高温合金差示扫描量热分析(DSC)的影响因素:粉末粒度和显微组织

Influencing Factors of Differential Scanning Calorimetry(DSC) Analysis of Superalloy: Particle Size and Microstructure

对固溶强化型625镍基高温合金粉末进行升、降温差示扫描量热分析(DSC)试验,研究了不同粉末粒度(<37,45~53,75~105,105~150,150~355μm)对相变温度的影响。采用场发射扫描电镜(FESEM)、电子探针(EPMA)和同步辐射X射线衍射(SXRD)对625合金粉末的形貌、元素分布和相组成进行表征。结果表明:不同粒径PM625粉末均为树枝晶结构,枝晶间距在2~10μm范围,元素Ni和Cr倾向分布于枝晶干,Mo和Nb偏析于枝晶间。不同粒度的PM625粉末中均仅存在基体γ相。PM625粉末DSC加热曲线固相线附近区域拐点尖锐,表现为合金开始熔化温度(偏离基线的拐点)与名义固相线温度(切线交点)差异很小,不同粒度间的差异仅为2~5℃。合金完全熔化后重新冷却的过程中原始粉末的低偏析特性消失,冷却曲线固相线区域圆弧较大,名义固相线和终凝温度差较大,为53~65℃。DSC试验升温过程中不同粒径粉末的固、液相线以及初熔温度最大差异分别为3, 2和2℃,降温过程不同粒径粉末固、液相线温度差分别为6和2℃。0~355μm粉末粒径范围内,粒径对固溶强化型PM625高温合金粉末相变温度无明显影响。

Differential scanning calorimeter(DSC) experiments were performed on a solid-solution strengthening Ni-base superalloy 625,considering the effects of powder particle size(<37,45~53,75~105,105~150,150~355 μm) and microstructure on the phase transformation temperature. The alloy powders were characterized by FESEM, EPMA and synchrotron XRD. The results indicate that the dendritic structure is evident in powders with different particle sizes and the dendritic arm spacing is in the 2~10μm range. Elements Ni and Cr are rich in dendritic core whereas the Mo and Nb tend to be distributed in the interdendritic region. Only the matrix y phase exists in the PM625 powders with different particle size ranges. The PM625 powders with weak segregation tendency exhibit a sharp inflection point in DSC heating curves in the region near solidus temperature, and there is only a 2~5 ℃ gap between the incipient melting temperature of the alloy(deviation from the baseline inflection point) and the nominal solidus temperature(tangent-onset intersection) for different particle sizes. However, the gaps between the nominal solidus and the end of the solidification temperatures are relatively large,which is in 53~65 ℃ range, in DSC cooling curves,because the low segregation characteristic of original powders has been removed during the full remelting and re-solidified process. The differences in solidus, liquidus and incipient melting temperatures in DSC heating curves are maximum 3, 2 and 2 ℃, respectively among different particle size powders, whereas they are 6 and 2 ℃ for the solidus and liquidus temperatures of the alloys in the cooling curves, respectively. Therefore, the particle size has minor effect on phase transformation temperature of solid-solution strengthening PM625 alloy powder.