Local Crack-Tip Process and Its Effect on Fracture Toughness

研究了裂纹尖端附近位错和移动的杂质等晶体缺陷的局部活动，特别关注了屏蔽/反屏蔽对断裂韧性的影响。在<IC>77 ~ 300 K范围内，NaCl等离子晶体的断裂韧性K值随温度的升高而降低，其变化规律<IC>与高温时相反<IC>，随温度的升高而升高。计算机模拟结果表明，在低温区，由内源产生的反屏蔽位错更活跃，随温度的升高而降低K<IC>值，而在高温区，屏蔽位错因热激活而大量从裂尖发射，对K值的提高有贡献<IC>。电子态计算表明，水分子在离子晶体中的化学吸附促进了位错从表面产生，并可能降低脆塑转变温度。当金属晶体中存在可动间隙杂质时，杂质在裂纹尖端附近聚集，使尖端局部k场增强，表现出杂质的反屏蔽效应。裂纹尖端场的增强促进了裂纹的扩展，从而降低了K<IC>值。使用充有轻惰性气体元素如氦的Fe-Si合金晶体验证了杂质的抗屏蔽效果。

Local activities of crystal defects such as dislocations and mobile impurities near a crack tip were investigated with particular attention to the shielding/anti-shielding effects on fracture toughness. The fracture toughness, K_<IC>, in ionic crystal such as NaCl is decreased with increasing temperature in the range between 77 and 300K.The change of the K_<IC> is opposite to that at high temperatures where K_<IC> values increase with increasing temperature. Computer simulation showed that, in the low temperature range, anti-shielding dislocations generated from intenal sources are more active and serve to decrease K_<IC> values with increasing temperature while, at high temperatures, shielding dislocations are much emitted from a crack tip by thermal activation and contribute to the increase of K_<IC> values. A comutation of electron states suggested that the chemical adsorption of water molecules in ionic crystals promote the dislocation generation from the surface and may lower the temperature of brittle-ductile transition. In case of mobil interstitial impurities in metallic crystals such as iron, the impurities concentrate neara crack tip to intensify the local k field at the tip, indicating the anti-shielding effect due to impurities. The intensification of the crack-tip field promotes the crack extension so as to decrease K_<IC> values. The anti-shielding effect by impurities was verified using Fe-Si alloy crystals charged with light inert-gas elements such as helium.