Understanding of crystallization behaviors in laser 3D printing of bulk metallic glasses

Abstract

In 3D printing of bulk metallic glasses (BMGs) by selective laser melting (SLM), partial crystallization normally occurs in heat-affected zones (HAZs), which significantly deteriorates the properties of 3D-printed BMGs. To understand which factor dominates amorphous-to-crystalline conversion of 3D-printed BMGs, two representative BMG alloys with different glass forming ability (GFA), i.e., Zr55Cu30Ni5Al10 (named as Zr55 with high GFA) and Zr60.14Cu22.31Fe4.85Al9.7Ag3 (named as ZrAg with low GFA), were selected for SLM 3D printing, and their crystallization behaviors were comparatively studied. It is revealed that the 3D-printed ZrAg BMG with low GFA always possessed a higher amorphous phase content than the Zr55 BMG with a high GFA under different laser energy inputs. Thermal analysis and TEM observations revealed that crystal growth rate is the crucial factor for the resistance against amorphous-to-crystalline conversion in 3D-printed BMGs, as the ZrAg system exhibited much lower crystal growth rate although it has a lower GFA. The different crystal growth behavior of the two BMGs is associated with their different crystallization mechanisms: ZrAg BMG follows the mode of primary-type crystallization with multiple phases formation, leading to a low crystal growth rate, while Zr55 BMG follows the mode of polymorphous-type crystallization, which is characterized with formation of a simple crystalline phase, thus leading to a high crystal growth rate. The present work sheds light on the understanding of crystallization mechanism in 3D-printed BMGs and provides a guide for selection or design of BMG compositions for 3D printing.