高温合金差示扫描量热分析(DSC)的影响因素研究III：合金状态和升降温速率

Influencing Factors on Differential Scanning Calorimetry (DSC) Analysis of Superalloy III: Alloy state and Heating/Cooling Rate

摘要:

摘 要：对固溶强化型镍基高温合金625进行升、降温差示扫描量热分析（DSC）试验，研究了同一合金不同状态（粉末态、粉末+热等静压态和铸态）以及升/降温速率（5-10℃/min）对相变温度的影响。采用场发射扫描电镜（FESEM）、电子探针（EPMA）对不同状态625合金的微观组织和元素分布进行表征。结果表明：（1）铸态比粉末态合金的枝晶间距大2个数量级，而热等静压态合金为无枝晶偏析的细等轴晶结构。（2）升、降温速率对DSC曲线中加热时基体开始熔化（等于固溶强化型合金的初熔温度）和冷却时开始凝固温度（偏离基线的拐点）无影响，但对合金加热熔化结束、冷却时大量凝固析出温度（峰位）和终凝温度（拐点）有明显影响。采用加热、冷却曲线相应相变温度平均值的方法可减少DSC试验和样品条件的影响，获得相对固定且更具可比性的合金相变温度。（3）合金状态对初熔温度和DSC升温曲线固相线温度附近的圆弧段有明显影响。根据DSC加热曲线固相线温度附近的圆弧大小可以判断合金的偏析倾向，弱偏析倾向的粉末态和热等静压态PM625合金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，分别仅为5℃和6℃；偏析倾向较大的铸态IN625合金的DSC加热曲线中固相线温度附近区域为较大圆弧，开始熔化温度与名义固相线温度差异可达52℃。铸态625合金的初熔温度比热等静压态和粉末态分别低45℃和40℃，在实际热处理和热等静压等热工艺参数选择时应注意圆弧段较大的合金降低初熔温度的影响。在所有DSC冷却曲线中，由于完全熔化重新凝固消除了合金原始显微组织特征，不同状态625合金固相线温度区域附近曲线形态相似，均为较大的圆弧。

Abstract:

Abstract: Differential scanning calorimeter (DSC) experiments were performed on solid-solution strengthening Ni-base superalloy 625, considering the effects of the alloy state (powder, powder+HIPed and as-cast) and heating/cooling rate (5-10°C/min) on the phase transformation temperatures. The alloys with different states were characterized by FESEM and EPMA. (1) The results indicate that the dendritic arm spacing of the as-cast alloy is 2 orders of magnitude higher than that of the 625 alloy powders, whereas the hot isostatically pressed (HIPed) alloys possess a fine equiaxed grain structures without dendritic segregation. (2) The heating/cooling rates have no significant influence on the onset of matrix melting ( equal to the incipient melting temperature for the solid-solution strengthening superalloy) and onset of solidification temperatures (inflection point), but have an obvious effect on the melting end, matrix mass solidification temperatures (peak position) and solidification end temperature (inflection point) of the alloy. Using the average phase change temperature on heating and cooling curve average method can reduce the influence of DSC experiment or sample conditions and obtain a relatively fixed and comparable phase transformation temperature. (3) The alloy state has obvious effect on incipient melting temperature and the arc shape near soidus temperature of DSC heating curve. The segregation tendency of alloy can be determined by the radian near the solidus reigion of DSC heating curve. The powders and HIPed PM625 alloys with weak segregation tendency exhibit a sharp inflection point in DSC heating curves in the region near solidus temperature, there is only a 5-6°C gap between the onset of matrix melting temperature of the alloy (deviation from the baseline inflection point) and the nominal solidus temperature (tangent-onset intersection). The DSC heating curve of the as-cast IN625 alloy with a high segregation tendency exhibit a larger radian near the solidus region, the difference between the onset of matrix melting temperature and the nominal solidus temperature (tangent-onset intersection) can reach 52°C. The onset of matrix melting temperatures of as-cast 625 alloys are 45°C and 40°C lower than that of the HIPed and powder 625 alloy respectively. The parameter of the process such as HT or HIP should be selected concerning the effect of the alloy state with large radian near solidus region of DSC heating curve on decreasing the incipient melting temperature. The DSC cooling curves for the different state of 625 alloys are similar, which all possess a large radian near the solidus region, because the alloys were completely remelted which eliminated the original microstructure features and re-solidified from the full liquid state.