Metastable Phase And Non-equilibrium Solidification In Hypercooled Bulk Fe-B Eutectic Alloys

Adopting molten glass denucleating technology combined with high frequencyvacuum melting and cycle superheating, the bulk Fe-rich side Fe-B eutectic alloy meltswere undercooled. The main factors affecting melt purifying and the micro-purificationmechanism were investigated. The optimized purification processing and parameters forhigh undercooling and hypercooling were given out. Using ICP, DSC, SAED, SEM,XRD analytical methods, the relations between the microstructure evolution andundercooling were systematically revealed. With the aid of the classic nucleation theory,time-dependent nucleation theory and dendrite growth theory, the microstructureformation, phase selection and competitive nucleation mechanism of metastable and stablephases were analysed. The forming mechanism of anomalous eutectic under the conditionof high undercooling is described. The thermal stability of the undercooled bulk metastablephase alloys and its soft magnetism properties were also investigated. The main results areas follows:1. The high undercooling and hypercooling of bulk Fe-B eutectic system alloy meltscan be achieved using the molten B₂O₃ glass denucleating technology combined with cyclesuperheating after preevacuation and filling back with Ar gas. The hypercoolings of324K～460K have been obtained successfully in bulk Fe₈₃B₁₇ eutectic alloy melts, and485K in Fe₈₀B₂₀ hypereutectic alloy melts. So the primary nucleation undercoolings of(0.3～0.4) T_m were obtain in bulk Fe-B eutectic system alloy melts.2. The regularity of the microstructure evolution with undercooling in bulk Fe₈₃B₁₇eutectic alloy is as follows: forâ–³T＜50K, regular lamellarα-Fe/Fe₂B eutectic is obtained;for 50K＜△T＜300K, the above structure is replaced by the hypereutectic structure withFe₂B as the primary phase; for 300K≤△T＜386K,α-Fe/Fe₂B eutectic is observed onceagain, and anomalous eutectics is formed progressively; forâ–³T≥386K, metastable Fe₃B isformed directly from the hypercooled melts andα-Fe/Fe₃B anomalous eutectic results, then metastable Fe₃B phase is preserved in the as-solidified microstructure at roomtemperature and no other solid phase transformation occurs.3. The competitive growth betweenα-Fe and Fe₂B phases in the undercooledFe₈₃B₁₇ eutectic alloy is calculated in terms of LKT/BCT dendrite growth theory. Then theeutectic couple zone and microstructure evolution map in the range of wide undercoolingis given accurately.4. Once metastable phase Fe₃B appears in Fe-B eutectic microstructure, theremarkable grain-coarsening occurs. The main mechanism of the coarsening is that thealloy melts have been hypercooled when metastable phase Fe₃B forms. At highundercooling, the nucleation process is mainly controlled by interfacial atomic diffusion,so atomic diffusion becomes difficult with increasing undercooling, which will result in thedecreasing of nucleation rate. Whereas, the growth velocity of crystal is sufficiently highunder the condition of the hypercooling. Therefore, the grain-coarsening results.5. The main mechanism of metastable phase formation in undercooled Fe₈₃B₁₇eutectic alloy melts is that the metastable phase Fe₃B defeats stable Fe₂B phase in theircompetitive nucleation. That is to say, after both of metastable and stable phases satisfynucleation thermodynamics condition in the alloys with definite composition, themetastable phase Fe₃B will suppress stable phase Fe₂B nucleating and grow preferablyonce the incubation time of metastable phase is less than that of stable phase.6. The theoretical and experimental results about the competitive nucleation in theundercooled Fe₈₃B₁₇ eutectic alloys indicated that nucleation kinetics condition is moreimportant than thermodynamics condition for the metastable phase formation in thecompetitive nucleation between metastable Fe₃B and stable phases Fe₂B which satisfynucleation thermodynamics simultaneously. Moreover, the crystal structure of metastablephase is also an importmant cause of preferable nucleation. And the incubation time basedon time-dependent nucleation theory is more available to predict competitive nucleation of metastable phase in comparision with the classic nucleation theory using the samethermodynamic parameters.7. An anomalous eutectic forming mechanism in the undercooled Fe-B eutecticalloys is described. It is that the regular lamellar eutectic structures disintegrate, ripen andform anomalous eutectic, or Fe₂B (Fe₃B) phase nucleates firstly and built up aninterpenetrating eutectic frame, within whichα-Fe phase completes nucleation and growth,so anomalous eutectic is formed.8. Metastable phase Fe₃B forming directly from the hypercooled alloy melts cankeep stable in the temperature range of 1223K-1273K for a period of time, i.e.Fe₃B (?)α-Fe+Fe₂B, Fe₃B phase is more stable than that precipitatedfrom amorphous alloys by heat-treatment.9. The measurements of soft magnetism properties suggest that the bulk metastableeutectic alloy prepared by hypercooling rapid solidification technique possesses moreexcellent soft magnetism properties than that of the corresponding quasi-stable eutecticalloy and amorphous alloy.