Suppression of Ductile-to-brittle Transition and Low-temperature Deformation Mechanisms in Bcc High-entropy Alloys - Insights from In-situ Neutron Diffraction Studies

Description

Most alloys with a body-centered cubic (bcc) structure undergo an abrupt ductile-brittle transition (DBT) when the test temperature decreases. For instance, polycrystal Nb shows a drastic drop in ductility at around -100 ℃. High-entropy alloys (HEAs) are multi component solid-solution alloys with nearly equiatomic ratio. In spite of the complex chemistry, HEAs based on refractory elements can crystallize into an incredibly simple lattice, e.g., the bcc structure. Some of these bcc HEAs display remarkable mechanical properties. For example, the equiatomic TiZrHfNbTa alloy exhibits a room temperature yield strength greater than 900 MPa and 10-20% ductility. Earlier studies by transmission electron microscopy (TEM) indicated that the room-temperature deformation is governed by the glide of screw dislocations. However, analysis of in situ neutron diffraction data suggested that edge dislocations also play a role. More strikingly, as the test temperatures is lowered, the strength of TiZrHfNbTa increases prominently without sacrificing the ductility, manifesting an unusually low DBT temperature. TEM analysis of samples retrieved from interrupted tensile tests has identified the activation of nano-twinning and deformation-induced phase transformation at 77 K. However, unlike face-centered-cubic (fcc) HEAs (e.g., CrCoNiFeMn), the bcc HEA displays almost negligible work hardening. This is markedly different from the characteristics of well-established low temperature deformation mechanisms, e.g., transformation-induced plasticity (TRIP), twinning-induced plasticity (TWIP), or stacking faults. The above experimental results show that the deformation in bcc HEAs, especially at low temperatures, remains poorly understood. We propose to conduct in-situ neutron diffraction study of the deformation behavior of bcc HEAs. The goal is to determine the physical mechanisms governing the excellent mechanical properties, particularly at low temperatures. With in situ neutron diffraction measurements, we aim to identify new deformation mechanisms (e.g., twinning or phase transformation) and to determine the competition between different types of dislocation activities, and use these data to elucidate the dramatic suppression of the anticipated DBT. In-situ neutron diffraction is a powerful method for study of mechanical behaviors and has made major contributions to our understanding of deformation in advanced alloys, most recently in low temperature deformation of fcc HEAs. The neutron diffraction results will be corroborated with TEM analysis and molecular dynamics simulations. Together, these data will provide unique insights into the origin for the lack of DBT in bcc HEAs. The knowledge gained from the proposed study will facilitate the design of new structure materials with superior mechanical properties.