Investigation of Creep Behavior and Microstructure in Nanocrystalline Materials

University Of California-Irvine, Irvine CA

Investigators

Abstract

TECHNICAL: This proposal describes a fundamental investigation into the deformation behavior of bulk nanocrystalline (NC) materials, which are characterized by a grain size in the range 1-100 nm. An understanding of the origin and nature of deformation processes in NC-materials especially under creep conditions is essential. In general, the identification or development of deformation processes requires guiding information that can be obtained from comprehensive experimental creep measurements. It has been found that the creep characteristics including the stress exponent and its variation with stress, the activation volume, and the magnitude of the creep rates cannot be accounted for by current deformation processes. This project will devise new experiments and perform careful analysis that aim at systematically characterizing the deformation behavior of NC-materials. In particular, these experiments and analysis are designed to address several questions including the following: (a) whether the activation volume for deformation in NC-material is constant or depends on variables such as grain size and temperature; (b) whether the strain measured during creep mostly arises from boundary sliding; (c) whether dislocations activity exists during deformation; (d) whether ductility can be improved in NC-Ni while maintaining nearly the same strength; (e) whether the creep characteristics reported in NC-Ni are representative for the behavior of other NC-materials such NC-Cu; and (f) whether NC-Cu exhibits a transition from conventional Hall-Petch strengthening to nano-scale softening, In seeking answers for the aforementioned questions, novel approaches will be adopted. For example, in investigating boundary sliding, the technique of atomic force microscopy that was very recently used to study sliding in ultra-fine-grained materials will be adopted. Also, for addressing the question of dislocation activity, the following new discovery will be utilized: As a result of preliminary research efforts at UCI, it is possible to introduce in the microstructure of NC-Ni coherent Sigma-3 annealing twins that can serve as barriers to dislocation motion. The presence these twins in NC-Ni will be used as a marker to check whether dislocation activity occurs during creep. Furthermore, combinations of annealing treatment, mechanical testing, and microstructural examination will be attempted to define the conditions under which the presence of coherent Sigma-3 annealing twins can enhance ductility while maintaining nearly the same strength and avoiding significant grain growth. In the initial phase of the program, NC-Ni and NC-Cu prepared by pulsed current electro-deposition will be used. Later studies will include Ni-W alloys. NON-TECHNICAL: NC materials offer interesting possibilities related to many structural applications. In order to explore some of these possibilities, an understanding of the origin and nature of deformation processes in NC-materials especially under creep conditions is essential. In general, the identification or development of deformation processes requires guiding information that can be obtained from comprehensive experimental creep measurements. In addition to providing improved fundamental understanding of deformation in NC-materials and identifying factors that control creep strength and ductility in these materials, the program will contribute to the education and professional careers of graduate and undergraduate students.