【作者】 吴宏；

【导师】 黄伯云； 刘咏； Ian Baker；

【作者基本信息】 中南大学 ， 材料学， 2011， 博士

【摘要】 作为新型材料的非晶合金由于独特的原子结构,被赋予了诸多不同于传统金属材料的优异性能。本文通过室温压缩和摩擦磨损两方面实验对Zr-Cu-Ni-Al-Ti块体非晶的力学行为展开了研究。总结分析文献中报道的不同体系块体非晶合金的弹性模量常数与室温压缩延展性数据,探索并建立两者之间的联系,并添加关键实验对其考证。对比去除表面软化层(铸造过程中冷却速率差异诱发)前后块体非晶的室温压缩性能,讨论表面软化层对块体非晶宏观力学行为的影响。从影响材料摩擦性能的内因(晶化度)和外因(载荷)出发,讨论在干摩擦条件下块体非晶合金的摩擦磨损行为及相关机理。借助聚焦离子束和透射电镜等先进材料微观结构表征技术,深入研究和分析摩擦气氛与摩擦对偶对块体非晶合金摩擦磨损行为的影响。主要结论如下：(1)块体非晶合金的力学性能与弹性模量之间存在本质联系。通过采用Pugh的一个理论参数,B/G,体积模量与剪切模量的比值,研究了其与室温压缩塑性的对应关系,发现其比泊松比具有更好的关联性。在Fe基和Zr基块体非晶中该参数均具有很好的预测性。此外,发现在该理论参数中,对非晶塑性起主要作用的还是剪切模量,因为该参数与原子的集群式运动相关。剪切模量越小,原子团越容易进行运动,因而变形能力增加。(2)室温压缩时,除去表面软化层的试样由于均匀的组织结构,表面萌生大量致密、均匀分布的剪切带,表现出较好塑性并呈现相对平滑的断面；而未除去软化层的试样仅萌生少量剪切带,剪切带快速扩展形成裂纹,导致灾难性断裂,表现出断面形貌的多样性。除去软化层的试样变形后自由体积数增加,压缩过程中发生应变软化。(3)块体非晶摩擦是结晶硬化与应变软化相互竞争的过程。随着载荷的增加,非晶的磨损量增大,摩擦系数减小,磨损机理由磨粒磨损向粘着磨损转变。块体非晶的硬度随晶化度增加而增大,其耐磨性同时取决于硬度和韧性。当晶化度为34%时,非晶试样具有最佳综合力学性能,耐磨性最好。随晶化度增加,主要磨损机理自粘着磨损向磨粒磨损转变。(4)块体非晶的磨损速率随环境中氧含量增加而显著增大。有氧环境中,非晶表层由于摩擦升温发生氧化,硬度与脆性均发生变化,表面微凸体(asperities)受剪切应力作用被剥落形成磨粒,摩损面呈现大量裂纹、蚀坑,属于磨粒磨损；氩气中,非晶摩擦表面塑性流变,与对偶表层机械混合作用,非晶纳米晶化,形成表面复合覆盖层。利用Archard模型计算磨损系数K说明主要磨损机理为轻微滑动磨损。(5)换钢作摩擦对偶时,块体非晶试样的摩擦系数与磨损速率均显著增大。由于钢的硬度远比氧化锆小(4H钢≈氧化锆),且小于非晶试样,摩擦时钢对偶受压迫表面变形,滑动受阻(高摩擦系数),摩擦副接触不完整,摩擦面局部剪切应力增大,应变集中(剪切带)、裂纹萌生与扩展,最终断裂。磨损机理为剧烈滑动磨损。更多还原

【Abstract】 Bulk metallic glasses (BMGs) have been emerging as a type of novel materials, due to their excellent mechanical, physical and chemical properties compared to crystalline metals. In the present study, the mechanical behaviors of Zr-Cu-Ni-Al-Ti BMG were investigated not only in compression test, but also in tribotest. The elastic moduli and compressive ductility for various BMGs were compiled and analyzed, in order to identify key physical factor controlling the deformation and fracture behavior of BMGs. The soft surface of as-cast Zr-based BMG that induced by high cooling-rate during solidification was machined for the comparison of the compressive behavior with the cast one. The friction and wear behaviors of the Zr-based BMG were studied using a ball-on-flat measurement at different loads, and the effect of crystallinity on the wear performance was also examined in the same equipment. To understand the effect of the oxygen in test environment, other wear tests were conducted in three different atmospheres, i.e. air, oxygen and argon, using a home-built pin-on-disk friction system. Different counterface was employed in the study as well. The microstructures of worn specimens were characterized by focus ion beam and transmission electron microscopy. The following conclusions are drawn:(1) The ductility of metallic glasses has a good correlation with the ratio of bulk modulus to shear modulus, BIG. For individual BMGs, there seems to be a critical BIG ratio for the ductility, such as 2.6 for Fe-based BMGs,3.4 for Zr-based BMGs. Since the shear modulus is very sensitive to the atomic structural change, it may dominate the plastic flow behavior. The smaller shear modulus corresponds to a looser atomic packing, and thus higher plasticity.(2) The as-machined small specimens without the cast-softened surface exhibites highly dense and intersecting shear bands, and extensive plastic deformation, in contrast to the catastrophic failure and low deformability in the cast specimens with the softened surface. More free volume was detected in the small as-fractured specimens, indicating the occurrence of strain softening during the compressive process. Compared to the relatively smooth fracture surface of the small specimens, the large specimens showed more diverse features on the fracture surface due to the graded structures.(3) The friction coefficient of metallic glass with a steel counterpart is in the range of 0.24-0.32. Both surface softening and crystallization occur on the surface of metallic glass during wear, and wear curve is not as stable as the crystalline materials due to the interaction of the two processes. The wear mechanism of metallic glass may change with wear conditions and the crystalline phase content. Fully amorphous material shows an abrasive wear at a low load, then adhesive wear at a high load. Increasing the crystalline phase results in more abrasive wear. The wear behaviors of metallic glass and its crystalline composites do not follow the Archard’s equation. Only a good combination of the hardness and the toughness can the metallic glass be wear resistant.(4) The wear rate of the specimens increased dramatically with increasing oxygen content in the testing environment. A number of cracks and pits were present on the worn surface of the pin tested in the oxygen-containing environments, whilst a relatively smooth worn surface and a mixed layer with a thickness of about 2-10μm were observed in the specimens tested in argon. For the tests in oxygen, abrasive particles induced by oxidation protruded and peeled off from the glassy matrix together with the debris come from the conterface, resulting in a combination of two-body and three-body abrasion, thus indicating an abrasive wear controlled process. In the oxygen-free environment, the plastic flow took place presumably accompanied by work-softening due to the frictional heat and the local stress concentrations. This led to the formation of the mixed layer on the pin and a material-transfer film on the disk. The wear coefficient K of the pin obtained via Archard’s model implied that a mild sliding wear was dominated.(5) For wear tests against steel, both the pin and the disk showed more roughly worn surface, thus exhibiting a much higher wear rate than that of the pin tested against zirconia. The possible reason may be due to the significantly low hardness and relatively good plasticity of the steel, which leads to the severe adhesion between the disk and the pin. This may induce large shear stress on the wear surface of the pin, resulting in local deformation and fracture.更多还原