Morphologies of silicon crystals solidified on a chill plate

Electromagnetically levitated liquid droplets of pure Si or a Si-Ge alloy were cooled to different temperatures and then dropped onto a chill plate of Cu. Droplet oscillations mark the solid/liquid interface during solidification and permit the different crystal morphologies of silicon to be observed on the quenched surface by scanning electron microscopy (SEM). A spherical morphology found on the quenched surface represents the initial stage of crystal growth. Further growth leads to octahedral crystals bounded by {111} faces near equilibrium and to other polyhedra or even faceted dendrites further from equilibrium. The spherical growth can be observed only when the initial melt undercooling is moderately high. The critical size at which spherical crystals start to develop dendritic growth is much bigger than that calculated from the Mullins and Sekerka model, and is bigger than the Coriell and Parker model when kinetic undercooling is taken into account.     

Growth direction of columnar crystals solidified in flowing melt

Aluminum-copper alloys containing 0.5–8.8 wt% Cu were unidirectionally solidified. Columnar crystals were grown in flowing melts at flow rates ranging from 4 to 90 cm/s, and the deflection angle of the crystal growth direction in the upstream direction was measured as functions of flow rate and copper content. The columnar crystals originated from nuclei or solid particles with the most suitable crystallographic orientation for growing in flowing melt. The growth direction of the columnar dendrites was nearly equal to that of the columnar grains comprising the columnar dendrites. The deflection angle for the columnar grains increased only slightly with an increase in flow rate or copper content within the ranges investigated, while the inclination angle of the growth direction of columnar dendrites from the [100] direction increased with an increase in flow rate and with a decrease in copper content.     

Development of rapidly solidified 6.5 wt% silicon steel for magnetic applications

The 6.5wt% Silicon steel has always a technological importance owing to its soft magnetic properties and low core losses. In this research, this steel is prepared in the form of continuous ribbons of 25 mm width and 35 mm thickness by rapid solidification technique using melt spinner. The as-cast ribbons show single phase of α-FeSi as observed from x-ray diffraction. By transmission electron microscopy (TEM), the alloy shows disordered A2 structure in as-cast ribbon while B2 ordered phase forms from the A2 matrix at annealed condition. Two magnetization temperatures are observed at 670 and 760°C due to the phase transformation of B2 and A2 structures, respectively. The alloy showed enhancement in the soft magnetic properties with reduction in coercivity from 1.90Oe in the as-cast state to 0.58Oe when annealed at 850°C. The lowering of coercivity is due to the formation of ordered B2 phase during annealing.     

Recrystallization of solidified columnar crystals in an Fe-36%Ni austenitic alloy

The recrystallization behavior of the 21 columnar grains in an Fe-36%Ni austenitic (f.c.c.) alloy during annealing at 973 K after 70% cold-rolling, whose rolling behavior had been studied in the coupled paper, has been studied with emphasis on the effect of the initial orientation. The recrystallization rate and the recrystallized grain size of the columnar-crystal specimen were hardly dependent on the initial orientation. This result is in contrast to that reported in the Fe-19%Cr (b.c.c.) columnar crystals. The reason for this difference was discussed in connection with the easiness of cross-slip in the two alloys. On the other hand, the orientation of recrystallized grains was dependent on the initial orientation. The columnar-crystal specimen showed the (001)〈310〉 recrystallization texture. The (001)〈310〉 recrystallized grains preferentially appeared from the shear bands which formed in the (001)[110] initially oriented columnar grains. TEM observation showed that some elongated cells within the shear bands have the (001)〈310〉 orientation in the as-rolled state.     

Faceted growth of silicon crystals in Al-Si alloys

Primary silicon crystals in hypereutectic aluminum-silicon alloy castings frozen at moderate rates have been studied metallographically. Etched growth traces parallel to external facets were used to demonstrate that the majority of the crystals, though of widely varying external shape, grow in basically octahedral form, enclosed in {111} facets. Equations are developed that predict the limits within which true octahedral or spinel-type growth can occur. The equations are tested by measurements of growth trace spacings. Outside the predicted limits, there can be accelerated corner growth, which may result in hollow hopper crystals or even a dendritic form. Alternatively, a corner may be truncated by formation of a facet other than {111}.     

Dislocation mobility in heavily doped silicon single crystals

Dislocation mobility has been studied in heavily doped silicon single crystals by kilohertz internal friction measurements. The dopant effect on dislocation mobility previously reported in germanium and silicon under plastic deformation has now been seen in silicon under the influence of much lower applied stresses. The low strains and only moderately elevated temperatures used in the present investigation allow the dislocations to oscillate only slightly about their equilibrium positions. The results from the slight dislocation excursions have led to a unified kink model of dislocation damping in silicon. The dopant effect on dislocation mobility is attributed directly to the surplus electronic carriers in the immediate vicinity of the dislocation kink. The intrinsic carrier concentration in the local vicinity of the dislocation has been found to be 100 times greater than the corresponding bulk value.     

Effect of wave motion on chill cast surfaces

The formation of exterior surface ripples, and their dependence on filling rate and superheat, was studied in bottom-poured ingots of low melting Sn-Pb alloys. It was found that surface waves in the liquid metal during mold filling are the basis for an important mechanism leading to formation of the primary exterior ripples. Imposition of a steady D. C. magnetic field strongly dampens the surface waves in the liquid metal, essentially eliminating the primary ripple marks throughout the bottom two-thirds of the ingots. The magnetic field also enhances columnar grain growth. E. N. ZULUETA, formerly Research Assistant at Massachusetts Institute of Technology.     

Niobium carbide heaters used in growing silicon carbide single crystals

Conclusions Silicon carbide single crystals can be grown by sublimation by using a furnace with niobium carbide heaters. The working life of these heaters is much greater than that of graphite ones. The crystals are suitable for making light-emitting diodes.Translated from Poroshkovaya Metallurgiya, No. 2(278), pp. 104–106, February, 1986.     

Microstructure stabilization in a rapidly solidified

Tensile properties and their relationship with microstructural features were investigated for a rapid solidification processed (RSP) type 304 stainless steel (SS) extruded powder material and compared with those of a conventionally processed type 304 SS. Significant improvements in tensile strength were observed up to 800 °C (maximum test temperature) for the RSP alloy. Stable and fine microstructural features, including grain size, small matrix precipitates, high residual dislocation density, and a high population of nanosized void/cavities, were observed in the RSP specimens after heat treatments to0.9T _m . The microstructural features directly re- sponsible for strengthening the RSP alloy were small grain size and the residual dislocation density. Formerly with Formerly with