Papers by Keyword: Zener Pinning

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Authors: Roger D. Doherty, Elizabeth Hoffman, Christopher Hovanec, Arnaud Lens
Abstract: The prior literature on abnormal grain coarsening (AGC) at low volume fractions (f) of stable second phase particles in high purity Al alloys is reviewed and reanalyzed in the light of developments in modeling particle inhibition of grain boundary migration. With the usual assumptions (i) of incoherent particles that retain their shape on contact with the grain boundaries and (ii) that all the grain boundaries are equally mobile, it appears impossible to account for process of AGC. Normal grain coarsening (NGC) is shown to be less inhibited by the particles than is AGC. This idea is explored using a new but simple model of particle inhibition by curvature removal. The curvature of the smallest grains is always larger than that of the larger grains. Two possible hypotheses to overcome this difficulty are proposed: First the possible change of shape of particles on slowly moving grain boundaries, of grains with near 14 neighbors should, after a small increment of NGC, promote AGC at low values of the volume fraction f. The second hypothesis involves the observed high density of immobile, low angle grain boundaries (LAGBs) found in recent experiments on high purity Al-Fe-Si alloys cast with very coarse grain sizes. These alloys undergo rapid AGC even at higher values of f (> 0.01). These LAGBs are expected to inhibit the shrinkage of many of the small grains, whose loss is the fundamental mechanism of NGC.
Authors: S.C. Hogg, I.G. Palmer, Patrick S. Grant
Abstract: This work describes the microstructure and properties of a range of Al-(4-6)Mg- (1.2-1.6)Li-(0.3-0.4)Zr-(0-0.2)Sc alloys produced at Oxford University by spraycasting. Follow- ing hot isostatic pressing of the as-spraycast billets to close any porosity and to precipitate a ¯ne, coherent dispersoid population, forging to a true strain of 1 at 250 and 400±C led to a substantial re¯nement of the microstructure with grain sizes in the range 0.8 to 5¹m. A large intra-granular orientation gradient with distance measured using EBSD showed that at 250±C, partial dynamic recrystallisation by progressive lattice rotation led to a `necklace' structure of very ¯ne grains surrounding larger deformed grains. At 400±C, dynamic recrystallisation oc- curred by nucleation of new grains at prior grain boundaries and triple points. The strength of as-forged alloys was 200-350MPa and the high ductilities of up to 30% rendered the alloys amenable to post-forging cold work. A proof strength of 460MPa with 9.5% elongation was achieved in a non-heat-treatable spraycast Al-6Mg-1.3Li-0.4Zr alloy, matching the best prop- erties of similar mechanically alloyed AA5091, and exceeding the properties of AA7010-T74. The as-forged alloys showed excellent thermal stability up to » 0.9Tm, with no abnormal grain growth and grain size stagnation due to Zener pinning. Finally, strain rate sensitivity testing revealed the potential for superplasticity at 400 and 500±C and strain rates of 0.001-0.05s¡1.
Authors: Xiao Fei Ma
Abstract: Based on cellular automata, a model of simulating grain growth is established to study the effects of the second phase particle’s size and volume fraction on grain growth. The simulation results show that the smaller is the volume fraction of second phase particle, the finer is the grain of pinned matrix, and the pinning force of bigger second phase particle is stronger than that of smaller one. The correlative laws obtained from the simulation is in accordance with the theoretical models.
Authors: N. Burhan, Michael Ferry
Abstract: Severe plastic straining is an established method for producing submicron grain (SMG) structures in alloys. However, the development of such a fine grained structure in single-phase alloys is usually futile if they are to be exposed or processed at elevated temperatures. This is a direct consequence of the natural tendency for rapid and substantial grain coarsening which completely removes the benefits obtained by grain refinement. This problem may be avoided by the introduction of nanosized, highly stable particles in the metal matrix. In this work, a SMG structure was generated in an Al-0.3 wt.% Sc alloy by Equal Channel Angular Pressing (ECAP). The alloy was prepared initially to produce a fine grained microstructure exhibiting a large fraction of high angle grain boundaries and a dispersion of nanosized Al3Sc particles. The evolution of microstructure during annealing at temperatures up to 550 °C was examined in detail and grain size distributions generated from the data. It was shown that grain coarsening is rapid at temperatures above 450 °C and the initial log-normal grain size distribution exhibiting low variance and skewness was altered considerably. The statistical information generated from the grain size distributions confirms that discontinuous grain coarsening occurs in this alloy only at temperatures greater than 500 °C.
Authors: Oleg V. Rofman, Pete S. Bate
Abstract: The effect of deformation on the microstructure of a Al-4wt.%Cu alloy has been investigated. Evaluation of the microstructural changes was made by comparing results after both static annealing and tensile testing (deformed and non-deformed regions) at 450 °C. Uniaxial perturbed-rate tests showed that the Al-4wt.%Cu has a low value of the strain rate sensitivity index (m ~ 0.22) and cannot be considered as a superplastic material. It was found that in the deformed regions, specimens showed a significant increase in the grain and particle size. These changes were accompanied by an increase in the aspect ratio of the matrix grains. Tensile tests carried out at constant strain rates and stopped at intermediate strains helped to estimate the rate of the grain and particle growth and the contribution of deformation to it. To examine in detail the mechanism of the particle coarsening during deformation, additional tensile tests were made using the Al-4wt.%Cu alloy annealed at temperature conditions leading to abnormal grain size. Results of these tests also showed strain-induced particle coarsening, so that dynamic particle coarsening was not simply caused by boundary migration effects.
Authors: K.B. Hyde, Pete S. Bate
Authors: Pete S. Bate, K.B. Hyde, S.A. Court, John F. Humphreys
Authors: G. Couturier, Claire Maurice, R. Fortunier, R. Doherty, Julian H. Driver
Abstract: An original model, based on a variational formulation for boundary motion by viscous drag, is developed to simulate single grain boundary motion and its interaction with particles. The equations are solved by a 3D finite element method to obtain the instantaneous velocity at each triangular element on the boundary surface, before, during and after contact with one or more particles. After validation by comparison with some simple, analytical and numerical cases, it is adapted to model curvature driven grain growth. For single phase material, the single grain boundary model closely matches the grain coarsening kinetics of a 3D multi boundary vertex model. In the presence of spherical incoherent particles the growth rate slows down to give a growth exponent of 2.5. When the boundary is anchored there is a significantly higher density, by a factor of 4, of particles on the boundary than the density predicted by the classic Zener analysis, and many particles exert less than this Zener drag force. As a result the Zener drag is increased by a factor of about 2.2. The limiting grain radius is compared with some experimental results.
Authors: Hidenori Terasaki, Yuichi Komizo, Fumihiro Nishino, Masahiko Ikeda
Abstract: Microstructure formation of CP-Ti and TiB reinforced titanium were in-situ observed during the thermal cycle simulated for Tungsten Inert Gas (TIG) welding, by using laser scanning confocal microscopy. Under the in-situ observation of TiB reinforced titanium, heterogeneous nucleation of α-phase at inclusion was clearly detected and plate growth was shown in high timeresolution. Furthermore, it was observed that grain boundary of β -phase was pinned by the inclusions. Microstructure difference between pure and TiB reinforced titanium was explained based on those in-situ observations.
Authors: Xiao Fei Ma
Abstract: Based on cellular automata, a model of simulating grain growth is established to study the effects of the second phase particle’s size distribution on grain growth. The simulation results show that the second phase particles in the matix pin the grain boundary and then inhibit the grain growth. Different size distributions of the second phase particles have different pinning effect on the grain boundary, and the relationship of average grain size for the material with the second phase particles is RLognormalRUniformRNormal. The correlative laws obtained from the simulation is in accordance with the theoretical models.
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