测定了GH4738合金在650、700、750及800 ℃空气环境下的疲劳裂纹扩展速率da/dN-ΔK曲线及疲劳裂纹扩展寿命a-N曲线,得出了温度对合金疲劳裂纹扩展的影响规律,并结合组织性能、疲劳特征、高温及室温下晶界氧化情况等分析了温度对合金疲劳裂纹扩展的影响。结果表明,随着温度升高,GH4738合金的疲劳裂纹扩展速率(FCGR)增加,合金的断裂方式由沿晶和穿晶混合型断裂向完全沿晶断裂转变;在初始应力强度因子幅度ΔK为40 MPam^1/2、晶粒尺寸为30~40 μm时,合金的疲劳裂纹扩展寿命在650~700 ℃内显著下降,存在一个温度敏感区间,其原因并不是材料的组织和力学性能的变化,主要是高温下的氧化作用所致;O通过裂纹尖端、滑移带间接进入晶界或O直接渗入晶界的方式,与晶界处的活性元素Co、Ti、Al反应生成脆性氧化物,从而降低了晶界强度,使合金的抗疲劳性能显著下降。

The low cycle fatigue (LCF) experiments of nickel-based turbine disc alloy GH4738 have been carried out at different temperatures in air to investigate the influence of temperature on fatigue crack growth (FCG) behavior of GH4738 alloy. The FCG curves (da/dN-ΔK and a-N) and their regularity have been obtained. The results show that there is a sensitive range of temperature in which the fatigue life for GH4738 decreases sharply. The microstructures and fracture surface morphologies of the GH4738 samples tested at different temperatures were observed by FE-SEM, and changes of the mechanical properties of GH4738 at high temperature were also taken into account through modifying the stress intensity factor amplitude, ΔK. The interruption experiments were carried out at 700 ℃ and room temperature, respectively, to investigate the crack growth mode and oxidation degree at the crack tip and grain boundary of the samples. And the essential reason of temperature influence on FCG behavior of GH4738 was discussed. The result showed that as the temperature increases, the fatigue crack growth rate (FCGR) of GH4738 accelerates, the fracture surface tends to coarse, and the failure mode converts from a mixed transgranular and intergranular fracture to totally intergranular fracture. The fatigue crack growth lifetime decreases remarkably at 650~700 ℃, existing a temperature-sensitive region under ΔK=40 MPam^1/2 and 30~40 μm grain size conditions, which is mainly caused by the oxidation at elevated temperature, independent of the microstructure and mechanical property. Oxygen diffuses into the grain boundary through crack tip and slip band, or penetrates directly into the grain boundary, reacts with active elements (Co, Ti, Al) and generates brittle oxides. These brittle oxides result in weakening of grain boundary and significant decrease of fatigue property of GH4738.