Showing papers on "Equiaxed crystals published in 1970"

On the nature of freckles in nickel base superalloys

Abstract: Freckles are chains of equiaxed grains that have previously been observed in macroetched ingots of both iron and nickel base alloys formed by the consumable arc process. These defects have been observed in nickel base superalloys in both directionally solidified polycrystalline and single crystal ingots. Such ingots exhibit a relatively periodic distribution of vertical freckle trails around their circumference which are particularly suitable for study. This paper investigates the nature of “Freckles,” examines the conditions under which they occur and specifies a set of consistent observations which any successful model for freckling must explain. Visual, chemical, electron probe, metallographic, and X-ray diffraction data are presented which establish the appearance, composition and orientation differences between freckled and normal regions in single crystals of the nickel base superalloys Udimet 700 and Mar-M200. It is shown that the freckle lines are linear assemblies of small random equiaxed grains which are enriched in all but the inversely segregated solute species. Excessive interdendritic porosity and feeding shrinkage are observed in the vicinity of a freckle line. Some correlations of the tendency toward freckling and the geometric distribution of freckle lines are made with growth rate, thermal gradient, and interface shape. The dependence of freckling tendency on composition is examined in the Ni-Al system.

Effect of Transformation Substructure on the Strength and Toughness of Fe-Mn Alloys

Abstract: Phase transformations in Fe−Mn alloys containing up to 9 pct Mn were studied by optical and electron transmission microscopy. Either equiaxed ferrite, massive ferrite, or massive martensite can form on cooling from austenite. The particular type of transformation product formed was found to depend on the alloy content, austenite grain size, and cooling rate. The mechanical properties of all the transformation products were evaluated using tensile and impact testing and are discussed in terms of the observed microstructural features. Yield strength and impact transition temperature were found to be relatively insensitive to manganese content but were strongly influenced by the transformation substructure and grain size of the transformed phase. In martensite it has been shown that the structural unit analogous to grain size in ferrite is the martensite packet size, which in turn is controlled by the prior austenite grain size. The fracture surface of broken impact specimens and the fracture profile were examined by means of electron and optical microscopy techniques. These fractographic observations were correlated with impact test data and microstructural observations of the various transformation products.

The development of macrostructure in ingots of increasing size

Abstract: A series of Al-Cu ingots ranging in volume from approximately 50 to 500 and to 5000 cc has been conventionally prepared in two ways, by normal chill casting or by using a magnetic field to control the convective currents. Comparisons allow us to differentiate among the various mechanisms proposed for the origin of equiaxed grains and for the columnar-to-equiaxed transition in castings: i) In normal casting, with the natural convection associated with standard superheats, the majority of grains is provided by the mechanism suggested by Chalmers (and also known as the Big Bang), or by crystal settling from a free surface as suggested by Southin. ii) If new grains, no matter how they may form, are inhibited from mixing with the rest of the liquid, no significant equiaxed structure appears. iii) Unless some special or preselected conditions prevail, other mechanisms such as those involving constitutional supercooling or dendrite remelting, do not seem operative in the formation of a central equiaxed zone.

Grain refinement in undercooled copper

Abstract: : It was shown possible to obtain a fine equiaxed grain structure throughout a copper ingot containing 0.015 wt % oxygen if prior to nucleation the ingot is undercooled by 55 deg C. After undercoolings of 38 deg C or less the ingots had structures consisting of two separate zones of fine and coarse grains, and the proportion of fine grains decreased with increasing temperature of nucleation. The fine grain structure is attributed to a dendrite remelting process which arises during recalescence in the presence of approximately 150 ppm oxygen. In contrast to earlier experiments involving copper of higher oxygen content and nucleated at larger undercoolings, little evidence of recrystallization was found in these present ingots. It is therefore suggested that recrystallization in high oxygen ingots is connected with the large amount of undercooling applied rather than with the presence of oxygen. (Author)

Crystal structure and morphology of the alloys for cast permanent magnets

Abstract: The structure of the Ticonal-type alloys was investigated using a transmission electron microscope technique. Two-dimensional square macrolattice was observed in the section normal to the magnetic field which was applied at heat treatment. The translation vectors of that lattice coincide with the \langle100\rangle directions of the crystal lattice. There were equiaxed regions of various contrasts at the corners and center of the unit cell and flat intermediate layers on the sides. Thus the two-phase model of the structure is only an approximation. The prolonged tempering at high temperature (800°C, 20 hours) of specimens after they are subjected to the thermal magnetic treatment gives rise to the development of the complex modulated structure.

The effects of experimental rolling on microstructure and mechanical properties of a Ni-Cu-Cb steel

Abstract: The effects of hot rolling and warm working on mechanical properties and microstructure of Ni-Cu-Cb steel were investigated. In the hot rolled condition optimum strength and toughness resulted from rolling in the upper portion of the austenite plus ferrite phase field. This treatment produced a fine-grained structure consisting of equiaxed and acicular ferrite. Unusually high strength-toughness combinations were realized by warm working the hot rolled steels at 1200°F.

Improvements in or relating to Articles with Controlled Grain Structure and Methods of Making Same

Abstract: 1,181,278. Casting processes; moulds. TRW Inc. 29 June, 1967, No. 30045/67. Heading B3F. [Also in Division F1T] A turbine blade is cast in a refractory shell mould 22 using a copper chill 29 (which may be cooled by circulating coolant) so that the blade root has a columnar grain structure and the vane has an equiaxed structure. The chill 29 and mould 22, with downsprue 28 and ingates 24, 25 to the vane and root portions 26, 27, rests on a base 17, together with an enclosing furnace having a heating element 18 and a graphite &c. susceptor 21 for preheating the mould. The columnar grains are ¢-1 inch in length and the equiaxed grains average #-¢ inch. The mould is preheated to 1800-2000‹ F. and a nickel-base alloy is poured at 2800- 3100‹ F.

Porosity and anomalous recrystallization behavior in doped tungsten wire

Abstract: An anomalous recrystallization behavior has been observed in 0010 in diam K−Si−Al doped tungsten wire Recrystallization to relatively equiaxed grains occurred only in the core at temperatures of 1400° to 1600°C, but not at lower temperatures or higher temperatures up to 1900°C The anomalous recrystallization reaction was suppressed when the wire was preannealed at 1700° to 1900°C, but not by preanneals at temperatures between 500° and 1100°C Critically shadowed electron fractographs indicated that linear arrays of microporosity (≈100A diam) were responsible for the stabilization of the fibrous structures, Recrystallization occurred when these strings of pores were absent