Zr-Ti-Cu-Al块体金属玻璃的断裂韧性

IMR OpenIR

	Zr-Ti-Cu-Al块体金属玻璃的断裂韧性
	贺强
学位类型	博士
导师	徐坚
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	金属玻璃玻璃转变温度弹性常数力学性能断裂韧性 Metallic Glasses Glass Transition Temperature Elastic Constants Mechanical Properties Fracture Toughness
摘要	"金属玻璃拥有优于传统晶态金属材料的诸多性能，如高强度、高弹性极限等。然而块体金属玻璃（BMG）在单轴拉伸载荷下没有宏观塑性，作为一种准脆性材料，断裂韧性是BMG工程应用的关键。在已发现的BMG合金体系中，Zr基BMG具有低成本、高玻璃形成能力、良好的综合力学性能等优势，然而其断裂韧性有待于提高。本工作通过调整合金成分来优化Zr-Ti-Cu-Al四元合金玻璃形成能力以及力学性能，研究玻璃转变温度，弹性常数等因素对力学性能的影响。在获得兼具高GFA以及良好弯曲塑性变形能力BMG的基础上进一步研究合金的断裂韧性包括缺口韧性，预制疲劳裂纹试样的断裂韧性以及抵抗裂纹扩展的能力（R曲线）。另外还研究了Zr基BMG断裂韧性与疲劳性能之间的关系。主要结论如下： 1.在Zr-Cu-Al三元合金系的富Zr端发现可形成块体金属玻璃（BMG）的新成分区域，其中Zr60Cu28Al12和Zr58Cu30Al12合金铜模浇铸形成BMG棒材的临界直径为6 mm。在三元基础上利用“3D法”由Ti元素部分置换Zr元素，可进一步增强合金的玻璃形成能力（GFA），GFA最佳的Zr61Ti2Cu25Al12（ZT1）合金形成BMG的临界直径可达到10 mm。 2. Zr(Ti)-Cu-Al四元BMG具有弯曲变形的能力。变形的弯曲位移和剪切带间距与玻璃转变温度（Tg）相关，Tg越低，弯曲位移越大，剪切带间距越小，弯曲变形能力越强。拉伸和压缩实验表明Zr(Ti)-Cu-Al BMG的变形受控于单一剪切带的萌生与扩展，导致这两种载荷模式下Zr61Ti2Cu25Al12（ZT1）和Zr61.6Ti4.4Cu24Al10（ZT3）BMG试样尺寸从2 mm到4 mm，应变速率从1×10-4 s-1到2×10-1 s-1都不具有任何宏观塑性。Tg强烈依赖于合金的成分并与弹性常数之间相关，可作为设计具有易变形的BMG的依据。 3. Zr61Ti2Cu25Al12 BMG在最大载荷下的断裂韧性Kmax超过100 MPam1/2，是目前预制疲劳裂纹试样断裂韧性最高的Zr基BMG之一。Zr基BMG的断裂韧性大体上与合金的泊松比ν、剪切模量与摩尔体积的乘积GVm以及约化温度TR/Tg相关。对于Zr-TM-Al（TM 为 Co、 Ni、Cu）三元BMG，用Cu来替代Ni或Co元素有助于提高断裂韧性。高Zr含量和低Al含量的BMG表现出高韧性。 4. Zr61Ti2Cu25Al12 BMG具有明显的裂纹扩展阻力，表现出阻力曲线（R-curve）的特征。裂纹的扩展分为由多重剪切带萌生与扩展诱导的剪切滑移（Shear-off）和超过最大载荷后的真实稳态扩展。在稳态扩展区，初始裂纹发生明显偏转，使裂纹尖端的应力大大降低，同时裂纹尖端局部载荷模式转变为I型和II型混合模式（此时远场的载荷模式仍为Mode-I）。采用最大载荷对应的KJmax值（裂纹刚开始扩展时的韧性值）作为ZT1 BMG的本征断裂韧性，KJIC，为130 ±20 MPam1/2，与目前韧性最好的Pd-Ag-P-Si-Ge BMG以及传统晶态合金相当。高泊松比并不完全是BMG具有高韧性的必要条件。因此，亦可用普通工程合金元素代替稀贵金属元素来设计具有高断裂韧性的BMG。 5.利用四点弯曲疲劳实验方法确定的ZT1块体金属玻璃的四点弯曲疲劳极限超过400 MPa，其疲劳极限与抗拉强度的比值为0.273，为目前疲劳性能最好的Zr基金属玻璃之一。ZT1 BMG的断裂韧性和疲劳性能相关，高断裂韧性将导致更好的疲劳性能。ZT1 BMG的疲劳寿命由裂纹的萌生和初始扩展控制，裂纹萌生表现为单一剪切带的萌生和扩展，只有当剪切带达到失稳临界长度时才会转变为疲劳裂纹进行扩展。"
其他摘要	"In contrast to conventional metals, Bulk metallic glasses (BMGs) exhibits high strength and high elastic limits. However, as often classified to be quasi brittle materials, BMGs do not exhibit global tensile plasticity. As a result, fracture toughness is the key mechanical property needed for their practical applications. Zr-based BMGs are of low cost and show high glass-forming ability (GFA) and good mechanical properties, while their fracture toughness needs to be enhanced. In the present work, the GFA and mechanical properties of quaternary Zr-Ti-Cu-Al BMGs are optimized by carefully tuning the chemical composition. The effects of glass transition temperature Tg, the elastic constants on the mechanical properties are revealed. On the basis of high GFA and good malleability, the toughness including the notch toughness, fracture toughness with fatigued sample as well as the crack resistance curve (R-curve) of these BMGs are investigated. In addition, the correlation of fatigue behavior with fracture toughness for Zr-based BMGs is also elaborated. 1. A new glass-forming composition area in Zr-rich region of Zr-Cu-Al ternary system was discovered. Zr60Cu28Al12 and Zr58Cu30Al12 alloys could be cast into fully glassy rods of 6-mm in diameter. By partially substituting Zr by Ti, the critical diameter Dc of BMG formation of Zr(Ti)-Cu-Al quaternary alloys was further upgraded with “3D pinpointing approach”. The Dc of Zr61Ti2Cu25Al12 (ZT1) alloy with the highest GFA is up to 10 mm. 2. Zr(Ti)-Cu-Al quaternary BMGs are malleable under three point bending. The bending deflection and shear band spacing are both correlated with Tg. Lower Tg results in higher bending deflection and narrower shear band spacing thus higher malleability. Compression and tension tests show the deformation of Zr(Ti)-Cu-Al BMGs is mainly controlled by a single shear band, resulting in none global compressive or tensile plasticity of Zr61Ti2Cu25Al12(ZT1) and Zr61.6Ti4.4Cu24Al10(ZT3) BMGs with various sample sizes (Dia.=2~4mm) under strain rates of 1×10-4 s-1 and 2×10-1 s-1. Tg is sensitively compositional dependent and correlated well with the elastic constants in Zr(Ti)-Cu-Al system. As a result, Tg serves as an sensitive and convenient indicator to detect malleable BMGs. 3. Fatigue pre-cracked Zr61Ti2Cu25Al12 (ZT1) BMG sample exhibits a fracture toughness (Kmax) in excess of 100 MPam1/2, which makes ZT1 one of the Zr-based BMGs with highest toughness to date. The toughness of Zr-TM-Al (TM=Co, Ni, Cu) ternary BMGs shows a general correlation with the Poisson’s ratio (n), the product of shear modulus and molar volume (GVm) and homologues temperature (TR/Tg). As for Zr-TM-Al BMGs, substituting Ni or Co by Cu will improve the toughness. Higher toughness is expected at relatively higher Zr content and lower Al content. 4. Zr61Ti2Cu25Al12 (ZT1) BMG exhibits a remarkable resistance to crack growth (R-curve). The internal crack propagation includes a shear-off zone ahead of the pre-crack tip and substantial subcritical crack growth well beyond the highest load. In the subcritical crack growth region, crack deflection occurs which reduces the stresses (driving force) at crack tip and introduces Mode-II loading to the crack tip field. (While the sample is subjected to far-field Mode I loading conditions). Using the KJmax value corresponding to the highest load and the onset of crack extension, the critical toughness KJIC of ZT1 BMG is determined to be ~130± 20MPam1/2, which is comparable to not only the toughest Pd-Ag-P-Si-Ge BMG but also conventional crystalline alloys. Exceptionally high Poisson’s ratio is not the only factor leading to high-toughness of BMGs. Normal engineering metallic elements instead of noble elements can also be used to design BMGs with potential high fracture toughness. 5. The four-point bending fatigue limit of ZT1 BMG is over 400 MPa, together with a fatigue limit/tensile strength of 0.273 which ranks it one of the Zr-based metallic glasses with the best fatigue properties. The fracture toughness and fatigue properties of ZT1 BMGs are correlated with each other, i.e., higher fracture toughness leads to better fatigue properties. The fatigue crack initiation and early propagation determine the fatigue life of ZT1BMG. In addition, the fatigue crack initiation is implemented by a single shear band initiation and propagation, and the fatigue crack would not propagate until a critical length of shear band is reached. "
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64449
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	贺强. Zr-Ti-Cu-Al块体金属玻璃的断裂韧性[D]. 北京. 中国科学院金属研究所,2012.