Phase Refinement Research Articles

Tuning microstructure, transformation behavior, mechanical/functional properties of Ti–V–Al shape memory alloy by doping quaternary rare earth Y

The microstructure features, martensitic transformation behavior and mechanical/functional properties of Ti–V–Al alloy were tailored by changing rare element Y content in the present investigation. The results showed that Y doping resulted in the grain refinement and formation of Y-rich phase mainly distributing along grain boundary in Ti–V–Al alloys. The martensitic transformation temperatures of Ti–V–Al alloys slightly increased due to the variation of matrix composition induced by the presence of Y-rich phase. The mechanical and functional properties of Ti–V–Al alloys doped moderate Y addition were significantly improved, which can be ascribed to grain refinement, solution strengthening and precipitation strengthening. The 1.0 ​at.%Y-doped Ti–V–Al alloy exhibited the highest ultimate tensile stress of 912 ​MPa and largest elongation of 17.68%. In addition, it was found that the maximum recoverable strain of 5.42% can be obtained in Ti–V–Al alloy with adding 1.0 ​at.%Y, under the pre-strain of 6% condition, which is enhanced by approximate 0.6% than that of Ti–V–Al alloy without Y addition.

Electric Transport and Dielectric Studies of CoFe2O4-BaFe12O19 Nanocomposite

Electric transport properties of cobalt ferrite and barium ferrite composite were studied in the frequency range of 1 Hz to 1 MHz with temperature varying from 100 to 500 °C. The composite with weight ratio 1:1 (CoFe2O4:BaFe12O19) was synthesized by chemical co-precipitation route. Rietveld refined X-ray diffraction analysis revealed pure phase formation of cobalt ferrite, barium ferrite and the prepared composite. For the prepared composite, dual phase refinement procedure was employed. Average crystallite size of CoFe2O4 and BaFe12O19 was observed as 19 nm and 51 nm respectively. The conduction and polarization mechanism in ferrites was explained on the basis of Maxwell–Wagner theory and Koop’s model which reflects the contribution of grain and grain boundaries present in the specimens. The conductivity analysis confirmed the presence of overlapping large polaron tunneling (OLPT) model for conduction.

Coupling between Deformation and Mass Transport in an Immiscible Alloy at High Shear Strains

Forced mixing to a single phase or supersaturated solid solution (SSS) and its prerequisite microstructure evolution in immiscible systems has been a focus of research for both fundamental science and a variety of applications. Controlling the formation of SSS by shear deformation assisted processing could enable a material design beyond conventional equilibrium microstructure in immiscible systems. Here, a highly immiscible (mixing enthalpy of ~ 20 kJ·mol-1) Cu-50 at. % Cr binary alloy was employed to investigate the microstructure evolution and localized tendencies of SSS during severe shear deformation. The present results demonstrate distinctive defect mediated microstructure refinement process in each phase and how it leads to localized SSS as a function of shear strain. Preferential dynamic recrystallization occurs in the softer Cu phase due to strain localization, leading to substantial grain refinement. The refinement of Cr phase in the top-most layer, however, is enabled by the progressive evolution of grain lamination, splitting, and fragmentation as a function of shear strain. The eventual SSS is found to be strongly dependent on the local environments that affect the dislocation activity, including the level of microstructure refinement, the interfacial orientation relationship, hardness difference, and supposed stability of oxidation. Ab initio simulations confirm that it is more favorable to oxidize Cr than Cu at incoherent Cu/Cr interfaces which then limits the mass transport on an incoherent boundary. Our results shed light on the underpinning mechanism for non-equilibrium mass transport in immiscible systems upon severe deformation that can be applicable to a variety of processing techniques aimed at producing immiscible alloys with superior mechanical properties.

Microstructures and tensile properties of as-cast Mg-5Sn-1Si magnesium alloy modified with trace elements of Y, Bi, Sb and Sr

The microstructures and mechanical properties of as-cast Mg-5Sn-1Si magnesium alloy modified with trace elements Y, Bi, Sb and Sr were investigated and compared. Results show that the microstructure of the as-cast Mg-5Sn-1Si alloy consists of α-Mg, Mg2Si, Mg2Sn and Mg2(SixSn1−x) phases. After adding 0.8wt.% Y, 0.3wt.% Bi, 0.9wt.% Sb and 0.9wt.% Sr, respectively into the Mg-5Sn-1Si magnesium alloy, Mg24Y5, Mg3Bi2, Mg3Sb2 and Mg2Sr phases are precipitated accordingly. Trace elements can refine α-Mg grain and Chinese script-shaped Mg2Si phase. Refinement efficiency of different trace elements on α-Mg grain and Mg2Si phase is varied. Sr element has the best refinement effect, followed by Sb and Bi, while Y has the least refinement effect. Mg-5Sn-1Si-0.9Sr alloy has higher tensile properties than the other three modified alloys. The refinement mechanism of Y, Bi and Sr elements on Mg-5Sn-1Si magnesium alloy can be explained by the growth restriction factors and the solute undercooling. For Mg-5Sn-1Si-0.9Sb alloy, the heterogeneous nuclei of Mg3Sb2 phase is the main reason for the refinement of grains and second phases.

Dual-phase cohesive zone modelling and experimental validation for hydrogen-assisted cracking of 2205 duplex stainless steel

The hydrogen-assisted cracking (HAC) behavior of 2205 duplex stainless steel is investigated via experimental testing and computational modelling, with specific focus on developing a predictive methodology for the coupled effects of hydrogen diffusion and stress. The effects of duration and stress level on hydrogen-assisted fracture in the dual-phase microstructure are characterized via tensile testing of single-edge notch specimens under hydrogen diffusion conditions. Crack initiation and growth is shown to occur predominantly in the ferrite phase with the austenite phase acting to retard crack growth, leading to discontinuous crack patterns. Mixed brittle and ductile fracture characteristics were identified, due to the competitive effects of hydrogen-induced decohesion and localized plasticity. A key novelty is the development and verification of a sub-modelling finite element methodology of the dual-phase microstructure, incorporating hydrogen diffusion, coupled hydrogen-stress effects and cohesive zone cracking. The model consistently predicts observed crack length and mixed-phase induced crack morphology. Increased refinement of austenite phase is shown to increase fracture resistance of the dual-phase steel, consistent with published findings.

Superplastic Deformation of Al–Cu Alloys after Grain Refinement by Extrusion Combined with Reversible Torsion

The binary as-cast Al–Cu alloys Al-5%Cu, Al-25%Cu, and Al-33%Cu (in wt %), composed of the intermetallic θ-Al2Cu and α-Al phases, were prepared from pure components and were subsequently severely plastically deformed by extrusion combined with reversible torsion (KoBo) to refinement of α-Al and Al2Cu phases. The extrusion combined with reversible torsion was carried out using extrusion coefficients of λ = 30 and λ = 98. KoBo applied to the Al–Cu alloys with different initial structures (differences in fraction and phase size) allowed us to obtain for alloys (Al-25%Cu and Al-33%Cu), with higher value of intermetallic phase, large elongations in the range of 830–1100% after tensile tests at the temperature of 400 °C with the strain rate of 10−4 s−1. The value of elongation depended on extrusion coefficient and increase, with λ increasing as a result of α-Al and Al2Cu phase refinement to about 200–400 nm. Deformation at the temperature of 300 °C, independently of the extrusion coefficient (λ), did not ensure superplastic properties of the analyzed alloys. A microstructural study showed that the mechanism of grain boundary sliding was responsible for superplastic deformation.

Microstructural and Hardness Evolution in a Duplex Stainless Steel Processed by High-Pressure Torsion

The duplex stainless steel 2205, designated DSS2205 and having a duplex structure comprising ferrite and austenite phases, was processed by high-pressure torsion (HPT) and the microstructural and hardness evolutions were investigated after various HPT revolutions and at different positions within the specimens. The results show that the grain refinement induced by severe deformation processing is different in the ferrite and austenite phases such that the ferrite grains are refined via dislocation subdivision, whereas grain refinement in the austenite phase depends mainly on the interaction of dislocations and twin boundaries at relatively low strains. When the numbers of revolutions increases, the grain refinement in austenite restricts the occurrence of deformation twinning so that dislocation slip becomes dominant. During HPT processing, the effect of the phase boundaries on the mechanical properties of the alloy is very significant. The results show the average width between two adjacent phases and the hardness of the alloy are generally consistent with the classical Hall–Petch relationship.

Ultrasonic refining mechanism of ternary phase in Al–Sc based alloys prepared through molten salt electrolysis

Aluminum alloys are widely used in cutting-edge technologies and emerging strategic industries, namely aerospace, high-speed rail transportation, electric vehicles, advanced functional materials, new energy storage, and conversion devices. The processability as well as the mechanical properties of aluminum alloys can be improved via the addition of trace scandium. The ultrasonically assisted molten salt electrolysis is a promising, short technical route for large-scale preparation of low-cost, Al–Sc-based alloys characterized by uniform and fine strengthening phases. At present, it is still unclear if that is the case for the ultrasonic refining mechanism of the Sc-bearing ternary phase. This study aims at clarifying the ultrasonic refining mechanism on the strengthening phase containing scandium. Two Al–Sc based alloys were prepared using ultrasonically assisted molten salt electrolysis while the effect of ultrasound on the morphology and size of the Sc-bearing ternary phase was studied using optical microscope, scanning electron microscope, and X-ray diffraction meter. The results show that the synergetic ultrasound facilitates the transformation of the ternary AlSiSc phase from the coarse rhombic tubes (~205 μm) to the short rod (40 μm). The cluster size of ternary AlCuSc phase is also greatly reduced from ~100 μm to ~30 μm. The ultrasonic refining mechanism is mainly related to the increase of the nucleation rate of the primary Al3Sc particles which are greatly refined and dispersed in the alloy melt before the solidification stage. The refinement of the Sc-bearing ternary phase is considered to be caused by the fine and disperse Al3Sc particles serving as nuclei. Furthermore, ultrasound can also aid the uniform distribution of solute field and prevent the precipitation of coarse Al3Sc phase. The effect of ultrasonic refinement of the ternary rhenium-containing phase is mainly present at the solidification stage after electrolysis.

Enhancement of chip breakability of aluminium alloys by controlling the solidification during direct chill casting

Refinement and uniform dispersion of the free machining elements within the matrix is always desirable for aluminium billets produced by direct chill (DC) casting. The conventional grain refiner addition cannot ensure uniformity of microstructure and homogeneous distribution of insoluble or intermetallic phases. Melt conditioned direct-chill (MC-DC) casting is a key technology for producing uniform microstructure and refinement of secondary phases. This physical approach combines conventional vertical direct chill (DC) casting with a high shear device directly immersed in the sump for in situ microstructural control. Increased heat extraction rate due to forced convection, a larger temperature gradient at the solidification front and a shallower sump contribute to fine equiaxed dendritic grains with fine dendritic arm spacing that favor the fine and uniform distribution of second phase particles in the as-cast billet. This trend of the microstructural features was preserved even after thermomechanical downstream processing, giving rise to much improved machinability for the extruded solid bars.

Adult Dermal Stem Cells for Scaffold-Free Cartilage Tissue Engineering: Exploration of Strategies.

Dermis-isolated adult stem (DIAS) cells, abundantly available, are attractive for regenerative medicine. Strategies have been devised to isolate and to chondroinduce DIAS cells from various animals. This study aimed to characterize DIAS cells from human abdominal skin (human dermis-isolated adult stem [hDIAS] cells) and to compare and to refine various chondroinduction regimens to form functional neocartilage constructs. The stemness of hDIAS cells was verified (Phase I), three chondroinduction pretreatments were compared (Phase II), and, from these, one regimen was carried forward for refinement in Phase III for improving the mechanical properties of hDIAS cell-derived constructs. Multilineage differentiation and mesenchymal stem cell markers were observed. Among various chondroinduction pretreatments, the nodule formation pretreatment yielded constructs at least 72% larger in diameter, with higher glycosaminoglycan (GAG) content by 44%, compared with other pretreatments. Furthermore, it was found that culturing cells on nontissue culture-treated surfaces yielded constructs (1) on par with constructs derived from aggrecan-coated surfaces and (2) with superior mechanical properties than constructs derived from cells cultured on tissue culture-treated surfaces. After the nodule formation pretreatment, combined supplementation of TGF-β1, IGF-I, and fetal bovine serum significantly enhanced aggregate modulus and shear modulus by 75% and 69%, respectively, over the supplementation by TGF-β1 alone. In summary, human skin-derived DIAS cells are responsive to chondroinduction for forming neocartilage. Furthermore, the mechanical properties of the resultant human constructs can be improved by treatments shown to be efficacious in animal models. Advances made toward tissue-engineering cartilage using animal cells were shown to be applicable to hDIAS cells for cartilage repair and regeneration.

Distinction in electrochemical behaviour of Ti6Al4V alloy produced by direct energy deposition and forging

The relationship between microstructural characteristics and anodic dissolution behaviour plays a crucial role in the process control of the electrochemical machining technology (ECM). An additive-manufactured Ti6Al4V alloy produced by direct energy deposition (DED) exhibited a finer lamellar α+β structure within the considerably coarser columnar prior-β grains as compared to the traditional forged Ti6Al4V alloy consisting of equiaxed α and transformed β phases. The electrochemical anodic behaviour of DED-produced Ti6Al4V alloy was investigated in a 15-wt.% NaCl solution and compared with that of the forged Ti6Al4V alloy. Electrochemical measurements showed that the DED-produced Ti6Al4V exhibited higher resistance to corrosion and lower electrochemical machinability than did the forged Ti6Al4V alloy. The high phase boundary density, originating from the refinement of constituent phases, increased the stability of the passive film formed on the surface of the DED-produced Ti6Al4V, thereby improving the corrosion resistance, while the high fraction of β phase in the Ti6Al4V deposit decreased the rate of anodic dissolution. The lower composition difference of solutes of DED-produced Ti6Al4V was beneficial to its higher resistance to corrosion. The fine and lath-type α phases led to a competitively high level of microscopic flatness of the Ti6Al4V alloy after the electrochemical dissolution.

Columnar grains formation and mechanical properties improvement of Ti44Al alloy by cyclic directional heat treatment

Ti44Al alloys were conducted cyclic directional heat treatment (DHT) for 1, 4 and 6 cycles at 1750 K to obtain directional microstructure and research their microstructure evolution. Microstructure was observed and mechanical properties was tested. Results show that grains of all treated Ti44Al alloys grow up obviously, the width of grains increases and the number of grains decreases with increasing of cycles. More columnar grains are obtained and spacing of lamellar γ and α2 phases decreases to 0.60 μm approximately after cyclic DHT for 4 cycles. Refinement of lamellar phases is caused by increasing of nucleation sites for γ phases. And it is noted that the spacing of lamellar phases is homogenized after DHT, compared with that of as-cast Ti44Al alloy. In addition, there are about 80% columnar grains with small angles (less 60°) between their growth directions and orientation of lamellar phases after DHT. Micro-hardness of Ti44Al alloys increases firstly and then decreases with increasing of cycles. Micro-hardness value is the highest about 312.9 HV for Ti44Al alloy after 4 cycles of DHT, which increases by 28% compared with that of as-cast alloy (244.8 HV), due to thin and uniform lamellar phases. Compression strength of Ti44Al alloy also increases from 900 MPa to 1200 MPa after DHT, due to directional microstructure and jagged grain boundary.

Tribological behavior of friction stir processed SiP/ZA40 in-situ composites

Abstract The effect of friction stir processing (FSP) at different rotation speeds (400, 630, 800, and 1000 r/min) and traverse speeds (25 and 50 mm/min) on the tribological properties of a Si particle reinforced Zn−40Al−2Cu-based in-situ composite was investigated. After preliminary optimization, 800 r/min and 25 mm/min were selected as optimum FSP parameters. According to the results, multi-pass FSP improved the tribological properties. For instance, at an applied pressure of 0.75 MPa, the wear rate and average coefficient of friction (COF) of four-pass FSPed composite were lower than those of base composite by 53% and 50%, respectively. SEM examinations of worn surfaces, wear debris, and worn subsurfaces revealed that the intensive refinement and uniform distribution of microstructural phases, especially the coarse Si particles, reduced Si particles interspacing, and elimination of casting defects were the most important factors enhancing the substrate resistance against sliding-induced deformation. This led to the formation of stable tribolayers that improved the tribological properties.

Mechanical Behavior of As-Cast and Extruded Mg-Si-Ni-Ca Magnesium Alloys

The mechanical performance of as-cast and hot extruded Mg-4Si-4Ni-xCa alloys was studied. In the as-cast microstructures, the dendritic primary Mg2Si particles, eutectic α-Mg + Mg2Si constituent, blocky χ-phase (Mg8Ni7Si5 intermetallic), and α-Mg phase were observed. Moreover, the needle-shaped CaMgSi intermetallic at high Ca additions was detected. The modification effect of Ca was discussed based on the size refinement and morphological enhancement of Mg2Si phase. Accordingly, the tensile properties of the as-cast alloys were enhanced by Ca addition no more than 1 wt.%. However, large additions (3 wt.%) resulted in the deterioration of strength and ductility due to the appearance of the needle-shaped CaMgSi intermetallic. Compared to the cast ingots, the wrought alloys (obtained by hot extrusion) showed superior mechanical performance with much larger work-hardening exponents and tensile toughness values. For instance, the tensile toughness of the extruded alloy with 1 wt.% Ca was determined as 78.65 MJ/m3, which is much higher than the value of 9.32 MJ/m3 for the as-cast 0 wt.% Ca alloy. As the best combination of tensile properties, the ultimate tensile strength and the total elongation of the extruded alloy with 1 wt.% Ca were 348.4 MPa and 16.6%, respectively.

Ultra high temperature high-entropy borides: Effect of graphite addition on oxides removal and densification behaviour

The introduction of 0.5–1.0 wt.% graphite to the powders prepared by Self-propagating High-temperature Synthesis (SHS) is found to be highly beneficial for the removal of oxide impurities (from 2.7-8.8 wt.% to 0.2–0.5 wt.%) during spark plasma sintering (1950°C/20 min, 20 MPa) of (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 and (Hf0.2Mo0.2Ta0.2Zr0.2Ti0.2)B2 ceramics. Concurrently, the consolidation level achieved is enhanced from about 92.5% and 88%, respectively, to values exceeding 97%. While a further increase of graphite slightly improves samples densification, final products become progressively richer of the unreacted carbon.It is assumed that graphite plays a double role during SPS, e.g. not only as a reactant during the carbothermal reduction of oxides contaminant, but also as lubricating agent for the powder particles. The latter phenomenon is likely the main responsible for the densification improvement when 3 wt.% or larger amounts of additive are used. Another positive effect is the crystallite size refinement of the high-entropy phases with the progressive abatement of oxides, to confirm that their presence promotes grain coarsening during the sintering process.

Nano-crystallization behavior and magnetic domain evolution in commercial Fe–Si–B metallic glass

Soft ferromagnetic properties of nano-crystalline metallic glass considerably depend on microstructure and its magnetic domain structure. Here, we investigated the microstructure evolution and its impact on magnetic domain structure, coercivity (Hc), and saturation flux density (Bs) by manipulating nano-crystallization behavior of Fe–Si–B commercial metallic glass. We revealed that mean grain size of system is affected by both annealing temperature and resulting crystalline phases owing to the competition between grain growth of α-Fe(Si) matrix phase and grain refinement by secondary phase Fe2B formation. As a result, the changed grain sizes played a significant role in deciding the Hc and Bs. However, Hc of nano-crystalline structure increased than that of amorphous state. Our findings will give insight to producing soft ferromagnetic metallic glass by controlling microstructure and domain structure.

Refinement of the Low-Temperature Phase with Nano SnO Doping in Ba-Ca-Cu-O Ceramics

The SnO nano dopant effects on to the low-temperature phase have studied by the selected composition of Ba2Ca3Cu6-xSnxOy (x = 0.0; 0.5; 1.0; 1.5; 2.0). The samples produced by Melt-Growth Method (MGM). As well-known, the BaCaCuO superconductor has two phases; one of them occurs in the high superconducting transition temperature at 126 K, but this phase is unstable and requires high pressure. Because of these unfavorable conditions, Ba-Ca-Cu-O superconductors out of practically be an application to technology. The other is the low-temperature phase, in which superconducting transition temperature is nearly at 90-95 K. In this work, we focused on investigating the low transition temperature phase. The phase has a Tetragonal crystal structure with a P4 / mmm symmetry and lattice constants a = 6.517 Å and c = 8.410 Å. The effects investigated by the differential scanning calorimetry, energy dispersive X-ray, scanning electron microscope, and X-ray powder diffraction method results. Structural analysis has shown that the matrix has infiltrated by the use of the SnO as a dopant. Namely, the SnO doping has resulted as the low-temperature phase has a growing percentage of 73.875% within the volume fraction of the matrix.

Mechanical behavior of in-situ alloyed Ti6Al4V(ELI)-3 at.% Cu lattice structures manufactured by laser powder bed fusion and designed for implant applications

In the present study, cellular lattice structures for implant applications are reported for the first-time incorporating copper directly by in-situ alloying in the laser powder bed fusion process. The aim to incorporate 3 at.% Cu into Ti6Al4V(ELI) is selected for improved antibacterial properties while maintaining appropriate mechanical properties. Previously, topologically optimized Ti6Al4V(ELI) lattice structures were successfully designed, manufactured and studied for implant applications. The development of a new alloy produced by in-situ alloying of elemental powder mixture of Ti6Al4V(ELI) and pure Cu powders was used here for the production of identical lattice structures with improved antibacterial properties. One of the same as-designed CAD models was used for the manufacturing of these lattices compared to previous work on pure Ti6Al4V(ELI) lattices, making direct comparison of mechanical properties possible. Similar manufacturability highlights the applicability of this alloying technique to other lattice designs. Microstructural characterization was performed by optical and electron microscopies, as well as microCT. Mechanical characterization was performed by means of compression tests and hardness measurements. Results showed that in-situ alloying with copper leads to the formation of localized Cu-rich regions, refinement of martensitic phase and the formation of CuTi2 intermetallic precipitates, which increased the hardness and strength of the material. Deviations in wall thickness between the as-designed and as-manufactured lattices led to anisotropy of the mechanical properties of the lattices. Higher compressive strength values were obtained when thicker walls were oriented along the loading direction. Nevertheless, alloying with Cu had a higher impact on the compressive strength of lattice structure than the wall thickness deviations. The direct in-situ alloying of copper in Ti6Al4V(ELI) is a promising route for direct manufacturing of antibacterial implants.

Effect of friction stir processing on microstructure and properties of AZ91 magnesium alloy CMT cladding layer

AZ91 magnesium alloy cladding layer obtained by cold metal transfer technique was subjected to friction stir processing (FSP). The results showed that, after FSP, the grains of the deposited layer were significantly refined, and the coarse β-Mg17Al12 phase was broken and dispersed into the α-Mg matrix, leading to the microstructure modification. Compared with the unprocessed cladding layer, the FSPed samples exhibited better corrosion resistance. Due to the grain refinement and disintegration of the second phase, the tensile strength and elongation of the cladding layer after FSP increased from 233.4 MPa and 5.6% to 278.5 MPa and 13.4%, respectively.

Stabilized asynchronous fast adaptive composite multigrid using additive damping

SummaryMultigrid solvers face multiple challenges on parallel computers. Two fundamental ones read as follows: Multiplicative solvers issue coarse grid solves which exhibit low concurrency and many multigrid implementations suffer from an expensive coarse grid identification phase plus adaptive mesh refinement overhead. We propose a new additive multigrid variant for spacetrees, that is, meshes as they are constructed from octrees and quadtrees: It is an additive scheme, that is, all multigrid resolution levels are updated concurrently. This ensures a high concurrency level, while the transfer operators between the mesh levels can still be constructed algebraically. The novel flavor of the additive scheme is an augmentation of the solver with an additive, auxiliary damping parameter per grid level per vertex that is in turn constructed through the next coarser level—an idea which utilizes smoothed aggregation principles or the motivation behind AFACx: Per level, we solve an additional equation whose purpose is to damp too aggressive solution updates per vertex which would otherwise, in combination with all the other levels, yield an overcorrection and, eventually, oscillations. This additional equation is constructed additively as well, that is, is once more solved concurrently to all other equations. This yields improved stability, closer to what is seen with multiplicative schemes, while pipelining techniques help us to write down the additive solver with single‐touch semantics for dynamically adaptive meshes.