ROLE PLASTIC DEFORMATION ON DUCTILE-BRITTLE FRACTURE TRANSITION

在韧性-脆性断裂过渡区，钢的韧性既涉及脆性又涉及延性。因此，必须从这两种机制来研究韧性的温度依赖性的来源。韧性由韧性裂纹稳定扩展的阻力即R曲线来表示。用不同组织和成分的钢进行了精密的R曲线测量。结果表明，铁素体的R曲线对温度的依赖性很小，铁素体的晶粒度和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是显著的，相应地，韧性的温度依赖性起源于脆性断裂的起始阶段，而导致起始的过程构成了韧性值。通过断裂力学计算的分析表明，在低韧性水平下，裂纹前锋区的局部拉应力控制着裂纹的萌生。另一方面，在韧性水平，裂纹尖端钝化程度较大，裂纹稳定扩展，脆性起裂受局部应变控制。这表明塑性变形是确定韧脆转变区韧性的关键过程。脆性断口附近区域的透射电子显微镜显示，即使在很低的韧性水平下，也可以在胞壁上形成复杂的位错晶胞构型和微孔洞。结果表明，引起脆性断裂的原因不是二次粒子的开裂，而是变形组织。

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.