Showing papers on "Equiaxed crystals published in 1980"

Grain structures in aluminum alloy GTA welds

Abstract: The grain structures in two dimensional GTA welds of a number of commercial aluminum alloys have been studied in order to clarify the mechanisms of grain refinement in welds. At low welding speeds and heat inputs the structures were either axial (continuous grains along the weld centerline) or stray (intermittent new grains). At higher speeds and heat inputs the structures were generally either columnar to the weld centerline, or contained some equiaxed grains at the center. Regression analyses indicated that both stray grains and equiaxed grains were favored by increased titanium content. In several alloys, titanium-rich compounds, and in one alloy, zirconium-rich compounds were found at the centers of dendrites. It is concluded that both stray and equiaxed grains originate by heterogeneous nucleation, with possible secondary effects due to constitutional undercooling.

Rapidly solidified nickel-base superalloys

Abstract: The structures of rapidly solidified APK1, In 100 and low-carbon In 792 are described and compared with that of Nimonic 80A. Under identical processing conditions, cellular, dendritic and homogeneous equiaxed structures can be obtained. This is not due to either the influence of cooling conditions or to any single alloying addition, but depends on the combined effects of the Ti, Cr and C contents. The spinodal-type formation of γ′, proposed for Nimonic 80A, cannot be suppressed in these alloys by pendant drop melt extraction or melt spinning techniques. However, detailed atom-probe field-ion microscopy suggests that the γ′ formation in APK1 does develop by a similar mechanism. Although not directly attributable to a modulated microstructure or to the presence of disordered particles, the extremely high strength levels observed in this alloy after heat treatment are due to the subsequent development of small, ordered, γ′ precipitates in a fine-grained matrix, together with the absence of deleterious grain boundary carbide precipitation.

Structural and temperature variations during rolling of aluminium slabs

Abstract: A finite-difference technique has been developed to predict the temperature profile during the rolling of aluminium slabs and experimental results are presented which show substantial agreement with the theory. It is shown that such variations affect the mechanisms determining the development of structure and that structural variations exist throughout the slabs on exit from the roll gap. Qualitatively, these variations agree with predicted variations in temperature-compensated strain rate. The effect of Mg as an alloying element is to reduce the possibility of formation of equiaxed subgrains resulting in an elongated substructure.

Cross‐Sectional Transmission Electron Microscopy For Polycrystalline Silicon Films

Abstract: SUMMARY The grain structure and the surface roughness of undoped polycrystalline silicon films were studied by TEM of cross-sectional specimens. The grain diameter parallel to the surface could be directly determined as a function of the distance from the interface. An almost linear increase was found for all deposition temperatures between 923 K and 1123 K. For low deposition temperatures, the grains are columnar, while for high temperatures they are equiaxed. Cross-sectional TEM-specimens further allow a quantitative determination of the surface roughness. The roughness is not correlated with the grain diameter at the surface. A sharp increase of the roughness at 1123 K can be explained by the disappearance of a preferred orientation of the grains.

Turbine wheel with integral DS blades and equiaxed hub

Abstract: A cast metal turbine wheel is made comprising a cylindrical disc with equiaxed grains and integral blades with columnar grains oriented substantially parallel to the leading and trailing blade edges. The blades are solidified unidirectionally by withdrawing heat through chill means located adjacent the mold at the blade tips. Cooling is inhibited from other blade surfaces by retaining cast metal in mold portions located above and below the blades. The disc portion of the mold is insulated so that it cools slowly to promote grain nucleation throughout the metal therein and equiaxed grain growth.

Recrystallization characteristics of oxide dispersion strengthened nickel-base alloys

Abstract: Electron microscopy was employed to study the process of recrystallization in two oxide dispersion strengthened (ODS) mechanically alloyed nickel-base alloys, MA 754 and MA 6000E. MA 754 contained both fine, uniformly dispersed particles and coarser oxides aligned along the working direction. Hot rolled MA 754 had a grain size of 0.5 microns and high dislocation densities. After partial primary recrystallization, the fine grains transformed to large elongated grains via secondary (or abnormal) grain growth. Extruded and rolled MA 6000E contained equiaxed grains of 0.2 micron diameter. Primary recrystallization occurring during working eliminated virtually all dislocations. Conversion from fine to coarse grains was triggered by gamma prime dissolution; this was also a process of secondary or abnormal grain growth. Comparisons were made to conventional and oxide dispersion strengthened nickel-base alloys.

Processing and properties of high-purity, fine-grain-size depleted-uranium, deep-drawn shapes

Abstract: Unalloyed uranium is a candidate material for shaped charge liners used in conventional ordinance applications. For test purposes, it was decided that a high-grade uranium material with good toughness should be used. This report describes a process for producing a high-purity (less than 500 ppM total impurities) wrought material consisting of a recrystallized, equiaxed grain structure with a 10 micron (ASTM-8) average grain size. The fabrication process is discussed in detail. In all, six material conditions having a wide range of mechanical and structural properties were investigated. The tensile, hardness, and microstructural properties of these six material conditions are reported in detail.

Mechanical properties of hot-pressed silicon nitride with different grain structures

Abstract: During hot-pressing of α-Si3N4 powders, the equiaxed α micro-structure gradually transforms into a β structure characterized by needle-shaped prismatic grains which are closely entangled and linked together. With increasing amounts of the β fraction, the bend strength, fracture toughness, and work of fracture increase significantly, then decrease as grain growth occurs. The Klc, improves by a factor of >2 and the change in γF by a factor of >4. The crack resistance to achieve the same crack velocity in materials of different β contents shows a similar trend. The dependence of the mechanical properties on the microstructure is explained by linking and pullout of the β crystals and by grain coarsening.

Ultrasonic inspectability improvements of austenitic stainless steel welds after thermal-mechanical processing

Abstract: A thermal-mechanical process was applied to Type 304/308 stainless steel GTA welds. The process improved the ultrasonic transmission properties in the weld region. These improvements are attributed to the microstructural changes observed after processing of the austentitic stainless steel welds. Some of the microstructural changes observed were conversion of cast dendrites into equiaxed austenitic grains, and the reduction in size and number of stringers, inclusions, and delta ferrite.

Fracture toughness and structure of the fractures of alloy VT9 after heating in the α+β and β ranges

Abstract: 1. Alloy VT9 with a lamellar structure, as compared with an equiaxed structure, is more capable of delaying fracture under single loads with low ductility and strength. 2. The superior fracture characteristics (double) of alloy VT9 with a lamellar structure are revealed by impact loading. 3. Titanium alloys with a lamellar microstructure are better able to delay crack development than alloys with an equiaxed structure, which is confirmed by operating tests of parts with a complex shape.