The Reversibility Of Liquid Structure Transition Of Cu-Sn/Sb And Pb-Bi/Sb Alloys And Its Effects On Solidification

Researches on the phenomena and regularities of solidifications are not only realted to the development of basic subjects, by also to the inovations of development and processing technolgies of materials. It has been recognized that melt structures have great effects on solidified microstructures and properties of solids, namely "the correlations between liquid to solid". This field attracts more and more attentions and become a new breaktrough of solidification theories and techniques. In the past, people changed the melt thermal history by melt overheating treatments, and further improved the solidified microstructures and properties of materials. However, due to the exact mechanism of the melt overheating is still not fully understood, the actual effects of melt overheating treatments are tend to not as good as the expectations.Recently, more and more attentions have been paid to liquid polyamorphism, and the liquid structure transitions are observed and verified to occur with temeperature or pressure. With the fact that temperature-induced liquid-liquid structure transitions (TI-LLSTs) have been found in more and more liquid metals and alloys, the regularity and mechanism of liquid structure changes have been understood more and more clearly. At the same time, the above mentioned facts provide a new viewpoint to investigate the liquid-solid correlation. In the present paper, the reversibility of TI-LLSTs of Cu-Sn, Pb-Bi and Pb-Sb alloys was explored, and the influences of TI-LLSTs with different characteristics (reversible and irreversible) on the solidified behaviors and solidified microstructures of those alloys were investigated. The present work aimed at deepening the understand of the essence and mechanism of the liquid-solid correlationship, and furthermore, providing a scientific and technical basis for controlling the solidified beaviors, solified microstructures and properties of as-cast by manipulating the liquid structural states. The innovative points and mian results of this paper are listed below.(1) The high temperature XRD results of InSn49.1(wt.%) melt proved that the methods of exploration the liquid structure by measuring their resistivity, thermal power, and internal friction are effective. In addition, in present paper, a theoretical discussion on the correlation between structure and physical properties of InSn49.1 melt was also given. It is believed that, during the TI-LLST, the sudden changes of electron density n_e and mean free path of conduction electrons L₀ lead to the abnormal change of resistivity and thermal power, and anomalous changes of internal friction and viscosity might related to the discontious variation of pair distribution functions and mean nearest neighbor distance.(2)The temperature dependences of resistivity(p-T) and thermal power(S-T) of Cu-Sn/Sb and Pb-Bi/Sb melts were investigated with the DC four probes method, and compared with the results of DSC/DTA and internal friction(Q^-1). The p-T, S-T, DSC/DTA and Q^-1-T curves of those melts also changed within certain temperatures, which suggested the TI-LLSTs occurred in those melts. In addition, the structure changes of different alloys have different features, which reflected on the occurring temperature ranges, pattern and types of those TI-LLSTs were different.Theρ-T curves of CuSn30, PbBi56.1, PbBi80 and PbSb5.8 melts changed unlinearly in the heating procedure, but changed linearly in the cooling process, which suggested the irreversible TI-LLSTs occurred in the heating procedure of those melts. It is believed that the TI-LLSTs were resulted from the destruction of solid-like clusters in the certain high temperature ranges.The results of resisitivity experiments show that the reversible TI-LLSTs were occurred in the heating process of CuSn80 and CuSb70 melts. After the melts experienced irreversible TI-LLSTs, the structures of them changed into new ones, and the TI-LLSTs occurred in new melt are reversible. By analysis the pattern ofρ-T curves and the results of solidification experiments, it is believed that the irreversible TI-LLSTs is related to the destruction of the solid-like clusters corresponding to Cu₆Sn₅ and Cu₃Sn, but the reversible TI-LLST may be related to Cu₃₃Sn₆₇ or Sn clusters. The reversible TI-LLST occurred in CuSb70 melt may be related to the rebuliting/destruction of Cu₂Sb clusters.(3) Different types of TI-LLSTs have different influences on the solidification of alloy. The results of solidification experiments show that, the irreversible TI-LLSTs have great effects on solidification of CuSn30, PbBi56.1 and PbBi80 melts, such as the enlarged undercooling, increased nucleation rate, advanced dendrite coherency point, refined microstructures, and modified morphologies.However, to the melt which experienced reversible TI-LLST, we should discuss it from two aspects. If the reversible clusters return to the state of beforehand, then the reversible TI-LLST has no obvious effects on solidification behaviors and microstructures. If we can prevent the reversible clusters not return to the state of beforehand, the solidification behaviors and microstructures would be apparently changed.These results revealed the physical nature of "correlation between solidified microstructures and the melt thermal history", and also suggested that, to control the solidification behaviors and solidified microstructures more effectively by manipulating the melt thermal history, whether and how the melt structures change should be kwown in advance. Or else, due to the blindness, the melt thermal treatments may not be effective for controlling solidification.(4) We also investigated the influences of TI-LLST on the directional solidification microstructures and the composition segregation patterns of the alloy PbSb5.8. It was found that the TI-LLST affected greatly on the solute redistribution and the dendrite growth. Firstly, the TI-LLST would lead to the reduction of solute redistribution coefficient. Secondly, it results in the aggravation of solute enrichment and composition undercooling in front of the liquid-solid interface. Furthermore, TI-LLST gives rise to an obvious reduction of the secondary dendrtic spacing and composition decrease along the dendrite core. The effects of TI-LLST on the primary dendrtic spacing revealed some regularities, although the TI-LLST has different effects on the primary dendrtic spacing under different drawing velocitys.