Heat flow parameters affecting the unidirectional solidification of pure metals

Unidirectional solidification of pure metals has been studied as a function of liquid superheat, heat transfer coefficient of the metal/mold interface(h _i ), and mold material. Experimental results show that the position of the solid/liquid interface (X) varies with time (l) following the relationshipX(t) = A . t ^1/2 -B, where, for a fixed metal,B is a constant only dependent on the superheat through a parabolic law andA is another constant that is independent of the superheat but dependent on the mold heat sink capacity,i.e., mold material and coefficienth _i At the same time, it has been calculated that the constantA varies withh _i through an error-function type law tending asymptotically to Lyubov’s exact analytic solution.     

Amorphous solidification of pure metals in submicron spheres

Submicron droplets of high purity elemental metals have been sprayed in high vacuum by electrohydrodynamic atomization. It is found that under these extreme conditions of high cooling rate and very small liquid volume, some pure metals solidify from the melt as an amorphous phase. A systematic study has been conducted for 15 metals plus silicon and germanium of the distribution of the amorphous phase with droplet size, f.c.c., b.c.c. and h.c.p. metals have been studied and it is found that the b.c.c. metals form an amorphous phase much more readily. The observed maximum diameter of amorphous spheres is less than 100 nm in all cases and is used as experimental input to an analytical simulation to determine the glass transition temperatures of the metals in each case. In general, amorphous phases are found in metals that have a reduced glass transition temperature of greater than 0.44. The dependence of glass transition temperature on other physical properties such as enthalpy of fusion and enthalpy of vaporization is also presented.     

Feasibility of surface carbidization of refractory metals by electric-spark alloying

Conclusions A study was made of the conditions of formation of alloyed layers during the ESA of transition metals with graphite. Negative cathode weight gains were observed together with the formation on the metal specimen surfaces of 5– to 15-m-thick layers with a changed structure, consisting of carbides and oxides of the metals and of the basis material. Carbidization during ESA with graphite was found to be intensified for Group IV metals under soft conditions (process No. 2) and for Groups V and VI metals, under hard conditions (processes Nos. 3 and 6).Translated from Poroshkovaya Metallurgiya, No. 6(222), pp. 42–47, June, 1981.     

The breakdown of single-crystal solidification in high refractory nickel-base alloys

The breakdown of single-crystal solidification has been studied over a wide range of solidification conditions in ten superalloys with large variations in Re, Ta, and W content. Over the range of experimental conditions investigated, grain defect formation was sensitive to local thermaland solutal conditions. For a fixed alloy composition and withdrawal rate, the transition from single-crystal to equiaxed solidification did not occur abruptly. Instead, as thermal gradients were decreased in a series of experiments, isolated, highly misoriented columnar grains with the same composition as that of the base alloy developed in the presence of positive (stabilizing) thermal gradients with increasing frequency until the advance of the single-crystal front was completely blocked. The onset of columnar grain formation occurred when the primary dendrite arm spacing exceeded a critical value, corresponding to a morphological transition in the dendritic array. The onset of “freckling” was observed at the same primary dendrite arm spacing where misoriented columnar grains began to appear. In experiments with varying levels of refractory alloy content, there was also a strong correlation between the onset of grain formation and freckle formation. These observations strongly suggest that in high refractory content superalloys, the breakdown of single-crystal solidification and the formation of misoriented grains as well as freckle-type defects are sensitively dependent on thermosolutal convection processes.     

Columnar to equiaxed transition of metals during solidification by using electric current pulse

 A new approach of applying the Electric Current Pulse (ECP) with parallel electrodes to promotion of the transition from columnar to equiaxed crystals and improvement of macrosegregation was introduced. And the experiments of application of ECP in the different stage of solidification have been carried out. The results show that applying ECP in both the initial and the late stage of solidification can promote the columnar to equiaxed transition (CET). Analysis shows that during solidification application of ECP induce a large number of nuclei around the upper surface, which subsequently showers down into the melt and impinges growth front of the columnar crystal. Therefore the CET occurs. In addition, this method is also employed to influence the solidification processing of bearing steel, the results show that the structrue is changed from columnar to equaixed, indicating that ECP can enhance the homogeneity of structure and composition of bearing steel.     

Surface quality and meniscus solidification in pure chill cast metals

Evaluation of groove formation for static casting conditions. Correlation of groove depth and spacing with meniscus solidification. Simple model for prediction of grooves under various casting conditions i.e. casting speed, superheat of melt, heat extraction at meniscus.     

Heat transfer-solidification kinetics modeling of solidification of castings

A close examination of the recent developments in the field of computer simulation of solidification process reveals that a combination of both macroscopic and microscopic models is necessary in order to accurately describe the solidification of castings. Currently available macroscopic models include models that describe heat transfer from metal to mold, fluid flow of liquid metal during mold filling, and stress field in the casting. At the microscopic level, the models should include more intricate issues such as solidification kinetics and fluid flow in the mushy zone. Although significant progress has been accomplished over the years in each field, the task of including all of these models into a comprehensive package is far from being complete. This paper describes the state of the art on coupling the macroscopic heat transfer (HT) and microscopic solidification kinetics (SK) models and introduces thelatent heat method as a more accurate method for solving the heat source term in the heat conduction equation. A new method for calculation of fraction of solid evolved during solidification based on computer-aided cooling curve analysis (CA-CCA), as well as a method based on nucleation and growth kinetics laws, is discussed. A new nucleation model based on the concept of instantaneous nucleation, which is used to describe equiaxed eutectic solidification of commercial alloys, has been introduced. It is demonstrated that the instantaneous nucleation model agrees well with the experimental results in terms of cooling curves and of evolution of the fraction of solid during solidification. Validation results are also shown for SK models that are based on CA-CCA coupled with HT models for eutectic Al-Si and gray cast iron alloys.     

Hot pressing of oxides of refractory metals

Conclusions A study was made of the sintering process in the hot pressing of several powdered oxides of transition metals, namely, TiO₂, ZrO₂, V₂O₅, Nb₂O₅, and Cr₂O₃. It was established that, at any given treatment duration, raising the temperature steadily increases the rate of densification and density of specimens. At each temperature, the densification process is characterized by a specific final density.Translated from Poroshkovaya Metallurgiya, No. 11 (107), pp. 22–25, November, 1971.     

Measurement of liquid metal diffusion coefficients from steady-state solidification experiments

Steady state solidification experiments were carried out on Sn?Bi alloys of compositions from zero to 3.5 at. pet Bi with tube diameters from 1.9 to 5 mm. Solute profiles were determined utilizing a radioiostope, Bi²⁰⁷, and a microtome slicing technique. Results indicate the technique offers a relatively precise method for determining liquid metal diffusion coefficients at the solidus temperature. Convection does not appear to be a problem for tube diameters under 3 mm. It is shown that neglect of thermal transport may give errors in the measured diffusion coefficients as high as 10 to 30 pet.     

Fluctuation mechanism of normal crystal growth during solidification of metals

A theory is developed, describing the solidification of a one-component metallic melt, based on the model of a two-phase transitional zone separating the crystalline phase and the melt. The concentrations of solid state atoms within each mono-atomic layer of the transitional zone are assumed to fluctuate due to thermal fluctuations. A crystal growth law has been derived expressing the crystallization velocity in terms of probability functions describing these concentration fluctuations. For small supercoolings the theory predicts that the crystallization velocity should be proportional to supercooling (normal crystal growth law). The kinetic coefficient of the normal crystal growth law has been calculated in terms of a material dependent parameter, the number of mono-atomic layers comprising the transitional zone, and the total number of atoms within a single mono-atomic layer. The material dependent parameter is determined by the fraction of an atom's nearest neighbors within a single mono-layer to the same number within the volume, the melting heat of the material in question, and the temperature.     

The preparation of aluminides of the refractory metals

Summary A study of the conditions of preparation of aluminides showed that the latter can be produced by direct synthesis, using the hot pressing method; however, this process can only be employed when a high purity of the materials is not essential.High-purity aluminides can be obtained by arc or vacuum melting, using very pure starting materials. In this case, in view of the volatibility of aluminum and its higher vapor pressure compared with that of the transition metals, an excess of aluminum must be provided.Aluminides produced by arc or induction melting should be subjected to grinding, followed by hot pressing for the preparation of dense specimens.     

High-temperature compatibility of refractory metals with scandium oxide

Conclusions Chemical reaction between Sc₂O₃ and Ta, Mo, and W is not observed when they are heated to 2100°C in a protective medium for 4 h. The depth of diffusion of the metal into the oxide is less than the resolving power of the microscope (1). It is, therefore, theoretically possible to use scandium oxide as a dispersion-strengthening phase for metal artifacts of Ta, Mo, and W, for coating these artifacts with Sc₂O₃, etc.Translated from Poroshkovaya Metallurgiya, No, 1, pp. 83–87, January, 1968.     

The siliconizing of refractory metals under nonequilibrium conditions

Summary It is shown that, during the vacuum siliconizing of refractory metals in the absence of an equilibrium silicon concentration at the phase boundaries, the growth of the silicide layer as a function of time may obey, a linear law. This is observed during the initial period of siliconizing, when new phases are formed, as well as in the presence of impurities whose vapor pressure is much higher than that of silicon.