【作者】 彭鹏；

【导师】 郭景杰；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2009， 硕士

【摘要】 本文以Ti-53at.%Al包晶合金为研究对象,在5μm/s~35μm/s速度区间内开展定向凝固实验研究。通过光学金相等手段对从初始过渡到稳态凝固组织的演化规律进行了研究。测量了凝固组织特征尺度如一次枝晶间距等,对后续固态相变形成的片层结构取向也进行了分析。同时选取典型生长速度,进行淬火前的保温实验,研究了L +α→γ包晶反应动力学;测量了包晶相层厚度的变化,重点考察了合金在包晶凝固过程中的溶质传输和包晶相的生长机制。随生长速度增加,固液界面形态由粗胞状、细胞状、胞状枝晶再到枝晶变化,胞/枝晶间距随生长速度的增加而呈近似指数减小。凝固组织由α和γ两相组成。随着生长速度的增大,初生α相由胞晶生长逐渐演变为枝晶生长,三相接触面积逐渐增大,包晶反应程度相应增大;且高速时自液相析出的γ相增多。包晶反应及自液相析出造成γ相的含量持续增加。对α2/γ片层取向进行了分析,当α相在熔体中自由形核时,α2/γ片层取向与生长方向严格垂直,α相以铸态原有晶粒为形核衬底时,α2/γ片层保留了原始铸态晶粒的片层取向。根据TGZM理论解释保温实验中糊状区内溶质传输,理论计算结果与实际相符。抽拉速度对保温结果影响很大,在低速时,糊状区内液相溶质富集程度相对较小,液固两相向α单相平界面转变所需保温时间较短。但在高速时,需较长的保温时间才能实现α单相平界面。通过包晶反应界面间接测量了凝固过程的温度梯度,发现温度梯度基本恒定。实际测量了随速度变化的包晶层厚度,并利用St.John和Hogan的包晶转变模型计算了包晶转变获得的包晶相厚度,发现此包晶转变模型不适用于Ti-53at%Al合金。结合包晶相厚度的变化,分析速度变化对包晶相生成机制的影响,表明,当α相由胞晶向枝晶生长转变时,通过包晶反应获得的包晶相γ增多,但包晶相的形成主要是通过自液相直接析出,即L→γ。更多还原

【Abstract】 Directionally solidified experiments were conducted for Ti-53at.%Al peritectic alloy in a growth rate range from 5μm/s to 35μm/s. The microstructure evolution was investigated from initial transient to steady-state solidication by optical metallographic. Characteristic scales of solidification microstructure were measured, such as dendritic spacing. The orientation of lamellar structure was also analyzed formed during the subsequent solid-state phase transformation. Based on the above experimental results, temperature-stabilization experiment before quenching was carried out to study the kinetics of peritectic reaction of L +α→γat some selected growth rates. The variation of thickness of peritectic phase was measured, and the solute transportation and growth mechanism of peritectic phase were mainly investigated during peritectic solidification process.With the increase of growth rate, the morphology of solid-liquid interface changes from coarse and shallow cells, fine and deep cells, to cellular dendrite and dendrite, and the cellular/dendrite spacing decreases exponentially approximately. The solidification microstructure is composed of both phases ofαandγ. With increase of growth rate, primaryα-phase grows gradually from cells into dendrites.Correspondingly, three-phase contacting regions increase which causes increase of degree of peritectic reaction. Andγphase precipitated directly from liquid increase at high growth rates. So, the volume fraction ofγ-phase increases due to the peritectic reaction and direct precipitation from liquid. The lamellar orientation ofα2/γwas analysed. Whenαphase nucleated freely in the melt, the lamellar orientation is strictly vertical to the growth direction, but ifα-phase nucleated on the substrates of the original cast grains, the final orientation retains the lamellar orientation of original cast grains.Using the TGZM theory, the solute transportation was analyzed in mush zone during the temperature-stabilization experiments, and the theoretical calculation results was in good agreement with the experimental ones. The growth rates affect final resuts much. At a low growth rates, solute enrichment in the liquid of mush zone is relatively poor, and it takes a relatively short time to finish the transformation from the liquid-solid phase region to single phase ofαplanar. But at a high rate, a relatively long temperature-stabilization time is needed to finish the above transformation.The temperature gradient of the above directioanl solidification was measured through peritectic reaction interface indirectly and was found to be approximately constant. The thickness of peritectic phase was measured at different growth rate, and the model by St.John and Hogan was used to calculate thickness of peritectic phase in the peritectic transformation, which was found that the above model was not appropriate to present alloy. Combined with change of the thickness of peritectic phase, the formation mechanism of peritectic phase was analysed at different growth rates. It is found that peritecticγphase increases obtained through the peritectic reaction whenαphase grows from cells to dendrites, and theγphase forms mainly by direct precipitation from liquid, i.e. L→γ.更多还原