高温合金差示扫描量热分析(DSC)的影响因素研究:升降温速率和取样部位

Influencing Factors on Differential Scanning Calorimetry(DSC) Analysis of Superalloy: Heating/Cooling Rate and Sampling Position

对定向凝固DZ22镍基高温合金在5~40℃/min不同速率下进行升、降温差示扫描量热分析(DSC)试验,通过线性外推和取平均值的方法确定合金近平衡态的相变温度。对不同取样部位的DZ22合金进行10℃/min下的升、降温DSC曲线的测定。结果表明,(1)DSC试验中的升、降温速率对DZ22合金的相变温度测试结果包括液相线、MC碳化物、固相线、共晶γ′和次生γ′均产生明显影响,其中加热曲线随着升、降温速率的升高向高温方向偏移,冷却曲线则向低温方向偏移,峰的高度随着升、降温速率的升高而增大,但升、降温曲线对应相变温度点平均值趋于一致,接近合金的平衡相变温度。除液相线外,采用升、降温曲线外推法获得的平衡态相变温度存在一定的差异,而取升、降温曲线对应的相变温度平均值较为固定,是确定合金近平衡相变温度的有效方法。(2)取样部位对DSC加热曲线低温段中的γ+γ′共晶和固相线温度产生明显影响,因显微偏析导致的组织差异造成同炉定向凝固试棒的底端和顶端的共晶γ+γ′和固相线温度分别相差17和20℃,而对曲线高温段的液相线和MC碳化物溶解温度影响不大;不同取样部位的合金加热完全熔化后组织趋于一致,再次凝固冷却过程中相变温度也趋于一致,因此DSC冷却曲线基本重合,取样部位对DSC冷却曲线无明显影响。

The differential scanning calorimetry（DSC） tests with different heating/cooling rates in the range of 5～40 ℃/min were performed on a directionally solidified（DS） Ni-base superalloy DZ22. The near equilibrium transformation temperatures（zero-heating/cooling rate） of the alloy were obtained by linear extrapolating the different heating/cooling rates or averaging specific peak temperatures of both heating and cooling DSC curves. The DSC tests with 10 ℃/min heating/cooling rate were carried out on samples cut from different positions of DS testing bar. The results indicate that the heating and cooling rate has effect on the DSC results, including the phase transformation temperatures of liquidus, MC carbides, solidus, eutectic γ＋γ′ and secondary γ′. As the heating and cooling rate increases, the peak of transformation temperature on the heating DSC curve shifts to high temperature direction, whereas the cooling curve tends to deviate to lower temperature. The peak height increases with the heating/cooling rate increasing. However, the average values of heating and cooling curves corresponding to the phase transformation temperature points are consistent. Except liquidus, the equilibrium transformation temperatures of alloy acquired by linear extrapolating the different heating/cooling rates will result in some differences for the result, whereas to average specific peak temperature of both heating and cooling DSC curves is an effective method to determine the near equilibrium phase transformation temperatures of superalloys. In addition, the sampling position also has obvious effect on the eutectic γ＋γ′ dissolving and solidus temperatures of heating DSC curve in relatively low temperature range. There is a 17 ℃ and 20 ℃ gap for eutectic and solidus temperatures, respectively between samples cut from the top and bottom part of the same directionally solidified test bar due to the micro-segregation and microstructure difference. However, this difference is absent in liquidus and MC carbides dissolving temperatures in high temperature range. Upon cooling, the sampling position has minor effect on phase transformation temperature of DSC curve because the similar microstructure of the different sampling parts of the alloy formed in the following solidification cooling process after heating to a full liquid state and phase transformation temperature tends to be consistent. For a superalloy with the same composition, the DSC test results are only meaningful as the microstructure of the sample is similar.