【作者】 潘杰；

【导师】 柳林；

【作者基本信息】 华中科技大学 ， 材料学， 2011， 博士

【摘要】 本论文利用X射线衍射分析仪(XRD)、光学显微镜(OM)、示差扫描量热仪(DSC)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、力学试验机、纳米压痕设备、显微硬度计、X射线小角散射(SAXS)及静态热分析仪(TMA)等实验手段系统研究了具有两种不同非均匀微结构(纳米尺度的相分离及二十面体中程有序结构)对Zr基块体非晶合金室温塑性及变形行为的影响,并从理论上分析了非晶合金的变形机理。采用电弧熔炼/水冷铜模吸铸的方法制备了ZrCuAlX(X=Ta,Fe)块体非晶合金。实验结果表明：微量正混合热元素Ta和Fe的添加能有效改善合金的室温变形能力,使Cu46Zr47A17非晶合金的室温塑性由0.59%分别提高至2.62%和4.81%。通过研究Fe含量对合金塑性变形行为的影响,发现塑性开始随Fe含量的增加而增加,到1at%时达到最大,为7.47%。随后塑性开始减小,直至2.5at%时不能形成完全的非晶结构。TEM结果表明Fe的添加使ZrCuAl合金中形成纳米尺度的相分离结构,形成富Cu和富Fe的非晶相,当相分离的尺寸在一定尺寸时,能很好地提高非晶合金的室温变形能力。通过调整合金中Zr含量,系统性地比较研究了Zr65Cu17.5Ni10Al7.5和Zr69.5Cu12Ni11Al7.5块体非晶合金的微观结构和力学性能。结果表明,这两种成分相近的合金表现出完全不同的晶化行为和力学性能。前者表现为单步晶化,形成稳定的CuZr2相,而Zr69.5Cu12Ni11Al7.5非晶合金则分两步晶化,优先析出准晶相进而形成CuZr2相,表明该合金中具有二十面体中程有序结构。热力学分析表明Zr695Cu12Ni11Al7.5块体非晶合金具有较小的晶化激活能和形核激活能,说明了合金中具有较强的团簇结构,这与SAXS的结果相一致。具有二十面体中程有序结构的Zr69.5Cu12Ni11Al7.5块体非晶合金具有很好的室温变形能力,其单轴压缩塑性高达25%而不发生断裂,缺口断裂韧性达到86MPm1/2,直径为1mm的合金样品弯曲120度以上也不发生断裂,表明增加Zr含量有利于形成二十面体中程有序结构,提高非晶合金的室温变形能力。通过研究应变速率对Zr695Cu12Ni11Al7.5块体非晶合金变形行为的影响时发现,该合金存在一个由“冷剪切”到“热剪切”的临界应变速率。在低应变速率下(≤10-1s-1),合金沿着一条主剪切带滑移,表现为典型的冷剪切行为。而当应变速率达到1s-1时,剪切带内合金因发生热剪切而表现为脆性断裂。通过绝热温升模型对其进一步分析发现,合金的变形行为不仅与应变速率有关,也与样品尺寸相关。此外,锯齿流变行为也强烈地依赖于应变速率,并随着应变速率的增大而减小,在应变速率达到10-s-1时消失。此过程类似与摩擦过程中的滑动行为,符合Stick-Slip模型。通过在Zr69.5Cu12Ni11Al7.5块体非晶合金上形成单一的剪切带,对剪切带的性质及非晶合金的变形机理进行直接研究。纳米压痕实验结果表明,在剪切带处存在明显的软化,其最小硬度(即为剪切带的硬度)为3.5GPa,且不随着塑性变形量的增加而改变。但剪切带的宽度随着塑性变形量的增加而增大,当塑性变形为6%时,其宽度为160μm,远大于文献中利用TEM观察的10-100nm。TMA和DSC实验结果表明,剪切带内有剧烈的体积膨胀,最大体积膨胀和自由体积分别为1.14%和1.40%。自由体积的最大值不随塑性变形量的增加而改变,表明剪切带内的自由体积在塑性变形为2%时即达到饱和。利用自由体积模型对该块体非晶合金的变形行为进行分析。研究发现,剪切带内的最大自由体积为1.20%,与实验结果完全吻合,说明非晶合金的剪切变形是自由体积产生的过程,符合自由体积模型。基于硬度值和自由体积含量,可以定量表征两者之间的关系,为H=0.56+0.04/ξ,并得到活化体积εov0=1.61×10-28m3。更多还原

【Abstract】 In this dissertation, the effect of heterogeneous microstructure on room temperature plasticity and deformation behavior in Zr-based bulk metallic glasses have been inverstigated by the means of X-ray diffraction (XRD), optical microscopy (OM), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), mechanical testing system, nanoindenter, thermomechanical analysis (TMA) and mcrohardness measurement, and then the deformation mechanism of metallic glasses is also discussed.Zr-based bulk metallic glassees (BMGs) were synthesized by addition an amount of Ta and Fe into ZrCuAl alloys by copper mould casting. It is found that the plasticity can be enhanced by the addition of positive elements, from 0.59% to 2.62% and 4.81% for Cu46Zr47Al7, Zr45Cu46Al7Ta2 and Zr47Cu44Al7Fe2 alloys, respectively. The effect of Fe content on the deformation behavior had been further studied. The plasticity increased with Fe content at frist, and reached the maximum of 7.47% for Zr47Cu45Al7Fe1, and the decreased until to the appearance of crystalline with the Fe content of 2.5%. TEM showed the occurrence of phase separation with Fe-riched and Cu riched phases in ZrCuAlFe metallic glasses is the reason of enhancement of plasticity. The size increased with the Fe content, and the plasticity is best with the Fe content of 1 at%.The mechanical properties of Zr65Cu17.5Ni10Al7.5 and Zr69.5Cui2Ni11Al7.5BMGs have been comparatively studied. The crystallization process of the two BMGs during continuous heating shows quite difference:Zr65Cu17.5Ni10Al7.5BMG undergoes one-step crystallization with the formation of intermetallic compounds of Zr2Cu and small amount Zr2Ni, while Zr69.5Cu12NinAl7.5 BMG follows two-step crystallization with a preferential formation of icosahedral phase, indicating that Zr6.5Cu17.5Ni10Al7.5 BMG may involve a strong icosahedral midium-range ordering structure. It was also found crystallization activation energy and nucleation activation energy were nuch lower in Zr69.5Cu12NinAl7.5 BMG, which further proved the existence of strong clusters in the alloy, which is also confirmed by SAXS results. Comparing with the former BMG,Zr69.5Cu12Ni11Al7.5 BMG exhibited large plastic strain and extremely good toughness. The compressive strain is more than 25%, while the bending degree can reach as large as 120 degree without failure. The noth toughness is as high as 86MPam1/2, while the Zr65Cu17.5Ni10Al7.5 BMG is only 60MPam1/2. It is suggested that the existence of icosahedral midium range ordering clusters could cause a heterogeneous distribution of free volume, which contributed the enhancement of plasticity and toughness of Zr69.5Cu12Ni11Al7.5 BMG.The deformation behavior of Zr69.5Cu12Ni11Al7.5 BMG was investigated at different strain rate, from 10-4 to 1s-1. It is found that there is critical strain rate form cold shear to hot shear. When the strain rate is higher than Is-1 for the 2 mm samples, the alloys fractured with litter plastictity due to the temperature rise above glass transition temperature, which can be well explained by temperature rise model. The serrated flow is also dependent on the strain rate, which decreased with the increasing of strain rate, and disappeared at 10-1s-1, which is consistent with the Stick-Slip model.Through the above studied, we have conducted a controlled experiment to form a single shear band in the specimen which enabled us to probe shear induced dilatation and softening directly on the shear band itself. Extreme dilatation and free volume increase as high as 1.14% and 1.40% respectively, have been observed resulted from a drastic structure change due to severe plastic flow in the band. The nano-indentation on the individual shear band revealed significant softening of 36% and unexpected wide width up to 160μm, three magnitudes higher than what has been reported. The minimum hardness was independent of plastic strain, which indicated that the free volume reached saturated with the plasti strain of only 2%. These prove beyond doubt the dilatation as the mechanism for softening rather than temperature rise in this study. The correlation between the free volume content and softening is discussed, and can be expressed as H=0.56+0.04/ζwith the activation volume of 1.61×10-28 m3.更多还原