Grain growth

About: Grain growth is a research topic. Over the lifetime, 19901 publications have been published within this topic receiving 447044 citations.

As-Fabricated and Heat-Treated Microstructures of the Ti-6Al-4V Alloy Processed by Selective Laser Melting

Abstract: Selective laser melting (SLM) is a rapid manufacturing process that enables the buildup of very complex parts in short delays directly from powder beds. Due to the high laser beam energy during very short interaction times and the high solidification rates of the melting pool, the resulting microstructure is out-of-equilibrium and particularly textured. This type of as-fabricated microstructure may not satisfy the aeronautical criterion and requires post heat treatments. Optimized heat treatments are developed, in one side, to homogenize and form the stable phases α and β while preventing exaggerated grain growth. In the other side, heat treatment is investigated to relieve the thermal stresses appearing during cooling. This study is aimed at presenting the various types of microstructure of the Ti-6Al-4V alloy after postfabrication heat treatments below or above the β transus. Tensile tests are then carried out at room temperature in order to assess the effect of the microstructures on the mechanical properties. The fine as-fabricated microstructure presents high yield and ultimate strengths, whereas the ductility is well below the standard. A strong anisotropy of fracture between the two loading directions is noted, which is attributed to the manufacturing defects. Conventional and optimized heat treatments exhibit high yield and ultimate strengths while the ductility is significantly improved. This is due to a new optimization of the process parameters allowing drastic reduction of the number of defects. These two heat treatments enable now a choice of the morphology of the grains between columnar or equiaxial as a function of the type of loading.

Sintering: Densification, Grain Growth and Microstructure

Abstract: Basis of Sintering Science Solid State Sintering Models and Densification Grain Growth Microstructure Development Sintering of Ionic Compounds Liquid Phase Sintering

Grain-size effect on structure and phase transformations for barium titanate

Abstract: We report the results of an investigation into the grain-size dependence of lattice structure for barium titanate (${\mathrm{BaTiO}}_{3}$) ceramics prepared by a sol-gel method. Raman and infrared spectroscopy, x-ray diffraction, and differential scanning calorimetry were used in combination with electron microscopy to study the evolution of lattice structure and phase transformation behavior with heat treatment and grain growth from the nano scale to the micron scale for ${\mathrm{BaTiO}}_{3}$ polycrystals. Raman spectroscopy and optical second-harmonic-generation measurements indicated the onset of local room-temperature acentric crystal symmetry with heat treatment and crystallite growth, well before the observation of any tetragonal structure by x-ray diffraction. Analysis of the room-temperature Raman spectra for ultrafine grain (grain size 0.1 \ensuremath{\mu}m) polycrystals suggested that a locally orthorhombic structure preceded the globally tetragonal form with grain growth. In support of this observation, differential scanning calorimetry suggested the orthorhombic-tetragonal phase transformation shifts up through room temperature with decreasing grain size. Hot-stage transmission electron microscopy studies revealed that fine grain (grain size \ensuremath{\approxeq}0.1 \ensuremath{\mu}m) ceramics, which showed a thermal anomaly associated with the cubic-tetragonal phase transformation, were untwinned at room temperature, as well as on cycling through the normal Curie temperature, suggesting a single-domain state for individual grains. The findings are discussed in light of a number of possible causes, including the presence of processing-related hydroxyl defects and the effect of elastic constraints on phase transformation behavior for ${\mathrm{BaTiO}}_{3}$ grains in a polycrystalline microstructure. \textcopyright{} 1996 The American Physical Society.

Rheology of synthetic olivine aggregates: Influence of grain size and water

Abstract: The influence of grain size and water content on the high-temperature plasticity of olivine aggregates was studied, using a gas-medium high-pressure deformation apparatus. The specimens used were hot-pressed, dense olivine aggregates with controlled grain size ranging from a few to 70 μm, with or without added water. Mechanical tests were made at 1573 K and 300 MPa confining pressure and at strain rates of 10−3 to 10−6 s−1. The results reveal two distinct mechanisms of deformation, depending on stress level and grain size. At relatively high stress and large grain size, the strain rate is proportional to about the cube power of the stress and is nearly independent of grain size. In this regime, microstructural observations gave evidence of intragranular deformation involving dislocation motion. At low stress and small grain size, the strain rate depends almost linearly on stress and decreases markedly with increase in grain size. In the latter regime, little evidence was found for intragranular deformation. These observations suggest that the deformation mechanism in the grain size insensitive regime is dislocation creep, while that in the grain size sensitive regime is diffusion creep. In both regimes, water was found to enhance the creep rate. The absence of grain size sensitivity in the dislocation creep regime and comparison with single-crystal data indicate that the water weakening effect is mainly an intragranular process. However, the existence also of a water weakening effect in the diffusion creep regime indicates that water also enhances diffusion. The extrapolation of the present results to coarser grain sizes indicates that the transition from dislocation to diffusion creep occurs at 0.1–1 MPa for 10-mm grain size. Therefore it is suggested that this transition may occur in the upper mantle and that, in both regimes, the presence of trace amounts of water will result in significantly lower creep strength than under strictly “dry” conditions.