Effect Of Imprinting Process On Fracture Behavior Of A Zr Based Bulk Metallic Glass

Bulk metallic glasses (BMGs) are relatively new amorphous materials that have received much attention in recent years. Rapid cooling methods from the liquid state provides the possibility to bypass crystallization during solidification. Due to the lack of crystal defects such as grain boundaries and dislocations, BMGs can have some special properties such as high yield strength and high elastic strain limit. Fracture toughness is also an important mechanical property for engineering design. Understanding the fracture behavior of BMGs is necessary for mechanical applications and material specifications. In recent year studies, it has been shown that the fracture toughness of different BMGs varies significantly from brittle to quite high damage tolerance. However, BMGs have been reported to have large variability in mode I fracture toughness even for a single composition. The primary aim of this thesis project is to understand the effect of one thermomechanical method on the fracture behavior of BMGs. In this study, alternating soft and hard regions were created in a Zr52.5Ti5Cu18Ni14.5Al10 (at.%) BMG via mechanical imprinting at room temperature. The results showed that only 50% of as-cast samples demonstrated plastic deformation during mode I fracture tests while 100% of imprinted samples demonstrated measurable plasticity and the scatter in measured mode I fracture toughness was significantly reduced. Besides the improved fracture reliability reflected from J-integral and KJ values, imprinted samples also showed more tortuous crack trajectory than as-cast samples. By studying the fracture surfaces, it was found that high toughness samples showed four distinct regions on the fracture surface originating from the end of fatigue pre-crack which are crack blunting region (I), Taylor meniscus instability region (II), stair-like steps region (III), and flat dimple pattern region (IV). However, the low toughness samples only show three regions with no evidence of region (III). It was found that dimple sizes in the fracture surfaces vary from 2 to 12[micro]m by the line intercept method for low to high toughness samples, respectively. Finally, by correlating fracture toughness values (KJ) and dimple sizes (w), a linear association between KJ2 and w was constructed. Overall, mechanical treatments show promise for improving fracture reliability of BMGs.