Covalent semiconductor crystals such as silicon and diamond have demonstrated ultralarge elastic strains at micro and nanoscales, enabling desired figures of merit for strain engineered electronic and optoelectronic devices. However, the underlying origin of their elasticity—whether it arises from pure lattice displacements or atomic rearrangements (defects or phase transformations)—remains unclear. Here, we directly observed in situ elastic-lattice responses of microfabricated single-crystalline silicon and diamond bridges at room temperature under uniaxial tensile loading by employing high-resolution transmission electron microscopy and four-dimensional scanning transmission electron microscopy. We quantified the distribution of deep-strained atomic coordinates in real time and mapped elastic lattice strains across the entire sample with sub-pixel-precision and a wide field of view, indicating pure lattice elongation without extended defects or phase changes. Additionally, we established a quantitative linkage between macroscopic strain and lattice deformation. This Letter not only elucidates the nature of ultralarge elasticity in covalent materials but also provides a guideline for intelligent design of silicon and diamond electronic and photonic devices with disruptive physical properties via deep elastic strain engineering.

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• Elastic deformation
• Diamond
• Electron microscopy
• Mechanical testing
• Strain engineering
• Transmission electron microscopy