties

In this work, we focus on the modification of the Cu element in commercial N40M magnets toward the composition and structure of the grain boundary phase. The magnetic properties and microstructure of Ce-magnets with low and high Cu content were investigated. The Ce-magnet with high Cu-content has higher coercivity and a more continuous and homogenous grain boundary phase. The grain boundary phase’s structural shift is found to be the source of the coercivity change. The transmission electron microscopy study shows that the PrNd23Ce7Gd1.5FebalB0.93Cu0.35M1 magnet contains fcc and hcp type (Ce, Nd)-oxide phases, each with a different O concentration. The formation of the superlattice has been examined, and the superlattice spots show that the Cu-rich phases have an ordered structure.

Evaluation on static mechanical stability of wrought Inconel 718 superalloy; effect of grain configuration and δ precipitation

Wrought nickel alloys are widely applied in hot static components for aero-engines, acquisition for the underlying mechanism of microstructural evolutions controlling mechanical degradation during elevated-temperature exposure is fundamentally vital to processing optimization as well as serving life prediction. In present work, the differential hot tensile characteristics during 500–2000 h exposure at 650 °C of wrought Inconel 718 alloy having specific microstructures designed via different processing routes were comparatively evaluated. The experimental results showed the static mechanical stability of Inconel 718 alloy was predominantly tailored by the complicated interaction of γ-matrix grain configuration and δ precipitation on grain boundaries (GBs). It mainly involved continuous GB-δ precipitation during the 500–2000 h exposure at 650 °C, which played opposite roles in homogeneous or heterogeneous grain configurations. GB-δ precipitation can improve hot ductility but deteriorate strength for homogeneous grain configuration, adequate GB-δ particles can improve the deformation compatibility between GBs and γ-matrix to achieve good balance between the strength and ductility as well as smaller mechanical fluctuations. The heterogeneous duplexing grain configuration exhibited enhanced mechanical sensitivity to sluggish GB-δ precipitation since the hot ductility was rapidly reduced by tiny δ particles with fracture mode transition. In comparison, appropriate sub-solvus solution prior to aging cycles can introduce GB-δ particles pre-existed in homogeneous grain configuration to benefit in superior long-term static mechanical stability for Inconel 718 alloy.

Analysis of the magnetization reversal mechanism of heterogenous grain boundary diffused Nd-Fe-B sintered magnets

The grain boundary diffusion (GBD) method could effectively improve the coercivity of Nd-Fe-B sintered magnets, but the uneven distribution of heave rare earth (HRE) elements would lead to the decrease of squareness factor of the demagnetization curve of the magnets, which limited the application of this method. In this paper, three typical-structure regions were summarized based on microstructure characterization, and the magnetic properties and magnetization reversal mechanism were analyzed. The differences in nucleation positions and magnetization reversal processes of the three regions were discussed, and it found that the remarkable differences among them resulted in the low squareness factor of GBD Nd-Fe-B sintered magnets. In addition, the efficiency of Tb elements could be further improved to enhance the coercivity of Nd-Fe-B sintered magnets by forming homogeneous and complete core-shell structure grains, which could strengthen the nucleation field and even transfer the nucleation position from the grain surface defect layer to the grain interior region. Our results could provide guidance and reference for further improving the permanent magnetic properties of Nd-Fe-B sintered magnets by optimized GBD method.

Fabrication of new gradient AZ91-bioactive glass composite using friction stir back extrusion

This study investigated the microstructure, mechanical characteristics, and in vitro corrosion behavior of gradient AZ91-bioactive glass composite wire fabricated by friction stir back extrusion. The results showed that by decreasing the extrusion speed from 40 to 20 mm/min, the distribution factor of bioactive glass increased from 0.31 ± 0.04–0.45 ± 0.07, and more homogenous grain size was formed at the surface and center zone of the composite wire. Under the influence of heat and plastic deformation during friction stir back extrusion, despite the formation of the β-phase in separate islands, the α + β eutectic phase was also formed in the composite wire. In composite wire processed with a rotational speed of 800 rpm and extrusion speed of 20 mm/min, a gradient distribution of bioactive glass was formed in the cross section of the wire. Compared to an as-cast AZ91 alloy, friction stir back extrusion conducted at a rotational speed of 800 rpm and an extrusion speed of 20 mm/min resulted in a 44% increase in corrosion resistance in simulated body fluid (SBF).

TiNi-Based Material with Shape-Memory Effect for Surgical Treatment of Diseases of Small Intestine in Newborn and Young Children.

Alloys based on TiNi are widely used in various fields of technology and medicine. In the present work, we report on the preparation of TiNi-alloy-based wire with the shape-memory effect, which was used for compression clips for surgery. The composition and structure of the wire and its martensitic and physical-chemical properties were studied using SEM, TEM, optic microscopy, profilometry, mechanical tests, etc. The TiNi alloy was found to consist of B2 and B19' and secondary-phase particles of Ti2Ni, TiNi3 and Ti3Ni4. Its matrix was slightly enriched in Ni (50.3 at.% of Ni). A homogeneous grain structure was revealed (an average grain size of 19 ± 0.3 μm) with equal quantities of grain boundaries of special and general types. The surface oxide layer provides improved biocompatibility and promotes the adhesion of protein molecules. Overall, the obtained TiNi wire was concluded to exhibit martensitic, physical and mechanical properties suitable for its use as an implant material. The wire was then used for manufacturing compression clips with the shape-memory effect and applied in surgery. The medical experiment that involved 46 children demonstrated that the use of such clips in children with double-barreled enterostomies permitted improvement in the results of surgical treatment.

Corrosion behavior and mechanical properties of extruded low-alloyed Mg−0.5Bi−0.5Y−0.2Zn alloy

AbstractA novel low-alloyed Mg−Bi−Y−Zn alloy system was developed and extruded successfully at 673 K. The corrosion behavior and tensile properties of the extruded alloy were investigated through scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), electrochemical tests and tensile test. The extruded alloy exhibits a nearly fully recrystallized grain structure with a scattered extrusion fiber texture and some sub-micron precipitates. In SBF solution, the dominated corrosion mode changed from galvanic induced pitting corrosion at initial stage to filiform-like corrosion and finally mixed pitting corrosion and local fall-off of grains during long-term corrosion processes. The tensile properties and corrosion resistance of the extruded Mg−0.5Bi−0.5Y−0.2Zn (wt.%) alloy are: tensile yield strength of 237 MPa, ultimate tensile strength of 304 MPa, elongation to failure of 31% and mean corrosion rate of 0.14 mm/a. The great balance between tensile properties and corrosion resistance is mainly attributed to the homogeneous grain structure and the presence of sub-micron scaled precipitates, indicating a promising potential in the application in biomedical field.

Simultaneous enhancement of strength and ductility in friction stir welded AZ31 alloy via multi-pass hot-rolling and subsequent annealing

Friction stir welding (FSW) is a new solid-state joining process for joining light metals, such as magnesium (Mg) alloys. However, the texture-induced softening mechanism (in stir zone, SZ) tends to cause the worsening of mechanical properties in FSW Mg alloys. In this work, multi-pass hot-rolling and subsequent annealing is used to ameliorate the FSW AZ31 alloy plate. The results show that multi-pass hot-rolling leads to the c-axes of most grains in the SZ approximately perpendicular to the normal direction (ND) transforming into these almost parallel to the ND, accompanied with multiple twin types. After subsequent annealing, twins, as nucleation sites, promote the formation of new grains with off-basal texture, resulting in the weak basal texture. Compared with the FSW plate and the initial plate, the rolled-annealed FSW plate exhibits a relatively uniform hardness distribution and a “strength-ductility” synergy, which are mainly attributed to an appropriate texture feature and a homogeneous grain structure. This work provides a new idea for the development of new high-performance FSW Mg alloys.

Optimizing interlayer cooling for SUS316L thin wall fabricated by directed energy deposition

The direct energy deposition (DED) process requires proper interlayer cooling (IC) to avoid geometric failure caused by overheating of the midsection. This study suggests an optimum IC step based on a constitutive equation, instead of trial and error, to ensure the geometric stability of DED-processed 316 L stainless steel within a short period. The temperatures after cooling (TC) were acquired per layer of building and precisely measured using a constitutive model. Subsequently, a cooling period to maintain a target TC was calculated for the 30-layered DED specimen using the model. The optimum IC step varied with the number of deposited layers: (i) non-IC up to the fourth layer, (ii) IC step of 1.05 s for the fifth layer, and (iii) IC step of 2.21 s for the subsequent layers. The developed approach resulted in a remarkable improvement in geometric stability (geometric error of 5.9%) compared with the DED specimen fabricated without an IC step (error of 33.5%). Furthermore, the processing time was reduced by 30% compared with a conventional IC step with a fixed interval of 5 s. The developed approach also led to homogeneous grain refinement and a resulting increase in microhardness.

Influence of ECAP Parameters on the Structural, Electrochemical and Mechanical Behavior of ZK30: A Combination of Experimental and Machine Learning Approaches

Several physics-based models have been utilized in material design for the simulation and prediction of material properties. In this study, several machine-learning (ML) approaches were used to construct a prediction model to analyze the influence of equal-channel angular pressing (ECAP) parameters on the microstructural, corrosion and mechanical behavior of the biodegradable magnesium alloy ZK30. The ML approaches employed were linear regression, the Gaussian process, and support vector regression. For the optimization of the alloy’s performance, experiments were conducted on ZK30 billets using different ECAP routes, channel angles, and number of passes. The adopted ML model is an adequate predictive model which agreed with the experimental results. ECAP die angles had an insignificant effect on grain refinement, compared to the route type. ECAP via four passes of route Bc (rotating the sample 90° on its longitudinal axis after each pass in the same direction) was the most effective condition producing homogenous ultrafine grain distribution of 1.92 µm. Processing via 4-Bc and 90° die angle produced the highest hardness (97-HV) coupled with the highest tensile strength (344 MPa). The optimum corrosion rate of 0.140 mils penetration per year (mpy) and the optimum corrosion resistance of 1101 Ω·cm2 resulted from processing through 1-pass using the 120°-die. Grain refinement resulted in reducing the corrosion rates and increased corrosion resistance, which agreed with the ML findings.

Breaking the strength–ductility trade-off in additively manufactured aluminum alloys through grain structure control by duplex nucleation

Achieving a homogeneous equiaxed grain structure and breaking the strength–ductility trade-off in additively manufactured aluminum alloys is a great challenge. In this paper, we propose a novel duplex nucleation mechanism that combines ex situ TiB2 and in situ Al3Ti for controlling the grain structure of additively manufactured AlCuMgTi-TiB2 composites. We conducted thermodynamic calculations and phase-field simulations to elucidate the duplex nucleation-based grain structure control. The Al3Ti-coated TiB2 inoculant system formed via duplex nucleation during solidification enabled the formation of a homogeneous ultrafine equiaxed microstructure in both the as-fabricated and heat-treated states. Different from the AlCuMgTi alloy, the TiB2-reinforced AlCuMgTi composites produced via laser powder bed fusion were amenable to the simultaneous enhancement of strength and ductility. The proposed alloy design approach and duplex nucleation mechanism can guide the tailoring of the microstructure and mechanical properties of additively manufactured aluminum parts.

Microstructure and mechanical properties of carbon nano-onions reinforced Mg-based metal matrix composite prepared by friction-stir processing

In consideration of the increasing demand for lightweight structural metal materials, it is essential to develop Mg-based metal matrix composite (MMC) with well-matched strength and ductility. In this study, carbon nano-onion (CNO) reinforced AZ31B Mg alloy MMC was successfully fabricated via friction stir processing (FSP). The microstructure and mechanical properties of the FSP CNO/AZ31B MMC were investigated. The obtained results showed that a homogeneous fine grain structure was achieved in the stir zone without gaps and microcracks. The grain refinement mechanisms were attributed to continuous and discontinuous dynamic recrystallization. The CNOs also played a critical role in the activation of {10−12} twinning behaviour. {10−12} twins can not only further refine the recrystallized grains but also reduce the basal texture intensity. The FSP CNO/AZ31B MMC exhibited enhanced yield strength due to the combined effects of grain boundary strengthening, dislocation strengthening, Orowan strengthening, and load bearing mechanism of reinforcements. Owing to the {10−12} twins and CNO induced back-stress strengthening, the FSP CNO/AZ31B MMC preformed improved strain hardening capacity. The yield strength and fracture elongation were enhanced by 14 % and 11.8 % compared with those of the matrix material, respectively, indicating that the strength and ductility were simultaneously improved due to the addition of CNO reinforcement.

Semiconducting metamorphism in Cu2+ substituted structurally enriched SnO2 nanocrystals

The manuscript portrays structural, microstructural, and electrical characteristics of pristine and Cu2+ doped SnO2 nanocrystals [Sn(1-x)CuxO2 NCs, where x = 0, 0.05, and 0.10] synthesized using conventional sol–gel technique. X-ray diffraction (XRD) results revealed that all the compositions possess single phase tetragonal rutile-type crystal configuration with space group P4/2mnm as confirmed by Rietveld refinement studies. Microstructural characterizations depicted homogeneous grain distribution with well interlinked grains and it was observed that the average grain size decreased from 30.29 nm to 27.47 nm with increasing Cu2+ concentration and the findings were consistent with structural studies. The concentric rings marked with Miller planes (110), (101), (200), and (211) observed in Selected Area Electron Diffraction (SAED) pattern. Hall effect measurements depicted that the concentration and mobility decreased with Cu2+ thereby transforms the system from n-type semiconductor to p-type.

Decoupling the effects of texture and composition on magnetic properties of Fe-Si sheet processed by shear deformation

Soft magnetic Fe-Si alloys (electrical steels) possess exceptional functional properties such as high permeability, low coercivity, and low core loss, which generally improve with increasing Si content in the alloy. However, Fe-Si alloys containing > 3.5 wt% Si are also characterized by prohibitively low workability and poor ductility that have prevented their efficient commercial production in sheet form by rolling. This has limited their use for improving efficiency of motors and transformers. In this study, hybrid cutting-extrusion (HCE) is used as a single-step thermomechanical processing method to produce continuous Fe-Si alloy sheet with high Si compositions of 4 wt% to 6.5 wt%. HCE sheet is shown to have a homogeneous annealed grain structure and simple-shear crystallographic textures. By controlling the HCE deformation path, varied crystallographic shear textures are created in the sheet. Quasi-static magnetic properties of the HCE sheet are evaluated to decouple the effects of sheet texture and Si composition on resultant permeability and coercivity properties. The results suggest that HCE, with suitable process scaling, is a viable route for production of high-Si content electrical steel sheet for next-generation motors and transformers.

A critical review on effect of nanoparticles and process parameters on metallurgical properties of friction stir processing of dissimilar aluminum alloys

Friction stir welding (FSW) and friction stir processing (FSP) are unique solid-state joining and surface modification techniques that offer several advantages over conventional fusion welding and surface treatment methods. The ability to modify the microstructure and properties of the weld and surface through the control of processing parameters is a key advantage of FSW/FSP. This has led to a great deal of research on understanding the microstructural evolution during FSW/FSP. One important aspect of microstructural control in FSW/P is the development of grain structures and textures. The high strain rates and temperatures during FSW/P can lead to dynamic recrystallization and the formation of refined and homogeneous grain structures. The degree of grain refinement and texture development can be controlled through the selection of processing parameters such as tool geometry, rotation speed, traverse speed, and applied pressure. Another important aspect of microstructural control in FSW/P is the formation of phases, phase transformations, and precipitation. The ability to control grain structures, phases, and IMCs, as well as the potential for local refinement and the formation of MMCs, makes FSW/P an attractive option for a wide range of applications. Continued research and development will be critical for further advancing the capabilities and potential of FSW/P.

Effect of the post annealing cooling rate on the martensitic transformation and the magnetocaloric effect in Ni-Mn-Sn ribbons

We report the modification of the martensitic transformation in Ni-Mn-Sn ribbons by controlling the post annealing cooling rate. Isothermal holding above the order-disorder temperature for 24 h ensured homogeneous grain growth and uniform microstructural features in all the samples. However the atomic order was tailored by varying the cooling rate. The furnace-cooled sample exhibited the highest austenite stabilization and the lowest martensitic transformation temperature of 200.2 K, whereas the air-cooled sample exhibited the highest transformation temperature of 298.5 K. The martensite evolved as ferromagnetic clusters in an anti-ferromagnetic matrix in the samples with high atomic disorder, whereas long-range anti-ferromagnetic co-relations developed in the highly ordered samples. The martensitic transformation in the as-spun ribbons generated a significant magnetocaloric effect and a large entropy change of 4.02 J/kg/K, for a ΔH of 2 T. Furthermore, furnace cooling increased the relative cooling power of the ribbons by 23.3 J/kg over the as-spun specimens.

Some Mineralogical Characteristics of the Egyptian Black Sand Beach Ilmenite Part I: Homogeneous Ilmenite and Titanhematite-Ferriilmenite Grains

The high-grade Egyptian beach ilmenite concentrate contains various mineral textures in addition to the main homogeneous ilmenite grains (63%), which may contain solid solutions of geikielite (MgTiO3) and pyrophanite (MnTiO3) mineral components. Few homogeneous ferrilmenite grains (2%) associated with the concentrated ilmenite grains are detected. The contents of Fe2O3, MgO, Al2O3, and Cr2O3 in the ferrilmenite grains range between 7.3% and 22.8%, 3.4% and 6.6%, 0.2% and 0.7%, and 0% and 1.2% respectively. The detected hematite-ilmenite exsolved intergrowths (21.4%) have titanhematite exsolutions of different shapes, sizes, and orientations. They occupy 5%-40% of the whole intergrowth and may show one or two distinct generations. In some ferrilmenite components, MnO ranges between 1.5% and 8.6%. The Cr2O3 and Al2O3 contents range between 0% and 1.2% and 0% and 3.2% respectively. They are mostly between 0% and 0.1% for either of the ferrilmenite components, while relatively greater content is present in the titanhematite components. In some grains, the titanhematite exsolution bodies are replaced by goethite or hydrated iron oxides. In others, the ferrilmenite intergrowth may be partially or completely altered into leucoxene. Some minor composite grains are detected in the concentrate, where each grain consists of two parts, one part is titanhematite-ferrimenite and the other is ferrilmenite-titanhematite. The titanhematite exsolved components have relatively lower TiO2 content (5.8%-23.8%). Both MgO and MnO are positively correlated with FeO rather than Fe2O3. The presence of sphenes in the obtained ilmenite concentrate may be responsible for the recorded amounts of SiO2 (30.1%-30.8%) and CaO (28.1% and 28.8%). The Cr2O3 content is relatively much higher in sphene spots than in ilmenite spots, ensuring that Cr2O3 neither follows TiO2 nor FeO. The nature of the problem of the relatively lower Ti content and the relatively higher Fe and Cr contents of the obtained ilmenite concentrates is the target of the article. The problem is related to the mineralogy of ilmenite or to the used physical concentration flowsheet of the separated concentrate and the ability to improve the ilmenite concentrate’s specifications. It is concluded that although the homogeneous ilmenite is characterized by low Cr2O3 content, some of the other exsolved texture components, e.g. titanhematite and sphenes, have relatively higher Cr2O3, in addition to Fe2O3, SiO2, or CaO. They can negatively affect the marketable specifications of the separated Egyptian black sand ilmenite concentrate.

A novel crystallographic location of rattling atoms in filled Eu x Co4Sb12 skutterudites prepared under high-pressure conditions

Abstract Thermoelectric M x Co4Sb12 skutterudites are well-known to exhibit a reduced thermal conductivity thanks to the rattling effect of the M-filler at the large cages occurring in the framework, centered at the 2a sites of the I m 3 ‾ $Im\overline{3}$ space group. A novel Eu-filled skutterudite has been synthesized under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. The structural refinement from high-angular resolution synchrotron X-ray diffraction (SXRD) patterns unveils an unusual position for Eu filler atoms. By difference Fourier synthesis they are found at 12d sites, conforming statistically occupied octahedra within the mentioned cages around 2a positions. The Debye temperature was estimated by averaging the isotropic displacements by the atomic masses, leading to θ D ${\theta }_{D}$ of 273(2) K. Oftedal plots concerning the y and z Sb fractional positions, the unit-cell parameter a and M filling fraction include the novel Eu specimen in the trend observed for other filled materials prepared under high-pressure, including rare-earths, alkali or alkali-earth elements, all accepted as rattlers in filled skutterudites. A total thermal conductivity (κ) of 0.82 W m−1 K−1 is measured at 773 K for Eu0.02(1)Co4Sb12, below that of other filled skutterudites, which is promoted by the enhanced phonon scattering of Eu located at 12d sites. FE-SEM images showed large, homogeneous grains, well compacted after the high-pressure synthesis.

Superior fracture toughness with high yield strength in a high-Mn steel induced by heterogeneous grain structure

Heterogeneous grain structures were designed in a high Mn steel (Fe28Mn10Al1.00C), and the tensile properties and fracture toughness were investigated and compared with those for homogeneous structures. The heterogeneous grain structures display larger tensile ductility, stronger strain hardening and higher fracture toughness at the similar yield strength level. Hetero-deformation-induced hardening is found to play an important role in the heterogeneous grain structures, resulting in better mechanical properties. The size of plastic zone and the strain hardening capacity around the crack tip for the heterogeneous grain structures are found to be much larger/higher than those for the homogeneous grain structures at the same level of yield strength, resulting in better fracture toughness. High density of geometrically necessary dislocations and grain refinement are induced at the adjacent area of the main crack path, and numerous microvoids are also observed besides the main crack for the heterogeneous grain structures, resulting in more energy dissipation for higher fracture toughness. The deformation mechanisms around the crack tip are highly dependent on the magnitude of plastic strain and the grain size. The observed higher fracture toughness in the heterogeneous grain structures can be partly attributed to the formation of microbands.

Hot workability and microstructure of Mg–6.0 Zn–2.0 Yb–0.5 Zr magnesium alloy

Mg–6.0 Zn–2.0 Yb–0.5 Zr (wt%) alloy exhibits excellent strength and ductility synergy after thermomechanical and heat treatment. Unfortunately, the hot workability and microstructure evolution under specific deformation regimes remain uncertain to date. In this work, the constitutive model and hot processing maps were established based on a series of isothermal compression tests. A detailed examination of deformation microstructures at different processing regimes revealed that the hot workability and correlated microstructure development were deformation condition dependent. When compressed at lower strain rates and higher deformation temperatures, the softening was promoted by dynamic recrystallization (DRX) and the prevalence of discontinuous dynamic recrystallization (DDRX) generally resulted in a complete DRX process with coarse grains and much-intensified texture. On the contrary, work hardening was improved with increasing dislocation density, and the continuous DRX (CDRX) may control the partial DRX progress, often presenting a much-refined microstructure with a more dispersed texture. Interestingly, a low strain rate of 0.001 s−1 and a moderate deformation temperature of 300 °C effectively balanced the nucleation rate and grain growth to finally achieve a much refined and relatively homogeneous grain structure. Further investigation concerning the activation energy suggested that the workability improved with increasing strain rate and temperature. Enhanced hot workability at most deformation regimes was achieved due to a lower average activation energy Q value in comparison with that of 1.0 wt% Yb alloyed ZK60 counterpart, which was likely due to a higher activity of non-basal slips achieved in this alloy.

Effects of drying temperature of corn from the center and extremities of the corncob on drying parameters, protein and starch properties, and carotenoid profile

The corn grains from the extremities of corncob are known to have a spherical shape and the grains from the center of corncob have higher length and lower thickness. It is understood that these differences in grain dimensions can affect post-harvest processes and the properties of the grains. Therefore, the objective of this study was to evaluate the effects of drying temperature (60, 80, and 100 °C) of corn from the center and extremities of corncob on drying parameters, protein and starch properties, and carotenoid profile. At 60 °C, the drying rate and effective moisture diffusivity of corn from the center and extremities of the corncob did not differ. However, at 80 and 100 °C these parameters were higher in the corn from the center. Corncob part and separation did not significantly affect corn pasting properties. However, they caused significant changes in the properties of the grain proteins, mainly the reduction of the solubility and inactivation of the lipase enzyme, and the reduction of the lutein and β-carotene contents in the grains from the center and separated after drying. The results of this research show the need to evaluate the effects of these drying conditions during grain storage. In addition, the implementation of an industrial separation step and/or the development of new corn cultivars with more homogeneous grains should be studied.