Stress asymmetry in cyclic deformation of b.c.c. metals

Asymmetric slip in b.c.c. metals is related to both the symmetry about screw dislocations and elastic anisotropy. The elastic anisotropy differs quite substantially from one b.c.c. metal to the next and this anisotropy has been observed in measurements of asymmetric stresses in plastic deformation experiments. In this paper, data from cyclic deformation of b.c.c. metals, with special emphasis on tungsten which is elastically isotropic, are used to demonstrate the role of screw dislocation geometry and elastic anisotropy on stress asymmetry. The interaction of dislocation geometry and thermal activation of screw dislocation motion is discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30320/1/0000722.pd

Surface film effects during cyclic deformation of dislocation-mobility-limited materials

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30047/1/0000415.pd

Short Fiber Reinforced Thermoplastics: Prediction of Stiffness in Injection Molded PS-PPO Blends

The prediction of stiffness in short fiber reinforced thermoplastics is stud ied as a function of fiber length using injection molded blends of PS and PPO. The theoret ical models for predicting composite stiffness are reviewed. The results are first compared with the theoretical models advanced for uniaxially aligned composites. These models predict higher than experimental values. However, agreement between the predictions and experimental values improves when the effect of fiber orientation distribution in the injec tion molded samples is taken into account and as the ductility (or the PPO content) of the matrix increases. Cox's model when used with the "laminate analogy" gives the closest prediction to the experimental stiffness. Reinforcement efficiency factor for stiffness is a strong function of retained fiber lengths. The dependence of composite stiffness on the matrix ductility and the effects of compatibility on the mechanical properties of PS-PPO blend system are also discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68613/2/10.1177_089270579100400205.pd

Cyclic deformation of tungsten single crystals

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26038/1/0000111.pd

Mechanical behavior and interface design of MoSi2-based alloys and composites

The mechanical behavior of hot pressed MoSi2-based composites containing Mo5Si3, SiO2, CaO and TiC as reinforcing second phases was investigated in the temperature regime 1000-1300 [deg]C. The effects of strain rate on the flow stress for Mo5Si3-, SiO2- and CaO-containing composites are presented. Effects of several processing routes and microstructural modifications on the mechanical behavior of MoSi2---Mo5Si3 composites are given. Of these four composite additions, Mo5Si3 and CaO produce strengthening of MoSi2 in the temperature range investigated. SiO2 greatly reduces the strength, consistent with the formation of a glassy phase at interface and interphase boundaries. TiC reduces the flow stress of MoSi2 in a manner that suggests dislocation pumping into the MoSi2 matrix. The strain rate effects indicate that dislocation creep (glide and climb) processes operate over the temperature range investigated, with some contribution from diffusional processes at the higher temperatures and lower strain rates. Erbium is found to be very effective in refining the microstructures and in increasing the hardness and fracture properties of MoSi2---Mo5Si3 eutectics prepared by arc melting. Initial results on microstructural modeling of the deformation and fracture of MoSi2-based composites are also reported.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29976/1/0000339.pd

Epitaxial growth and characterization of GaAs/Al/GaAs heterostructures

We report on transmission electron microscopy, secondary ion mass spectroscopy, X-ray and Raman scattering studies of GaAs/Al/GaAs heterostructures grown by migration enhanced epitaxy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28626/1/0000440.pd

The Clustering of Oxygen in Solid Solution in Niobium

129 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1965.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Microstructural deformation behaviour in polystyrene-based compatible polymer blend systems

Microstructural deformation behaviour in polystyrene-based compatible polymer blend systems was studied using transmission electron microscopy (TEM) and microdensitometry. Four different binary compatible blend systems were employed and characterized in this investigation: polystyrene (PS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), PS and poly(vinyl methyl ether) (PVME), PS and poly([alpha]-methylstyrene) (P[alpha]MS), and PPO and P[alpha]MS. Individual craze and shear deformation zone (DZ) microstructures were examined by TEM. For TEM observations, specimens deformed in situ on a TEM grid (loaded condition) were utilized. TEM micrographs showed that, for PS/PPO blends, deformation mode transition from crazing to shear DZ occurred around 25% PPO inclusion. For PS/PVME blends, this transition occurred around 20% PVME inclusion. For PS/P[alpha]MS blends, the deformation mode was totally controlled by crazing regardless of composition. For PPO/P[alpha]MS blends, deformation mode transition from shear DZ to crazing occurred around 25% P[alpha]MS inclusion. Quantitative analyses of these crazes and shear DZs were conducted utilizing microdensitometry of the TEM negatives in the manner developed by Lauterwasser and Kramer. From the microdensitometry, molecular parameters such as fibril extension ratios ([lambda]s) were determined. Microdensitometry results showed that [lambda] decreased as the PPO content increased in the PS/PPO blends, and, for 100% PPO, only shear DZs were observed. For PS/PVME blends, [lambda] also decreased as the PVME content increased. For PS/P[alpha]MS and PPO/P[alpha]MS blends, [lambda] increased as the P[alpha]MS content increased. These results were analysed in terms of existing entanglement and intermolecular interaction models in compatible blends. From this analysis, it is concluded that the overall microstructural deformation behaviour of binary compatible blends cannot be fully explained by either entanglement density or intermolecular interaction model alone. Rather, the combined entanglement density and intermolecular interaction model can explain the microstructural deformation behaviour in binary compatible blends well.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31627/1/0000561.pd

Characteristics of dislocations at strained heteroepitaxial InGaAs/GaAs interfaces

The formation, interaction, and propagation of misfit dislocations in molecular‐beam epitaxial InGaAs/GaAs heterointerfaces have been studied by transmission electron microscopy. With the lattice mismatch less than 2%, most of the interfacial dislocations are found to be 60° mixed dislocations introduced by glide processes. Sessile edge‐type dislocations can also originate from the combination of two 60° mixed dislocations. The ratio of densities of edge dislocations to 60° dislocations was increased during the later part of the elastic strain relaxation. These sessile edge dislocations may be generated in appreciable numbers through a climb process. For large lattice‐mismatched systems, the majority of the misfit dislocations are pure edge dislocations and high threading dislocation density is generally found. The interfacial dislocation network is found to contain regions of dislocations with the same Burgers vector that extend over several micrometers. The results support a mechanism that involves misfit dislocation multiplication during the molecular‐beam epitaxial growth process.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71028/2/JAPIAU-66-7-2993-1.pd

Transmission electron microscopy of strained InyGa1−yAs/GaAs multiquantum wells: The generation of misfit dislocations

We have investigated the generation and propagation of misfit dislocations in strained InyGa1−yAs/GaAs multiquantum wells grown by molecular‐beam epitaxy, with cross‐sectional transmission electron microscopy. The samples are of excellent optical quality, with multiquantum wells having well widths of 100 Å, being characterized by excitonic linewidths and Stokes shifts of 1.5–2.5 and 1–2 meV, respectively. We have examined the growth of 2‐μm‐thick multiquantum‐well samples grown either directly on GaAs, or with an intermediate composition buffer layer, and for the cases of small (y=0.07) and large (y=0.16) misfits. It is seen that for the case of quantum wells with small misfit, grown directly on GaAs, metastable growth can be achieved. This is confirmed by low‐temperature absorption measurements and from transmission electron microscopy experiments performed both before and after post‐growth thermal annealing. In the case of quantum wells with large misfits directly grown on GaAs, dislocations are generated within the first few periods, and high optical quality is retained in the subsequent free‐standing quantum wells. In the case of quantum wells grown with an intermediate composition InxGa1−xAs buffer layer, dislocations are generated at the buffer‐GaAs interface, and the freestanding multiquantum well is again of very high quality.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71047/2/JAPIAU-65-9-3391-1.pd