Comprehensive Analysis of Ordering in CoCrNi and CrNi2 Alloys

Abstract

In the intricate realm of high and medium entropy alloys (HEAs/MEAs), the elusive nature of Chemical Short-Range Order (CSRO) has captivated researchers, creating intense debates on its impact on material properties. Rooted in the local arrangement of atoms, CSRO's pivotal role in shaping the structural and mechanical characteristics of HEAs remains a subject of fervent exploration. The challenges lie in confirming and quantifying CSRO, as its detection proves exceptionally demanding, contributing to conflicting data in the literature regarding its true effects on mechanical properties. Our work uses high-precision calorimetric data to unambiguously prove the existence and, coupled with atomistic simulations, quantify the type of CSRO. This methodology allows us to propose a mechanism for its formation and destruction based on the heat evolution during thermal analysis and facilitates a precise identification of local ordering in CoCrNi alloys. Samples of CoCrNi (Co33Cr33Ni33) and CrNi2 (Cr33Ni66) alloys are fabricated in varying ordered states, extensively characterized via synchrotron X-Ray diffraction, X-Ray absorption spectroscopy, and transmission electron microscopy. Samples with considerably different ordered states are submitted to tensile tests with in-situ synchrotron X-Ray diffraction. Despite inducing varied CSRO levels in CoCrNi, no significant alterations in overall mechanical behavior emerge. However, the CrNi2 alloy, which undergoes long-range ordering, experiences profound shifts in yield strength, ultimate tensile stress, dislocation density, and ductility. The novel methodology proposed in this work charts a course toward harnessing the nuanced effects of CSRO on material properties in complex alloys.