Cyclic Plasticity and Fatigue of Ultrafine Grain FCC Metals at Low Plastic Strain Amplitudes

Clarkson University, Potsdam NY

Investigators

Abstract

Cyclic Plasticity and Fatigue of Ultra Fine Grain FCC Metals at Low Plastic Strain Amplitudes This project is aimed at achieving a fundamental understanding of low plastic strain amplitude cyclic deformation and fatigue behavior in ultra fine grain FCC metals that have a low initial dislocation density. Ultra fine grain metals are those that have grain diameters in the submicron range, but generally greater than about 10 nm. Such materials are only beginning to be produced in large quantities and little is known about their mechanical behavior relative to conventional grain size materials, but ultra fine grain metals have been shown to exhibit exceptional strength with, in many cases, reasonable ductility. They are generally produced in one of three ways: by inert gas condensation and powder metallurgy, by severe plastic deformation of conventional grain size materials, and by electrodeposition. Materials produced by severe plastic deformation can exhibit both high strength and good ductility, but they have a very high dislocation density, a property that is very important in determining subsequent mechanical behavior. Materials produced by inert gas condensation and powder metallurgy generally exhibit poor ductility - they are somewhat brittle. Only materials produced by electrodeposition have both low dislocation density and exhibit high strength and good ductility. For this reason they offer potential for achieving good fatigue life characteristics relative to conventional grain size materials and ultra fine grain materials produced by other means. Because it is possible to produce large samples of ultra fine grain metals by electrodeposition, there is significant potential for commercial development for structural uses, but more needs to be known about the mechanical properties of these materials. The project will execute a focused experimental program using ultra fine grain nickel produced by electrodeposition. The objectives of the project are to develop and analyze a detailed cyclic plasticity database for ultra fine grain nickel at low plastic strain amplitudes. Fatigue experiments will be performed on specimens produced using electrodeposition of nickel. Various techniques such as optical microscopy, scanning and transmission electron microscopy, and x-ray diffraction will be used for characterizing the materials produced by electrodeposition and for studying the effects of fatigue loading. Additional objectives include the development of an understanding of fatigue crack initiation behavior in ultra fine grain nickel. Fatigue crack initiation is a precursor to the eventual loss of load-carrying capacity. The project will involve at least one graduate student in thesis work that encompasses elements of materials science, electrochemistry, solid mechanics and ferromagnetics. It will also involve undergraduate students, particularly those from underrepresented groups. The results will be disseminated through conference presentations and publications in referred journals and via a website maintained by the principal investigators and their students.