Inconel 740H合金原位高温拉伸微裂纹萌生扩展研究

Microcrack Nucleation and Propagation Investigation of Inconel 740H Alloy Under In Situ High Temperature Tensile Test

利用自主研发的SEM原位高温拉伸台,研究了750℃高温条件下镍基高温合金Inconel 740H单轴拉伸变形过程中微观组织演变规律及微裂纹萌生与扩展机制。结果表明,在室温和高温条件下,Inconel 740H合金变形过程中晶界是主要的裂纹萌生源,但是在室温时微裂纹也会在晶内萌生。通过对原位变形机制的分析表明,750℃高温不仅降低了滑移系的开启能量,使更多的滑移系容易开动,而且弱化了晶界强度,使晶界具有弯曲和滑移的变形特性,从而增强了合金的塑性协调变形能力,但是却降低了合金的屈服强度和抗拉强度,高温同时也使合金晶界的相对强度弱化,导致微裂纹更易从晶界处萌生并扩展。

As a result of increasing energy demands and accelerated environmental problems, there is an urgent need to improve the thermal efficiency of ultra supercritical （USC） power plants. To achieve this goal, advanced ultra-supercritical （A-USC） technologies with the main steam temperature of 700-750℃ and pressure of 35 MPa have been developed quickly in recent years. One of the most promising candidate Ni-based superalloys for the main steam pipe of 700℃ ultra-supercritical coal-fired power plants is Inconel 740H, which is a modified version of Inconel 740 developed by Special Metals Corp （SMC）. Compared with IN740, the Ti/AI ratio in IN740H is lowered in order to stabilise the microstructure at long ageing times. In addition, the Nb content is lowered to improve the weldability. In this work, the microstructure evolutions, the nucleation and propagation mechanisms of microcracks in the nickel base superalloy Inconel 740H at 750 ℃ high temperature were studied by the self-developed in situ high temperature tensile stage inside a SEM. The results showed that under the uniaxial tensile stress at 22 ℃ room temperature and 750 ℃ high temperature conditions, the grain boundaries of Inconel 740H alloy are al- ways the most primary crack sources. The strength of grain boundaries is higher than that of grains under the room temperature, and the microcracks will be nucleated at the grains as well, but the relative strength of grain boundaries will be weaken under the high temperature, which makes the microcracks tend to nucleate at grain boundaries. The experimental results also showed that the influence of high tem- perature on the mechanical properties is very significant, the high temperature reducing the activate energy of slip and weakening the strength of the grain boundaries, so that more slip systems activated and the grain boundaries occurring bending and sliding deformation, so further enhance the ability of plastic deformation of alloy. However, the reduction of relative strength of alloy grain boundaries leads to microcracks nucleation and propagation more easily from grain boundaries and lower the yield strength and tensile strength of alloy.