【作者】 黄永江；

【导师】 沈军； 孙剑飞；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 博士

【摘要】 本文采用单轴拉伸和压缩以及纳米压痕技术,系统研究了Ti40Zr25Ni3Cu12Be20块体非晶合金宽温度范围内(123 K～658 K)的力学行为。铸态非晶样品的尺寸对非晶合金的塑性变形能力产生显著影响。相比较大尺寸非晶试样而言,具有相同化学成分的小尺寸非晶试样体现更大的压缩塑性和较低的压缩强度,这表明非晶合金具有“愈小亦愈软”的趋势。在凝固过程中,小尺寸的试样经历更大的冷却速度,使得其拥有更多的自由体积,促进剪切带的形核并导致玻璃态合金在变形时产生更多剪切带,使得小尺寸试样在单轴压缩过程中体现更大的塑性变形能力。Ti40Zr25Ni3Cu12Be20非晶合金在低温环境下塑性和强度均显著增强。其塑性的提高可归因于密集的剪切带的形成及剪切带的扭转。低温环境大大降低原子的活动能力,使得原子扩散困难,进而抑制了剪切带的扩展,使得合金在远低于室温时体现塑性与强度的同时提高。低温压缩变形中,非晶合金产生“变形诱发纳米晶转变”现象,同时,低温变形试样的晶化程度较室温变形试样大大减轻。Ti40Zr25Ni3Cu12Be20非晶合金在过冷液相区体现优异的超塑性变形能力,高温压缩应变可高达0.83。非晶合金的超塑性流变行为强烈取决于测试温度和应变速率。在低温和高应变速率时,测量的粘度值随着应变速率的增加而不断降低,即,材料体现非牛顿型流变,且在应力应变曲线上可观察到峰值应力现象。而在低应变速率和高测试温度条件下,材料体现牛顿型流变。非晶合金在宽温度范围内测试温度与其强度之间符合下列关系:σ= aE ( b ? T / Tg)。对于本文研究的Ti基非晶合金而言,当测试温度在0～0.9Tg时,σ/ E= 0.0066T/Tg+0.0206;而当测试温度在0.9～1.1Tg时,σ/ E= 0.1163T/Tg+0.1278。研究了Ti40Zr25Ni3Cu12Be20非晶合金在不同测试温度下的单轴压缩断裂行为以及断口形貌特征。在高温变形时,压缩断口较室温断口更为粗糙不平。随着测试温度的升高,一个与脉状花样不同的特征,即大面积粘性介质层,出现在断口上,外加热场以及塑性流变导致的绝热升温被认为是导致粘性介质层的两个重要原因。升高测试温度,导致试样的剪切断裂角亦随之逐渐增加。非晶合金在不同测试温度下的断裂行为均遵循与正应力相关的Mohr-Coulumb准则。预塑性变形对Ti40Zr25Ni3Cu12Be20非晶合金的力学性能产生重要影响。预塑性变形前后合金试样在均体现非晶态结构特征的同时,塑性变形能力体现较大差别。随着预应变量的增加,试样在发生断裂破坏前的压缩塑性应变出现先增加后减小的趋势,在预应变为10 %时,试样体现最大的压缩塑性。同时,随着预应变量的增加,预应变试样在破坏后试样表面的剪切带数量及宏观断口的脉状纹络的密度亦体现先增大后减小的趋势,而预变形试样的硬度体现先减小后增加的趋势。热分析结果表明,在预应变为10%时,试样体现最多的自由体积。基于Spaepen提出的自由体积理论,对于预变形过程中,非晶体内自由体积出现饱和的现象进行了合理解释。研究了Ti40Zr25Ni3Cu12Be20块体及薄带非晶合金在宽最大载荷范围内(5～100 mN)室温纳米蠕变特性。应力指数n由载荷-位移曲线推算出来并作为一个衡量材料抗蠕变特性的重要指标。随着纳米压痕尺寸增加,所测得的应力指数急剧增加,体现明显的尺寸效应。相比薄带材料而言,这种尺寸效应在块体中体现得更为显著。纳米压痕蠕变行为的变形机理可以由基于“剪切转变区”的非晶合金变形理论来合理解释。采用循环加载纳米压痕测试研究了Ti40Zr25Ni3Cu12Be20非晶合金的反塑性行为。随着循环加载次数的增加,卸载曲线与重新加载曲线之间的差别越来越不明显。小的加载/卸载速度导致材料的卸载曲线与重新加载曲线之间体现更为显著的差别。同时,具有高玻璃转变温度的非晶合金体现更小的反塑性。更多还原

【Abstract】 In the present dissertation, the mechanical behaviors of Ti40Zr25Ni3Cu12Be20 bulk metallic glass (BMG) have been systematically investigated using unixial compression and tensile tests, as well as nanoindentation technique at different testing temperatures ranging from 123 K to 658 K.The dramatic effect of sample size on the plastic deformation capability of the glassy alloy tested was studied. Compared with the larger one, the smaller size glassy sample with the same chemical composition exhibits significantly pronounced plasticity, suggesting a“smaller is softer”trend of metallic glasses. The phenomenon is attributed to the fact that the smaller size alloy, which experienced a faster cooling rate during solidification, contains a larger amount of heats of relaxation and crystallization, favoring the preferential nucleation of shear bands and thus allowing enhanced plasticity upon compressive loading.The Ti-based BMG studied exhibits a significant enhancement in both compressive strength and plasticity at low temperatures. The ductilization of the BMG system can be evidently attributed to the formation of dense shear bands and the rotation mechanism of shear bands. The cryogenic surroundings can effectively slow down the mobility and diffusion of the atoms and consequently, suppress the nucleation and growth of nanocrystals during the deformation process, allowing the simultaneous improvement in the mechanical responses of the glassy alloy subjected to compressive loading far below the ambient temperature. The deformation-induced crystallization occurs during low temperature compression tests of the studied alloy while the crystallization in the low temperature deformed sample is not as profound as that in the room temperature deformed sample.The superplastic deformation behaviors of the studied alloy were examined. The alloy shows an extraordinary compressive superplastic formability in the supercooled liquid region, typically evidenced by a large compressive strain up to 0.83. The superplastic flow behaviors depend strongly on the testing temperatures and initial strain rates. At low temperatures or high strain rates, the viscosity measured was found to dramatically decrease with increasing strain rate (i.e. non-Newtonian flow) and a stress overshoot is detected in the first steps of strain. At low strain rates or high testing temperatures, the studied glass exhibits a Newtonian flow behavior. It is found that the relationship between the strength and the testing temperature of the metallic glasses obeys the following relation:σ= aE ( b ? T / Tg). For the testing temperature regime of 0～0.9Tg,σ/ E= 0.0066T/Tg+0.0206 whereas for the testing temperature regime of 0.9～1.1Tg,σ/ E= 0.1163T/Tg+0.1278.The fracture behaviors of the studied BMG subjected to uniaxial compression tests at high temperature have been investigated. At high temperatures, the compressive fracture surface of BMG becomes much rougher than that at room temperature. With increasing the testing temperature, a different pattern from the vein appeared on the fracture surface, which is viscous flow layer. This fracture feature is most likely due to the external heat fields and the adiabatic heating created by plastic flow. As the testing temperature increases, the fracture angle correspondingly increases as well. The normal stress-dependent yield criterion (i.e., the Mohr-Coulomb criterion) should be used to elucidate the controlling fracture mechanisms of metallic glass deformed under various testing temperatures.The mechanical properties of pre-deformed Ti-based metallic glass have been examined. With increasing pre-strain, the plasticity of the pre-deformed sample initially increases, and then decreases with further increasing pre-strain. The hardness of the pre-deformed samples exhibits the reverse trend. For the case of 10 % pre-strain, the pre-deformed sample exhibits the largest plastic strain prior to failure. At the same time, as the pre-strain increases, the quantity of shear bands observed on the outer surface and the density of vein patterns shown in macroscopical fracture surface also increases initially and then decreases. The thermal analysis results reveal that the 10 % pre-strained sample possesses the largest quantity of free volume. The saturation of free volume in metallic glassy sample during pre-deformation can be successfully interpreted based on the free volume theory proposed by Spaepen.The time-dependent plastic deformation properties of Ti40Zr25Ni3Cu12Be20 bulk and ribbon metallic glass were investigated using nanoindentation technique at room temperature. The stress exponent n, defined asε? =Aσn, has been derived from the load-displacement curve and is used as a measure of the creep resistance. It was found that the measured stress exponent increases rapidly with increasing indentation size, exhibiting a positive size effect. The size effect in n obtained from the bulk sample is more pronounced than that obtained from the ribbon sample.The reverse plasticity of Ti-based bulk metallic glasses was investigated using cyclic nanoindentation tests. The deviation between the unloading and reloading curves gradually diminishes with the increasing number of cycles for the Ti40Zr25Ni3Cu12Be20 metallic glass. Smaller loading/unloading rate leads to a larger deviation between the unloading and reloading curves for the Ti40Zr25Ni3Cu12Be20 metallic glass. Metallic glasses with high Tg show smaller deviation between the unloading and reloading curves.更多还原