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非晶合金理想裂纹预制方法及其在小试样K_\rm Ic测试中的应用

A GENERAL METHOD TO INTRODUCE PRE-CRACK IN BULK METALLIC GLASSES FOR PLANE STRAIN FRACTURE TOUGHNESS TEST

  • 摘要: 介绍了一种简单、低成本且可靠的方法在非晶合金中预制理想裂纹并应用于小试样平面应变断裂韧性的测试.近年来,非晶合金由于高弹性、高强度、耐磨及软磁性等优异性能 展示了广泛的应用前景.断裂韧性作为材料工程应用的一个重要指标,也引起了非晶合金领域的广泛关注. 然而,由于非晶合金的亚稳态结构以及最大可铸造尺寸的限制,目前关于非晶合金断裂韧性的测试还存在较大的挑战.一方面,铸造工艺造成的非晶合金热历史的差异、内部微孔洞和杂质等缺陷以及裂纹预制方式等都会显著影响其断裂韧性测试的可靠性;另一方面,非晶合金可铸造尺寸的限制使得目前绝大多数报导的断裂韧性值都是非平面应变的断裂韧性,导致即使是对于同种非晶合金,所报导的断裂韧性值也存在较大偏差.本文利用非晶合金在过冷液相温度下具有可热塑性成型的特性,对预制有缺口的非晶合金试样进行局部压缩成型,使得预制的缺口裂纹重新闭合形成类似疲劳裂纹的理想裂纹面.基于该方法对Zr基非晶合金进行断裂韧性测试,实验结果表明,随着试样厚度的增加,测试值迅速降低并趋向于一个定值.需要指出的是,通过设计实验使得试样在理想裂纹面区域形成局部凹陷,使得趋于定值的试样厚度远小于平面应变断裂韧性测试标准中的试样厚度要求.

     

    Abstract: We report a simple, low-cost but robust method to introduce pre-crack in bulk metallic glasses (BMGS) for plane strain fracture toughness test. In recent years, bulk metallic glasses have shown potential and promising engineering application due to their excellent properties such as high elasticity, high strength, wear resistance and soft magnetism. As an important material parameter for engineering application, fracture toughness has also attracted great attention in the BMG community. However, there is still challenge on the fracture toughness test of BMGs because of the metastable nature and limited maximum castable size of BMGs. On the one hand, the casting induced thermal history difference and the defects such as internal micropores and impurity inclusions in BMGs, and the way of crack prefabrication will significantly affect the reliability of the fracture toughness test. On the other hand, limited by the sample size, most of the measured fracture toughness of BMGs are not the plane strain fracture toughness, resulting in the reported values showing large deviation, even for the same amorphous alloy. In this work, pre-notched BMG samples were thermoplastically compressed at the notched region to form ideal crack by creep flow. As an example, the fracture toughness of Zr-based amorphous alloy is tested by this method. The experimental results show that as the sample thickness increasing, the measured toughness decreases quickly and tends to a constant value. It should be pointed out that in the experiments, the local thermoplastic compression is designed to obtain a neck shaped pre-crack, which makes the minimum thickness of the specimen being far less than the required thickness by the plane strain fracture toughness testing standard.

     

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