抽拉速率对定向凝固Mar-M247LC合金微观结构及蠕变性能的影响

From Withdrawal Rate to Creep Life: Microstructure-Mediated Performance in Directionally Solidified Mar-M247LC

摘要:

定向凝固镍基高温合金由于消除了与应力方向垂直的横向晶界，具有优异的蠕变性能。考虑到其蠕变性能与初始凝固组织的强关联性，本文重点研究了抽拉速率对Mar-M247LC定向凝固镍基高温合金组织和持久性能的影响。结果表明：随着定向凝固试样抽拉速率的提高，一次枝晶间距逐渐减小（从479 μm降至322 μm），γ'相平均尺寸减小（枝晶间区域从460 nm降至345 nm，枝晶干区域从298 nm降至203 nm）。此外，碳化物形态由块状转变为汉字状。热处理后试样中形成规则的立方状γ'相，且热处理后试样中的γ'相含量较定向凝固试样有显著增加。在抽拉速率为40 μm/s条件下凝固的合金会形成细长状γ'筏形组织、狭窄的基体通道及规则的位错网络，这些因素协同作用使蠕变断裂寿命延长至96.6小时。断口分析确认其为穿晶韧性断裂，裂纹萌生于分解的MC碳化物处。

Abstract:

Directionally solidified (DS) nickel-based superalloys exhibit excellent creep resistance due to the elimination of transverse grain boundaries perpendicular to the stress axis. Given the strong dependence of creep performance on the initial solidification microstructure, this study reveals how withdrawal rate governs the as-cast microstructure of DS Mar-M247LC and dictates its post-heat-treatment creep life at 980 °C/220 MPa. It was found that an increase in the withdrawal rate of the directionally solidified specimens led to a reduction in the values of primary dendrite arm spacing (from 479 μm to 322 μm) and the average size of γ' precipitates (from 460 nm to 345 nm in interdendritic regions and from 298 nm to 203 nm in dendritic core). In addition, the carbide morphology changes from blocky to script-like. The heat treatment led to the formation of distinct cuboidal γ' precipitates and was accompanied by a significant increase in the γ' volume fraction compared to the directionally solidified microstructure. The alloy solidified at 40 μm/s exhibits elongated γ' rafts with narrowed matrix channels and regular dislocation networks, synergistically extending creep rupture life to 96.6 h. Fractographic analysis confirmed transgranular ductile, with fracture initiation occurring at decomposed MC carbides.