Near-eutectic Composition Research Articles

Microstructural Evolution in Near‐Eutectic Yttrium Silicate Compositions Fabricated from a Bulk Melt and as an Intergranular Phase in Silicon Nitride

Near‐eutectic composition Y2O3─SiO2 melts were formed as bulk samples or as an intergranular phase in Si3N4. Upon cooling to room temperature the bulk material partially crystallized to γ‐Y2‐Si2O7 whereas the intergranular phase was glass. On heat‐treating at 1500°C the bulk material transformed to γ‐Y2Si2O7 whereas the intergranular glass crystallized first to γ‐Y2Si2O7 and then to β‐Y2Si2O7. Possible reasons for the different behavior are discussed.

Solidification of Undercooled Bulk Melts of Fe–Cr–C, Co–Cr–C and Ag–Ge Alloys of Near-Eutectic Composition

Solidification of Undercooled Bulk Melts of Fe–Cr–C, Co–Cr–C and Ag–Ge Alloys of Near-Eutectic Composition

Microstructure and crystallography of a rapidly quenched, eutectic Cu-P alloy

A Cu-P alloy with near-eutectic composition was quenched rapidly by the melt spin process. An investigation of the product by transmission electron microscopy revealed a microcrystalline, directionally solidified, lamellar eutectic of cubic (c) Cu and hexagonal (h) Cu3P, which are the equilibrium phases. Between these two phases there is a definite orientation relationship: (0110)h//(111)c and [001]h//[615]c. The interface is parallel to (0110)h and (111)c. There appears to be no preferred growth direction.

Mechanism of phase formation during quenching in the Al-Ga system

Through an evaluation of the bond energy in eutectic and metastable phases of different crystallographic structures but the same composition in the aluminumgallium system, an explanation is given of the mechanism of phase formation in this and other eutectic systems during quenching. It is shown that a metastable phase is formed in this system from the liquid, where preferred interaction between different atomic species should be expected to occur in the melt. An explanation is also given for the nonoccurrence of the expected expansion of the region of the solid solution based on aluminum above 65% (at.) gallium during ultrarapid cooling of the liquid. Finally, an explanation is offered for the well-known empirical fact that most metastable phases formed during quenching are formed in the region of eutectic or near-eutectic compositions.

Structural variation in near-eutectic zinc-aluminum alloys

The effects of cooling rate and titanium addition on the as-solidified structures of hypo- and hypereutectic Zn-Al alloys of near-eutectic composition have been investigated. It was found that increasing cooling rate refined the microstructure of hypoeutectic alloys (i.e., on the Zn-rich side of the eutectic) whereas, in the case of hypereutectic alloys, it could also modify the nature of the microconstituents present. The addition of titanium decreased the size of the primary aluminum solid solution α′ crystals in hypereutectic alloys and the eutectic grains in all alloys. However, there was no observable influence on the nucleation of primary zinc solid solution β crystals in hypoeutectic alloys. The results are interpreted in terms of the location of the coupled zone of two-phase eutectic growth and of the factors affecting nucleation in the Zn-Al system.

Structure and internal friction of alloys of the system Zr-B with near-eutectic composition

The authors employ electron and x-ray diffraction and scanning electron microscopy to investigate the internal friction and structure and crystallization parameters of zirconium boride systems under conditions of quenching at a cooling rate of 10,000,000 K per second.

High-energy-density pulsed ion-beam irradiation of Co/Si, Pt/Si, and Au/Si

Pulsed ion beams have been used to irradiate Pt/Si, Co/Si, and Au/Si samples. The thicknesses and compositions of the reacted layers were determined using Rutherford backscattering spectrometry. The phases in the reacted layers were identified using x-ray diffraction and transmission electron microscopy. Low-energy-density irradiation produced interfacial layers having near-eutectic compositions. High-energy-density irradiation produced interfacial layers having graded compositions (ranging between two solidus phases). The experimentally determined compositions, compositional gradients, and phases are consistent with interfacial melting at temperatures at or above the eutectic.

Endogenous-exogenous freezing characteristics of pure and impure as-cast Zn-Al alloys

AbstractA study was initially made of the cast structures of pure binary Zn-Al alloys having compositions within the eutectic horizontal. For the fixed casting conditions employed it was observed, in hypoeutectic alloys, that there is a coarsening of the macrostructure with increasing aluminium content accompanied by a change in the mode of freezing from endogenous (equiaxed grain structure predominating) to exogenous (columnar grain structure predominating). On the other hand, hypereutectic alloys were observed to freeze as completely equiaxed structures for all compositions examined. When Pb was added as an impurity to alloys in the range 3–10 wt.-%AI, the general effect was a coarsening of the as-cast macrostructure. However, two major differences were noted between the solidification of hypo- and hypereutectic alloys. In hypoeutectic alloys of neareutectic composition the mode of freezing changed from exogenous to endogenous, whereas the opposite effect occurred in the hypereutectic alloys. Furthermor...

Coupled and uncoupled growth in silver-copper alloys

An examination has been made of the structures of undercooled silver-copper alloys of eutectic and neareutectic composition. The results indicate that a coupled zone of the form predicted by the coupled zone theory exists for this alloy. Material outside the coupled zone grew either with an anomalous structure or with dendrites, depending on the relative amounts of undercooling of the two phases.