High-strain rate deformation caused by microparticles impacting at high velocities is used to refine the microstructure of metallic materials to the nanocrystalline regime. Under these conditions, metallic targets and particles show a gradient distribution of nanograins, with size increasing away from the impact surface. Some of the mechanisms responsible for the refinement process are still not fully understood. We present finite element simulations of single-crystal and polycrystalline aluminum 20μm particles impacting a sapphire substrate at velocities ranging from 50 m/s to 500 m/s. The model includes finite deformation crystal plasticity, a contact algorithm, and an equation of state for high strain rate response. We study the effect of crystal orientation on the restitution coefficient, the permanent deformation of the particle, and grain refinement. The extreme deformation of the particle during the impact results in a high dislocation density and lattice rotation that develop a microstructure with a gradient distribution of grains that are smaller near the impact surface. The size of the new grains scales linearly with the inverse of the average stress in the particle.

中文翻译：

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在高冲击速度下对金属微粒进行晶粒细化


利用微粒高速撞击引起的高应变速率变形，将金属材料的微观结构细化至纳米晶状态。在这些条件下，金属靶材和颗粒显示出纳米颗粒的梯度分布，尺寸在远离撞击表面的地方增加。一些负责优化过程的机制仍未完全理解。我们介绍了单晶和多晶铝 20μm 颗粒以 50 m/s 至 500 m/s 的速度撞击蓝宝石衬底的有限元仿真。该模型包括有限变形晶体塑性、接触算法和高应变率响应的状态方程。我们研究了晶体取向对恢复系数、颗粒永久变形和晶粒细化的影响。颗粒在撞击过程中的极端变形导致高位错密度和晶格旋转，从而形成一种微观结构，其颗粒梯度分布在撞击表面附近较小。新晶粒的大小与颗粒中平均应力的倒数呈线性关系。