A critical review of high entropy alloys (HEAs) and related concepts

The field of high entropy alloys (HEAs) is barely 10 years old. It has stimulated new ideas and has inspired the exploration of the vast composition space. Here we give a critical review of this body of work, with the intent of summarizing key findings, uncovering major trends and providing guidance for future efforts. Based on detailed analysis, the following major results emerge. An intense focus is apparent on alloys based on first row transition metal elements. These efforts have discovered an extended, single-phase solid solution phase field and are exploring the extent and properties of this phase field. As a result of this focus, the vast range of complex composition space remains relatively unexplored and still offers a compelling motivation for future studies. Seven distinct alloy families have been reported, and the design of alloy families by selecting elements to achieve a specific, intended purpose is becoming established. A major hypothesis is that configurational entropy in HEAs may favor solid solution phases with simple crystal structures. Detailed analysis of current experiments and calculations does not support this hypothesis. Increasing the number of alloy constituents increases configurational entropy slowly, but increases much more rapidly the probability of including a pair of atoms with interaction energies that are sufficiently negative to overcome configurational entropy. The hypothesis that diffusion may be unusually slow in HEAs is also not supported by the current data. Data for mechanical properties of HEAs will be reviewed and suggestions for new research directions will be offered

Atomistic simulations of dislocations in a model BCC multicomponent concentrated solid solution alloy

Molecular statics and molecular dynamics simulations are presented for the structure and glide motion of a/2(111) dislocations in a randomly-distributed model-BCC Co16.67Fe36.67Ni16.67Ti30 alloy. Core structure variations along an individual dislocation line are found for a/2(111) screw and edge dislocations. One reason for the core structure variations is the local variation in composition along the dislocation line. Calculated unstable stacking fault energies on the (110) plane as a function of composition vary significantly, consistent with this assessment. Molecular dynamics simulations of the critical glide stress as a function of temperature show significant strengthening, and much shallower temperature dependence of the strengthening, as compared to pure BCC Fe as well as a reference mean-field BCC alloy material of the same overall composition, lattice and elastic constants as the target alloy. Interpretation of the strength versus temperature in terms of an effective kink-pair activation model shows the random alloy to have a much larger activation energy than the mean-field alloy or BCC Fe. This is interpreted as due to the core structure variations along the dislocation line that are often unfavorable for glide in the direction of the load. The configuration of the gliding dislocation is wavy, and significant debris is left behind, demonstrating the role of local composition and core structure in creating kink pinning (super jogs) and/or deflection of the glide plane of the dislocation. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved