【作者】 喻更生；

【导师】 林建国；

【作者基本信息】 湘潭大学 ， 材料物理与化学， 2009， 博士

【摘要】 块体非晶合金具有优异的物理、化学及力学性能,在基础科学和实际应用领域越来越受到人们的关注。然而块体非晶合金室温下缺乏明显的塑性变形,从而限制了其作为结构材料在实际中的应用。因此,研究非晶合金的塑性变形机理和寻找提高塑性的途径是目前研究的热点问题。本文利用微纳米压痕方法和宏观单轴压缩方法,系统地研究了多种块体非晶合金体系的宏微观塑性变形行为。重点研究了非晶合金的压力敏感性(pressure sensitivity),探讨了其压力敏感因子和硬化指数的物理本质及其影响因素;分析了具有不同压力敏感因子的非晶合金的剪切带形貌与演变规律;研究了玻璃转变温度,结构弛豫对非晶变形的影响;初步研究了非晶的挤压变形行为。主要结论如下:1、非晶合金的变形表现出压应力敏感性,其压力敏感程度可以用压力敏感因子来表征,压力敏感因子与非晶结构密切相关,随着非晶结构弛豫程度的提高,压力敏感因子增加,压力敏感因子大小反映了非晶在变形过程中的原子运动摩擦力的大小;2、在压痕变形条件下,非晶剪切带的形貌和演化与非晶的压力敏感因子密切相关。在维氏压痕和洛氏压痕变形条件下,剪切带夹角的变化随着压力敏感因子增加而减小,而在纳米压痕法变形条件下,其深度载荷曲线的曲率随压力敏感因子变化而变化。通过对压痕周围剪切带夹角的测量,可确定非晶材料的压力敏感因子;3、非晶合金在纳米压痕试验过程中的锯齿流变现象与合金体系的玻璃转变温度有关,具有适中玻璃转变温度的合金体系表现出显著的锯齿流变特征,而锯齿流变现象的出现不仅与加载条件有关,还与非晶的结构有关。随着非晶结构弛豫程度的增加,出现锯齿流变的临界加载速率降低;4、在La基、Zr基和Ni基非晶合金的纳米压痕周围,可观察到明显的材料堆积(pile-up)形貌特征,其pile-up高度与非晶合金的硬化指数有关;5、含原位生长晶化相的Nd基和Zr基块体非晶合金表现出较高的强度和室温塑性,其宏观塑性变形和力学性能与晶化相的尺寸、体积分数等密切相关;6、Zr基块体非晶的挤压变形行为对应变速率和温度有明显的依赖性,在355～415℃温度范围内,保持平稳流变所需的力从1.5KN到0.5KN,当温度为395℃,挤压速率为0.05mm/s时,保持平稳流变所需的力高达4KN。更多还原

【Abstract】 Bulk metallic glasses (BMGs) have attracted a great attention on fundamental science and practical applications due to their promising physical, chemical and mechanical properties. However, the lack of any significant plastic deformation at room temperature of BMGs is one of the main obstacles to their application in practice. Thus, deeply understanding the macro- and micro- plastic deformation mechanisms of BMGs is always important to the improvement of the ductility of BMGs. In this thesis, by using micro- and nano- indentation methods and uniaxial compression test, the plastic deformation behavior of several BMG systems was investigated, focusing on the physical nature of the pressure sensitivity index and the strain hardening index of the BMGs. The morphology and evolution of the shear bands around the indentation obtained by the different indentation tests conditions were observed in the BMGs with the different pressure and the relationship between the pressure sensitivity index and the shear band patterns of the BMGs was set up. Moreover, the serrated flow feature at different loading conditions in several BMG systems was characterized by nanoindentation measurements, and the effect of structural relaxation on the plastic deformation behavior was investigated. Finally, the extrusion behavior of a Zr-based BMG was investigated. The primary conclusions are summarized as follows:1. The deformation of BMGs exhibits the pressure sensitivity which is reflected by the pressure sensitivity index. The pressure sensitivity index is strongly dependent on the structure of BMGs. With the increase of the degree of the structural relaxation, the pressure sensitivity index increased. Physically, the pressure sensitivity index reflects the atom friction.2. The morphology and evolution of the shear bands around the indentation correlate to the pressure sensitivity index: Under the Vickers and Rockwell indentation conditions, the include angle between two family shear bands decreases with the increase of the pressure sensitivity index, while, under the nano-indentation condition, the curvature of the load-depth curve changes with the change of the pressure sensitivity index. The pressure sensitivity index can be obtained through the measurement of the include angle between two family shear bands around the Vickers indentation.3. During nano-indentation test, some BMGs with the proper glass transition temperature exhibit the serrated flow feature. The appearance of the serrated flow phenomenon is not only dependent on the loading conditions, but also dependent on the structure of BMGs. With the increase of the structure relaxation degree, the critical loading rate for the appearance of the serrated flow decreases.4. Pile-up of materials was observed around the nano-indentation in La-, Zr- and Ni-based BMGs. The altitude of pile-up of materials related to the hardening index.5. The Nd- and Zr- based BMGs with the presence of the in-situ crystalline phases exhibit high strength and room-temperature ductility, compared to the monolithic BMGs. The feature of the deformation of the composites is strongly dependent on the shape and size of the crystalline phase in the BMGs.6. The extrusion deformation of a Zr-based BMG is strongly dependent on the temperature and the strain rate. At the temperature in the range of 355～415℃, the stress for the steady flow decreased from 1.5KN to 0.5KN with the temperature increasing. Under the temperature of 395℃and the strain rate of 0.05mm/s, the stress for the steady flow is up to 4KN.更多还原