Combined Effects Of Melt States And Cooling Rate On Solidification Of Pb-Te/Sb/Bi Alloys

Theories and experiments indicated that similarly to polymorphism structure change ofcrystalline, liquids and amorphous solids also existing structure polymorphism and itschange phenomenon. In recent years, many groups have investigated and checked thephysic phenomenon on temperature induced liquid-liquid structure change of some alloysand simple substances. Based on liquid-liquid structure change, the effects and mechanismof alloy melt states on crystallization and solidification structures were studied. It overcamethe blindness of exploratory changing solidification structure by melt overheating and gotan encouraging progress. However, it is undoubtedly need to further studying, especially onthe interaction, regularity and cognize of melt states and cooling conditions. It needs asystematic exploration and it is the start point and aim of this paper.On the basis of backgrounds and objectives above, this paper investigated differentkinds of concentrations of Pb-Sb/Te/Bi three systems, based on resistivity reveals differentconditions, this study adopt copper mold, iron mold, crucible air cooling, furnace programcontrolled cooling, and thermo insulator etc. to obtain different cooling velocities, thenusing thermal analysis, structure inspect and X-ray diffraction to explore the influence andregularity of melt states as well as cooling velocity on the solidification behavior,phase/texture scale and distribution, patter of crystal grow and morphology of crystal. Inaccordance with liquid theory and fundamentals of solidification, the mechanism of thisphenomenon was discussed. The main conclusions and results just as follows:（1） Two cycles heating and cooling resistivity testing of Pb-Sb30investigated thatthere are two different temperature regimes abnormal behavior of resistivity-temperaturecurve (ρ-T) during first heating, the after cooling and heating ρ-T curves reveal linearrelation. It indicates that a temperature induced structure change had happened during thefirst heating procedure, and it is irreversibility.（2） Solidification thermal analysis and its data indicates that compared withsolidification before states change, its solidification behavior changed obviously afterstructure transition, it is mainly reflected in primary phase and eutectic undercoolingincrease, the rise of primary phase precipitation exothermic peak proclaim nucleation rateincreased, solidification velocity improved. The influence of solidification cooling ratemainly reflected in although improve cooling rate can increase undercooling, the effects ofmelt states change on increase undercooling is weakened by cooling rate rise.（3） Analysis on solidification structures demonstrate no matter primary phase oreutectic texture, its solidification textures refined to varying degrees after melt states changed; rise cooling rate also notably refine crystalline texture just as anticipated.Nevertheless, the interaction between cooling rates and melt states are not identically same,it reveals different regularity in accordance with different alloy systems or compositions.With regard to some alloys, the effect of refine texture by melt states changing is enhancedalong with cooling rate rise, e.g. Pb-Sb90, Pb-Bi70; To some other alloys, the effect isopposite, e.g. Pb-Sb30. Regardless it doesn’t have a satisfactory theory interpretation tillnow, the discordance of cooling rate and melt state interaction effect solidificationundercooling prompt that solidification undercooling is not the unique factor to texturerefine.（4） Texture morphology and the crystalline orientation revealed by X-raydiffraction indicate that melt state and cooling rate both can effect growth pattern ofprimary phase like semimetal Sb, melt state change or cooling rate rise make crystal fromfacet growing gradually turn to non-facet growing, simultaneously crystalline morphologyturned from outer edge angular bulk or dendrites to out edge passivation dendrites. In otherwords, both can weak anisotropy during crystal growth, and the interaction make effectremarkable.（5） Analysis indicate that melt experience structure change make its state change,its short-order cluster diminish, its disorder increased, so it needs large undercooling duringcooling process, make nucleation rate increased and crystal refined. On the other hand, theeffect of melt state change or cooling rate rise weak crystal anisotropy is due to the solidliquid interface diffusion degree increased, by the same time interface roughness increased,therefore solid liquid interface structure changed from facet to non-facet.