沉淀强化奥氏体合金氢脆机理及相关性能研究

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	沉淀强化奥氏体合金氢脆机理及相关性能研究
	郭子峰
学位类型	博士
导师	戎利建
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料加工工程
关键词	沉淀强化奥氏体合金硼 Γ′相氢脆机理持久性能 Precipitation-hardened Austenitic Alloy Boron Gamma Prime Phase He Mechanism Stress Rupture Property
摘要	"本文以一种沉淀强化奥氏体合金J75为研究对象，系统研究了硼对合金组织、力学及抗氢脆性能的影响，结合扫描电镜（SEM）、三维原子探针（3DAP）、二次离子质谱（SIMS）、透射电镜（TEM）等技术揭示了合金具有高氢脆敏感性的原因，提出了J75合金氢脆机理并进行了验证，同时考察了该合金的持久性能。峰值时效处理后，无硼J75合金晶界上极易析出片层状h相，在变形过程中该相会导致应力集中，从而降低合金的拉伸塑性；由于h相与基体界面为非共格关系，使得h/g界面成为强的氢陷阱，能捕获大量的氢原子，导致合金氢脆敏感性增加；在合金中加入适量的硼，通过硼在晶界上的偏聚，可抑制h相的析出，提高合金力学及抗氢脆性能。对于过时效态合金，适量硼的加入，不仅抑制晶界h相的析出，而且阻碍晶界碳化物的粗化，这是由于偏聚的硼原子占据h相、碳化物的形核位置，并抑制合金元素沿晶界扩散；过量硼的添加，降低游离态硼原子的有益作用，促进晶界硼化物、h相的析出，合金的氢脆敏感性增加。与单相奥氏体合金相比，即使晶界无h相的析出，J75合金的氢脆敏感性仍较高，氢致塑性损减达到40%以上，现有的氢脆机理不能解释其原因。3DAP结果显示，氢原子未在g¢相内及g¢/g界面上富集，表明g¢和g¢/g界面均不是强的氢陷阱；HRTEM观察表明，发现在整个拉伸变形过程中g¢/g界面始终保持共格关系；位错组态观察结果显示，氢和有序相g¢促进基体中位错的平面滑移，使得氢易于被运输至较强的氢陷阱处并产生聚集。根据以上研究结果，提出沉淀强化奥氏体合金氢脆机理：在强化相g¢与氢的共同作用下，位错的平面化滑移严重，氢易被位错运输到晶界等强陷阱处，引起合金发生沿滑移带开裂和沿晶开裂，氢脆敏感性增加。对单相奥氏体合金的氢脆敏感性研究表明，即使晶界上析出大量碳化物，但由于变形过程中位错严重缠结，氢难于通过位错运动被运输到晶界上造成局部的富集，合金表现出很好的抗氢脆性能；J75合金在低温下（低于-50℃）具有良好的抗氢性能，是由于低温下氢的扩散系数低，氢不能随平面化滑移的位错运动，因而氢也难于在强陷阱处富集。上述实验结果支持本研究提出的氢脆机理。对含η相的J75合金在不同温度和应力下的持久性能研究表明，合金持久性能受到η相的影响，在低温高应力下，η相促进晶界处应力集中并引起开裂，降低合金持久寿命及塑性；高温低应力状态下，η相通过阻碍晶界滑动及迁移，显著提高合金的持久塑性。"
其他摘要	"In the present work, the effect of boron on the microstructure and mechanical properties of precipitation-hardened austenitic alloy J75 was investigated detailedly. Also, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and secondary ion mass spectrometer (SIMS) and three-dimensional atom probe (3DAP) were used to study the essence of hydrogen embrittlement (HE) in J75. Based on experimental results, a new mechanism of HE has been proposed. Furthermore, the stress rupture properties of the alloy are studied. The main results are summarized as following: After the solution and peak-aged treatment, there is a mount of η phase at grain boundaries in the alloy without boron. Because of high stress concentration around the η phase during deformation, the ductility is decreased remarkably. At the same time, it is found that the η/g interface is incoherent, and the HE sensitivity of the J75 without boron is increased. In the alloy with boron addition, the precipitation of η phase is retarded through the segregation of boron atoms at grain boundaries, which improves the ductility and hydrogen resistant ability. When the boron addition is over the critical content, the borides are formed at grain boundaries and the HE sensitivity increases slightly, due to the quantity of the free boron atoms decreases. After the solution and over-aged treatment, no η phase is found and the carbides become fine and discontinuous in the alloy with proper boron, due to the segregated boron atoms which can occupy the nucleation sites of precipitates and retard the grain-boundary diffusion. The addition of 0.01wt.% boron results in the intergranular precipitation of the borides and η phase, and the HE sensitivity is increased again. Compared with the single-phase austenitic alloy, the HE sensitivity of the precipitation-hardened J75 alloy without η phase is much higher and the percent loss of reduction of area is about 40%. It is shown by 3DAP that no segregation of hydrogen atoms is found at the coherent γ′/γ interface or in γ′ phase, which indicates that the interface is not a strong hydrogen trap. The HRTEM results show that interface coherency between γ′ phase and matrix is maintained during the deformation, even tensile to fracture. Based on experimental results, the HE mechanism of J75 can be proposed: with the combination effects of γ′ phase and hydrogen, serious dislocation planar slip occurs which promotes the transportation of hydrogen atomes to strong hydrogen traps, as a results, macroscale slip band rupture and intergranular fracture are promoted，and become the predominant feature in the tensile-to-fracture sample after hydrogen charging. HE sensitivity in the sensitized treated single-phase J20 alloy is quite low, even the grain boundaries are covered with amount of carbides, because dislocation planar slip can not occur and hydrogen concentration at the interface of carbide with matrix can be avoided. HE sensitivity of the aged alloy J75 is decreased dramatically at the temperature below -50oC, and this phenomenon can be interpreted as a result of the poor diffusive ability of hydrogen at low temperature, which can not follow the movement of planar slip dislocations. The proposed HE mechanism is supported by the above-mentioned results. It is found that the stress rupture property J75 alloy will be influenced by the η phase. In the condition of the low temperature and high stress, it is found that the η phase will induce the stress concentration at grain boundary which promotes the origin of microcrack, and is deleterious to the rupture life and ductility. While, in the condition of the high temperature and low stress, the η phase can low the stress accumulation at grain boundary throughh the preventing slip and migration of grain boundaries effectively, and the rupture ductility is improved. "
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64495
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	郭子峰. 沉淀强化奥氏体合金氢脆机理及相关性能研究[D]. 北京. 中国科学院金属研究所,2012.