Showing papers on "Equiaxed crystals published in 1990"

Prediction of equiaxed grain density in multicomponent alloys, stirred electromagnetically

Abstract: A phenomenological model, which numerically simulates nucleation and growth of equiaxed grains in a stirred, is presented. Process parameters are stirring intensity and alloy composition. The main results are the volume grain density and the maximum undercooling, together with intermediate parameters such as undercooling at the beginning of nucleation, nucleation rate, grain growth rate .... The model is used to discuss the effect of compositional changes. In particular it explains the influence of low level additions (for instance titanium), independently of the effect of nucleating agents.

The effect of alloy composition and welding conditions on columnar-equiaxed transitions in ferritic stainless steel gas-tungsten arc welds

Abstract: The columnar-equiaxed transition (CET) was investigated in full penetration gas-tungsten arc (GTA) welds on ferritic stainless steel plates containing different amounts of minor elements, such as titanium and aluminum, for a range of welding conditions. In general, the fraction of equiaxed grains increased, and the size of the equiaxed grains decreased with increasing titanium contents above 0.18 wt pct. At a given level of titanium, the equiaxed fraction increased, and the size of the equiaxed grains decreased with increased aluminum content. The CET was ascribed to heterogeneous nucleation of ferrite on Ti-rich cuboidal inclusions, since these inclusions were observed at the origin of equiaxed dendrites in the grain refined welds. Titanium-rich cuboidal inclusions, in turn, were found to contain Al-Ca-Mg-rich inclusions at their centers, consistent with observations by previous investigators for other processes. The welding conditions, in particular, the welding speed, were observed to affect the occurrence of the CET. Increasing the welding speed from 3 to 8 mm/s increased the equiaxed fraction noticeably, but a further increase in speed to 14 mm/s had a smaller additional effect. A finite element model (FEM) of heat transfer was used to examine the role of the welding conditions on the local solidification conditions along the weld pool edge. The results are compared with existing models for the CET.

Microstructure and steady state creep in Ti-24Al-11Nb

Abstract: The steady state creep behaviour of the two-phase Ti3Al-based alloy, Ti-24Al-11Nb, has been examined as a function of microstructure at temperatures ranging from 798 to 998 K and stress levels ranging from 30 to 400 MPa. Three microstructural conditions corresponding to 90% equiaxed α2, 40% equiaxed α2, and 100% lath α2 structures have been studied. A low-stress Coble creep regime has been identified, with the lath α2 structure showing the greates creep resistance in this regime. The lath α2 structure is also stronger in the dislocation creep regime. The creep strength of this ordered alloy is shown to derive from frequency factors for diffusion, which are about two to three orders of magnitude lower than those for disordered alloys. Activation energies for creep in both the diffusional and dislocation creep domains are similar to values obtained in disordered alloys.

Investment casting a titanium aluminide article having net or near-net shape

Abstract: A TiAl alloy base melt including at least one of Cr, C, Ga, Mo, Mn, Nb, Ni Si, Ta, V and W and at least about 0.5 volume % boride dispersoids is investment cast to form a crack-free, net or near-net shape article having a gamma TiAl intermetallic-containing matrix with a grain size of about 10 to about 250 microns as a result of the presence of the boride dispersoids in the melt. As hot isostatically pressed and heat treated to provide an equiaxed grain structure, the article exhibits improved strength.

Directional recrystallisation in Inconel MA 6000 nickel base oxide dispersion strengthened superalloy

Abstract: The directional recrystallisation process which leads to the growth of highly anisotropic ‘new’ grains in an oxide dispersion strengthened nickel base superalloy is investigated to elucidate the mechanism of recrystallisation and the process variables important in controlling microstructural evolution. It is found that after extrusion and hot deformation, the alloy consists of an ultra fine equiaxed grain microstructure characteristic of a material which has undergone dynamic primary recrystallisation. Thus, the directional recrystallisation which follows when the hot rolled sample is annealed in a moving temperature gradient (zone annealed) is really a secondary recrystallisation phenomenon. By comparing grain boundary mobility with the rate of zone annealing, it was theoretically predicted and experimentally observed that directional recrystallisation should give way to equiaxial recrystallisation when the interface velocity is less than the speed of zone annealing. The variables involved in the...

Grain structures in gas tungsten-arc welds of austenitic stainless steels with ferrite primary phase

Abstract: The grain structures were investigated in full penetration gas tungsten-arc (GTA) welds in sheets of 304 and 321 austenitic stainless steels for a range of welding conditions. In type 321 steel welds, fine equiaxed ferrite dendrites were observed in the ferrite phase. The equiaxed structure was ascribed to heterogeneous nucleation of ferrite on Ti-rich cuboidal inclusions present in this steel, since these inclusions were observed at the origin of equiaxed dendrites. In type 304 welds, the ferrite grains were columnar, except in less complete penetration specimens, where a few coarse equiaxed dendrites appeared to originate from the weld surface. The secondary austenitic grain structure was columnar in both steels. In type 304 steel, the columnar austenitic grain structure did not necessarily correspond to the primary ferrite grains. In type 321 steel, the secondary austenite was columnar despite the equiaxed structure of the primary ferrite. Factors which affect the columnar-to-equiaxed transition (CET) are discussed. The failure to form equiaxed austenitic grains in type 321 steel is ascribed to austenite growing across the space between ferrite grains instead of renucleating on the primary equiaxed ferrite.

Electromagnetic stirring with alternating current during electroslag remelting

Abstract: An alternating current (AC)-operated electromagnetic stirring (EMS) device, using line frequency, was designed and built to operate on a laboratory electroslag remelting (ESR) furnace for 150-mm-diameter ingots. Laboratory-scale experiments were conducted employing both 4340 alloy steel and INCONEL 718 alloy as electrode material. The initiation of stirring is accompanied by a thin strip of segregated material and favors the formation of spot segregation. Changes produced in the fluid flow conditions in the liquid pool ahead of the solidification front result in a transition from a highly directional columnar to an unoriented, branched structure. Except for small pockets of segregated liquid, the flow of molten metal does not penetrate into the mushy zone. Both electrode material and molten metal pool shape play an important role on the extent of promoting an equiaxed structure.

Anisotropic Effect of Magnetohydrodynamics on Metal Solidification

Abstract: Anisotropic effect of magnetohydrodynamics on solidification of Pb-10%Sn alloy has been studied in a magnetic field generated by a superconducting magnet. Experimental castings have been conducted with a mold designed to solidify the alloy unidirectionally. It has been observed that equiaxed structure changes to columnar with increasing the intensity of magnetic field parallel to the solidifying direction. On the other hand, a structural change has been slightly obtained with the field perpendicular to the direction of the solidification.Transport of heat in a mercury pool examined under the various conditions of magnetic fields has revealed that temperature distribution in mercury pool is affected strongly by the direction of a magnetic field through the anisotropic effect of magnetohydrodynamics on the convection. This has demonstrated that anisotropic effect of a magnetic field gives rises to the structural change of the solidification of the metal alloy.

Tantalum and chromium containing titanium aluminide rendered castable by boron inoculation

Abstract: A method for providing improved castability in a gamma titanium aluminide is taught. The method involves adding inclusions of boron to the titanium aluminide containing chromium and tantalum. Boron additions are made in concentrations between 0.5 to 2 atomic percent. Fine gain equiaxed microstructure is formed from solidified melt. Property improvements are also achieved.

Orientation effects on the elevated temperature fatigue of copper single crystals

Abstract: Effects of orientation on dislocation structures which evolve during the elevated temperature fatigue of copper single crystals have been studied using crystals oriented for double slip. The resulting dislocation reactions produce sessile jogs, Cottrell-Lomer locks, or cells formed by coplanar slip. The relative strengths of the reaction products vary markedly with increasing temperature. At room temperature coplanar slip crystals are strongest, crystals forming Cottrell-Lomer locks weakest. At 678 K (0.5 T m ) Cottrell-Lomer lock crystals are strongest, those forming sessile jogs weakest. The orientation dependence of the saturation stress is much greater at 678 K than at room temperature. A plateau in the saturation stress of about 14 MPa is observed in sessile jog and Lomer lock crystals cycled at 523 K. Persistent slip bands (PSBs) with the familiar ladder structure are observed by electron microscopy in these crystals. No saturation stress plateau or TEM evidence of PSB formation was found at 678 K. Only cells, usually equiaxed, were seen.

Effect of Impurity Content on Cavitation in the Superplastic Zn-22 Pct Al Alloy

Abstract: Micrograin superplasticity refers to the ability of some materials to exhibit neck-free strains of several hundred percent upon deformation. The two basic prerequisites for the observation of micrograin superplasticity are (a) a temperature greater than about one half of the melting point,Tm, and (b) a fine, stable, and equiaxed grain size that does not undergo significant growth during elevatedtemperature deformation. In addition to these two prerequisites, grain boundaries need to be mobile, high-angled, and able to resist tensile separation.

Nature of Grain Refinement in Titanium Alloy Welds by Microcooler Inoculation

Abstract: The influence of microcooler inoculation on the solidification and fusion zone beta grain structure of gas tungsten- arc welds in titanium has been investi­ gated by the addition of Ti-6Al-4V pow­ der to the weld pool in Ti-15V-3Al-3C r- 3Sn sheet. The complete melting of a large proportion of the added powder particles effectively lowered the fusion zone melt temperature and reduced temperature gradients across this region, thereby pro­ moting the growth of equiaxed dendrites both from partially melted powder parti­ cles and in a more conventional manner from dendrite fragments. The equiaxed dendrites subsequently blocked the growth of epitaxially nucleated columnar grains throughout a large portion of the fusion zone. The fine, equiaxed beta grain structure exhibited within the fusion zone at room temperature differed markedly from the beta grain structure that existed at the completion of solidificatio n due to appreciable grain boundary migration and possible dynamic recrystallization during cooling through the beta phase field. Powder additions to gas tungsten arc (CTA) welds in Ti-6AI-4V sheet promoted a similar, predominantly equiaxed fusion zone beta grain morphology. The simul­ taneous application of EMS at a moderate field strength during welding with powder additions reduced the extent of grain re­ finement.

Niobium and chromium containing titanium aluminide rendered castable by boron inoculations

Abstract: A method for providing improved castability in a gamma titanium aluminide is taught. The method involves adding inclusions of boron to the titanium aluminide containing chromium and niobium. Boron additions are made in concentrations between 0.5 and 2 atomic percent. Fine grain equiaxed microstructure is found from solidified melt. Property improvements are also achieved.

Microstructure and thermal stability of a rapidly solidified Al-4Er alloy

Abstract: The effect of the rapid-solidification melt-spinning process on an Al-4Er alloy has been studied Compared to the normally solidified master alloy, rapid-solidification processing promotes the formation of a eutectic suppressed, relatively refined microstructure Varying relative proportions of microstructures occupy the ribbon longitudinal transverse cross section: the featureless zone at the contact surface, the columnar zone in the middle and the equiaxed grains zone with degenerated dendrites at the free surface The presence of a finely dispersed, coherent Al3Er secondary equilibrium phase in the contact is evident Ribbon microhardness remains fairly stable up to 200 ° C and at further elevated temperatures retains higher mean values than the master alloy

Process for preparing titanium and titanium alloy materials having a fine equiaxed microstructure

Abstract: According to the present invention, titanium and titanium alloy materials having a fine equiaxed microstructure are produced. A titanium, α titanium alloy or (α+β) titanium alloy material is hydrogenated in an amount of 0.02 to 2% by weight. If necessary, the hydrogenated material is subjected to pretreatment [i.e., heated above 700° C. (β transformation point)] and/or working (i.e., working at 450° to 950° C., or temperatures above β transformation point and below 1100° C.). The material is then aged at 10° to 530° C. or 10° to 700° C. (in the case of working at temperatures above β transformation point), and finally dehydrogenated and recrystallized to prepared a material having a fine equiaxed microstructure.

Structures and magnetic properties of a Nd15Fe77B8 alloy produced by twin‐roller quenching

Abstract: The relationship between the magnetic properties and microstructures of flakes of a Nd15Fe77B8 alloy produced by twin‐roller quenching was studied. Flakes produced at low to intermediate roller speeds (1.4–8 m/s) showed a dendritic structure of Nd2Fe14B oriented normal to the chilled surface, while flakes produced at higher roller speeds showed fine, equiaxed Nd2Fe14B grains. The flakes with a dendritic structure were found to be magnetically anisotropic and showed a high‐energy product (BH)max (15.2 MGOe) in the direction perpendicular to the chilled surface. The flakes that had fine equiaxed grains were magnetically isotropic. The maximum coercivity of the flakes exceeded 20 kOe. The microstructural transition from columnar dendrites to equiaxed grains is explained in terms of morphological changes in the solidification of undercooled melts. Optimization of twin‐roller quenching parameters for the production of desirable microstructures in Nd‐Fe‐B magnets is discussed.

Structures and Properties of Gamma-Tial Alloys Containing Interstitial Elements

Abstract: This paper reports on the effects of carbon, nitrogen, boron, and oxygen on the microstructure and properties of gamma-TiAl alloys. It has been found that nitrogen and boron are capable of altering the solidification structure, whereas the others are not. The additions of >1 at.% B or 0.5 at.% N changed the cast structure from columnar to equiaxed with fine grain sizes. As a result, the strength has been significantly increased. Additional results on phase and structural stabilities will be presented.

Titanium aluminide containing chromium, tantalum, and boron

Abstract: A method for providing improved ductility in a gamma titanium aluminide is taught. The method involves adding inclusions of boron to the titanium aluminide containing chromium and tantalum and thermomechanically working the casting. Boron additions are made in concentrations between 0.5 and 2 atomic percent. Fine grain equiaxed microstructure is found from solidified melt. Property improvements are achieved by the thermomechanical processing.

Metallographic study of the eutectic solidification of gray, vermicular and nodular cast iron

Abstract: A colour map of the segregation pattern can be revealed as a consequence of the development of a film of varying thickness formed by the etchant at the sample surface. When the light incises on this film isoconcentrates in the alloy are revealed in bright colours broght out by double reflection and interference. The techniques have been used extensively in the past in order to relate the segregation substructures with casting structures. As a consequence several contributions have been done to the understanding of crystal nucleation, instability of the solid-liquid interface, origin and development of chill and equiaxed zones as well as the stability of crystals growing freely in the bulk liquid. More recently the use of these techniques has been extended to the detection of the microsegregation associated with nodular cast iron solidification. On this frame, the present paper intents to contribute to the understanding of the solidification mechanisms associated with the eutectic solidification when gray, vermicular and, specially, nodular cast irons are considered.

Superplasticity of Non-Oxide Ceramics

Abstract: Superplasticity is a mode of deformation for fine-grained polycrystalline solids at elevated temperatures: included are metals, ionic polycrystals (e.g. oxides) and covalent polycrystals (e.g. non-oxide ceramics). Plastic deformation of covalent crystals by dislocation glide is not easy because of their high Peierls force. This paper describes high ductilities in Si3N4 and SiC, and also reports superplastic elongations in Si3N4/SiC composite. p]Fine-grained Si3N4/SiC composites (20 wt% SiC) were fabricated by hot-pressing amorphous Si-C-N powder with 6 wt% Y2O3 and 2 wt% A12O3 as sintering aids. The composites were composed of equiaxed grains (<200 nm) and elongated grains. A composite exhibited a superplastic elongation larger than 150 % at 1600 °C and at an initial strain rate of 4 × 10−5 s−1. The superplasticity of the composite is probably related to the presence of an intergranular liquid phase.

Thermomechanically processed structural elements of titanium aluminides containing chromium, niobium, and boron

Abstract: A method for providing improved ductility in a gamma titanium aluminide is taught. The method involves adding inclusions of boron to the titanium aluminide containing chromium, carbon, and niobium and thermomechanically working the casting. Boron additions are made in concentrations between 0.5 and 2 atomic percent. Fine grain equiaxed microstructure is found from solidified melt. Property improvements are achieved by the thermomechanical processing.

Nucleation in the multicrystalline Polix silicon ingot

Abstract: An analysis of nucleation from solid nuclei in Polix photovoltaic polycrystalline silicon is reported. The bulk texture is studied by neutron diffraction using a transmission method. The crystallographic orientation is determined for three adjacent samples along the growth axis in the lower central part of the ingot. The size and orientation of all crystallites are obtained in the domains showing different morphologies: a nucleation zone with equiaxed crystallites, an upper domain with columnar structure, and an intermediate part where both types of crystallization are present. The crystallites are counted in terms of their volume in the three parts. The mean grain size increases in the columnar domain, but a pronounced elimination of particular crystalline orientations is not observed as the morphology is changed.

Application of a Continuous Annealing Process to the Production of Superplastic High-Carbon Steels

Abstract: High-carbon steels with carbon content of more than 0.8wt% have been known to be superplastic by refining their microstructure. Many methods such as warm rolling have been proposed to refine the microstructure. But most of them are considered to be difficult to use as an actual production method. To make high-carbon steels superplastic and economical, application of a cold rolling and continuous annealing process has been examined. p]Changes in microstructure of high-carbon hot rolled steels during the cold rolling and continuous annealing was investigated. By cold rolling with a reduction of more than 50%, a lamellar pearlite structure is deformed and heavily sheared, and a large strain energy is stored. Rapid annealing at temperatures just below the A1 temperature(1000K) is shown to spheroidize the sheared pearlite and to recrystallize the ferrite grains. After processing, the final structure consists of very fine cementite particles with 0.05–0.2µm diameter in a very fine equiaxed ferrite matrix with 0.1–0.5µm diameter. The processed steel shows total elongations of more than 700% and a strain rate sensitivity index of about 0.5 when deformed at 973K.

The nature of metastable phase formation in the microsecond pulsed laser-melted pool of AlGe eutectic alloy

Abstract: The solidification behaviour of a microsecond pulsed neodymium-doped glass laser-melted AlGe eutectic alloy was studied. The results indicate the nucleation and columnar growth of a metastable monoclinic phase from the melt-substrate interface at a high power density. An equiaxed microstructure containing the same monoclinic phase is obtained at a low power level. It is shown that the requirement of solute partition acts as a barrier to eutectic regrowth from the substrate. The resultant interface undercooling favours nucleation of the monoclinic phase. Further, it is shown that this phase grows with very little solute partitioning at this composition, which imparts a growth advantage over other competitive phases.

Method of hot forming beryllium-copper alloy and hot formed product thereof

Abstract: A method of hot forming beryllium-copper alloy including from 1.60 to 2.00% by weight of Be, from 0.2 to 0.35% by weight of Co and the balance being essentially Cu, under specified conditions of a working temperature, a working rate, and an amount of work strain to produce a hot formed product of an equiaxed grain structure having a uniform stable grain size. 2