镁基金属玻璃的断裂韧性及其影响因素

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	镁基金属玻璃的断裂韧性及其影响因素
	王绍钢
学位类型	博士
导师	马恩 ; 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	金属玻璃弹性常数断裂韧性复合材料镁合金 Metallic Glass Elastic Constant Fracture Toughness Composite Magnesium Alloy
其他摘要	" 相对于普通的晶态镁合金，镁基金属玻璃具有高强度(800-1000 MPa)、高弹性极限(~2%)和耐腐蚀等优异性能。镁基金属玻璃具有良好的玻璃形成能力，临界尺寸可达到厘米量级。然而，镁基金属玻璃的脆性成为其工程应用的瓶颈。发展具有高玻璃形成能力并兼有良好韧性的新型玻璃态镁合金具有重要意义，而影响镁基金属玻璃韧性的因素有待于澄清。本工作以Mg-TM-RE (TM=Cu, Ag, Ni; RE=Y, Gd, Tb, Nd)合金为基础，从以下几个方面探讨解决镁基金属玻璃脆性的可能性：（一）镁基金属玻璃合金成分对韧性的影响；（二）以镁基金属玻璃弹性性能作为依据改进合金的韧性；（三）镁基金属玻璃的样品尺寸对屈服强度和断裂韧性的影响；（四）形成以B2结构AgMg相作为初生相的镁基金属玻璃内生复合材料。主要结论如下：在Mg-Cu-Ag-Y四元合金中，Mg56Cu21Ag14Y9金属玻璃泊松比达到0.332，但未发生明显的“脆-韧转变”。对于镁基金属玻璃，根据合金组元的晶体弹性常数权重计算得到的体模量、剪切模量和平均原子体积分别高于实际金属玻璃的8%、10%和4%。镁基块体金属玻璃的缺口韧性从Mg57Cu34Nd9的3.6 MPa·m1/2到Mg59.5Cu22.9Ag6.6Gd11的8.2 MPa·m1/2。增加泊松比或者降低剪切模量对镁基金属玻璃的韧性并没有明显的影响。在Mg-TM-RE (TM=Cu, Ni; RE=稀土)金属玻璃中，采用Ag部分替代Cu、Gd和Tb部分替代Y或Nd会使韧性有所改善。在所研究的合金中，Mg59.5Cu22.9Ag6.6Gd11金属玻璃的韧性较好并兼有高玻璃形成能力。由于存在铸造缺陷，Mg59.5Cu22.9Ag6.6Gd11铸态金属玻璃的压缩断裂强度受到样品尺寸的影响。圆柱直径从1 mm增加到10 mm，断裂强度下降约25%。由1 mm直径样品得到的断裂强度(sy»950 MPa)可视为材料的本征性能。断裂强度与样品直径之间的关系不符合Weibull分布或者Gumbel分布的假设。采用平板样品测量Mg59.5Cu22.9Ag6.6Gd11块体金属玻璃的缺口韧性KQ，平板厚度增加引起的KQ下降，源于样品内部的铸造缺陷而非平面应力和平面应变的混合模式向完全平面应变的转变。采用三参数Weibull分析可以得到缺口韧性的位置参数Km，低于Km将不发生裂纹扩展。忽略缺口钝化效应的影响，Mg59.5Cu22.9Ag6.6Gd11金属玻璃的断裂韧性约为10 MPa×m1/2。在MgCuAgY四元合金中，可通过调整成分和铜模浇铸，制备出以B2结构AgMg相作为初生相的内生复合材料。树枝晶形貌的AgMg相均匀分布于金属玻璃基体上，体积百分数可达到41%。与金属玻璃相比，含AgMg相内生复合材料的屈服强度(sy»1040 MPa)、断裂强度(sf»1200 MPa)和塑性应变(ep»2%)均得到提高。复合材料的强化归因于屈服强度的拉压非对称、偏离等原子化学计量比以及固溶强化三个因素。内生的AgMg相对于裂纹扩展具有阻挡作用，使得含AgMg相内生复合材料的韧性相对于基体金属玻璃增加约20%。; " In comparison with conventional crystalline magnesium alloys, Mg-based bulk metallic glass (BMG) manifests many attractive properties, such as high fracture strength (800-1000 MPa), high elastic limit (~2%) and excellent corrosion resistance. The glass-forming ability (GFA) of Mg-based bulk metallic glass is no longer a bottle-neck to fabricate the centimeter-scale bulk materials. However, their applications as structural materials are restricted due to a highly localized failure mechanism that results in catastrophic failure under unconfined loading and lower fracture toughness. Thus, it is of great importance to develop the new amorphous magnesium alloys with extraordinary GFA and high toughness, and understand the influencing factors for the toughness of Mg-based BMG. In this thesis, based on the Mg-TM-RE (TM=Cu, Ag, Ni; RE=Y, Gd, Tb, Nd) alloys, several possible aspects for alleviating the brittleness of Mg-based BMG were investigated: firstly, the effect of the alloy composition on the toughness; secondly, the improvement of the toughness under the direction of the elastic properties; thirdly, the effect of the sample dimensions on the yield strength and the fracture toughness of Mg-based BMG; finally, the formation of in-situ Mg-based BMG composites with the AgMg as the primary phase. The main results are as follows: By compositional tuning, a BMG with the ν of 0.332 is located at Mg56Cu21Ag14Y9, which ν value is in excess of the previously-suggested critical value (ν=0.31-0.32) for the brittle-to-tough transition in metallic glass. As indicated from fifteen representative Mg BMG, the predicted values of the bulk modulus, shear modulus and average atomic volume of BMG by weighting the volume fraction of the moduli and atomic volume of the constituent elements in crystalline form are, respectively, 8%, 10% and 4% greater than measured. For the Mg-based BMG with different chemistry, notch toughness varies between 3.6-8.2 MPa×m1/2. Clear tendency that the toughness improvement correlates with either the increase of the ν or the reduction of G multiplied by molar volume VMg-TM-RE BMG can be toughened, to some extent, by Ag substitution for Cu and by addition of Gd or Tb rather than Y and Nd. Among the investigated alloys, the Mg59.5Cu22.9Ag6.6Gd11 BMG manifests a good combination of toughness and GFA. m is not presented. Regarding the chemistry effects, Due to the presence of cast defects, Mg59.5Cu22.9Ag6.6Gd11 BMG manifests a significant specimen-dependency of compressive fracture strength. As the rod diameter increases in a range of 1-10 mm, fracture strength is reduced by 25%. The compressive fracture strength of about 950 MPa can be treated as the intrinsic property. It is inappropriate to describe the size dependence of fracture strength using the Weibull or Gumbel statistics. The decrease of notch toughness with the increase of the plate thickness of Mg59.5Cu22.9Ag6.6Gd11 BMG does not result from the transition from plane-stress to plane-strain in the specimen but from the extrinsic flaws. With three-parameter Weibull analysis, threshold toughness Km of as-cast Mg59.5Cu22.9Ag6.6Gd11 BMG can be determined, which represents a minimum toughness value below which the crack arrests. Ignoring the notch blunting effect, intrinsic fracture toughness of Mg59.5Cu22.9Ag6.6Gd11 BMG is estimated to be about10 MPa×m1/2. The ductile B2-type AgMg phase reinforced MgCuAgY BMG composites were fabricated by tuning the alloy composition and using the conventional copper mold casting method. The B2 AgMg phase in the form of dendrites distributed homogenously on the glass matrix with the volume fraction as high as 41%. Compared with the monolithic glass, the yield strength(sy), fracture strength (sf) and plastic strain (ep) of the composites were all enhanced to about 1040 MPa, 1200 MPa and 2%, respectively. The asymmetry of sy under tension and compression, together with both the stoichiometry deviation hardening effect and solid solution hardening effect are responsible for the strengthening of the composites. The Mg-based BMG was toughened with an increase of 20% by introducing the AgMg phase into the glass matrix. Such toughening comes from the crack-resistant role of the AgMg phase.
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64425
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	王绍钢. 镁基金属玻璃的断裂韧性及其影响因素[D]. 北京. 中国科学院金属研究所,2012.