Effect Of The Structure Transition On The Solidification Microstructure And Property Of PbSnBi Alloys

In recent years, the studies of effect of heat history on solidification have made some achievements, it has been found that the structures and properties of some alloys could be obviously improved by such superheat treatment,but some deep problems about the essence of effect of heat history on solidification behavior still needs further discussion.Prior investigation suggested that a temperature–induced liquid-liquid structure transition(TI-LLST) could occur in Pb26Sn42Bi32 ,Pb36.32Sn58.68Bi5 and Pb3.82Sn6.18Bi90 alloys. In this paper, on the basis of the previous studies, the three components alloys were experienced different heat treatment, the solidification behaviors,microstructures and property of melt with reversible TI-LLST and melt with irreversible TI-LLST were studied. At the same time, we do some research about the solder containing Pb PbSnBi Innoxiousness by use of heat-treatment.The major contents and conclusions are as follows:First,The Pb26Sn42Bi32 melt with reversible TI-LLST were experienced different heat treatment, the solidification microstructures of Pb26Sn42Bi32 alloy is invariant in the air,due to cooling speed is very big, inhibit the reversibility of atom clusters in Pb26Sn42Bi32 melt,So that the microstructures of melt became finer and the Vickers hardness of the sample is reduced after solidified from rapid solidification. The Pb36.32Sn58.68Bi5 and Pb3.82Sn6.18Bi90 melt with irreversible TI-LLST were experienced different heat treatment, due to irreversible TI-LLST, irreversibility of atom clusters in melt completely disappear,the melt structure becomes more homogeneous and disorder after structure transition,so the microstructures became finer, the components became homogeneous in the air cooling and rapidly cooling conditions, especially in rapid solidification microstructures,the differences is very significant.Second,the directional solidification experiments of three components PbSnBi alloys were carried out to investigate the effect of the TI-LLST on directional solidification microstructures by the using of self-made unidirectional solidification device.the results show that the directional solidification microstructures are changed obviously after TI-LLST, the directional solidification microstructures of sample became finer that experienced structure transition,and the directional is extraordinarily consistent.Third, the solder containing Pb Pb36.32Sn58.68Bi5 undergo irreversible TI-LLST after superheat treatment, compared with pre-structure transition,we found the evaporation of Pb in more difficult after structure transition; the relative corrosion rate and corrosion rate of the samples in acid, alkali, salt become smaller; the formation of passivation film of the sample experienced structure transition earlier than the sample unexperienced structure transition in H2SO4,the corrosion current of the sample experienced structure transition at each time should be less than the sample unexperienced structure transition in the NaOH, the passivation film of the sample experienced structure transition will be generated eventually in NaCL, while no passivation film is generated in the sample unexperienced structure transition.According to the studies that the influence of liquid-liquid structure transition on solidification behaviors, microstructure and property of alloys , not only to avoid the blindness of traditional melt heat treatment process, but also to provide effective method for improvement of the preparation technology of alloys,and provide scientific basises for more in-depth understanding of liquid material structure.