【作者】 魏超锋；

【导师】 坚增运；

【作者基本信息】 西安工业大学 ， 材料加工工程， 2008， 硕士

【摘要】 许多金属材料的制备过程都包含液-固转变,作为母相的液体结构和性质对所形成的固体材料的组织和性能有重要影响。虽然人们对液态金属熔体的结构进行了大量的实验和理论研究,但是,国内外的研究主要集中在熔体在一定温度下的结构及其随温度的变化上,未见有温度变化后其结构随时间的变化规律研究。本文主要研究熔体温度变化后,其电阻率、形核过冷度和凝固组织随时间的变化规律。取得的主要研究结果主要有:采用四电极法对连续升降温和非连续升降温后过热和过冷液态镓的电阻率进行了测定。结果发现温度变化后液态镓的电阻率随保温时间的延长而发生变化。温度快升后,液态镓的电阻率随保温时间的延长而增大,温度快降后,液态镓的电阻率随保温时间的延长而减小。这说明镓熔体结构的变化滞后其温度的变化。这一滞后时间超过20分钟。在连续升降温过程中,在同一温度下液态镓的在升温过程中的电阻率小于其在降温过程中的电阻率,这与温度快变后液态镓电阻率随时间的变化规律是一致的。首次发现镓熔体电阻率随温度的变化在熔点温度附近具有不连续性。加热过程中,过冷镓熔体的电阻率在接近镓的熔点时有一个突变。连续加热时,完成突变所需的时间为6分钟左右;急速加热时,完成突变所需时间为7分钟左右。经过过热的镓熔体冷到其熔点温度以下时,其电阻率的突变存在非常明显的滞后性,镓熔体的电阻率开始发生突变的滞后时间超过67分钟,镓熔体电阻率发生突变所用的时间为7分钟。采用DSC热分析技术研究了过热的镓熔体降温后的保温时间与形核过冷度及放热峰面积的关系。结果发现,镓的形核过冷度及放热峰面积均随降温后的保温时间的延长而减小,这与镓熔体电阻率实验所得结果是一致的。建立了预测液态金属原子团尺寸的理论模型。根据此模型可确定液态金属中最大原子团尺寸与温度的关系。从此模型中预测的液态镓中最大原子团尺寸随温度变化规律符合其电阻率、过冷度和结晶潜热随温度的变化实验规律。研究了熔体加热温度和保温时间对Al-11.6%Si合金显微组织及性能的影响。结果发现:合金液的加热温度和保温时间不仅影响合金凝固的组织,而且也影响其力学性能。将合金液温度从720℃升到900℃,共晶硅由粗大的针片状变为点状或短棒状、合金抗拉强度提高了6.33%、延伸率提高了10.83%。升温后的保温时间对合金组织和力学性能的影响时间不同于降温后的保温时间,随升温后保温时间的延长,合金的组织变细、抗拉强度和延伸率提高;随降温后保温时间的延长,合金的组织变粗、抗拉强度和延伸率降低。更多还原

【Abstract】 For many metal materials,the preparation process involves a transformation from liquid to solid.So the structure and character of liquid inevitablely influence the structure and property of solid.Many experimental and the theoretical researches have been done to the structure of metal melt.However,these researches mainly concentrate on the melt structure at a certain temperature or the effect of temperature on the melt structure.It has not been reported that the effect of the time after changing the melt temperature on its structure.This thesis mainly deals with the effects of the time after changing the melt temperature on the resistivity,the nucleation undercooling and the solidification structure of metal melts.The resistivitiese of the overheated and the undercooled liquid gallium during continuous and discontinuous heating and cooling processes have been determined by using four electrodes method.It is found that the resistivity of liquid gallium varies with the time after changing the melt temperature.For the discontinuous heating process,the resistivity of liquid gallium increases with the time after changing the melt temperature.For the discontinuous cooling process,the resistivity of liquid gallium decreases with the time after changing the melt temperature.These mains that structure transformation of melt lags behind its temperature.The lagging time for liquid gallium is beyond 20 minutes.For the continuous heating and cooling processes,the resistivity of liquid gallium at a temperature during the continuous heating processes is lower than that during continuous heating processes,which is in agreement with the result of discontinuous heating and cooling processes.It is found that the dependence curve of resistivity on temperature is discontinuous around the melting point.For the heating process,the resistivity of undercooled liquid gallium increases suddenly when the temperature approaches the melting point.The time for accomplishing the sudden change of resistivity is 6 minutes for the continuous heating processe, and 7 minutes for the discontinuous heating processe.For the cooling process,the resistivity of superheated liquid gallium decreases suddenly when the temperature is lower than the melting point.The lagging time for the resistivity to start sudden change is as long as 67minutes,and the time for accomplishing the sudden change of resistivity is 7 minutes.The effects of the time holding melt in a temperature after a heating process on the nucleation undercooling and the exothermic peak area of gallium have been determined by using DSC.It is found that the nucleation undercooling and the exothermic peak area decrease with increasing the time holding the melt in a temperature after a heating process,which is in agreement with the result of resistivity.A model to predict the size of atom cluster in liquid metal has been made.In terms of this model,the dependence of the largest atom cluster in liquid metal on temperature can be determined.The results of the dependence of the largest atom on temperature predicted from present model is in agreement with the experimental results of the resistivity,nucleation undercooling and the exothermic peak area obtained in liquid gallium.The effects of melt temperature and holding time on the microstructure and the mechanical property of Al-11.6%Si alloy have been studied.It is found that the melt temperature and the holding time influence the microstructure and the mechanical property of alloy.When the melt temperature increases from 720℃to 900℃,the eutectic silicon transforms from large plate to short rod or small point,and the tensile strength and extensibility increase 6.33%and 10.83%, respectively.The effect of the time holding melt in a temperature after a heating process on the microstructure and the mechanical property of Al-11.6%Si alloy is different from that of a cooling process.For the heating process,increasing the holding time can refine the microstructure and increase the mechanical property of alloy.For the cooling process,the microstructure becomes large and the mechanical property of alloy decreases with increasing the holding time.更多还原