| 双相钢的循环形度与断裂 | |
| 孙正明 | |
| Subtype | 硕士 |
| Thesis Advisor | 王中光 |
| 1988 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 中国科学院金属研究所 |
| Abstract | 时马氏体含量均为23%，形貌与分布不同的三种铁素体/马氏体双相组织进行了单向拉伸、循环形度、疲劳纹萌生和断裂行为的研究，以及磁性能与循环形变关系的探讨，并对循环形变引入的位据结构进行了详细的透射电镜分析。得到的主要结果如下。马氏体弥散分的细片状组织具有最佳的拉伸强度和延性，具有较高的疲劳极限和较低的循环硬化潜力。马氏体呈网状分布的组织具有最差的拉伸性能，最低的疲劳极限，却有最高的循环硬化潜力。马氏体呈网状分布的组织具有最差的拉伸性能，最低的疲劳极限，却有最高的循环硬化潜力。马氏体块状分布的组织拉伸性能与循环硬化潜力介于上述二者之间，疲劳极限与细片状组织相迫但具有更高的低寿命疲劳抚力。在网状与块状组织中，疲劳裂纹多萌生于相界与晶界，而在细片状组织中多萌生于铁素体滑移集中处。在较高应变幅下循环形度，三种组织都呈现快速初始硬化，随后出现微量软化并过渡过稳定的饱和阶段。循环软化是由于位错低能结构的形成而引起的。稳定的应力响应来源于特殊的位错组态。不同应变幅下循环饱和的位错组态与面心立方金属的疲劳位错结构相似。循环形变时，尤其在低应变下，相界一般不影响铁素体的位错组态，这意味着应度在相界周围的均匀分布。循环形变引入的高密度位错使得双相钢的交流磁化率降低，应变幅或应力幅越高，降低的幅度越大。但是在应变幅∈_(ph)高于2.0－3.0 * 10~(-3)下循环形变饱和，由于位错胞结构的形成，磁化率的下降小得多，或者说其磁化率比较低应变下循环形变化的高深得多。 |
| Other Abstract | Both monotonic and cyclic deformation of a steel with three duplex microstructures the fine debris structure was the best in terms of strength and ductility while the network structure showed the wrost due to the early cleavage of ferrite phase. However, the network structure has the highest cyclic hardening potentiality because of the shell structure of martensite surrounding ferrite grains, and the fine debris has the lowest because of the good continuity of ferrite matrix. When cycled at rather high strain amplitudes, specimens of all the three microstructures hardened rapidly at first followed by a slight softening before the stable saturation stages, whereas the specimens hardened gently for a large number of cycles to saturate at relatively low strain amplitudes. It was concluded from the transmission electron microscopic observations that the cyclic softening was caused by the dislocation rearrangement into low energy dislocation structures(LEDS). Dislocation structures observed in cyclically deformed duplex microstructures at various strain amplitudes were found quite similar to those observed in fatigued f.c.c. metals. Especially, the dislocation parrallel wall structure produced by cyclic deformation at a plastic strain amplitude Δεp/2 of about 2.2 * 10~(-3) were similar to PSB structures. Such well arranged dislocation structures were believed to be responsible for the stable stress response as in f.c.c. metals. The similarities were considered to be due to the silicon solution strengthening and the effect of other alloy elements. Generally it was found that martensite/ferrite interfaces did not affect the dislocation structures in ferrite significantly, which indicate a uniform strain distribution around interfaces. Meanwhile, hetrogeneous distributions were also observed in differen ferrite regions as well as in some interfaces, which would become the main sources of fatigue crack initiation on the surfaces of specimens. Scanning electron microscopic observations on fatigued specimens' surfaces and fracture topographies demonstrated that the fatigue crack initiate mainly from the extrusion and intrusions in ferrite grains in the fine debris structure while mainly from the phase boundaries and ferrite grain boundaries in the other two microstructures. The S/N curves of the three duplex microstructures and the magnetic susceptibility of the fatigued specimens were also determined. The network structure showed the lowest fatigue limit and the block structure, which showed monotonic properties between network and fine debris structure, showed the best fatigue behavior. Because of the low resistance to fatigue crack propagation the fine debris structure did not show the best fatigue behavior despite the combination of best strength and ductility. Cyclic deformation induced high density dislocations caused decreases in alternating magnetic susceptibility. The higher the cyclic strain amplitude or the peak stress at which the specimens were fatigued, the greater the decrease in the magnetic susceptibility was. It is surprising that, a cyclic deformation at a Δεp/2 of about 2 to 3 * 10~(-3) to saturation caused a much smaller decrease in magnetic susceptibility in comparasion with cyclic deformation at a lower strain amplitude, which was explained in terms of the concepts of the formation of LEDS and internal stress. Deformation at even higher strain amplitude caused a further reduction of susceptibility because of the pinning effect of dislocations on the movement of magnetic domain walls. |
| Pages | 65 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/17341 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 孙正明. 双相钢的循环形度与断裂[D]. 中国科学院金属研究所. 中国科学院金属研究所,1988. |
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