CAREER: Linking Processing Practice to the Performance of Materials in Design

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9702017 Miller The overall objective of this project in materials processing / mechanics of materials is to establish tangible links between the processing of a material and its in- service performance. In general, the method which will be employed in both the educational and research components of the project is to use experiments to understand various aspects of material behavior and microstructural evolution. Guided by these observations, models for in-service performance which incorporate processing effects will be developed and implemented. The research component of this project, An Investigation of the Effects of Metal Forming Practice on In-Service Cyclic Performance, is motivated by the need for a unifying constitutive model framework for small strain cyclic plasticity which explicitly incorporates large strain processing-induced effects. It is well known that flat rolling produces strong internal structure gradients through the thickness of rolled plate material. In this project, the dependence of small strain inelastic cyclic deformation on processing induced initial structure (in the form of crystallographic texture and large strain dislocation structures) will be investigated by taking uniaxial and planar biaxial test specimens at various locations through the thickness of rolled aluminum plate. Both the mechanical response as well as the microstructural evolution will be monitored in these experiments and a determination made as to which of the large strain effects have the most profound influence on the subsequent small strain cyclic behavior. This information will then be incorporated into unifying small strain cyclic models which explicitly account for prior large strain metal forming episodes. While both involve inelastic strains, the large deformations incurred during a metal forming episode differ sharply from the small strain cyclic history which a component may encounter during its service life. These differences produce a discontinuity with respect to model structure. Large strain models place more of an emphasis on the characterization of slowly evolving material behavior due to processes such as crystallographic texture evolution while models for cyclic plasticity must capture quickly evolving phenomena such as that related to the nucleation and evolution of dislocation cells and lattice structures. Even though the internal structure produced during forming is distinct from small strain cyclic structure, the evolution of structure is continuous across the forming/service interface. It is this continuity of state evolution which offers hope for developing a unifying model framework. The initial model will build on a currently-existing crystal plasticity framework while the final modeling effort proposes an new micromechanical formulation. The educational plan, Designing with processed materials, shares the overall goal of bridging the processing / in- service design interface and focuses primarily on undergraduate and graduate curriculum development as well as laboratory development and implementation. A stronger emphasis on the effects of processing on design will be interjected into existing materials processing courses. On the undergraduate level, students will investigate experimentally what effects forming and thermal processing practice have on the properties of the processed material and how the microstructure evolves during processing On the graduate level, a new experimental course in materials processing is being developed taking the general theme of incorporating processing effects into design to a greater depth by investigating issues such as effects of deformation- induced anisotropy, rate and temperature dependence, instabiliti es, and superplasticity. Development of the instructional laboratory which serves the materials processing curriculum in Mechanical Engineering at Cornell consistent with the proposed curriculum changes is also proposed.