Explication of the degradation mechanism of the strength of heat-resistant alloys under random loading at elevated temperatures and the establishment of the prediction method of their lifetime

本研究的主要目的是：1)阐明材料在高温蠕变-疲劳载荷作用下加速退化的机理；2)建立随机载荷条件下材料损伤的定量评估方法。制作了原始的哑铃形小试件，既可以观察载荷分布下的退化过程，又可以在扫描电子显微镜上直接观察合金局部结晶度的变化。采用自行研制的电子显微镜间歇微拉伸试验方法，连续监测了具有代表性的耐热合金镍基高温合金在高温蠕变-疲劳载荷下的结晶度退化过程。连续观察了材料结晶度和强度的退化过程，并考虑了机械应力对原子扩散激活能局部降低的影响，应用修正的Arrhenius方程对其进行了解释。研究还发现，空位和位错仅在晶界附近加速积累，因此，加载和卸载时的应变速率越快，晶间裂纹的萌生时间越短。加速的主要原因是合金在高温下的粘弹性被激活。

The main objectives of this research are 1) Clarification of the acceleration mechanism of the degradation of materials under creep-fatigue loading at elevated temperatures, and 2) Establishment of the quantitative evaluation method of the damage of the materials under random load conditions. Original small dumbbell-shape specimens were manufactured for both the observation of the degradation process under the distribution of the applied load and the direct observation of the change of the local crystallinity of the alloy in a scanning electron microscope. The degradation process of the crystallinity of the representative heat-resistant alloy, Ni-base superalloy, under creep-fatigue loading at elevated temperatures was monitored continuously by using our original intermittent micro tensile test method with electron microscopy. The degradation process of the crystallinity and strength of the material was continuously observed, and it was explicated by applying the modified Arrhenius equation by considering the effect of mechanical stress on the local decrease in the activation energy of atomic diffusion. it was also found that the accumulation of vacancies and dislocations was accelerated only around grain boundaries and thus, the initiation time of intergranular cracking became shorter when the strain rate during loading and unloading became faster. The main reason for the acceleration was found to be the activation of viscoelasticity of the alloy at the elevated temperature.