Hf和凝固冷速对定向凝固高温合金低周疲劳性能的影响

对DZ22及其不含Hf的改型合金用快冷和缓冷两种凝固工艺制备出定向凝固试样，在760℃和1．0～1．4％总应变条件下测定其低周疲劳（LCF）性能，用光学金相、定量金相及扫描电镜观测了试样原始组织、疲劳过程组织的变化以及断口特征。结果表明：加Hf显著提高了合金的LCF寿命，缓冷凝固则严重损害合金的LCF性能；LCF裂纹大多产生于疲劳寿命的后半期，主要出现在柱晶界、枝晶间、滑移带界面以及MC碳化物处；滑移带的出现是LCF损伤的先兆，在滑移作用下MC碳化物群体开裂并向周围扩展留下疲劳条痕，是这类合金中温LCF断裂的重要特征。本研究还讨论了减少或细化碳化物对延长LCF寿命的实用意义。

Effect of Element Hf and Cooling Rate during Solidification on Low Cycle Fatigue Properties of Directionally Solidified Superalloys

The directionally solidified specimens of DZ22 superalloy and its Hf-free version were prepared by rapid or slow cooling solidification. The low cycle fatigue (LCF) properties of specimens at 760℃ and 1.0~1.4% total strains were determined. The optical metallography, quantitative metallography and scanning electron microscopy (SEM) were applied to observe the original microstructure, the changes of LCF microstructure and the fracture surface characteristics of specimens. The results show that the addition of Hf significantly increases LCF life of DZ22 alloy and the slow cooling solidification seriously impairs LCF properties of alloys. Most of LCF cracks appear in the late half lifetime and mainly distribute at the columnar grain boundaries, interdendrites, interfaces of slipping bands and MC carbide. The appearance of slipping bands can be considered to be the early signal of LCF damage. It is important characteristics of LCF rupture at intermediate temperature for this kind of alloy that the cluster of MC carbides cracks by slipping impact, than cracks propagate to periphery and leave the fatigue striations. In present paper, the practical operations to prolong the LCF lifetime by means of decreasing or refining MC carbides are also discussed.