A. Zamiri

The role of elastic and plastic anisotropy of Sn on microstructure and damage evolution in lead-free solder joints

The elastic, thermal expansion, and plastic anisotropy of Sn is examined to assess how anisotropy affects the microstructural evolution and damage nucleation processes in SAC305 solder joints. Examination of all joints in a package indicates that upon solidification, crystal orientations are nearly randomly distributed. Initial studies of cracked joints after thermal cycling showed that orientations with the c-axis parallel to the joint interface (red orientations) are more likely to crack arising from tensile stresses during the hot part of the cycle. Subsequent studies show that package design has a large influence on how the microstructure evolves; higher strain designs stimulate recrystallization at earlier times. Recrystallization appears to be strongly correlated with crack nucleation and propagation processes, as red orientations often develop and lead to crack nucleation and propagation. The details of the recrystallization process depend strongly on the plastic slip and recovery processes arising from the specific crystal orientation / temperature / strain history that makes microstructural evolution of each joint unique. The unique history for each joint implies that worst case scenarios need to be identified and models developed that can predict microstructural evolution that leads to worst case scenarios.

Mechanical Properties of High RRR Niobium With Different Texture

High purity bulk niobium RRR ~ 300 is the standard material used for SRF cavities since this has been the most promising material for over 10 years. Mechanical properties of the high RRR niobium play a critical role in the physical integrity of these structures. It is well known that the mechanical properties of polycrystalline high RRR niobium are affected by a variety of possible contributors: crystallographic texture, grain size, and impurity concentration. Tensile tests were carried out using three groups of niobium samples from MSU SRF cavity and Fermi International Linear Collider projects with different texture and grain size, which came from two different manufacturers. For some of the groups the tensile tests were done with different orientations with respect to the sheet. Orientation Imaging Microscopy (OIM) data were collected in these samples. The relationships between mechanical properties, texture, and grain size in these samples are analyzed.

A quantitative study of the effect of surface texture on plasticity induced surface roughness and dislocation density of crystalline materials

Microscale simulations are used to study the effects of the surface texture and plastic deformation on surface roughness and dislocation density, which are important parameters controlling some surface physical properties such as electron work function (EWF) and phonon emission of crystalline materials. The results of the simulations on superconducting niobium show that the intensity and the components of the surface texture have significant effects on the plasticity induced surface roughness and dislocation density. A weak surface texture develops a rough surface after plastic deformation, which is due to the different plastic “shear rates and directions” behavior in the grains with different orientations. Some grains with specific orientation experience more plastic deformation, and therefore develop an intragrain surface roughness due to the development of microtexture and inhomogeneous plastic deformation inside the grain. Due to an inhomogeneous plastic deformation, the dislocation density not only i...