Showing papers on "Grain growth published in 2014"

On the Texture Formation of Selective Laser Melted Ti-6Al-4V

Abstract: Selective laser melting (SLM) has been shown to be an attractive manufacturing route for the production of α/β titanium alloys. The relationship between the SLM process parameters and the microstructure of titanium alloys has been the object of several works, but the texture formation during the SLM process has yet to be understood. In the present study, the texture formation of Ti-6Al-4V components was investigated in order to clarify which microstructural features can be tailored during the SLM process. The microstructural characterization of the as-built components was carried out using various microscopy techniques. Phase and texture analysis were carried out using backscattered electron imaging and diffraction. It was found that as-built components consist exclusively of α′ martensitic phase precipitated from prior β columnar grains. The texture of the prior β phase was reconstructed and discussed in relation to the used SLM process parameters. It was found that the β grain solidification is influenced by the laser scan strategy and that the β phase has a strong 〈100〉 texture along its grain growth direction. The α′ martensitic laths that originate from the parent β grains precipitate according to the Burgers orientation relationship. It was observed that α′ laths clusters from the same β grain have a specific misorientation that minimizes the local shape strain. Texture inheritance across successive deposited layers was also observed and discussed in relation to various variant selection mechanisms.

Annealing twin development during recrystallization and grain growth in pure nickel

Abstract: A 99.995% pure Ni sample, compressed to 25%, was annealed in a SEM chamber and changes in the density of annealing twins were monitored in situ during recrystallization and grain growth. In addition to average microstructural measurements, the evolution of individual grains was also observed. Both the average annealing twin density in the recrystallized domain and the annealing twin density per grain increased during recrystallization. The rate of increase in twin density correlates with the velocity of the recrystallization front. During grain growth, however, the average annealing twin density decreased. The in situ EBSD observations showed both the formation of new twins and the extension of existing twins during annealing. The observations reported here suggest that the existing models for annealing twin formation are incomplete.

Influence of grain size distribution on the Hall-Petch relationship of welded structural steel

Abstract: The strength of polycrystalline metals increases with a decrease in grain size according to the Hall–Petch relationship. However, heterogeneous microstructures deviate from this relationship depending on the distribution of grain sizes. This paper introduces a rule of mixtures based approach for determining the characteristic length of the microstructure for heterogeneous weld metal. The proposed grain size parameter, the volume-weighted average grain size, is measured experimentally for nine structural steel weld metals and two base materials. The weld metals are found to have a large variety of grain size distributions that are noticeably broader than those of the base material due to differences in phase contents. The results show that the volume-weighted average grain size is able to capture the influence of grain size distribution on the strength of welded structural steel. Based on the experimental results, a modified Hall–Petch relationship is formulated for the strength prediction of heterogeneous microstructures. The modified relationship is also found to be applicable to data from the literature.

Densification behaviour and microstructural development in undoped yttria prepared by flash-sintering

Abstract: Conventional sintering of undoped Y 2 O 3 requires temperatures above 1400 °C for a few hours. We show that it can be sintered nearly instantaneously to nearly full density at furnace temperature of 1133 °C under a DC applied field of 500 V/cm. At 1000 V/cm sintering occurs at 985 °C. The FLASH event, when sintering occurs abruptly, is preceded by gradually accelerated field-assisted sintering (FAST). This hybrid behaviour differs from earlier work on yttria-stabilized zirconia where all shrinkage occurred in the flash mode. The microstructure of flash-sintered specimens indicated that densification was accompanied by rapid grain growth. The single-phase nature of flash-sintered Y 2 O 3 was confirmed by high-resolution transmission electron microscopy. The non-linear rise in conductivity accompanying the flash led to Joule heating. It is postulated that densification and grain growth were enhanced by accelerated solid-state diffusion, resulting from both Joule heating and the generation of defects under the applied field.

Fundamentals of materials science: the microstructure–property relationship using metals as model systems

Abstract: Introduction.- electronic structure of the atom.- chemical bonding in solids.- crystallography.- the crystal imperfection lattice defects.- analysis of the microstructure analysis of lattice imperfections.- phase equilibria.- diffusion.- phase transformations.- recovery, recrystallization and grain growth.- mechanical strength of materials.

Recovery and Recrystallization: Phenomena, Physics, Models, Simulation

Abstract: Recovery, recrystallization and grain growth are among the most important metallurgical heat treatment processes to soften cold worked metals and design desired microstructures and textures. Specifically the reduction in grain size can be efficiently achieved by recrystallization. While plastic cold working increases the stored energy of metals, mainly through dislocation accumulation, recovery and specifically recrystallization lead to it reduction. While recovery describes the gradual re-ordering and annihilation of the stored dislocations, primary recrystallization proceeds discontinuously by the formation and motion of high angle grain boundaries which discontinuously sweep the deformation substructure. Grain growth describes the process of competitive capillary driven coarsening of the average grain size. This chapter reviews the main mechanisms, lattice defects, and driving forces associated with recovery, recrystallization and grain growth and provides an introduction to the simulation of these phenomena.

Flash Sintering of Nanocrystalline Zinc Oxide and its Influence on Microstructure and Defect Formation

Abstract: We report the sintering behavior of nanocrystalline zinc oxide under external AC electric field between 0 and 160 V/cm. In situ acquisition of density by means of laser dilatometry, evaluation of specimen temperature, real-time measurement of electric field and current help analyze this peculiar behavior. Field strength and blocking electrodes significantly affect densification and microstructure, which was evaluated in the vicinity of the flash event and for the fully sintered material. High current densities flow through the sample at high electric fields, entailing a sudden increment of the temperature estimated to several hundreds of K and an exaggerated grain growth. In contrast, low current density flows through the sample at lower electric fields, which guarantees normal grain growth and highest final density. Macroscopic photoluminescence measurements give insights into the development of the defect structure. Electric fields are expected to enhance defect mobility, explaining the high densification rates observed during the sintering process.

Microstructural Evolutions During Thermal Aging of Alloy 625: Impact of Temperature and Forming Process

Abstract: The microstructural evolutions occurring upon thermal aging of alloy 625 sheets were studied in the 823 K to 1173 K (550 °C to 900 °C) temperature range and for durations up to 2000 hours. TTT diagrams of the δ and γ″ phases were established based on high-resolution scanning electron microscopy and associated quantitative image analysis approaches. The evolutions of secondary carbide volume fraction were also characterized. It was observed that the precipitation domains of the γ″ and δ phases are, respectively, 823 K to 1023 K (550 °C to 750 °C) and 923 K to 1173 K (650 °C to 900 °C) and that the γ″ coarsening follows the LSW theory once these particles have an ellipsoidal morphology. The onset of grain growth, accompanied with an increase of the texture index, was observed at a temperature as low as 1173 K (900 °C). It results from the progressive dissolution of grain boundaries’ secondary carbides (especially M6C carbides) at this temperature, a process that favors a greater mobility of grain boundaries. It is also shown that the forming process (shear spinning), even after a relaxation heat treatment, enhances and stabilizes the precipitation of the δ phase compared to as-rolled + solution heat-treated sheets. It hence slows down the precipitation of the γ″ phase, a result that is in good agreement with a thermal aging that was performed under load (i.e., during a creep test).

Alkali-metal-enhanced grain growth in Cu2ZnSnS4 thin films

Abstract: The highest efficiency solar cells based on copper zinc tin sulfide (CZTS), a promising photovoltaic material comprised of earth abundant elements, are built on soda lime glass (SLG), a substrate which contains many impurities, including Na and K. These impurities may diffuse into CZTS films during processing and affect film structure and properties. We have investigated the effects of these impurities on the microstructure of CZTS films synthesized by ex situ sulfidation of Cu–Zn–Sn alloy films co-sputtered on SLG, Pyrex, and quartz. CZTS films synthesized on SLG were found to have significantly larger grains than films grown on the other substrates. Furthermore, we show that by including a bare additional piece of SLG in the sulfidation ampoule, the grain size of films grown on nominally impurity-free quartz increases from 100's of nm to greater than 1 μm. This demonstrates conclusively that impurities in SLG volatilize in S-containing atmospheres and incorporate into nearby CZTS films synthesized on other substrates. Impurity concentrations in these CZTS films were examined using depth profiling with time-of-flight secondary ion mass spectrometry (TOF-SIMS). Of all the impurities present in SLG, the TOF-SIMS experiments implicated Na, K, and Ca as possible elements responsible for the enhanced grain growth. To investigate the effects of these impurities individually, we introduced very small and controllable amounts of Na, K, or Ca into the sulfidation ampoule during CZTS synthesis. Impurity amounts as low as 10−6 moles of Na or 10−7 moles of K resulted in a dramatic increase in grain size, from 100's of nm to several microns, for films deposited on quartz, while Ca loading had no visible effect on the final microstructure. Based on this vapor transport mechanism, we thus demonstrate an approach for delivering precisely controlled amounts of specific impurities into CZTS films on arbitrary substrates to facilitate large-grain growth.

High-Strength Low-Alloy (HSLA) Mg–Zn–Ca Alloys with Excellent Biodegradation Performance

Abstract: This article deals with the development of fine-grained high-strength low-alloy (HSLA) magnesium alloys intended for use as biodegradable implant material. The alloys contain solely low amounts of Zn and Ca as alloying elements. We illustrate the development path starting from the high-Zn-containing ZX50 (MgZn5Ca0.25) alloy with conventional purity, to an ultrahigh-purity ZX50 modification, and further to the ultrahigh-purity Zn-lean alloy ZX10 (MgZn1Ca0.3). It is shown that alloys with high Zn-content are prone to biocorrosion in various environments, most probably because of the presence of the intermetallic phase Mg6Zn3Ca2. A reduction of the Zn content results in (Mg,Zn)2Ca phase formation. This phase is less noble than the Mg-matrix and therefore, in contrast to Mg6Zn3Ca2, does not act as cathodic site. A fine-grained microstructure is achieved by the controlled formation of fine and homogeneously distributed (Mg,Zn)2Ca precipitates, which influence dynamic recrystallization and grain growth during hot forming. Such design scheme is comparable to that of HSLA steels, where low amounts of alloying elements are intended to produce a very fine dispersion of particles to increase the material’s strength by refining the grain size. Consequently our new, ultrapure ZX10 alloy exhibits high strength (yield strength R p = 240 MPa, ultimate tensile strength R m = 255 MPa) and simultaneously high ductility (elongation to fracture A = 27%), as well as low mechanical anisotropy. Because of the anodic nature of the (Mg,Zn)2Ca particles used in the HSLA concept, the in vivo degradation in a rat femur implantation study is very slow and homogeneous without clinically observable hydrogen evolution, making the ZX10 alloy a promising material for biodegradable implants.

Hot deformation characteristic and processing map of superaustenitic stainless steel S32654

Abstract: The hot deformation behavior of superaustenitic stainless steel S32654 was studied in the temperature range of 950–1200 °C and strain rate range of 0.001–10 s −1 employing hot compression tests. The results show that peak stress increases with decreasing of temperature and increasing of strain rate. The apparent activation energy of this alloy is about 469 kJ/mol. The processing maps for hot working were developed on the basis of flow stress data and the dynamic materials model. It is found that the features of the maps obtained in the strain range of 0.2–1.0 are fundamentally similar, indicating that the strain does not have a significant influence on processing map. The maps exhibited two domains. The first domain occurs in the strain rate range of 0.01–0.4 s −1 and temperature range of 1030–1150 °C with a peak efficiency of about 49%, which is considered as the optimum window for hot working. The microstructure observations of the specimens deformed in this domain showed the full dynamic recrystallization (DRX) structure with finer and more homogeneous grain sizes. The second domain occurs at the temperatures higher than 1160 °C and strain rates lower than 0.1 s −1 with a peak efficiency of about 41%, the microstructure observations in this domain also indicated the typical DRX structure accompanied with grain growth. A instability domain occurs at temperatures below 1175 °C and strain rate above 0.1 s −1 .

Phase-transition-driven growth of compound semiconductor crystals from ordered metastable nanorods

Abstract: In polycrystalline semiconductors, grain boundaries are often sites with prevalence for electron-hole recombination and various strategies have been followed to minimize grain boundary areas. Generally, large grains or epitaxial films can be obtained at high temperatures. However, high growth temperatures limit the choice of substrate materials and can prove elusive for semiconductors comprising volatile elements such as kesterite Cu2ZnSnS4. Here we show that this limitation can be overcome by a transition of a matrix of densely packed metastable nanorods into large stable grains. Real-time analysis reveals that the grain growth is driven by a direct, isocompositional solid-state phase transition. Following this route, semiconductor films with a large-grained microstructure can be achieved within a few seconds at relatively low temperatures. Grain size as well as electrical and optical properties of the resulting films can be controlled via the heating rate. This synthesis route opens new possibilities for the fabrication of semiconductor crystals for photoelectric devices with tailored microstructures.

Single-source-precursor synthesis of dense SiC/HfCxN1−x-based ultrahigh-temperature ceramic nanocomposites

Abstract: A novel single-source precursor was synthesized by the reaction of an allyl hydrido polycarbosilane (SMP10) and tetrakis(dimethylamido)hafnium(IV) (TDMAH) for the purpose of preparing dense monolithic SiC/HfCxN1−x-based ultrahigh temperature ceramic nanocomposites. The materials obtained at different stages of the synthesis process were characterized via Fourier transform infrared (FT-IR) as well as nuclear magnetic resonance (NMR) spectroscopy. The polymer-to-ceramic transformation was investigated by means of MAS NMR and FT-IR spectroscopy as well as thermogravimetric analysis (TGA) coupled with in situ mass spectrometry. Moreover, the microstructural evolution of the synthesized SiHfCN-based ceramics annealed at different temperatures ranging from 1300 °C to 1800 °C was characterized by elemental analysis, X-ray diffraction, Raman spectroscopy and transmission electron microscopy (TEM). Based on its high temperature behavior, the amorphous SiHfCN-based ceramic powder was used to prepare monolithic SiC/HfCxN1−x-based nanocomposites using the spark plasma sintering (SPS) technique. The results showed that dense monolithic SiC/HfCxN1−x-based nanocomposites with low open porosity (0.74 vol%) can be prepared successfully from single-source precursors. The average grain size of both HfC0.83N0.17 and SiC phases was found to be less than 100 nm after SPS processing owing to a unique microstructure: HfC0.83N0.17 grains were embedded homogeneously in a β-SiC matrix and encapsulated by in situ formed carbon layers which acted as a diffusion barrier to suppress grain growth. The segregated Hf-carbonitride grains significantly influenced the electrical conductivity of the SPS processed monolithic samples. While Hf-free polymer-derived SiC showed an electrical conductivity of ca. 1.8 S cm−1, the electrical conductivity of the Hf-containing material was analyzed to be ca. 136.2 S cm−1.

An atmospheric structure equation for grain growth

Abstract: We present a method to include the evolution of the grain size and grain opacity κgr in the equations describing the structure of protoplanetary atmospheres. The key assumption of this method is that a single grain size dominates the grain size distribution at any height r. In addition to following grain growth, the method accounts for mass deposition by planetesimals and grain porosity. We illustrate this method by computation of a simplified atmosphere structure model. In agreement with previous works, grain coagulation is seen to be very efficient. The opacity drops to values much below the often-used "interstellar medium opacities" (~1 cm2 g–1) and the atmosphere structure profiles for temperature and density resemble that of the grain-free case. Deposition of planetesimals in the radiative part of the atmosphere hardly influences this outcome as the added surface is quickly coagulated away. We observe a modest dependence on the internal structure (porosity), but show that filling factors cannot become too large because of compression by gas drag.

Facile Synthesis of Hierarchical Porous TiO2 Ceramics with Enhanced Photocatalytic Performance for Micropolluted Pesticide Degradation

Abstract: In this research, hierarchical porous TiO2 ceramics were successfully synthesized through a camphene-based freeze-drying route. The well-dispersed TiO2 slurries were first frozen and dried at room temperature, followed by high-temperature sintering. The ceramics were analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. Results indicated that the obtained TiO2 ceramics could inhibit undesirable anatase-to-rutile phase transformation and grain growth even at temperatures as high as 800 °C. In this experiment, optimal compressive strength and porosity of the TiO2 ceramics were produced with the initial TiO2 slurry content of ∼15 wt %. The resultant TiO2 ceramics performed excellently in the photodegradation of atrazine and thiobencarb, and the total organic carbon removal efficiency was up to 95.7% and 96.7%, respectively. More importantly, the TiO2 ceramics were easily recyclable. No obvious changes of the photocatalytic performance were obse...