318 / 2023-05-04 10:51:57

Mapping Extremal Stresses of Diamond from First Principles

高压物理与材料科学 > 氢与高能量密度材料-I

Fundamentally, stress-strain relations define intrinsic mechanical properties of materials^1,2. From a theoretical perspective, stress responses to various small strains on the equilibrium structure of a crystal determine elastic constants and moduli that describe structural stability and deformability of the crystal, but such results do not adequately describe the full range of mechanical properties, especially the extremal stresses at large strains that set ultimate limits on structural stability and strength under extreme loading conditions. Recent years have seen increasing interest and efforts in exploring stress responses to large strains^3-6, but most studies are limited to a few deformation paths in each case due to computational constraints. There remain major unresolved issues concerning the range and distribution of extremal stress fields that are crucial to gaining insights and guiding property optimization. Accurate descriptions of material behaviors under extreme loading conditions present a challenging frontier in materials physics research, and pertinent studies become increasingly more relevant and pressing as advances in materials synthesis are producing nearly defect free specimens in nanoscale or nanocomposite forms⁷.



In this work, we pursue an extensive mapping of the extremal stress fields in diamond from first-principles stress-strain calculations along many deformation paths. The results show intriguing patterns and trends of stress responses to large-range strains in a rich variety of tensile directions and shear planes, revealing previously unknown behaviors and offering a more comprehensive view on the peak stress distributions and associated bonding variations. Notably, we find highly contrasting direction dependent stress variation patterns under tensile strains. Results in Fig. 1 show that peak stresses are extremely sensitive and fast-changing for deformation paths aligned around the [001] direction where the global maximal tensile stress appears, while the peak stresses are nearly direction independent in a large range around the [111] direction where the global minimal tensile stress is found. These results can be elucidated by analyzing stress responses (Fig. 2) in various crystal directions with different bonding arrangements. Moreover, we identify a surprisingly large number of easy-slip directions in many shear planes with nearly equal peak stresses compared to the established and often quoted (111)[11-2] easy-slip direction, and we uncover strikingly anisotropic shear stresses with the maximal value greatly exceeding the previously known highest shear stress along the (111)[-1-12] shear strain path. The newly created benchmarks on extreme mechanical behaviors establish a quantitative basis for evaluating sustainable structural changes and threshold strengths at large deformations. This more in-depth assessment introduces a robust protocol for characterizing essential structural and mechanical properties of crystalline solids over the full elastic ranges of diverse deformation modes, which are crucial to elucidating crucial benchmark material behaviors and guiding rational performance optimization.





Figure 1. A 3D mapping of extremal tensile stresses of diamond along various crystalline directions.





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