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ROLE PLASTIC DEFORMATION ON DUCTILE-BRITTLE FRACTURE TRANSITION

塑性变形对延性-脆性断裂转变的作用

基本信息

批准号：
03650531
负责人：
NAGUMO Michihiko
金额：
$ 1.41万
依托单位：
WASEDA UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1991
资助国家：
日本
起止时间：
1991 至 1992
项目状态：
已结题

项目摘要

In toughness of steels at the ductile-brittle fracture transition region, both the brittle and ductile natures are involved. The origin of the temperature dependence of toughness must be thus examined from both mechanisms. The ductile nature is represented by the resistance against the stable ductile crack propagation, i.e. R curve. By using steels with various microstructures and compositions, precise measurements of R curves were conducted. It was revealed that the temperature dependence of R curve is small and that factors such as grain size and toughness of ferrite matrix have almost no effects. However, carbon con-tent and particularly grain boundary carbides which act as obstacles of slip propagation were found to have remarkable effects.Accordingly, the origin of the temperature dependence of toughness is at the initiation stage of brittle fracture, while the processes leading to the initiation constitute the toughness value. By referring to the analyses by means of fracture mechanics calculation, it was shown that the initiation is controlled by the local tensile stress in the crack front area at a low toughness level. On the other hand, at the toughness level where the crack tip blunting is substantial and the stable crack advances, the brittle fracture initiation is controlled by local strain. It implies that the plastic deformation is the key process for the determination of toughness at the ductile-brittle transition region.Transmission electron microscopy of the area near the brittle fracture ini-tiation site revealed complicated dislocation cell configurations and formation of micro-voids at cell walls even at a very low toughness level. It suggests that deformation structure rather than the cracking of secondary particles induces the brittle fracture initiation.

钢在韧脆断裂转变区的韧性，包括脆性和韧性两种性质。因此，韧性的温度依赖性的起源必须从这两种机制来研究。韧性性质由抵抗稳定韧性裂纹扩展的阻力表示，即R曲线。用不同组织和成分的钢进行了R曲线的精确测量。结果表明，R曲线的温度依赖性很小，晶粒尺寸和铁素体基体韧性等因素几乎没有影响。然而，碳含量，特别是晶界碳化物对滑移扩展的阻碍作用却很明显，因此，韧性的温度依赖性起源于脆性断裂的起始阶段，而导致脆性断裂起始的过程构成了韧性值。断裂力学计算分析表明，在低韧性条件下，起裂受裂纹前沿局部拉应力控制。另一方面，在裂纹尖端钝化和稳定裂纹前进的韧性水平下，脆性断裂起裂受局部应变控制。在韧脆转变区，塑性变形是决定韧性的关键过程，脆性断裂起始点附近的透射电镜观察显示，即使在很低的韧性水平下，位错胞的排列也很复杂，胞壁也有微孔洞形成。结果表明，脆性断裂的发生是由变形组织引起的，而不是由二次粒子的开裂引起的。