There is a high demand for low density and high strength materials to substitute heavy alloys that are currently used in the industry. The efficient solution to address this requirement is modifying the mechanical performance of available alloys via tailoring their microstructures. Gradient microstructure has exceptional potential to modify the properties of the materials without sacrificing other parameters. Mechanical incompatibility between different layers in this structure leads to extra strengthening and toughening mechanisms. Microstructural variation along the depth of thickness can be achieved via exerting desirable severe plastic deformation during the manufacturing and finishing process. The aim of this work is to develop gradient microstructure, with various statistical distributions of grain size, grain orientations, dislocation, and twins to optimize strength and ductility. To this end, several processing techniques are represented to apply static and dynamic severe plastic deformation such as high-pressure torsion (HPT) and severe shot peening (SSP). Then, the effect of various parameters in these procedures and on the creation of new microstructure such as twins, ultra-fine grains, SSD, and GND dislocation through the thickness was investigated precisely by utilizing electron backscatter diffraction (EBSD) data.
Next, the role of these introduced defects and UFG grains in local hardness and overall tensile behavior was determined experimentally to establish a relationship between the microstructure and the hardening mechanism. On the other hand, subsequent annealing of heavily cold deformed structure contributes to grain boundaries modification, dislocation rearrangement or annihilation, and consequently, ductility enhancement due. Furthermore, the structure would be unstable at the thermal condition with regards to incomplete recrystallization, high density of dislocation, and smaller grain size. Thus, microstructural evolution and thermal stability of created gradient microstructure are required attention during the thermal condition.
In the end, new surface treatment ``severe impact loading'' is introduced to overcome problems observed in the previous ones. In this process, the residual strain is created by a single impact wherein tensile twins formed without excessive grain refinement led to strength enhancement along with acceptable ductility.
|Advisor:||Field, David P.|
|Commitee:||Li, Qizhen (Katherine), Gozen, Arda|
|School:||Washington State University|
|School Location:||United States -- Washington|
|Source:||DAI-B 82/3(E), Dissertation Abstracts International|
|Subjects:||Mechanical engineering, Materials science, Plastics|
|Keywords:||Annealing, EBSD, Gradient microstructures, Mechanical properties, Severe shot peening, Strengthening mechanism|
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