【作者】 马超群；

【导师】 李小武； 张哲峰；

【作者基本信息】 东北大学 ， 材料物理与化学， 2010， 硕士

【摘要】 AL6XN超级奥氏体不锈钢作为一种应用背景广阔、性能优异的新型不锈钢材料,使用过程中必然要承受各种单向、循环等类型的载荷,有关它的力学性能以及疲劳行为的研究具有重要的实际意义。但是目前此方面的系统研究仍未开展。本论文选取AL6XN合金作为研究对象,考察了其在不同应变速率下的单向变形与损伤行为,并着重研究了此材料在恒应力幅控制和恒塑性应变幅控制下的循环形变及损伤行为。AL6XN不锈钢的单向变形行为的应变速率敏感性较低。单向变形均以位错滑移为主要变形特征。不同应变速率下的拉伸断口均由纤维区和剪切唇组成,纤维区由具有大小两相尺寸的韧窝组成。压缩屈服强度与拉伸屈服强度基本相近。AL6XN不锈钢的应力幅-疲劳寿命曲线满足Basquin关系,而塑性应变幅-疲劳寿命曲线满足Coffin-Manson关系。AL6XN不锈钢在较低塑性应变幅下材料表现出了持续的循环软化行为；当塑性应变幅达到5×10-3以上时,材料表现出了先循环硬化而后循环软化的特点,但均未发现明显的应力饱和现象。对于应力幅控制下的疲劳变形,当外加应力幅小于其拉伸屈服强度时,表面变形特征主要表现为在晶粒内或孪晶内的滑移以及连续穿越晶粒和孪晶的“之”字形滑移等；当外加应力幅稍大于其拉伸屈服强度时,还观察到形态各异的二次裂纹,有穿晶裂纹、沿晶裂纹、穿晶／沿晶混合裂纹等。对于塑性应变幅控制下的低周疲劳变形,二次裂纹的特征随应变幅的增加而呈现出一定的规律：在较低应变幅下表面裂纹以穿晶裂纹为主,裂纹沿滑移带方向扩展；中等应变幅下裂纹的扩展路径较为复杂,主要为沿晶-穿晶混合型裂纹。较高应变幅下的二次裂纹以沿晶为主要扩展特征。另外,当塑性应变幅高于3×10-3时,沿孪晶界萌生裂纹现象也变得明显。恒应力幅控制下的AL6XN不锈钢的疲劳断口形貌特征与外加应力幅有一定的关系。随应力幅从低于屈服强变化到高于屈服强度,疲劳源区的微观特征从脆性的解理台阶和脆性疲劳条纹逐渐转变为规则的塑性疲劳条带和二次裂纹的形成；裂纹扩展区均表现为塑性疲劳条纹的形成,而最终瞬时断裂区的特征均表现为韧窝的形成。恒应变幅控制下的AL6XN不锈钢的疲劳断口形貌呈现出类似的特征。利用SEM-ECC和TEM技术对低周疲劳样品表面位错组态进行观察发现,随着应变幅的增加,位错组态特征逐步由平面滑移型位错结构(如平面滑移带)向波状滑移型位错结构(如脉络、位错胞、PSB楼梯等)发生演变。一些变形集中区还有二次孪晶出现。另外,利用SEM-ECC技术对裂纹尖端位错组态进行了观察,发现在裂纹尖端及附近区域晶粒内形成了近似沿裂纹扩展方向发展的拉长的位错胞结构。更多还原

【Abstract】 As a kind of newly-developed stainless steel materials with a broad application background and excellent performance, the A16XN super-austenitic stainless steel would be inevitably subjected to uniaxial or cyclic loads during the actual service. Therefore, studies on its mechanical and fatigue behavior are of particular importance for its further development and practical applications. But until now, no systematical investigation in this area has yet been performed. In light of this, the A16XN super-austenitic stainless steel was selected in the present work as the target material to examine its uniaxial deformation and damage behavior at different strain rates, stressing upon the cyclic deformation and damage behavior under constant stress amplitude control and constant plastic strain amplitude control.A16XN stainless steel shows a low strain rate sensitivity of its uniaxial deformation behavior, and dislocation slip is the main uniaxial deformation feature. All fracture surfaces induced by tensile deformation at different strain rates can be divided into two parts, i.e., fibrous zone and shear lip zone. The fibrous zone consists of dimples with a bimodal size. It is noted that the compressive yield strength and tensile yield strength are basically comparable.The curve of fatigue life versus stress amplitude of the AL6XN stainless steel meets the Basquin relationship, while the curve of fatigue life versus plastic strain amplitude follows the Coffin-Manson relationship. This material exhibits monotonically sustained cyclic softening behavior at low plastic strain amplitudes. However, when the plastic strain amplitude is as high as 5×10-3s-1, the stress response curve of this material enters into an striking softening stage just after an initial hardening stage. No obvious stress saturation phenomenon appears.For the fatigue deformation under constant stress amplitude control, as the applied stress amplitude is below the tensile yield strength, the surface deformation feature is primarily manifested by slips in gains and twins, and zigzag slips crossing some grains and twins, and etc. However, when the applied stress amplitude is slightly higher than its tensile yield strength, the secondary cracks with different patterns are also found, such as transgranular cracks, intergranular cracks, transgranular/intergranular mixed-model cracks and so on. For the low-cycle fatigue deformation under constant plastic strain amplitude control, the secondary crack features exhibit a certain varying rule with increasing strain amplitude. At low strain amplitudes, the majority of cracks are transgranular ones, strictly propagating along the slip bands. At moderate strain amplitudes, the crack growth path is more complicated, and cracks are mainly intergranular/transgranular mixed-model ones. As the plastic strain amplitude is raised up to high level, secondary cracks extend mainly along the grain boundaries. In addition, when the plastic strain amplitude is higher than 3×10-3, the cracks initiating along twin boundaries also become apparent.The fatigue fracture features of the AL6XN steel under constant stress amplitude control are more or less related with the applied stress amplitude. As the stress amplitude changes from lower values than the yield strength to ones higher than the yield strength, the micro-features with brittle cleavage steps and brittle fatigue striations in the fatigue crack source zone changes gradually into features with regular fatigue striations and the formation of secondary cracks. The micro-characteristics in the fatigue crack growth zone are basically featured by the formation of ductile fatigue striations, and the micro-features in the final rapid fracture area for all samples correspond to the formation of dimples. The fatigue fracture features of the AL6XN steel under constant plastic strain amplitude control present quite similar situation.The low-cycle fatigue dislocation arrangements have also been observed using the electron channeling contrast (ECC) technique in scanning electron microscopy (SEM) and a transmission electron microscopy (TEM). It is found that with increasing strain amplitude, the dislocation structures with planar slip type (e.g. planar slip bands) are evolved gradually into those with wavy slip type (e.g. veins, dislocation cells and persistent slip band ladders and etc.). Secondary twins form in some deformation concentration areas. Furthermore, the dislocation at the crack tip has been detected by SEM-ECC. It is found that the major microstructural features around the crack (tip) are dominated by the elongated dislocation cells, which form in great number in the grains near the crack (tip). These elongated cells seemingly tend to develop away from the crack but towards the crack growth direction.更多还原