【作者】 申健；

【导师】 李德元；

【作者基本信息】 沈阳工业大学 ， 材料加工工程， 2013， 博士

【摘要】 单晶高温合金的高强度是多种强化机制和多种元素共同作用的结果。因此，单晶合金成分、工艺的改变对合金的组织与力学性能的关系的影响是一个较为复杂的问题。本文工作选用采用液态金属冷却定向凝固技术（LMC）制备的一种低成本无铼镍基单晶高温合金（DD265），该合金具有高强度、低成本、低密度和铸态直接使用的特点。主要研究内容包括：LMC法制备单晶铸件工艺研究；单晶合金的拉伸性能；单晶合金的持久性能和蠕变性能；长期时效对单晶合金组织与性能的影响等。液态金属冷却定向凝固过程中，增加选晶器起晶段激冷层晶粒数量是控制单晶取向的有效方法；型壳温度一定时，随着抽拉速率的增加合金组织细化，一次枝晶间距降低，共晶含量降低，型壳温度升高这种变化趋势更加明显；工艺参数对合金持久性能的影响不大，但是对室温拉伸性能影响较大。LMC工艺可以有效细化合金组织，采用LMC工艺制备的DD265单晶合金一次枝晶间距相对HRS工艺制备的合金缩小近1倍，共晶和碳化物尺寸为HRS工艺的一半，并且DD265合金的初熔温度升高。LMC工艺制备的合金最高固溶温度和处理时间都低于HRS工艺制备的合金。固溶处理过程中，LMC工艺制备的合金中铸态MC型碳化物全部发生溶解，而HRS工艺制备的合金中碳化物只有少量发生溶解。这是由于LMC工艺可以有效的降低显微偏析和减小碳化物尺寸所引起的。对LMC法制备的DD265单晶合金的拉伸性能研究发现，合金的屈服强度与抗拉强度随温度的变化规律一致，首先随着温度的升高略有降低，450℃后开始升高，在600℃达到最大值，然后缓慢下降，850℃后屈服强度与抗拉强度随着温度升高迅速下降。延伸率与断面收缩率随温度的变化规律也基本相同，但是变化趋势与强度正好相反。DD265单晶合金的持久强度与典型第一代单晶合金相当。同一温度下蠕变曲线的形状相似，不同温度下存在差异。低温区蠕变可以发现明显的蠕变三个阶段；高温区表现出明显的蠕变稳态和蠕变加速阶段。同时，本文从位错运动角度深入分析了不同温度蠕变过程。合金在975°C/255MPa的持久性能从大到小的顺序为：[11]>靠近[001]-[11]边界>[001]>靠近[001]-[011]边界>[011]。(111)[01]滑移系的Schmidt因子和取向旋转对不同取向单晶高温合金975°C/255MPa持久性能有很大影响。DD265合金在900℃和1000℃条件下经过2000小时时效处理，合金内部没有发现TCP相。900℃条件下，单晶合金的室温拉伸强度和屈服强度随着时效处理时间的延长，表现为先降低，再升高，最后再降低的趋势。在1000℃条件下，抗拉强度和屈服强度呈下降趋势；持久性能的变化规律与室温拉伸性能相似。更多还原

【Abstract】 The high strength of single crystal (SX) superalloys was resulted from the variousstrengthening mechanisms and the effect of alloying elements. Therefore, it is difficult tomake clear the correlation between the chemical compositions, processing parameters andthe microstructure or mechanical properties. A Re-free, low cost, low density nickel-basesuperalloy DD265was directionally solidified (DS) by liquid metal cooling (LMC) methodin the present study. The processing of LMC casting, tensile behavior, creep properties andthe effect of long term annealing on the microstructure stability and mechanical propertieswere investigated.During DS casting by LMC method, more grains generated in the starter resulted inbetter crystal orientation. With the increase of withdraw rate finer microstructure wasgenerated, which means fine primary dendrite arm spacing and low volume fraction ofeutectics. More obvious changes were observed with the increase of the shell temperature.With the change of processing parameters almost no change of creep properties could bedetected, while the room temperature tensile properties were affected dramatically.Very fine microstructure could be obtained by LMC method, the primary dendritearm spacing changed to one half of that in alloys cast by HRS, so did the sizes of eutecticsand carbides. The incipient melting temperature was increased. A simpler solution heattreatment with lower solution temperature and shorter time was employed in alloys cast byLMC than HRS. During the solution heat treatment all the as-cast MC carbides dissolvedin alloys cast by LMC, while only small amount of carbides dissolved in alloys cast byHRS. This was probably due to the lower micro-segregation and finer carbides in LMCalloys. With the increase of temperature the yielding strength and ultimate tensile strengthshow the same trend, i.e. decreased a little bit, and then increased from around450oC, amaximum value was achieved at600oC then decreased slowly, after850oC they decreasedsignificantly. The elongation and percentage of area reduction show the opposite trends.DD265alloy shows the same creep performance as the typical1stgeneration SXsuperalloys. During low temperature creep three stages were shown, while during hightemperature creep only steady stage and accelerate stage were shown. The creep propertieswere affected by the crystal orientation, during975oC/255MPa creep the creep rupture lifedecreased in the sequence of [11]> near [001]-[11]>[001]> near [001]-[011]>[011].Schmidt factor of (111)[01] slip systems and the crystal rotation should be the mainreason.No TCP phases were observed in DD265alloys after2000h annealing at900oC and1000oC. After900oC annealing, the yielding strength and ultimate tensile strengthdecreased, increased and then decreased again with the increase of annealing time. After1000oC annealing both the yielding strength and the ultimate tensile strength decreasedwith the increase of annealing time. The creep properties show the same trend as the tensileproperties.更多还原