Deformation Mechanisms Microstructure Evolution And Mechanical Properties Of Nanoscale Materials Volume 1297

Author : Claire E. Griesbach
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (14 download)

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Download or read book The Microstructural Evolution and Mechanical Behavior of Materials in the Extreme Environments of High-velocity Impact and Irradiation written by Claire E. Griesbach and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Understanding the process-structure-property relations is imperative when designing materials for critical applications, especially when exposed to extreme environments. Clever design of materials containing structural heterogeneities or features spanning multiple length scales can provide synergistically improved properties well beyond the limits of their homogeneous components or provide predictable and acceptable failure modes. In this work, we investigate the process-structure-property relations and the potential for improved performance of materials exposed to the extreme environments of high-velocity impact and irradiation.Similar to material processing techniques such as shot peening and cold spray which induce structural changes through high-velocity impact, we perform well-controlled single microprojectile impact tests of silver (Ag) single crystals which allows for clear correlation of the post-deformation microstructures to the impact-induced plasticity mechanisms. When comparing the impacted microsamples to their pristine single crystal counterparts, we find that the high-strain rates achieved during impact ([epsilon]~108 s-1) induce dramatic structural changes including extensive grain refinement, dislocation density gradients, and a martensitic phase transformation, while the quasi-statically ([epsilon]=10-2 s-1) compressed Ag microcubes remain single crystalline. The impacted samples show a synergistic improvement in strength and toughness, each on average over twice the respective properties of single crystal and bulk polycrystal Ag samples. Such synergistic improvements result from heterogeneous deformation induced stress and strain gradients. Progressive yielding of the gradient grain structure causes enhanced nucleation and pile-up of dislocations in the relatively softer domains to accommodate the elastic-plastic mismatch between grains. The observed dislocation accumulation-which is higher in larger grains-provides ultra-high strain hardening. Additionally, enhanced toughness is achieved through intergranular plasticity mechanisms such as nanograin rotation and grain boundary migration, leading to grain coalescence. These complementary inter- and intragranular plasticity mechanisms elicit improved mechanical properties. We demonstrate the ability to tune the dominant plasticity mechanisms-and thus the resultant properties-through control over the crystal orientation and impact velocity. Our findings provide new understandings of impact-induced nanostructural evolution and mechanistic pathways to improve mechanical properties through heterogeneous deformation, which can be used to improve high strain rate metal processing techniques. The second research thrust examines the effects of structural heterogeneity on the mechanical properties, performance, and failure of tristructural isotropic (TRISO) coated nuclear fuel particles. We examine the irradiation-induced densification and fracture behavior of the porous pyrocarbon buffer layer, which has pronounced effects on the overall particle's performance. Microstructural characterization of the initial as-fabricated buffer layer reveals a gradient of increasing porosity in the radial direction with the porosity reaching a maximum near the buffer-IPyC interface-which is commonly where circumferential tearing initiates in the buffer layer. Using the as-fabricated buffer structure as a basis to investigate the irradiation-induced structural changes, we study the influences of irradiation temperature and fluence on buffer layer response, by characterizing multiple TRISO particles from three different irradiation condition groups. The high temperatures, radiation damage, and mechanical stresses applied to the buffer layer during irradiation cause irradiation condition-dependent micro and nanostructural changes: localized densification near the kernel occurs in particles exposed to relatively lower temperatures, whereas significant changes in the entire pore microstructure occur in particles irradiated under high temperature and fluence. Intriguingly, a large proportion of the total buffer layer densification is accommodated through graphitization of the pyrocarbon rather than the changes in the pore microstructure. Significant nanostructural changes-including an increase in crystallite size, decrease in interplanar spacing, and formation of onion-like graphitic structures-contribute to densification and are most pronounced in the samples exposed to the highest temperature and fluence. Our findings provide a new detailed understanding of the irradiation-induced densification and fracture behavior of the pyrocarbon buffer layer in TRISO nuclear fuel particles, which will enable better predictions of buffer failure, aid in improved future designs, and provide guidance on the acceptable usage of these particles given different reactor conditions.