Garlapati Mohan Muralikrishna

Ni diffusion in the equiatomic CoFeMnNiV high-entropy alloy is measured over an extended temperature range that covers both single-phase (face-centered cubic, FCC, solid solution) as well as two-phase (FCC solid solution + σ-phase) states. In addition, Ni diffusion experiments were performed on a single-phase σ-alloy. The σ-phase precipitation is found to affect significantly the self-diffusion kinetics in the FCC solid solution. The measured tracer diffusion coefficients follow an Arrhenius-type relationship when estimated in the single-phase solid solution, and a characteristic enhancement of the Ni diffusivity is observed at lower temperatures when the σ-phase precipitates during the diffusion annealing treatments. The enhancement is significantly suppressed when the alloy is pre-aged to approach a nearly equilibrium σ-phase volume fraction at a given temperature. Since Ni volume diffusion in the σ-phase was found to be similar to that in the FCC equiatomic counterpart, the observed diffusion enhancement is attributed to an excess vacancy concentration, generated during the growth of σ-precipitates. This excess gets eliminated when approaching thermodynamic equilibrium, i.e., when precipitation ends. A simple model of the diffusion enhancement during precipitation is proposed.

Here, we explore component-dependent local lattice distortions in polycrystalline, equiatomic, face-centered cubic CrMnFeCoNi high-entropy alloys and their modifications induced by dilute interstitial carbon. Multi-edge extended X-ray absorption fine structure spectroscopy combined with reverse Monte Carlo analysis reveals that the Cr component experiences the most substantial local distortions, independent of the temperature of prolonged annealing treatments (993 K or 1373 K) and the nominal carbon content (0 to 0.8 at.%). The static disorder around Cr atoms was found to increase markedly and monotonically upon carbon alloying, whereas Mn, Fe, Co, and Ni demonstrate weaker and non-monotonic tendencies. The carbon-induced lattice distortions extend over several coordination shells, indicating the early stages of carbide-related local configurations specifically around Cr absorbers. First-principles density functional theory and finite-temperature molecular dynamics simulations confirm the preferential occupation of Cr-rich interstitial sites by dilute carbon. These results provide decisive hints towards the atomistic origin of the non-monotonic diffusion behavior previously reported for carbon-doped CrMnFeCoNi alloys, and are noticeable for understanding the carbon-induced phase transitions in compositionally complex systems.

High-entropy alloys (HEAs) have gained substantial attention over the past two decades, necessitating a comprehensive understanding of their intrinsic and extrinsic properties, including mechanical behavior, creep resistance, phase stability, environmental degradation, etc. Among these, atomic transport mechanisms, particularly along grain boundaries (GBs) play a pivotal role in determining material performance. This review critically evaluates the “sluggish” diffusion concept, focusing on its validity and applicability to GB diffusion in HEAs. It examines the influences of GB complexions, segregation phenomena, and precipitation processes on GB diffusion behavior in HEAs, comparing them with their counterparts in conventional binary and ternary alloys, both dilute and concentrated. The inherent challenges in accurately characterizing GB diffusion in multi-principal element alloys, given their broad compositional variability and complex microstructures are also highlighted. The contribution of inter-phase boundary diffusion in multi-component alloy systems is also identified and analyzed. Furthermore, the broader implications of GB diffusion on the mechanical and physical properties of polycrystalline HEAs are discussed in terms of their strength, ductility, and degradation resistance. By consolidating the current state of research on GB diffusion in HEAs and identifying the key research gaps, this review aims to catalyze focused and intensive research efforts into diffusion-related phenomena in HEAs and other compositionally complex alloys. Emphasis is placed on comprehensive understanding the interplay between GB structure, chemistry, and atomic transport phenomena to enable effective GB engineering stategies for these alloys. Insights from such studies will be instrumental in optimizing HEAs for advanced technological applications and in guiding the development of next-generation high-performance materials.