Nanocrystalline copper samples were produced and analyzed with the aim of elucidating how very small grain sizes affect mechanical properties of crystalline materials. The mechanical properties and microstructure of the material were investigated. Important mechanical properties explored include yield strength, hardness, and ductility. Such microstructural parameters as grain size, grain morphology, grain size distribution, and bulk density were also evaluated. Grain size and strength in conventional metals are related by the Hall-Petch relation, which is expected to lose relevance at low grain sizes. At average grain sizes of 10--100 nm, previous researchers often noted deviations from Hall-Petch predictions of strength. In contrast, samples in the same grain size regime tested in compression were found to adhere to the relation for both yield strength and hardness in this study. At the lowest grain sizes, a mild reversal in the trend of the Hall-Petch relation may be visible, suggesting a possible shift in the dominant deformation mechanism. In addition, Transmission Electron Microscopy (TEM) was used to observe deformation of nanocrystalline copper in situ. Grains of 40 nm and perhaps smaller sizes appear to deform by dislocation motion, though other mechanisms may also be active. Extensive examinations of loose powder were also performed via TEM. Wide grain size distributions were likely to have arisen during consolidation, because uncompacted powders have a more uniform and smaller distribution. Processing of samples was controlled to achieve a variety of grain morphologies for property evaluation.

Ultrafine grained (UFG) and nanocrystalline materials have attracted considerable interest because of their unique mechanical properties as compared with coarse grained conventional materials. The fabrication of relatively large amounts of these materials still remains a challenge, and a thorough understanding of the relationship between microstructure and mechanical properties is lacking. The objective of this study was to investigate the mechanical properties of UFG and nanocrystalline copper obtained respectively by a top down approach of severe plastic deformation of wrought copper and a bottom up approach of consolidation of copper nanoparticles using equal channel angular extrusion (ECAE). A critical assessment and correlation of the mechanical behavior of ECAE processed materials to the microstructure was established through the determination of the effect of strain level and strain path on the evolution of strength, ductility and yield anisotropy in UFG oxygen free high conductivity copper in correlation with grain size, grain morphology and texture. ECAE was shown to be a viable method to fabricate relatively large nanocrystalline consolidates with excellent mechanical properties. Tensile strengths as high as 790 MPa and fracture strain of 7 % were achieved for consolidated 130nm copper powder. The effects of extrusion route, number of passes and extrusion rate on consolidation performance were evaluated. The relatively large strain observed was attributed to the bimodal grain size distribution and accommodation by large grains. The formation of bimodal grain size distribution also explains the simultaneous increase in strength and ductility of ECAE processed wrought Cu with number of passes. Texture alone cannot explain the mechanical anisotropy in UFG wrought copper but we showed that grain morphology has a strong impact and competes with texture and grain refinement in controlling the resulting yield strength. Tension-compression asymmetry was observed in UFG wrought copper. This asymmetry is not always in favor of compression as reported in literature, and is also influenced by grain morphology through the interaction of dislocations with grain boundaries. Different prestrains in tension and compression should be experimented to have a better understanding of the encountered anisotropy in Bauschinger parameter in relation with the observed tension-compression asymmetry.

Dislocation microstructures formed during drawing of copper to strains up to 7, at deformation temperature between 77 and 473K, have been investigated, together with their relationship to the resulting mechanical properties. The changes in microstructure and tensile flow stress are explained by dynamic recovery and recrystallization processes; correlations of mechanical properties are made with parameters describing the dislocation microstructure. (Author).

This book concentrates on both understanding and development of nanocrystalline materials. The original relation that connects grain size and strength, known as the Hall-Petch relation, is studied in the nanometer grain size region. The breakdown of such a relation is a challenge. Why and how to overcome it? Is the dislocation mechanism still operating when the grain size is very small, approaching the amorphous limit? How do we go from the microstructure information to the continuum description of the mechanical properties?

This thesis presents a series of mechanical test methods and comprehensively investigates the deformation and damage behavior of Cu/Pb-free solder joints under different loading conditions. The fracture behavior of Pb-free joint interfaces induced by stress, deformation of solder and substrate are shown, the shear fracture strength of the Cu6Sn5 IMC is measured experimentally for the first time, and the dynamic damage process and microstructure evolution behavior of Pb-free solder joints are revealed intuitively. The thesis puts forward the argument that the local cumulative damage is the major cause of failure in solder joints. The research results provide the experimental and theoretical basis for improving the reliability of solder joints.