Showing papers on "Equiaxed crystals published in 1978"

Microstructure of SiC Prepared by Chemical Vapor Deposition

Abstract: Silicon carbide prepared by chemical vapor deposition at a substrate temperature of ∼1400425-429°C was investigated. Optical microscopy showed that the growth characteristic is dendritic; the deposits consist of columnar blocks within each of which the orientation of crystals is approximately the same. Small pores occur along the boundaries of these blocks. Transmission electron microscopy revealed finer details of the growth characteristics. The crystals were mostly 3C in structure, but crystals with 2H and one-dimensionally-disordered structures were also found. The 3C crystals are mostly dendritic, but some columnar growth with alternating twin bands occurs. Both 2H and one-dimensionallydisordered crystals exhibit a columnar habit, reflecting the difference in crystal symmetry from 3C. In all these crystals, the c axis (or one of the axes in 3C crystals) lies perpendicular to the substrate.

Method of making cast metal turbine wheel with integral radial columnar grain blades and equiaxed grain disc

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.

Development of high fracture toughness titanium alloys

Abstract: This paper describes the relationship between composition, processing history, and microstructure of a series of a + p and metastable β titanium alloys. The relation between a wide range of microstructures and the resulting properties of these alloys has been investigated by making extensive use of electron fractography and metallographic fracture path determinations. These results were obtained in connection with an alloy development program in which two alloy compositions ultimately were selected: Ti-5Mo-4.5A1-1.5Cr and Ti-8Mo-2.5A1-4.5Cr. These alloys were processed according to various schedules in order to achieve microstructures which produce good strength-toughness combinations. In the a + β alloy (Ti-5Mo-4.5A1-1.5Cr), the optimum property combinations were achieved in p-processed material which has an acicular microstructure and α-phase at prior β-grain boundaries, although excellent properties also were obtained in equiaxed a + β processed material. In the metastable β-alloy (Ti-8Mo-2.5A1-4.5Cr), significantly better strength-toughness combinations were achieved in a + β processed material which contains globular primary a, compared to p-processed material which contains grain boundary a. The metastable p-alloy exhibits a significant degree of directionality of properties in all microstructural conditions. It is suggested that such directionality is due to the elongated p-grain structure which causes more crack branching when the crack is propagating in the transverse direction compared to the longitudinal direction. Finally, a model is presented to account for the effect of grain boundary a on fracture toughness of a + p and metastable p alloy.

Discriminating method of casting structure by ultrasonic waves

Abstract: PURPOSE:To measure the ratios of colum nar grains and equiaxed crystal grains forming casting structures and let these be reflected in processes from continuous casting process up to final products by letting ultrasonic waves be propagated in the specimen and measuring the velocities of sound and ultrasonic damping constants.

TIG weld solidification structures in carbon sheet steels

Abstract: Two-dimensional TIG welds have been made in a number of sheet steels containing up to O·2%C. Five different grain structures have been identified. Both the welding speed and the composition, particularly the carbon content, affected the grain structures through the interaction between pool shape and the effects of composition and thermal conditions on the solidification microstructures. Equiaxed grains were found in some steels at welding speeds greater than 45 cm/min, but most nucleation of equiaxed grains appeared to take place at the gas/liquid surface of the welds.

Microstructures of electron beam deposited Nb3Sn and Nb3Sn-Al2O3 composites related to their superconducting properties

Abstract: A wide variety of Nb 3 Sn films has been produced by electron beam coevaporation of the elements. The correlation between the deposition parameters, including rate, substrate temperature and chemical composition, and the microstructural features of both pure Nb 3 Sn and coevaporated Nb 3 Sn-Al 2 O 3 films has been studied using transmission electron microscopy as the main tool. The corresponding superconducting properties have been characterized by measurements of the critical temperature, the residual resistivity and the initial slope of the upper critical magnetic field. Pure Nb 3 Sn films grow as fibers or columns with diameters from 250 A to 2000 A. Coevaporation of A1 2 O 3 causes a refined, stabilized and equiaxed growth morphology and also an extremely fine distribution of intragranular voids 25 A in diameter. The critical temperature is not degraded by the Al 2 O 3 .

Convection Analysis of the Dendrite Remelting Rocket Experiment

Abstract: T HE dendrite remelting rocket experiment (experiment 7421) was performed aboard a Black Brant VG sounding rocket during a period in which gravity levels of approximately 10 ~ 5 g prevailed. The objective of this experiment was to gain information on the origin of the equiaxed zone obtained in many metal castings. To observe the actual solidification process in lowg, however, the transparent metal-model material NH4C1-H2O (aqueous ammonium chloride solution) was used. Utilizing the crystallization of ammonium chloride from aqueous solutions as a model for the solidification behavior of metal alloys in any given circumstance requires a comparison of the convective flow velocities and patterns. Convective flow velocities can profoundly affect temperature and concentration distribution and, hence, cause various segregation phenomena. High convective velocities can also bend or break off delicate dendrite arms, thus possibly causing the equiaxed zone. The purpose of the present study was to assess the convective conditions in aqueous and metallic liquid systems under conditions of the flight experiment to help establish the relevance of the rocket experiment to metal casting phenomena. The results of the study indicate that aqueous or metallic convective velocities in the dendrite remelting rocket experiment ceil are of insignificant magnitudes at the 10~ to 10~ g levels of the experiment. The crystallization phenomena observed in the rocket experiment, therefore, may be indicative of how metals will solidify in low g.

Structure and phase transformations in cold-worked Fe−18Cr−9Ni steel

Abstract: 1. The structure of cold-rolled strip (e=97%) of hard magnetic Fe−Cr−Ni steel of the 18-9 type was examined in thin foil by means of transmission electron microscopy. Three elements of the structure occurring simultaneously were discerned — a biphase banded structure, biphase regions of fine fragments, and untransformed austenite. The sizes of these individual structural components were estimated. 2. It was concluded on the basis of the structural elements and electron-diffraction data that crystals of strain martensite are grouped in packets with interlayers of austenite during rolling of 18-9 steel. 3. It is possible for the isothermal martensitic transformation to occur in hard magnetic 18-9 steel during slow cooling from austenitizing temperature and rest. 4. It follows from an analysis of the dilatometric curves and measurements of the saturation magnetization that additional γ→α transformation may occur in cold-worked 18-9 steel during tempering at temperatures of 370–380°. Equiaxed particles of austenite free of dislocations are formed beginning at 480° and the original cold-worked structure is polygonized in the process of heating.