Maryam Ghazisaeidi, Ohio State University, High entropy alloys: mechanical properties and phase stability

Date: Mon. November 19th, 2018, 12:45 pm-1:45 pm
Location: Rockefeller 221 (Les Foldy Room)
Website: https://ghazisaeidigroup.engineering.osu.edu/

High entropy alloys: mechanical properties and phase stability
Maryam Ghazisaeidi, Department of Materials Science and Engineering, Ohio State University

The term “High entropy” alloys (HEA) refers to a relatively new class of multicomponent—usually
five or more—metallic alloys in equal or near equal atomic concentrations. Instead of ordered
intermetallics, expected from classical physical metallurgy, some HEA systems strikingly crystalize
as single phase solid solutions with simple crystal structures. The complex compositions of these
alloys, and their derivatives, lead to unique properties. They also encourage new ways of viewing
fundamentals of physical metallurgy, yielding new insights that are applicable to a wide range of
metallic alloys. In this talk I will present two aspects of these systems: mechanical properties and
thermodynamics.

In the first part, I present a phase transformation strengthening mechanism in CrCoNi, a ternary
derivative of the CrMnFeCoNi high entropy alloy. CrCoNi alloy exhibits a remarkable combination
of strength and plastic deformation, even superior to the CrMnFeCoNi high-entropy alloy. We
connect the magnetic and mechanical properties of CrCoNi, via a magnetically tunable phase
transformation. While both alloys crystallize as single-phase face-centered-cubic (fcc) solid
solutions, we find a distinctly lower-energy phase in CrCoNi alloy with a hexagonal close-packed
(hcp) structure. Comparing the magnetic configurations of CrCoNi with those of other equiatomic
ternary derivatives of CrMnFeCoNi confirms that magnetically frustrated Mn eliminates the fcchcp
energy difference. This highlights the unique combination of chemistry and magnetic
properties in CrCoNi, leading to a fcc-hcp phase transformation that occurs only in this alloy, and
is triggered by dislocation slip and interaction with internal boundaries.

In the second part, I present our Multi-cell Monte Carlo (MC)^2 method for predicting stable
phases of alloys from first principles calculations. Application of this method to the high entropy
HfZrTaNbTi HEA, confirms the experimental observations of phase separation in this alloy and
provides a powerful tool for predicting the thermodynamically stable phases of multicomponent
alloys. In addition, our prediction of phase separation, in line with experiments, cast new doubts
on the equilibrium stable phases of those HEAs which had been widely regarded as random solid
solutions.

host: Walter Lambrecht