Showing papers on "Grain growth published in 2017"

Change in microstructure of selectively laser melted AlSi10Mg alloy with heat treatments

Abstract: In the present study, we examined changes in the microstructure and mechanical properties of AlSi10Mg alloy, initially fabricated using selective laser melting (SLM) combined with a powder-bed system, by applying heat treatments at temperatures of either 300 or 530 °C. The as-fabricated samples exhibited a characteristic microstructural morphology and {001} texture. Melt pools corresponding to the locally melted and rapidly solidified regions were found to be composed of several columnar α-Al grains surrounded by fine eutectic Si particles. A fine dislocation substructure consisting of low-angle boundaries is present within the columnar α-Al grains. At elevated temperatures, fine Si phase precipitates within the columnar α-Al phase and coarsening of the eutectic Si particles occurs. These fine Si particles inhibit grain growth in the α-Al matrix, resulting in the microstructural morphology and [001] texture observed in the heat-treated samples. The dislocation substructure disappears in the columnar α-Al grains. Furthermore, the formation of a stable intermetallic phase occurs, reaching microstructural equilibrium after long-term exposure. The as-fabricated specimen exhibits a high tensile strength of approximately 480 MPa. The strength is independent of the tensile direction, that is, normal and parallel to the building direction. In contrast, the tensile ductility is found to be direction-dependent, and is therefore responsible for a fracture preferentially occurring at a melt pool boundary. The direction-dependence of the tensile ductility was not found in the specimen that had been heat-treated at 530 °C. The present results provide new insights into the control of the direction-dependence of the tensile properties of AlSi10Mg alloys fabricated by SLM.

SLM-processed Sc- and Zr- modified Al-Mg alloy: Mechanical properties and microstructural effects of heat treatment

Abstract: Traditionally 4xxx casting alloys are used for the additive manufacturing of structurally optimised lightweight parts in space, aerospace and automotive. However, for such applications there is a need for hardenable high-strength Al-alloys exceeding the properties of the 4xxx alloys family. The study analyses the hardness response of different heat treatment temperatures and hold durations applied to a Sc- and Zr-modified Al-Mg (5xxx-) alloy (Scalmalloy®) processed by Selective Laser Melting, and compares the mechanical properties and microstructure in the as-processed and annealed condition, and these properties are clearly related to the very fine grained microstructure. The results show that the static mechanical properties are exceptionally good with R m -values exceeding 500 MPa along with almost no build-orientation related anisotropic effects, and a high ductility even in the heat treated condition. These properties are clearly related to the very fine grained material, along with the good hardenability of the alloy. The stress-strain curves show the typical Portevin-Le-Chatelier (PLC) effect as known for other 5xxx alloys. Due to significant grain boundary pinning by different particles the very fine-grained bi-modal microstructure originating from the SLM-process can be maintained even in the heat treated condition, and only a HIP treatment leads to local grain growth only in coarser grained regions.

Grain Structure Control of Additively Manufactured Metallic Materials.

Abstract: Grain structure control is challenging for metal additive manufacturing (AM). Grain structure optimization requires the control of grain morphology with grain size refinement, which can improve the mechanical properties of additive manufactured components. This work summarizes methods to promote fine equiaxed grains in both the additive manufacturing process and subsequent heat treatment. Influences of temperature gradient, solidification velocity and alloy composition on grain morphology are discussed. Equiaxed solidification is greatly promoted by introducing a high density of heterogeneous nucleation sites via powder rate control in the direct energy deposition (DED) technique or powder surface treatment for powder-bed techniques. Grain growth/coarsening during post-processing heat treatment can be restricted by presence of nano-scale oxide particles formed in-situ during AM. Grain refinement of martensitic steels can also be achieved by cyclic austenitizing in post-processing heat treatment. Evidently, new alloy powder design is another sustainable method enhancing the capability of AM for high-performance components with desirable microstructures.

Demonstration of the cold sintering process study for the densification and grain growth of ZnO ceramics

Abstract: With the cold sintering process (CSP), it was found that adding acetic acid to an aqueous solution dramatically changed both the densities and the grain microstructures of the ZnO ceramics Bulk densities >90% theoretical were realized below 100°C, and the average conductivity of CSP samples at around 300°C was similar to samples conventionally sintered at 1400°C Frequently, ZnO is also used as a model ceramic system for fundamental studies for sintering By the same procedure as the grain growth of the conventional sintering, the kinetic grain growth exponent of the CSP samples was determined as N = 3, and the calculated activated energy of grain growth was 43 kJ/mol, which is much lower than that reported using conventional sintering The evidence for grain growth under the CSP is important as it indicates that there is a genuine sintering process being activated at these low temperatures and it is beyond a pressurized densification process

Microstructure evolution characteristics of Inconel 625 alloy from selective laser melting to heat treatment

Abstract: Superalloy Inconel 625 has been widely used in selective laser melting (SLM). Since SLM-induced microstructure with columnar grains, strong texture, porosity, and undesired properties, heat treatment is often used to tune the microstructure and mechanical properties. However, the microstructure evolution of IN 625 from SLM to heat treatment is poorly understood. In this study, IN 625 samples were SLMed and then heat treated at elevated temperatures. Microstructure evolution characteristics of the processed IN 625 alloy have been characterized using optical metallography, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction, and micro indentation. Fine dendrite microstructures with strong texture parallel to layer build-up direction was observed in the as-SLM samples due to rapid cooling and epitaxial growth. High dislocation density and high microhardness were found in the γ matrix which also contains high Z-contrast precipitates. After annealing at high temperatures, random grain growth accompanied by dislocation annihilation and twinning occurs. The decrease in lattice parameter and the prevalence of large grain boundary misorientation in the γ matrix suggests that SLM-induced residual stress be significantly reduced. In addition, the uncertainty of mechanical properties due to the process variations was quantified.

Reconciling grain growth and shear-coupled grain boundary migration.

Abstract: Conventional models for grain growth are based on the assumption that grain boundary (GB) velocity is proportional to GB mean curvature. We demonstrate via a series of molecular dynamics (MD) simulations that such a model is inadequate and that many physical phenomena occur during grain boundary migration for which this simple model is silent. We present a series of MD simulations designed to unravel GB migration phenomena and set it in a GB migration context that accounts for competing migration mechanisms, elasticity, temperature, and grain boundary crystallography. The resultant formulation is quantitative and validated through a series of atomistic simulations. The implications of this model for microstructural evolution is described. We show that consideration of GB migration mechanisms invites considerable complexity even under ideal conditions. However, that complexity also grants these systems enormous flexibility, and that flexibility is key to the decades-long success of conventional grain growth theories.

Microstructure and properties of a CoCrFeNiMn high-entropy alloy processed by equal-channel angular pressing

Abstract: A CoCrFeNiMn high-entropy alloy (HEA) was processed by equal-channel angular pressing (ECAP) for up to four passes at 673 K and the results show that the strength increases gradually with increasing straining up to ~ 1 GPa with an elongation to failure of ~ 35% after four passes of ECAP. In this condition, the microstructure is a single-phase ultrafine-grained (UFG) CoCrFeNiMn HEA with an average grain size of ~ 100 nm and a high dislocation density. This UFG HEA was subjected to post-deformation annealing (PDA) at temperatures of 673–1073 K for 60 min and it is shown that the hardness increases slightly due to precipitation to 773 K and then decreases to 1073 K due to a combination of recrystallization, grain growth and a dissolution of precipitates. The formation of brittle σ -phase precipitates improves the strength significantly but with a minor decrease in ductility. Annealing at the peak temperature of 773 K produces a very high yield strength of ~ 1015 MPa and an ultimate strength of ~ 1080 MPa together with an excellent elongation to failure of ~ 30%. An analysis of the data shows that grain boundary strengthening is the most important strengthening mechanism in these ECAP samples both before and after PDA.

Influence of heat treatment on the microstructure and tensile properties of Ni-base superalloy Haynes 282

Abstract: The effect of heat treatment on the microstructure and mechanical properties of Ni-base superalloy Haynes 282 was investigated. Applying a standard two-step ageing (1010 °C/2 h +788 °C/8 h) to the as-received, mill annealed, material resulted in a the presence of discrete grain boundary carbides and finely dispersed intragranular γ′, with an average size of 43 nm. This condition showed excellent room temperature strength and ductility. The introduction of an additional solution treatment at 1120 °C resulted in grain growth, interconnected grain boundary carbides and coarse (100 nm) intragranular γ′. The coarser γ′ led to a significant reduction in the strength level, and the interconnected carbides resulted in quasi-brittle fracture with a 50% reduction in ductility. Reducing the temperature of the stabilization step to 996 °C during ageing of the mill annealed material produced a bi-modal γ′ distribution, and grain boundaries decorated by discrete carbides accompanied by γ′. This condition showed very similar strength and ductility levels as the standard ageing of mill-annealed material. This is promising since both grain boundary γ′ and a bi-modal intragranular γ′ distribution can be used to tailor the mechanical properties to suit specific applications. The yield strength of all three conditions could be accurately predicted by a unified precipitation strengthening model.

Unveiling a Key Intermediate in Solvent Vapor Postannealing to Enlarge Crystalline Domains of Organometal Halide Perovskite Films

Abstract: Hybrid organic/inorganic perovskite solar cells (PSCs) have shown great potential in meeting the future challenges in energy and environment. Solvent-vapor-assisted posttreatment strategies are developed to improve the perovskite film quality for achieving higher efficiency. However, the intrinsic working mechanisms of these strategies have not been well understood yet. This study identifies an MA2Pb3I8(DMSO)2 intermediate phase formed during the annealing process of methylammonium lead triiodide in dimethyl sulfoxide (DMSO) atmosphere and located the reaction sites at perovskite grain boundaries by observing and rationalizing the growth of nanorods of the intermediate. This enables us to propose and validate an intermediate-assisted grain-coarsening model, which highlights the activation energy reduction for grain boundary migration. Leveraging this mechanism, this study uses MABr/DMSO mixed vapor to further enhance grain boundary migration kinetics and successfully obtain even larger grains, leading to an impressive improvement in power conversion efficiency (17.64%) relative to the pristine PSCs (15.13%). The revelation of grain boundary migration-assisted grain growth provides a guide for the future development of polycrystalline perovskite thin-film solar cells.

Origin of the low grain boundary conductivity in lithium ion conducting perovskites: Li3xLa0.67−xTiO3

Abstract: Although the bulk conductivity of lithium ion conducting Li3xLa0.67-xTiO3 electrolytes reaches the level of 10-3 S cm-1, the grain boundary conductivity is orders of magnitude lower; the origin of the low grain boundary conductivity should be thoroughly understood as a prerequisite to improve the overall conductivity. Samples with grain sizes ranging from 25 nm to 3.11 μm were prepared. According to SEM and TEM investigations, the grain boundaries are free of any second phase; however, the grain boundary conductivity is still ∼4 orders of magnitude lower than the bulk conductivity. The grain boundary conductivity decreases with decreasing grain size, indicating that the low grain boundary conductivity is not dominated only by the crystallographic grain boundary. Since electrons are attracted to the grain boundaries, as reflected by the dramatically enhanced grain boundary conductivity when electrons are introduced, the grain boundary core in Li3xLa0.67-xTiO3 should be positively charged, causing the depletion of lithium ions in the adjacent space-charge layers. The very low grain boundary conductivity can be accounted for by the lithium ion depletion in the space-charge layer.

Determination and controlling of grain structure of metals after laser incidence: Theoretical approach

Abstract: There are serious questions about the grain structure of metals after laser melting and the ways that it can be controlled. In this regard, the current paper explains the grain structure of metals after laser melting using a new model based on combination of 3D finite element (FE) and cellular automaton (CA) models validated by experimental observation. Competitive grain growth, relation between heat flows and grain orientation and the effect of laser scanning speed on final micro structure are discussed with details. Grains structure after laser melting is founded to be columnar with a tilt angle toward the direction of the laser movement. Furthermore, this investigation shows that the grain orientation is a function of conduction heat flux at molten pool boundary. Moreover, using the secondary laser heat source (SLHS) as a new approach to control the grain structure during the laser melting is presented. The results proved that the grain structure can be controlled and improved significantly using SLHS. Using SLHS, the grain orientation and uniformity can be change easily. In fact, this method can help us to produce materials with different local mechanical properties during laser processing according to their application requirements.

Electrical and hydrogen reduction enhances kinetics in doped zirconia and ceria: I. grain growth study

Abstract: The kinetics of mass transport is central to ceramic processing and device stability. In this work, the effect of electrical and hydrogen reduction on the grain growth behavior of doped zirconia and ceria has been investigated. Faster grain growth has been observed under reducing conditions in all cases. The results firmly establish that a depressed local oxygen potential can enhance cation kinetics in fluorite-structured oxide ceramics. Meanwhile, a large electrical current can generate a sharp, spatially varied oxygen potential profile, creating a graded microstructure with a dramatic grain size transition across the length of the sample.

Evidence for superplasticity in a CoCrFeNiMn high-entropy alloy processed by high-pressure torsion

Abstract: A CoCrFeNiMn high-entropy alloy was processed by high-pressure torsion to produce a grain size of ~10 nm and then tested in tension at elevated temperatures from 773 to 1073 K using strain rates in the range from 1.0×10−3 to 1.0×10−1 s−1. The alloy exhibited excellent ductility at these elevated temperatures including superplastic elongations with a maximum elongation of >600% at a testing temperature of 973 K. It is concluded that the formation of precipitates and the sluggish diffusion in the HEA inhibit grain growth and contribute to a reasonable stability of the fine-grained structure at elevated temperatures. The results show the activation energy for flow matches the anticipated value for grain boundary diffusion in nickel but the strain rate sensitivity is low due to the occurrence of some grain growth at these high testing temperatures.

Ultra-large-scale phase-field simulation study of ideal grain growth

Abstract: Grain growth, a competitive growth of crystal grains accompanied by curvature-driven boundary migration, is one of the most fundamental phenomena in the context of metallurgy and other scientific disciplines. However, the true picture of grain growth is still controversial, even for the simplest (or ‘ideal’) case. This problem can be addressed only by large-scale numerical simulation. Here, we analyze ideal grain growth via ultra-large-scale phase-field simulations on a supercomputer for elucidating the corresponding authentic statistical behaviors. The performed simulations are more than ten times larger in time and space than the ones previously considered as the largest; this computational scale gives a strong indication of the achievement of true steady-state growth with statistically sufficient number of grains. Moreover, we provide a comprehensive theoretical description of ideal grain growth behaviors correctly quantified by the present simulations. Our findings provide conclusive knowledge on ideal grain growth, establishing a platform for studying more realistic growth processes. Grain growth under ideal conditions is simulated by phase-field simulations in ultra-large time and space scales to elucidate the statistical behaviors. A team led by Tomohiro Takaki at Kyoto Institute of Technology in Japan performed large scale phase-field simulations to study ideal grain growth behavior. The time and space scales used in the simulations are more than ten times larger than those in previous reports, enabling them to reach a true steady-state with a statistically significant number of grains. A comprehensive theoretical description was derived to understand the ideal grain growth behavior based on the simulations. The knowledge provided by these findings may offer a model to understand the effects of complicated factors present in real materials and thus establish a platform to study more realistic grain growth phenomena in the future.