摘要
本文对K418及TiAl两种材料带有不同缺口根半径的平板及圆棒试样进行了拉伸断裂实验,通过宏观力学性能参数的测定；断口和金相观察；并结合不同缺口根半径试样的有限元(FEM)计算,对两种材料拉伸的宏观力学性能、缺口敏感度和断裂机理进行了研究。得到以下主要结果：(1)平板双缺口试件、圆棒缺口试件和新型多缺口试件力学性能测定结果表明K418和TiAl合金对缺口敏感性有较大不同：K418对缺口敏感性不大,但TiAl合金表现出较大的缺口敏感性。因而K418铸件容限标准不能直接应用到TiAl合金上。(2)SEM断口分析表明K418室温断裂是韧性断裂和脆性断裂混合型断裂,韧性断裂是由韧窝形成、扩展、融合形成的,其控制因素是塑性变形,试样断裂源于内部缺陷,缺陷组织主要为疏松；TiAl合金室温断裂是典型脆性解理断裂,其控制因素是正应力,缺口试件的起裂源位于缺口根部,基本无缺陷。K418和TiAl合金断裂机理不同,导致其缺口敏感性不同。(3)有限元计算结果表明：缺口前端的正应力syy/sy,三向应力度sm/σ和等效塑性应变e随外加载荷P的增加而增加；随缺口根半径的减小而增加。试样变形是从缺口根部开始的,而试样所受最大正应力却在距离缺口根部一定距离处。(4)在新型多缺口圆棒试件拉伸实验中发现：对TiAl合金,当缺口深度为试件直径的10%时断裂强度下降,当为20%时断裂强度突然急剧下降。
In this paper, tensile macroscopic mechanical properties and fracture mechanism of TiAl and K418 alloys were studied for different notch root radius flat and round bar specimens, based on the measurements of macro-mechanical properties and notch sensitivity, observation of the fracture surface and microstructure, and the detailed FEM calculations of the specimen with different notch root radius. The main results are as follows:(1) the mechanical properties analysis of the double-notched flat specimen, notched rod specimen, and new specimen with multi-notches indicated that:there are a great different in notch sensitivity for TiAl alloy and K418 alloy:K418 alloys have no notch sensitivity. However, a greater notched sensitivity appears in the TiAl alloys. Therefore, K418 cast defect tolerances standard cannot be directly used on TiAl alloy. (2) SEM fracture surface analysis showed that the fracture mode of K418 is composed of ductile and brittle fracture at the room temperature, ductile fracture is induced by dimple formation, expansion, integration, and its controlling factors is the plastic deformation. The specimen fracture finally due to propagation and connection of internal defects, such as shrinkage; the fracture mode of TiAl alloy is a typical cleavage fracture at room temperature, the controlled factor is the normal stress, and the crack initiation origins of the notch specimens are located at the root, no casting defects are found. So, the fracture mechanisms of TiAl and K418 alloys are different, which results in their different notch sensitivity. (3)FEM analysis results show that:the normal stress syy/sy, the stress triaxiality the equivalent plastic strain e increase with the increasing of the load P and decreasing of the root radius of notches. Specimen deformation is induced from the notch root, but the maximum normal stress is located at a distance from notch root. (4) The results of tensile test for new multi-notch rod specimens show that:when the notch depth is 10% of the diameter of the specimen, fracture strength decreased, while notch depth reaches 20% of the diameter of the specimen, the fracture strength is decreased sharply.
引文
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