Grain Structure Evolution Research Articles

Modeling Magnetron Sputter Deposition

A comprehensive model of the cylindrical magnetron sputtering system is described. Microstructural features that affect the performance of the coating are modeled and compared to experimental data obtained from atomic force microscopy. Dynamic scaling parameters are used to characterize the evolution of grain structure in physical vapor deposition models and in experimental data obtained from atomic force microscopy. Surfaces are described in terms of a roughness exponent α, growth exponent β, and dynamic exponent z. The scaling exponents provide a valuable metric for comparative analysis and as a means of optimizing parameters used in the simulation.

The Grain Structures in some 5000 Series Aluminium Alloys after Asymmetric Rolling and Annealing

The development of grain structures after asymmetric rolling (ASR) and annealing was investigated in Al-Mg alloys AA5754, AA5182 and AA5083. It has been demonstrated that a fine grain structure could be produced through continuous recrystallization, but it is strongly affected by the presence of large second phase particles. In AA5754 the volume fraction of large particles is relatively low and continuous recrystallization is able to occur throughout the sheet thickness, resulting in a fine grain structure of 2μm mean grain size. In AA5182 the fraction of large particles increases to a level that the continuous recrystallization occurs only in the sheet surface, whereas the sheet centre undergoes discontinuous recrystallization. The discontinuous recrystallization due to particle stimulated nucleation (PSN) is dominant in AA5083 so that no continuous recrystallization has been observed. The fully recrystallized grain structure is slightly finer in the ASR processed AA5083 than the conventionally rolled one.

Evolution of grain structure in AA2195 Al-Li alloy plate during recrystallization

The evolution of the grain structures in AA2195 Al-Li alloy plate warm-rolled by 80% reduction during recrystallization annealing at 500 °C was investigated by electron backscatter diffraction, scanning electron microscopy and transmission electron microscopy. It is found that the elongated grain structures are caused by the lamellar distribution of recrystallization nucleation sites, being lack of large second phase particles (> 1 μm), and dispersive coherent particles (such as δ′ and β′) concentrated in planar bands. The recrystallization process may be separated into three stages: firstly, recrystallization nucleation occurs heterogeneously, and the nuclei are concentrated in some planar zones parallel to rolling plane. Secondly, the grain boundaries interacted with small particles concentrate in planar bands, which is able to result in the elongated grain structures. The rate of the grain growth is controlled by the dissolution of these small particles. Thirdly, after most of small particles are dissolved, their hindrance to migration of the grain boundaries fades away, and the unrecrystallized zones are consumed by adjacent recrystallized grains. The migration of high angle grain boundaries along normal direction leads a gradual transformation from the elongated grains to the nearly equiaxed, which is driven by the tension of the grain boundaries.

Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL): A Project of the European Space Agency (ESA) - Microgravity Applications Promotion (MAP) Programme

The main objective of the research project of the European Space Agency (ESA) - Microgravity Application Promotion (MAP) programme entitled Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL) is the investigation of the formation of the transition from columnar to equiaxed macrostructure that takes place in casting. Indeed, grain structures observed in most casting processes of metallic alloys are the result of a competition between the growth of several arrays of dendrites that develop under constrained and unconstrained conditions, leading to the CET. A dramatic effect of buoyancy-driven flow on the transport of equiaxed crystals on earth is acknowledged. This leads to difficulties in conducting precise investigations of the origin of the formation of the equiaxed crystals and their interaction with the development of the columnar grain structure. Consequently, critical benchmark data to test fundamental theories of grain structure formation are required, that would benefit from microgravity investigations. Accordingly, the ESA-MAP CETSOL project has gathered together European groups with complementary skills to carry out experiments and to model the processes, in particular with a view to utilization of the reduced-gravity environment that will be afforded by the International Space Station (ISS) to get benchmark data. The ultimate objective of the research program is to significantly contribute to the improvement of integrated modelling of grain structure in industrially important castings. To reach this goal, the approach is devised to deepen the quantitative understanding of the basic physical principles that, from the microscopic to the macroscopic scales, govern microstructure formation in solidification processing under diffusive conditions and with fluid flow in the melt. Pertinent questions are attacked by well-defined model experiments on technical alloys and/or on model transparent systems, physical modelling at microstructure and mesoscopic scales (e.g. large columnar front or equiaxed crystals) and numerical simulation at all scales, up to the macroscopic scales of casting with integrated numerical models.

A discrete nonlinear mass transfer equation with applications in solid-state sintering of ceramic materials

The evolution of grain structures in materials is a complex and multiscale process that determines the material's final properties. Understanding the dynamics of grain growth is a key factor for controlling this process. We propose a phenomenological approach, based on a nonlinear, discrete mass transfer equation for the evolution of an arbitrary initial grain size distribution. Transition rates for mass transfer across grains are assumed to follow the Arrhenius law, but the activation energy depends on the degree of amorphization of each grain. We argue that the magnitude of the activation energy controls the final (sintered) grain size distribution, and we verify this prediction by numerical simulation of mass transfer in a one-dimensional grain aggregate.

Substructural and phase transformations during plastic deformations of materials obtained by intensive deformation

The paper presents the results of electron microscopy investigations of the defect structure evolution and the phase transformation of ultra fine-grained (UFG) Cu and Cu Al O alloy. The UFG Cu was prepared using the equal-channel angular (ECA) pressing method. The Cu Al O alloy was synthesized by mechanical milling and subsequent high temperature pressing. The initial grain size of the materials was 200 nm. The grain and dislocation structures were investigated qualitatively and quantitavely. In the Cu alloy, deformation twinning was revealed to occur in coarse grains together with slip deformation. The evolution of grain and dislocation micro structure in the presence or absence of solid solution and of dispersion hardening is analyzed, and differences are assessed.

Studies of structure and morphology of sputter-deposited stainless steel–nitrogen films

Stainless steel films doped with nitrogen were deposited on heated and unheated (100) silicon substrates by radio-frequency magnetron sputtering of an austenitic stainless steel target in argon and nitrogen gas mixtures, containing a range of nitrogen compositions. The evolution of phases, morphologies and grain structures in the resultant films was studied by X-ray diffraction and field emission scanning electron microscopy. It was found that, with increasing nitrogen composition in the gas mixture, the crystalline structure of the films deposited on the heated substrates changed from bcc ferrite (α), to nitrogen-stabilized fcc austenite (γ), then to distorted expanded austenite phase (γN) with nitrogen supersaturation, and finally to the newly discovered fcc ‘MN’ phase with ideal cubic symmetry and further enlarged lattice. On the unheated substrates, the phase-evolution trend was found to be different for % N2 above 10. For the 25% N2 film, amorphous phase formed along with the crystalline austenite and ferrite phases, while the percentage of amorphous content decreased when % N2 was increased to 50. This different trend was understood to be due to the role of increase in % N2 in decreasing the energy loss of sputtered species through collisions. The dependence of crystalline phase formation on the energy of sputtered species is less severe on the heated substrates. Although all the films deposited experienced three-dimensional fibrous growths, they exhibited different surface morphology and grain structure. There exists a correlation between film morphology and phase constituents, while grain size was influenced by the nucleation density and the energy and mobility of adatoms that are reduced due to nitrogen incorporation.

Evidence for Extensive Grain Boundary Meander and Overgrowth of Substrate Grain Boundaries in High Critical Current Density ex Situ YBa2Cu3O7−x Coated Conductors

It has been generally accepted that YBa2Cu3O7−x (YBCO) films deposited on deformation textured polycrystalline metal tapes result in YBCO grain boundary (GB) networks that essentially replicate the GBs of the underlying substrate. Here we report that for thicker YBCO films produced by a BaF2 ex situ process, this is not true. Using electron backscatter diffraction combined with ion milling, we have been able to map the evolution of the YBCO grain structure and compare it to the underlying template in several coated conductors. For thin (≤0.5 μm) YBCO films deposited on rolling-assisted biaxially textured substrates (RABiTS), the YBCO GBs nearly directly overlap the substrate GBs. For 0.7–1.4 μm YBCO films, the GBs were found to meander along the substrate GBs and along the sample normal, with displacements several times the film thickness. In very thick films (2.5–2.9 μm), the YBCO grains can completely overgrow substrate grains and GBs, resulting in a substantial disconnection of the YBCO and substrate GB networks. Similar behavior is found for BaF2 ex situ YBCO films on ion-beam-assisted deposition-type templates. The ability of the YBCO to overgrow substrate grains and GBs is believed to be due to liquid-phase mediated laminar grain growth. Although the behavior of the YBCO GB networks changes with YBCO film thickness, the samples maintained high critical current density (Jc) values of >2 MA/cm2 for films up to 1.4 μm thick, and up to0.9 MA/cm2 for 2.5–2.9-μm-thick films.

Computer Simulation of Grain Growth in Three Dimensions by the Phase Field Model and the Monte Carlo Method

Temporal evolution and morphology of grain structure in three dimensions were simulated by the phase field and the Monte Carlo simulations. In order to prevent impingement of grain of like orientation, a new algorithm was adopted for both simulations. Excluding the initial stage, the average area is found to be proportional to time in the phase field and the Monte Carlo simulations. The scaled grain size and the face number distributions become time-independent in both simulations. The scaled grain size and the face number distributions obtained by the phase field simulation are in good agreement with those by the Monte Carlo method. The nearest neighbor face correlation similar to the Aboav– Weaire relation is observed in simulated grain structures by both methods. The nearest neighbor face correlation for the phase field model is quite similar to that for the Monte Carlo method. The Allen–Cahn type equation for the phase field simulation can be derived from the master equation of the Monte Carlo Model.

Numerical Simulation of Grain Structure Evolution in Solidification of an Al-5.0wt%Cu Alloy under Electromagnetic Stirring and Its Experimental Verification

A two-dimensional computational model, coupling a hmacroscopic heat and fluid flow analysis with a cellular automaton model under electromagnetic stirring, was developed. The dynamic evolution of solidification grain structures and transport phenomena during solidification were presented successfully. The Gaussian distribution was chosen to deal with nucleation events both on the mold wall and in the bulk melt. The KGT (Kurz-Giovanola-Trivedi) model, based on the calculated temperature field, was adopted to evaluate the growth velocity of a dendrite tip. The material used was an Al-5.0wt%Cu alloy. Both the computational and experimental results showed that grain refinement was improved remarkably with electromagnetic stirring. The fully equiaxed grain structures were obtained with a proper choosing of the casting process variables, such as pouring temperature, cooling rate and stirring intensity.

Grain structure and texture of cast iron aluminides

AbstractThe grain structure and texture of cast iron aluminides have been investigated in the scanning electron microscope by orientation contrast imaging and electron back‐scatter diffraction, respectively. Depending on the aluminum concentration the alloys exhibit a fine‐grained chill zone, medium grain size columnar zone and/or coarse‐grained equiaxed zone. The texture is random except in the columnar zone which is characterized by a <100> fibre texture in the direction of heat flow, i.e parallel to the elongated grains. The development of grain structure and texture is discussed with respect to cooling rate, thermal and constitutional undercooling, and order. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of process parameters on grain structure formation during VAR of INCONEL alloy 718

The development of grain structure during Vacuum Arc Remelting (VAR) of INCONEL1 alloy 718, a nickel-based superalloy, is complex depending both on compositional variations and a range of process parameters. A multiscale model is presented which combines a macroscopic solution of the heat transfer, fluid flow and electromagnetism with a mesoscopic model of grain nucleation and growth. The model was used to investigate the influence of variations in process control parameters upon the macroscopic molten pool size and shape, together with the predicted grain structure. Simulations of structures produced for variations in melt rate, arc power and arc focus (both thermal and electrical) were compared with observations from instrumented and characterized plant-trials for steady state melting conditions; good agreement was achieved.

Evolution of non-uniform grain structure during hot defornnation of a Nb-Ti microalloyed steel

Recrystallisation and the evolution of the abnormally grown austenite grains were investigated for a continuously cast slab of a 0.13 C-1.41 Mn-0.027 Nb-0.012 Ti steel during reheating and after the subsequent deformation. The stability of the recrystallised structure and the uniformity of the final microstructure were also studied. The abnormally grown grains appear in a few minutes at reheating temperatures around 1200 °C. All grains in the bimodal grain structure recrystallised at 1100 °C for strains > 0.2 within about 40 s. The coarse grains are refined, while the fine grains become slightly larger. Some abnormal grain growth can occur again in the recrystallised structure within 10 min. After cooling at 1° C/s the coarse austenite grains transform into large areas of upper bainite, while the finer grains transform to fine ferrite and pearlite. The transformed microstructure in specimens cooled at l°C/s consists of large upper bainitic areas corresponding to the prior coarse austenite grains, surrounded by fine ferrite-pearlite grains.

Development of Grain Structure and Texture during Annealing in Asymmetrically Rolled AA5754

AA5754 sheet has been processed by asymmetric rolling and the development of grain structure and texture in subsequent annealing studied at 240-500°C. It has been found that asymmetric rolling facilitates the formation of ultra-fine grain structure (1-2µm grain size) by shear strain promoted continuous recrystallization, which is a process of extended recovery and subgrain/grain growth. The ultra-fine grain structure is not thermally stable, and when the annealing temperature or time increases, the grain size eventually grows to its Zener limit. The deformation texture is similar to the typical f.c.c. cold rolling texture but rotated about the transverse direction. Along with the formation of an ultra-fine grain structure and subsequent grain growth, the deformation texture is retained.

New Grain Formation in a Coarse-Grained 7475 Al Alloy during Severe Hot Forging

Strain-induced grain refinement in a coarse-grained 7475Al alloy was studied by means of multidirectional forging (MDF) carried out at T = 490oC under a strain rate of 3 x 10-4 s-1. Integrated flow curves exhibit significant work softening just after yielding, followed by steady-state-like behavior at high strains. The evolution of new fine grain structure during deformation can be assisted by grain-boundary sliding, resulting in frequent formation of high strain gradients and subsequently microshear bands in grain interiors. Microshear bands developed in various directions are intersected with each other, subdividing original grains into misoriented small domains. The number and the misorientation angle of microshear bands progressively increase during deformation, finally followed by their transformation into high-angle boundaries. It is concluded that grain refinement under hot MDF conditions occurs by a series of deformation-induced continuous reactions; that is essentially similar to continuous dynamic recrystallization.

Formation and evolution of grain structures in thin films

We describe an approach to studying grain structure formation and evolution in polycrystalline thin films using a “grain continuum” representation of the solid. Formation and evolution of grain structure are tracked using level-set-based geometry software we call PLENTE. We demonstrate the approach using two processes; (1) vapor deposition and (2) grain boundary migration during curvature-driven coarsening. We employ several codes in concert to track the development of grains during deposition while accounting for reaction kinetics, ballistic transport, and re-emission of reactive species.

Development of grain structure during friction stir welding

A stop-action friction stir weld has been prepared in Al–Li 2195 to “freeze in” the dynamic deformation field surrounding the FSW tool. Analysis of a plan-view section of this weld reveals important new details of the grain structure evolution and texture development occurring around the FSW tool.

Grain topology in Ti–6Al–4V welds—Monte Carlo simulation and experiments

The importance of topological features of grains in the evolution of grain structure is well recognized in isothermal systems. However, during fusion welding, strong spatial gradients of temperature exist in the heat-affected zone (HAZ), and this region undergoes rapid heating and cooling. The effects of spatial and temporal variations of temperature on the topological class distribution, relationship between size and topology of grains and the interdependence between grain topology and its neighbours are not known. Topological features of grains in the HAZ of Ti–6Al–4V alloy welds were measured for various heat inputs in the range 0.55–4.33 MJ m−1. The topological class distributions were also calculated using a three-dimensional Monte Carlo model utilizing thermal cycles computed from a well tested numerical heat transfer and fluid flow model. The computed results showed that the topological class distributions were unaffected by the spatial and temporal variations of temperature. Experimental investigations of a few sections confirmed the simulation results. The average grain size for each edge class varied linearly with the edge class number. The local topological environment, i.e. the average number of sides of neighbours, nn, varied linearly with the inverse of the number of sides of grains, 1/nr, at a given location in the HAZ. Locations with the same topological environment showed the same grain size, indicating the significant influence of grain topology on grain growth in the HAZ.

HAZ microstructure simulation in welding of a ultra fine grain steel

In the present work the evolution of grain structure in the weld HAZ (heat affected zone) under welding thermal cycle was simulated. Especially the grain growth in the HAZ of a SS400 ultra fine grain steel was investigated. An integrated 3-D Monte Carlo (MC) simulation system for grain growth of the weld HAZ was developed based on Microsoft Windows. The results indicate that MC simulation is an effective way to investigate the grain growth in weld HAZ. The method not only simulates the non-isothermal dynamics process of the grain growth in the weld HAZ, but also visualizes the austenite grains realistically. Moreover, the thermal pinning effect can be easily included in the simulation process. The grain sizes of the CGHAZ (coarse grain heat affected zone) obtained from MC simulation are basically in agreement with the experimental measurement of the real welded joints under different heat input. Furthermore, the simulation indicates that the grain growth degree is higher for the SS400 ultra fine grain steel compared to conventional steel. With the increase in the heat input, the grain growth of the CGHAZ rapidly increases. Because the activation energy of the grain growth is lower for the SS400 ultra fine grain steel, austenite grains can grow at a relatively lower temperature, hence the range of the CGHAZ becomes wider.

Development of grain structure during friction stir welding

A stop-action friction stir weld has been prepared in Al–Li 2195 to “freeze in” the dynamic deformation field surrounding the FSW tool. Analysis of a plan-view section of this weld reveals important new details of the grain structure evolution and texture development occurring around the FSW tool.