Showing papers on "Grain growth published in 2013"

Abnormal Grain Growth Induced by Cyclic Heat Treatment

Abstract: In polycrystalline materials, grain growth occurs at elevated temperatures to reduce the total area of grain boundaries with high energy. The grain growth rate usually slows down with annealing time, making it hard to obtain grains larger than a millimeter in size. We report a crystal growth method that employs only a cyclic heat treatment to obtain a single crystal of more than several centimeters in a copper-based shape-memory alloy. This abnormal grain growth phenomenon results from the formation of a subgrain structure introduced through phase transformation. These findings provide a method of fabricating a single-crystal or large-grain structure important for shape-memory properties, magnetic properties, and creep properties, among others.

Estimation of grain boundary segregation enthalpy and its role in stable nanocrystalline alloy design

Abstract: Grain boundary segregation provides a method for stabilization of nanocrystalline metals—an alloying element that will segregate to the boundaries can lower the grain boundary energy, attenuating the driving force for grain growth. The segregation strength relative to the mixing enthalpy of a binary system determines the propensity for segregation stabilization. This relationship has been codified for the design space of positive enthalpy alloys; unfortunately, quantitative values for the grain boundary segregation enthalpy exist in only very few material systems, hampering the prospect of nanocrystalline alloy design. Here we present a Miedema-type model for estimation of grain boundary segregation enthalpy, with which potential nanocrystalline phase-forming alloys can be rapidly screened. Calculations of the necessary enthalpies are made for ∼2500 alloys and used to make predictions about nanocrystalline stability.

Shear band-related recrystallization and grain growth in two rolled magnesium-rare earth alloys

Abstract: Two binary alloys, Mg–1Ce and Mg–1Gd (wt%), were subjected to large strain rolling and different annealing treatments. The onset of recrystallization in the shear banded microstructure was delayed to 300 °C in both alloys. Mg–1Ce showed virtually no potential for rolling texture modification during annealing at various temperatures, retaining the deformation texture, whereas Mg–1Gd revealed important deformation/recrystallization texture transition producing significant texture modification. This was attributed to favorable growth behavior. Correlations between deformation, annealing conditions and material composition were established.

What determines the grain size distribution in galaxies

Abstract: Dust in galaxies forms and evolves by various processes, and these dust processes change the grain size distribution and amount of dust in the interstellar medium (ISM). We construct a dust evolution model taking into account the grain size distribution, and investigate what kind of dust processes determine the grain size distribution at each stage of galaxy evolution. In addition to the dust production by Type II supernovae (SNe II) and asymptotic giant branch (AGB) stars, we consider three processes in the ISM: (i) dust destruction by SN shocks, (ii) metal accretion on to the surface of pre-existing grains in the cold neutral medium (CNM; called grain growth) and (iii) grain–grain collisions (shattering and coagulation) in the warm neutral medium and CNM. We found that the grain size distribution in galaxies is controlled by stellar sources in the early stage of galaxy evolution, and that afterwards the main processes that govern the size distribution changes to those in the ISM, and this change occurs at earlier stage of galaxy evolution for a shorter star formation time-scale (for star formation time-scales = 0.5, 5 and 50 Gyr, the change occurs about galactic aget ∼ 0.6, 2 and 5 Gyr, respectively). If we only take into account the processes which directly affect the total dust mass (dust production by SNe II and AGB stars, dust destruction by SN shocks and grain growth), the grain size distribution is biased to large grains (a ∼ 0.2–0.5 μm, where a is the grain radius). Therefore, shattering is crucial to produce small (a 0.01 μm) grains. Since shattering produces a large abundance of small grains (consequently, the surface-to-volume ratio of grains increases), it enhances the efficiency of grain growth, contributing to the significant increase of the total dust mass. Grain growth creates a large bump in the grain size distribution around a ∼ 0.01 μm. Coagulation occurs effectively after the number of small grains is enhanced by shattering, and the grain size distribution is deformed to have a bump at a ∼ 0.03–0.05 μ ma tt ∼ 10 Gyr. We conclude that the evolutions of the total dust mass and the grain size distribution in galaxies are closely related to each other, and the grain size distribution changes considerably through the galaxy evolution because the dominant dust processes which regulate the grain size distribution change.

Competitive influence of grain size and crystallinity on gas sensing performances of ZnO nanofibers

Abstract: In the present study, we investigated the effect of grain size and crystallinity on the gas sensing performances of ZnO nanofibers. The grain sizes in electrospun ZnO nanofibers ranged from 20 to 80 nm in diameter for different calcination temperatures. The sensing abilities of ZnO nanofibers with different grain sizes were investigated in terms of detection of CO. In addition to the size, the crystallinity of the grains in each nanofiber influenced its sensing performances synergistically. Two domains of influence, i.e., (1) the crystallinity dominant domain and (2) the grain size dominant domain, existed. In the crystallinity-dominant domain, in which lower calcination temperatures were used, the enhancement of crystallinity overcame the adverse effect of grain growth. In contrast, at higher calcination temperatures, the adverse effect of grain growth was intensified because the enhancement of crystallinity stagnated in the grain size-dominant domain. These results strongly suggest that the simultaneous optimization of the size and crystallinity of the grains is essential to maximize the sensing abilities of oxide nanofibers.

Solar water splitting: preserving the beneficial small feature size in porous α-Fe2O3 photoelectrodes during annealing

Abstract: We prepared mesoporous α-Fe2O3/FTO electrodes by a simple anodic precipitation and annealing method. The classic problem is that when annealed in air at >500 °C, the individual particle size increases from ≈20 nm to 40–60 nm. This grain growth is highly detrimental for the water splitting efficiency. In the present work we show that grain coarsening can efficiently be prevented by a two-step annealing method in Ar, leading to photocurrent densities for such electrodes (obtained in 1 M KOH solution) of up to 2.6 mA cm−2 at 1.6 V vs. RHE under AM 1.5 100 mW cm−2 illumination, which is significantly higher than 1.5 mA cm−2 for conventionally annealed samples.

Microstructure, ferroelectric and piezoelectric properties of Bi0.5K0.5TiO3-modified BiFeO3–BaTiO3 lead-free ceramics with high Curie temperature

Abstract: The effects of composition, sintering temperature and dwell time on the microstructure and electrical properties of (0.75 − x)BiFeO3–0.25BaTiO3–xBi0.5K0.5TiO3 + 1 mol% MnO2 ceramics were studied. The ceramics sintered at 1000 °C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and pseudocubic phases is formed at x = 0.025. The addition of Bi0.5K0.5TiO3 retards the grain growth and induces two dielectric anomalies at high temperatures (T1 ∼ 450–550 °C and T2 ∼ 700 °C, respectively). After the addition of 2.5 mol% Bi0.5K0.5TiO3, the ferroelectric and piezoelectric properties of the ceramics are improved and very high Curie temperature of 708 °C is obtained. Sintering temperature has an important influence on the microstructure and electrical properties of the ceramics. Critical sintering temperature is 970 °C. For the ceramic with x = 0.025 sintered at/above 970 °C, large grains, good densification, high resistivity and enhanced electrical properties are obtained. The weak dependences of microstructure and electrical properties on dwell time are observed for the ceramic with x = 0.025.

Effects of extrusion speed on the microstructure and mechanical properties of ZK60 alloys with and without 1 wt% cerium addition

Abstract: The effects of extrusion speed on the microstructure and tensile properties of the ZK60 and ZK60-1Ce alloys were investigated by performing indirect extrusion at three ram speeds (0.3, 1.0 and 3.0 mm/s). All of the extruded alloys showed a bimodal microstructure consisting of equiaxed fine recrystallized (DRXed) grains and elongated coarse unDRXed grains. With increasing extrusion speed, the exit temperature increased due to deformation heating, resulting in a larger grain and a higher DRXed fraction. The yield and ultimate tensile strengths and elongation at RT decreased with an increase of extrusion speed. The ZK60-1Ce alloys exhibited a finer grain size, a higher DRXed fraction, and weaker texture intensity than the ZK60 alloys at the same extrusion speed due to the inhibition of grain growth by the pinning effect and the promotion of DRX by particle-stimulated nucleation. The yield and ultimate tensile strengths at room and elevated temperatures were increased by the addition of Ce, while elongation was decreased due to cracking at the Mg–Zn–Ce particles.

The effect of rare earth elements on the behaviour of magnesium-based alloys: part 2 - recrystallisation and texture development

Abstract: The recrystallisation behaviour of two hot deformed magnesium alloys, AZ31 and Mg–1.5Gd, have been compared with particular emphasis on texture development. The alloy Mg–Gd, which contains the rare earth element gadolinium, statically recrystallised significantly faster than the conventional alloy AZ31. Despite faster recrystallisation kinetics, Mg–Gd developed the weaker recrystallisation texture associated with the addition of rare earth elements. It has been concluded that the weakening of recrystallisation texture that results from the addition of RE elements is not intrinsically linked with a reduction in the rate of recrystallisation. It is proposed that solute partitioning of RE elements to dislocations during deformation creates a microstructure from which weaker recrystallisation textures are favoured.

The stabilization of nanocrystalline copper by zirconium

Abstract: Alloys of copper (Cu) and zirconium (Zr) were generated by mechanical alloying via cryogenic, high-energy ball milling and then annealed to a maximum temperature of 1000 °C. The addition of only 1 at% Zr to Cu was found effective at stabilizing the grains in the nanocrystalline state to homologous temperatures in excess of 0.85. When Zr was added in concentrations of 2 and 5 at%, the alloys underwent substantial hardening during annealing, but grain size stability was not enhanced. The mechanism of grain size stabilization was investigated using thermodynamic and kinetic modeling. Zr is predicted to significantly reduce the grain boundary energy of Cu via segregation, but simplifications in the thermodynamic model do not capture high temperature behavior. Kinetically, good correlation between calculation and experimental observation was found by applying estimations for the limiting grain size and Orowan strengthening via second-phase pinning. Both thermodynamic and kinetic mechanisms may be active during annealing, but kinetic parameters appear to be sufficient in explaining the excellent stability of nanocrystalline Cu, even at low Zr concentration.

High strain rate superplasticity in friction stir processed ultrafine grained Mg–Al–Zn alloys

Abstract: Friction stir processing of a rolled AZ31 alloy and a high pressure die cast AZ91 alloy produced ultrafine grained materials with average grain size of 0.8 μm and 0.5 μm, respectively. Tensile testing was done in the temperature range of 210–360 °C and strain rate range of 1×10 −4 –3×10 −2 s −1 . Higher amounts of β-precipitates in AZ91 thermally stabilized the microstructure up to 330 °C. Friction processed ultrafine grain AZ91 exhibited high strain rate superplasticity at all tested temperatures. The best elongation of 1251% was achieved at a strain rate of 1×10 −2 s −1 and temperature of 330 °C. The kinetic analysis of the superplastic data revealed a large variation in the kinetic parameter due to difficulty in using proper grain size values because of concurrent grain growth.

Fabrication of transparent electro-optic (K0.5Na0.5)1−xLixNb1−xBixO3 lead-free ceramics

Abstract: Lead-free transparent electro-optic ceramics (K 0.5 Na 0.5 ) 1− x Li x Nb 1− x Bi x O 3 have been fabricated by hot-press sintering. Owing to the effective suppression of grain growth, the Li and Bi co-modified ceramics generally possess a dense and fine-grained structure. The co-modification also causes the ceramics to transform into a nearly cubic structure with minimal optical anisotropy. A diffuse phase transformation is also induced, causing the ceramics to become more relaxor-like and contain more polar nano-regions. These would reduce the light scattering by the grains, at the grain boundaries and at the domain walls, respectively, and thus making the ceramics become optically transparent. For the ceramic modified with 5 mol% Li + and Bi 5+ , the optical transmittance reaches a high value of 60% in the near-IR region. The ceramics also exhibit a strong linear EO response, giving a large effective linear EO coefficient in the range of 120–200 pm/V.