晶界氧化對高溫合金GH4738疲勞裂紋擴展的作用

晶界氧化對高溫合金GH4738疲勞裂紋擴展的作用Grain Boundary Oxidation Effect of GH4738 Superalloy on Fatigue Crack Propagation

測定了GH4738合金在650、700、750及800 ℃空氣環(huán)境下的疲勞裂紋擴展速率da/dN-ΔK曲線及疲勞裂紋擴展壽命a-N曲線，得出了溫度對合金疲勞裂紋擴展的影響規(guī)律，并結(jié)合組織性能、疲勞特征、高溫及室溫下晶界氧化情況等分析了溫度對合金疲勞裂紋擴展的影響本質(zhì)。結(jié)果表明，隨著溫度升高，GH4738合金的疲勞裂紋擴展速率（FCGR）增加，合金的斷裂方式由沿晶和穿晶混合型斷裂向完全沿晶斷裂轉(zhuǎn)變；在初始應力強度因子幅度ΔK為40 MPa·m1/2、晶粒度為30~40 μm的條件下，合金的疲勞裂紋擴展壽命在650~700 ℃內(nèi)顯著下降，存在一個溫度敏感區(qū)間，其原因并不是材料的組織和力學性能的變化，主要是高溫下的氧化作用所致；氧通過裂紋尖端、滑移帶間接進入晶界或氧直接滲入晶界的方式，與晶界處的活性元素Co、Ti、Al反應生成脆性氧化物，從而降低了晶界強度，使合金的抗疲勞性能顯著下降。

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 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 field emission scanning electron microscope (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. Besides, the interruption experiments were carried out at 700℃ and room temperature, respectively, to investigate the crack propagation mode and oxidation degree at the crack tip and grain boundary of the samples. According to the above analyses, the essential reason of temperature influence on FCG behavior of GH4738 was discussed. 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 for temperatures at 650~700℃, existing a temperature-sensitive region under ΔK=40 MPa·m1/2 and 30~40 μm grain size conditions, which is mainly caused by 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 strength and significant decrease of fatigue property of GH4738.

全文下載：https://pan.baidu.com/s/1i4BcKTJ