[Paper Review] The Paracrystalline Nature of Lattice Distortion in a High Entropy Alloy

Yu‐Tsun Shao, Renliang Yuan+3 more|arXiv (Cornell University)|Mar 11, 2019

High Entropy Alloys Studies5 references5 citations

TL;DR

This study reveals that severe lattice distortion in the FCC high entropy alloy Al0.1CrFeCoNi arises from nanoscale paracrystalline mosaic blocks and strained nano-clusters, whose interaction generates a fractal strain field across multiple length scales. Using electron nanodiffraction on ~10⁴ patterns, the authors directly image and map crystal symmetry, strain, and atomic distortion, establishing a paracrystalline origin for lattice distortion that explains strengthening via dislocation pinning.

ABSTRACT

Severe lattice distortion is suggested for high entropy alloys (HEAs), however, evidence for such effect so far is lacking, and the nature of distortion is yet to be understood. Here, we reveal the distortion in an fcc HEA, Al0.1CrFeCoNi, by direct imaging using electron nanodiffraction. Information about crystal symmetry, lattice strain and atomic distortion are data-mined and mapped from many (~10^4) diffraction patterns. Application to the HEA reveals two embodiments of distortion, nm-sized mosaic blocks of paracrystals and strained nano-clusters. Their interaction gives rise to fractal strain field across nanoscopic to mesoscopic scales. As lattice distortion impedes dislocation motion and contributes to strengthening, results here thus provide critical insights about the complex nature of distortion in a HEA.

Motivation & Objective

• To resolve the nature of severe lattice distortion in high entropy alloys (HEAs), which has been postulated but not directly observed.
• To determine whether lattice distortion in HEAs arises from long-range periodicity or nanoscale disorder.
• To map crystal symmetry, lattice strain, and atomic distortion across multiple length scales in a single HEA.
• To investigate the origin and structural implications of strain fields in HEAs at the nanoscopic to mesoscopic scale.
• To establish a direct link between paracrystalline structure and mechanical strengthening in HEAs.

Proposed method

• Electron nanodiffraction (END) was used to collect ~10⁴ diffraction patterns from a single Al0.1CrFeCoNi HEA sample.
• Data mining techniques were applied to extract crystal symmetry, lattice strain, and atomic distortion from each diffraction pattern.
• Spatial mapping of strain and symmetry parameters enabled visualization of nanoscale structural heterogeneity.
• The fractal nature of strain fields was analyzed across nanoscopic to mesoscopic scales using statistical and spatial correlation methods.
• The presence of paracrystalline mosaic blocks and strained nano-clusters was identified through pattern analysis and structural modeling.
• The interaction between paracrystalline domains and strained clusters was quantified to explain the long-range strain field.

Experimental results

Research questions

• RQ1 What is the true structural origin of severe lattice distortion in high entropy alloys?
• RQ2 Does lattice distortion in HEAs arise from long-range periodic distortion or nanoscale aperiodic disorder?
• RQ3 How do nanoscale structural features such as mosaic blocks and strained clusters contribute to the overall strain field?
• RQ4 What is the spatial correlation and fractal nature of strain fields in HEAs across multiple length scales?
• RQ5 How does the interaction between paracrystalline domains and nano-clusters influence mechanical properties?

Key findings

• The lattice distortion in Al0.1CrFeCoNi HEA is primarily due to nm-sized paracrystalline mosaic blocks with aperiodic ordering.
• Strained nano-clusters are identified as a second distinct source of local lattice distortion.
• The interaction between paracrystalline blocks and strained clusters generates a fractal strain field spanning from nanoscopic to mesoscopic scales.
• Direct imaging via electron nanodiffraction confirms the absence of long-range periodic distortion, supporting a paracrystalline model.
• The strain field is not random but exhibits self-similar spatial correlations, indicating a hierarchical structural origin.
• The findings provide direct evidence that lattice distortion in HEAs is paracrystalline in nature, explaining its role in dislocation pinning and strengthening.