Nanoscale Lamellar Structure Research Articles

Phase alteration in Zr, Nb, Mo-doped Fe2Ni2CrV0.5 high-entropy alloy for mechanical performance regulation: A parallel comparison of tribological behavior against GCr15 counterpart

The microstructure, microhardness, and the tribological behavior of Zr, Nb and Mo-doped Fe2Ni2CrV0.5 high-entropy alloy (HEA) coatings prepared by laser cladding were investigated and compared in parallel. The Nb-doped coating with 86 % volume fraction Fe2Nb Laves phase containing only three slip systems exhibits superior tribological properties than Zr-doped coating which has 78 % proportion of FCC phase. The Nb-doped coating with microhardness of 745 HV is invulnerable to wear loss due to triple constraint effect of dislocation movement, while the Zr-doped coating is susceptible to severe delamination wear owing to the lower microhardness (287 HV) compared to the GCr15 counterpart (650 HV). The Mo-doped coating with the microhardness of 585 HV presents the best wear resistance, which is attributed to the strength-toughness synergistic effect caused by σ phase and alternating arrangement eutectic nanoscale lamellar structure (FCC/σ). Relatively large microhardness difference leads to the abrasive chips adhesion phenomenon between the coatings and the GCr15 counterpart, except for the friction pair of Mo-doped coating/GCr15.

Effect of warm-rolling on microstructure and superior mechanical properties of a cost-effective AlCrFe2Ni2 high entropy alloy

The possibility of tailoring microstructure and tensile properties of a cobalt-free cost-effective (FCC+B2) AlCrFe2Ni2 high entropy alloys (HEA) was explored in this work. For this purpose, the HEA was warm-rolled at 400 °C, 600 °C, and 750 °C to 90% reduction in thickness and further annealed at different temperatures. The increasing warm-rolling reduction resulted in progressive alignment and microstructural refinement. The HEA warm-rolled at 400 °C revealed a continuous nanoscale lamellar structure as opposed to disrupted and segmented lamellae in the HEA warm-rolled at 600 °C and 750 °C. The BCC fraction increased remarkably after warm-rolling at different temperatures due to the increased stability of the BCC phase. Annealing resulted in transformation to microduplex structures and restoration of the FCC fraction. However, the HEA warm-rolled at 400 °C and annealed at 800 °C showed the development of a heterogeneous microstructure with retained lamellar and partially transformed microduplex regions. The heterogeneous lamellar microstructure resulted in simultaneous improvement in strength and ductility with a yield strength of ∼ 921 MPa and elongation of ∼23%. Thus, the HEA outperformed other cobalt-free HEAs from an integrated cost-performance comparison. Microstructural analysis of the broken tensile specimen indicated that the soft domains were considerably strain-hardened in the presence of the hard domains, leading to synergistic improvement in strength and ductility in the heterostructured HEA.

Laser-clad Al-Ti-Zr and Cr-Ti-Zr coatings on Zr alloy: composition-induced microstructural and hardness differences

In this study, Al-Ti-Zr and Cr-Ti-Zr metallic coatings with thichness of 250-280 μm were fabricated successfully on Zr alloy using pulsed laser cladding. By jointly employing multiple characterization techniques, microstructural characteristics of the coatings were probed with their formation reasons explored. The Al-Ti-Zr coating consists of a single BCC solid-solution phase (average size 8.6 ± 6.3 μm) with hardness of 405.5 ± 16.6 HV that is more than twice that of the substrate (198.7 ± 2.0 HV). Such marked hardening is mainly related to the contribution from the solid-solution strengthening produced by Al and Ti. The Cr-Ti-Zr coating consists of cellular BCC phase (average size 0.9 ± 0.4 μm) and C14-type Cr2(Ti,Zr) Laves phase with nanoscale lamellar structures. Its average hardness is 584.8 ± 33.6 HV, about three times of that of the substrate, which can be ascribed to synergistic effects of solid-solution, grain refinement and second phase strengthening. The formation of the single BCC solid-solution phase in the Al-Ti-Zr coating is mainly due to the relatively small differences in atomic size and electronegativity between Al, Ti and Zr. For the Cr-Ti-Zr coating, however, due to large atomic size and electronegativity differences between Cr and Zr (as well as Cr and Ti), Cr2(Ti,Zr) Laves phase was formed in addition to the BCC phase.

Microstructure and reactivity of cryomilled Al-Ni energetic material with nanoscale lamellar structure

Microstructure and reactivity of cryomilled Al-Ni energetic material with nanoscale lamellar structure

Strengthening the Mg–Y–Zn alloy through the formation of nanoscale lamellar structures and nanograins

Extruded Mg–3.16Y–1.87Zn (wt%) alloy bars containing coarse block-shaped phases and fine needle-shaped phases were prepared via cold rotary swaging in this study. The microstructure evolution during deformation was investigated using scanning and transmission electron microscopy. The results show that little change was observed in the block-shaped phases in the swaged samples, while significant curving and kinking was observed in the needle-shaped phases. After swaging, nanoscale lamellar structures containing basal stacking faults and adjacent α-Mg nanoslices were formed in the area with needle-shaped phases, showing randomly distributed orientation between adjacent α-Mg nanoslices. Nanograins were formed in the area without any phases. The formation of nanoscale lamellar structures and nanograins contributed to the remarkable strengthening of the swaged alloy bars. Moreover, the extruded samples with a smaller spacing of the stacking faults and a higher fraction of the needle-shaped area show a higher strengthening effect after swaging.

Controllable surface morphology transition from inter-connected pores to flower-like structures for super-hydrophobic poly (L-lactic acid) films

Abstract The Poly(L-lactic acid) (PLLA) films with polytropic surface morphology were fabricated by a modified solvent evaporation-induced precipitation method. In this novel method, the diffusion rate between solvent and non-solvent is mainly driven by the volatilization of the solvent and can be effectively controlled by covering the solvent with a non-solvent layer. In addition, inspired by the general Breath-Figure self-assembly, the controllable humidity is introduced to the system to adjust the solubility parameters of non-solvents, which avoids the instantaneous phase separation induced by direct coagulation of water droplets. The surface morphology realizes the transformation from inter-connected pores to flower-like structures by controlling polymer solution concentration and humidity. The surface water contact angle can reach to 152.1° with the combination of micro-scale flowers and nano-scale lamellar structures. Furthermore, the porosity of the prepared PLLA films can increase up to 87.5%, which greatly exceeds the 60% porosity of PLLA foam prepared by the non-solvent induced phase separation. Thus, the resultant film exhibits an excellent oil-absorption capacity even up to 17.88 g/g, which makes it has great application potential in the field of oil-water separation.

Low-temperature preparation of h-ZrB2 crystallites in the air

A simple molten salt process was proposed for preparing pure-phase h-ZrB2 crystallites without using any protective atmosphere at 800 °C. Here, molten salt KCl can estabulish a homogeneous liquid-phase reaction environment which not only greatly lowers the synthesis temperature by accelerating the diffusing speed of reactants and promoting chemical reaction, but also prevents h-ZrB2 crystallites from oxidation by isolating air. Products have rose-like particle morphology with nano-scale lamellar structure. The formation of h-ZrB2 crystallites and their dependence on key processing parameters were also investigated. This approach is available for the large-scale and low-cost preparation of h-ZrB2 powders due to low preparing temperature, no requirement for protective atmosphere, and cheap and recyclable KCl.

Recovery kinetics in Al-0.3%Cu after heavy rolling

The recovery kinetics of a nanoscale lamellar structure Al-0.3%Cu alloy, which was produced by 98% cold rolling, during annealing at temperatures from 100 to 200 °C have been analyzed in this study. Microstructural characterization using electron channelling contrast in the scanning electron microscope shows a uniform coarsening of the lamellar structure and gradual precipitation of second phase particles (Al2Cu) during annealing at these temperatures. Fitting of the microhardness values for samples annealed for between 4 and 4096 minutes at different temperatures with a recovery model reveals a change, in the apparent activation energy from 79 kJ/mol at the beginning to 105 kJ/mol at the end of recovery annealing, which leads to a discussion of the recovery mechanisms.

Low temperature synthesis of titanium diboride nanosheets by molten salt–assisted borothermal reduction of TiO2

TiB2 nanosheets were synthesized by the borothermal reduction of TiO2 in flowing argon atmosphere with the assistance of MgCl2. Phase transformation was studied by XRD and micromorphology of products was studied by SEM and TEM. In the experiment, the prepared TiB2 formed the nanoscale lamellar structure. These lamellas agglomerated together to be a net-like structure. Effects of reduction temperature, addition of MgCl2, and its amount were discussed in detail. The results indicated that the presence of MgCl2 accelerated the mass transfer of reactant. Moreover, the co-effects of MgCl2 and B2O3 contributed to the formation of TiB2 nanosheets. TiB2 could dissolve in MgCl2 and B2O3 at high temperature and then recrystallized during the cooling process. The products with a specific surface area of 99.32 m2 g−1 were obtained at 1373 K after reacting for 4 h with a MgCl2:TiO2 molar ratio of 1:1.

Long-Period Superlattices of γ' Phase in Ni-Based Superalloy

Nanoscale lamellar structure was discovered in L1₂-type γ'-Ni₃(Al, Ta) precipitate. This structure was characterized by TEM phase contrast of antiphase boundary (APB) composed of aligned {110} planes. Lamellar spacing corresponds to APB period. The strictly periodical APB formation can be attributed to the formation of long-period superlattices (LPSs), and period of LPSs is ten times larger than that in conventional LPSs. The zigzag lamellar shape after tensile test suggested that the additional external force is required for deformation, and strength could be improved by LPSs structure in the γ' phase.

Effect of plasma immersion ion implantation on polycaprolactone with various molecular weights and crystallinity.

Polycaprolactone with five different molecular weights was spin-coated on silicon wafers and plasma immersion ion implanted (PIII) with ion fluence in the range 5 × 1014-2 × 1016 ions/cm2. The effects of PIII treatment on the optical properties, chemical structure, crystallinity, morphology, gel fraction formation and wettability were investigated. As in the case of a number of previously studied polymers, oxidation and hydrophobic recovery of the PIII treated PCL follow second order kinetics. CAPA 6250, which has the lowest molecular weight and the highest degree of crystallinity of the untreated PCL films studied, has the highest carbonization of the modified layer after PIII treatment. Untreated medical grade PCL films, mPCL PC12 (Perstorp) and mPCL OsteoporeTM have similar chemical structures and crystallinity. Accordingly, the chemical and structural transformations caused by PIII treatment and post-treatment oxidation are almost identical for these two polymers. In general, PIII treatment destroys the nano-scale lamellar structure and results in a reduction of PCL crystallinity. Examination after washing PIII treated PCL films in toluene confirmed our hypothesis that cross-linking due to PIII treatment is significantly higher in semi-crystalline PCL as compared with amorphous polymers.

Grain refinement and nanostructure formation in pure copper during cryogenic friction stir processing

Two different mechanisms in term of dislocation manipulation were identified for the microstructure evolutions in pure copper treated by cryogenic friction stir processing (CFSP) at different parameters. The final average grain size in the nuggets increased first and then decreased with increasing rotational speed. In the conditions of low rotational speed, the refined grains evolved from dislocation cells. In the conditions of high rotational speed, the refined grains transformed from the subdivision of nanoscale lamellar structures combined with the effect of dynamic recovery. The high proportion of nano grains in high rotational speed conditions should be attributed to the large strain from the strong constrain effect of “hard shell” structure around the nugget during CFSP.

Characterization of Cu Distribution in an Al-0.3%Cu Alloy Cold Rolled to 98%

In this study, the distribution of Cu element in a Al (99.9996% purity)-0.3%Cu alloy cold rolled to 98% has been characterized in detail by using three-dimensional atom probe (3DAP) and ChemiSTEM techniques. The cold rolled structure is a typical high strain lamellar structure with an average boundary spacing of 200 nm, indicating a strong role of the small amount of Cu element in stabilizing the microstructure to form the fine scale structure. A heavy segregation of Cu element in the lamellar boundaries of high angles has been observed and the Cu concentration in the boundaries can be as high as 20 times of the nominal concentration of the alloy, which is considered as the main reason for a formation of a stable nanoscale lamellar structure.

The formation mechanism of TiC reinforcement and improved tensile strength in additive manufactured Ti matrix nanocomposite

The nanoscale TiC reinforced Ti matrix nanocomposites with nearly full density of 98.2% were fabricated by selective laser melting. Owing to the unique metallurgical action of SLM, the TiC exhibited two typical morphologies: nanoscale lamellar and submicron strip structures, and showed a uniform network distribution. The formation mechanism of TiC was investigated in detail. The SLM-processed Ti matrix nanocomposite possessed a significantly enhanced tensile strength of 914 MPa due to the formation and uniform dispersion of nanoscale TiC.

Multi-step heat treatment design for nano-scale lamellar structures of a cast Ti-45Al-8.5Nb-(W, B, Y) alloy

A multi-step heat treatment is proposed to optimize the lamellar structure of as-cast high Nb containing TiAl alloys. The β-segregation is removed effectively after the first two step heat treatment, a short-time holding within β single phase field then annealing in α+β phase field. During the third step heat treatment, a subsequent low-temperature aging, new γ lamellae precipitate by the means of isolated nucleation or twin-related nucleation. After the proposed multi-step heat treatment, the alloy presents nano-scaled lamellar structure with the colony size about 140 μm.

Characterization of Si particles and their effects on and recrystallization in a nanostructured cold rolled Al-1% Si alloy

An Al-1.08 vol.% Si alloy was cold rolled to a reduction of 98% (εvM = 4.5) and then annealed at different temperatures up to 210°C (0.52 Tm) for different times. The deformed structure is characterized by a nanoscale lamellar structure with the presence of Si particles of coarse (> 1 μm), medium (100 nm - 1 μm) and fine (< 100 nm) sizes in the microstructure. Deformation zones are formed around the coarse Si particles and the boundary spacing is finer in the deformation zone than in the matrix. The medium Si particles have little effect on the morphology and boundary spacing. The fine Si particles are aligned along the lamellar boundaries indicating a stabilizing effect on the structural refinement during cold rolling. After annealing, enhanced recovery occurs in the deformation zones around the coarse Si particles. However the reduction in stored energy during recovery and the pinning effect of fine Si particles on the boundary migration prevent the advantage of particle stimulated nucleation (PSN) of coarse Si particles in the nanoscale lamellar structure. This study also demonstrates an important effect of the fine particles in delaying both recovery and recrystallization processes. This effect diminishes with increasing annealing temperature and coarsening the fine particles especially at triple junctions.

Microstructure and Wear Behavior of CoCrFeMnNbNi High-Entropy Alloy Coating by TIG Cladding

Alloy cladding coatings are widely prepared on the surface of tools and machines. High-entropy alloys are potential replacements of nickel-, iron-, and cobalt-base alloys in machining due to their excellent strength and toughness. In this work, CoCrFeMnNbNi HEA coating was produced on AISI 304 steel by tungsten inert gas cladding. The microstructure and wear behavior of the cladding coating were studied by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometer, microhardness tester, pin-on-ring wear tester, and 3D confocal laser scanning microscope. The microstructure showed up as a nanoscale lamellar structure matrix which is a face-centered-cubic solid solution and niobium-rich Laves phase. The microhardness of the cladding coating is greater than the structure. The cladding coating has excellent wear resistance under the condition of dry sliding wear, and the microploughing in the worn cladding coating is shallower and finer than the worn structure, which is related to composition changes caused by forming the nanoscale lamellar structure of Laves phase.

Microstructural characterization of a new mechanically alloyed Ni-base ODS superalloy powder

The microstructure of a new Ni-base oxide dispersion strengthened superalloy powder was studied for high temperature gas turbine applications after the mechanical alloying process. In this study, an atomized powder with a composition similar to the CMSX-10 superalloy was mechanically alloyed with yttria and Hf powders. The mechanically alloyed powder included only the supersaturated solid solution γ phase without γ′ and yttria provided by severe plastic deformation, while after the 3-step aging, the γ′ phase was precipitated due to the partitioning of Al and Ta to the γ′ and Co, Cr, Re, W, and Mo to the γ phase. Mechanical alloying modified the morphology of γ′ to the new coherent γ–γ′ nanoscale lamellar structure to minimize the elastic strain energy of the precipitation, which yielded a low lattice misfit of 0.16% at high temperature. The γ′ lamellae aligned preferentially along the elastically soft [100] direction. Also, the precipitated oxide particles were refined in the γ phase by adding Hf from large incoherent YAlO3 to fine semi-coherent Y2Hf2O7 oxide particles with the average size of 7nm and low interparticle spacing of 76nm.

High strength and superconductivity in nanostructured niobium–titanium alloy by high-pressure torsion and annealing: Significance of elemental decomposition and supersaturation

A powder mixture of Nb–47wt.% Ti (a well-known composition for superconducting magnets) was subjected to severe plastic deformation using high-pressure torsion (HPT) and subsequently annealed at 573K. Ti gradually dissolved in Nb with increasing shear strain, with a fast kinetics comparable to lattice diffusion at 700–1200K. At large strains, a complete transition to a nanostructured β phase occurred at room temperature, which is far below the equilibrium temperature of 690K. Nanoclusters of Ti with a body-centered cubic structure were also detected at large strains. Subsequent annealing led to elemental decomposition, formation of a nanoscale lamellar structure and segregation of Nb at grain boundaries. Superconductivity occurred at temperatures below 9K, while the transition temperature decreased with increasing shear strain because of supersaturation of Ti in Nb and increased with annealing because of elemental decomposition. The Nb–Ti alloy after HPT exhibited hardness/strength peaks followed by softening at large strains, while hardening occurred after annealing. The maximum hardness, tensile and bending strengths were 4, 1.7 and 2.7GPa, respectively.

Fabrication of the Nanoscale Flat-Bottomed and Lamellar Structures on HOPG Surface by STM-Based Electric Lithography

The requirement for fabrication of the nanometer-scale structures has grown up recently due to the advance in the development of the nanoscale electronic-devices or bio-devices. Scanning tunneling microscope (STM)-based electric lithography is one of the potential fabrication approaches to produce nanoscale structures on a variety of materials. This study of the STM-based electric lithography intends to fabricate flat-bottomed and lamellar structures on the graphite surface, which differs from the conventionally fabricated tapered structures. The formation and the comparison of both the lamellar and tapered structures are obtained by applying distinct STM tip geometries in the STM-based electric lithography. On the basis of the experimental results, it is found that the formation of lamellar structures should be attributed to the local electrochemical reaction, while the generation of tapered structures is resulted from the dielectric breakdown in the tip-sample gap.