Uncovering electron scattering mechanisms in NiFeCoCrMn derived concentrated solid solution and high entropy alloys

Abstract: Whilst it has long been known that disorder profoundly affects transport properties, recent measurements on a series of solid solution 3d-transition metal alloys reveal two orders of magnitude variations in the residual resistivity. Using ab-initio methods, we demonstrate that, while the carrier density of all alloys is as high as in normal metals, the electron mean-free-path can vary from ~10 {\AA} (strong scattering limit) to ~10$^3$ {\AA} (weak scattering limit). Here, we delineate the underlying electron scattering mechanisms responsible for this disparate behavior. While spin dependent site-diagonal disorder is always dominant, for alloys containing only Fe, Co, and Ni the majority spin channel experiences negligible disorder scattering, thereby providing a short circuit, while for Cr/Mn containing alloys both spin channels experience strong disorder scattering due to an electron filling effect. Unexpectedly, other scattering mechanisms (e.g. displacement scattering) are found to be relatively weak in most cases.

• The paper reveals that elemental composition and disorder lead to distinct low and high resistivity behaviors in NiFeCoCrMn alloys.
• It employs the KKR-CPA method combined with density functional theory to quantify residual resistivity variations from 1.3 µW•cm to 124.8 µW•cm.
• These insights enable the design of high entropy alloys with tailored electron transport properties for advanced technological applications.

Overview of Electron Scattering Mechanisms in NiFeCoCrMn Alloys

This paper presents an in-depth analysis of electron scattering mechanisms within NiFeCoCrMn, a concentrated solid solution and high entropy alloy. The study leverages state-of-the-art ab-initio transport theory to unravel how different elemental compositions and disorder affect electron scattering, thus impacting electrical resistivity.

The researchers employed the KKR-CPA method combined with density functional theory to investigate residual resistivity across several subsets of the Cantor-Wu alloys. These alloys include elements like Ni, Fe, Co, Cr, and Mn that form equiatomic fcc solid solutions known for their maximal disorder and unique electron transport properties.

Key Findings

The study reveals that the residual resistivity, $r_0$, is highly dependent on the specific alloying elements present. Strong numerical results show a bifurcation into low-resistivity and high-resistivity groups, where alloys containing Mn and Cr display significantly higher resistivity. The variation across the Cantor-Wu alloys was two orders of magnitude, with the resistivity going from as low as 1.3 µW•cm in NiCo to as high as 124.8 µW•cm in NiFeCoCrPd.

Key findings indicate that the site-diagonal, spin-dependent potential scattering significantly influences resistivity. While the majority spin channel experiences negligible disorder in Ni, Fe, and Co-only alloys, the introduction of Mn and Cr leads to strong disorder scattering in both spin channels. This behavior is attributed to the electron filling effect, which is pronounced in alloys near a half-filled d-band, such as those with Cr and Mn.

Implications and Future Directions

The implications of these findings are critical for the design of alloys with tailored electronic properties. The identification of the disorder scattering mechanisms provides essential insights into optimizing alloy compositions for specific resistivity behaviors. In practical terms, this knowledge may aid in creating materials with either minimized resistivity for improved conductivity or those with controlled resistivity for applications requiring electrical resistance.

From a theoretical standpoint, the study enriches our understanding of how chemical disorder in high entropy alloys affects electron transport. This could pave the way for more sophisticated simulations incorporating additional scattering mechanisms like spin noncollinearity and short-range order effects.

As the field progresses, an interesting area for future research would be the exploration of electron correlation effects beyond the local density approximation. Moreover, further studies could investigate the impact of spin noncollinearity and spin-orbit interactions, especially in Pd-containing alloys.

Conclusion

In conclusion, the paper presents a comprehensive study of electron scattering mechanisms in NiFeCoCrMn-derived alloys. Through advanced computational methods, the authors clarify how elemental composition and disorder intricacies influence residual resistivity. These results hold significant potential for advancing the design and application of high entropy alloys in various technological domains. The exploration of electron scattering provides a crucial connection between alloy structural disorder and their transport properties, offering paths to enhance material performance through informed alloy design choices.