鑄造鎳基高溫合金K445的熱疲勞行為

鑄造鎳基高溫合金K445的熱疲勞行為THERMAL FATIGUE BEHAVIOR OF NICKEL BASE SUPERALLOY K445

利用開有V形缺口的平板試樣，研究了一種新型鑄造高溫合金K445在最高溫度分別為800、850、900℃，最低溫度為室溫的熱循環下的熱疲勞行為. 通過光學顯微鏡和掃描電鏡觀察合金的組織和熱疲勞裂紋形貌，研究熱疲勞損傷機制. 結果表明，熱疲勞主裂紋主要從V形缺口處萌生，沿晶界擴展，而二次裂紋則穿晶擴展. 當最高循環溫度為800℃時，碳化物的組成和分布起主要作用，(Ti, Ta)C的開裂處以及碳化物與基體分離的界面處是裂紋優先形成區域. 當最高循環溫度為900℃時，高溫氧化的作用占主導地位. 應力輔助作用下的晶界氧脆是此時的主要損傷機制.

A study of thermal fatigue (TF) behavior of cast nickel-base superalloy K445 was conducted between the temperatures ranging from 800℃ to 900℃ and RT, using plate specimens notched to induce crack initiation. Conventional optical microscopy (OM) and scanning electron microscopy (SEM) examinations were conducted to investigate the damage mechanisms of thermal fatigue. Almost all the primary fatigue cracks at elevated temperatures propagate intergranularly and the secondary crack is transgranular. At 800℃, coarse MC carbides either at grain boundaries or within the grain interior, as well as decohesion of MC/matrix interface, are the preferential sites of crack initiation and propagation. At 900℃, crack propagation is an oxidation-dominant and carbide-assisted process. Stress assisted grain boundary oxygen (SAGBO) embrittlement is the principle mechanism of the environmental degradation to the alloy at the crack-tip region.

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