“Non-equilibrium” grain boundaries in additively manufactured CoCrFeMnNi high-entropy alloy: Enhanced diffusion and strong segregation

Abstract

Grain boundary diffusion in an additively manufactured equiatomic CoCrFeMnNi high-entropy alloy is systematically investigated at 500 K under the so-called C-type kinetic conditions when bulk diffusion is completely frozen. In the as-manufactured state, general (random) grain boundaries are found to be characterized by orders-of-magnitude enhanced diffusivities and a non-equilibrium segregation of (dominantly) Mn atoms. These features are explained in terms of a non-equilibrium state of grain boundaries after rapid solidification. The grain boundary diffusion rates are found to be almost independent on the scanning/building strategy used for the specimen’s manufacturing, despite pronounced microstructure differences. Grain boundary migration during diffusion annealing turned out to preserve the non-equilibrium state of the interfaces due to continuous consumption of the processing-induced defects by moving boundaries. Whereas the kinetic “non-equilibrium” state of the interfaces relaxes after annealing at 773 K, the non-equilibrium segregation is retained, being further accompanied by a nano-scale phase decomposition at the grain boundaries. The generality of the findings for additively manufactured materials is discussed.

Impact of Titanium Addition on Microstructure, Corrosion Resistance, and Hardness of As-Cast Al+6%Li Alloy

“…This concentration gradient enables solute atoms to be carried and deposited at the sinks. The accumulation of excess solute atoms at the grain boundary leads to non-equilibrium segregation at the grain boundaries [ 44 , 45 , 46 ].…”

Internal Interfaces in Severely Deformed Metals and Alloys: Coupling of Kinetics, Structure and Strain with Properties and Performance

“…5) This term, as well as the proposed model, have been intensively and partially controversially discussed in the literature. 1,68) For the purpose of the present paper, we are not reverting back to this discussion, but acknowledge that severe plastic deformation processing has been shown conclusively to alter the local structure of (a fraction of ) internal interfaces as well as of macroscopically averaged properties of different metals and alloys in a consistent manner. 4,912) For describing the modified internal interfaces, we shall use the term "nonequilibrium interface" 5,6) or "deformation-modified interface" 8) synonymously.…”

“…6,66) Very recently, grain boundary tracer diffusion was measured in an additively manufactured equiatomic CoCrFeMnNi high-entropy alloy. 67,68) The radiotracer experiments were performed at a relatively low temperature of 500 K, at which no measurable tracer penetration would be observed in annealed coarse-grained polycrystals 69,70) of equal composition. Since additive manufacturing is known to induce cracks and pores in the material, 71) the tracer penetration in the asprinted state was monitored, too, triangles in Fig.…”

“…The tracer diffusion experiment at 500 K has been repeated after pre-annealing of the sample at 773 K for 1 h. 68) As a result, no tracer penetration was resolved in such case (squares in Fig. 12).…”

Solute segregation in a moving grain boundary: a phase-field approach

“…Segregation reduces grain growth through two mechanisms: (i) solute segregation at GBs lowers GB energy [7][8][9], and (ii) solute drag force by GB solute impedes GB migration [10,11]. Moreover, this solute drag phenomenon in migrating GB depends on various factors such as GB mobility, solute diffusivity, misfit strain, GB structure, etc [12][13][14][15].…”