ABSTRACT

This award by the Division of Materials Research to New Mexico Institute of Mining and Technology is to carry out research on superalloys driven by the desire to increase their high temperature capability thereby enabling higher efficiency turbine engines. There is also a need for improved life prediction methodology that considers long-term microstructural stability. The overall goal of this research is to understand and model the anisotropic deformation behavior of single crystal nickel-base superalloys that contain a high volume fraction of gamma prime precipitates embedded coherently in the disordered gamma matrix. These nanostructured materials exhibit excellent high temperature properties. There is considerable debate regarding the evolution of internal stresses and mechanisms of dislocation movement, and their influence on the creep response and microstructural stability of these alloys. In-situ neutron diffraction studies to be carried out in collaboration with Los Alamos Neutron Science Center is to probe the internal elastic strain state in the gamma and gamma prime phases of single crystal and columnar grain alloys obtained by directional solidification techniques. Such direct measurements should help confirm or reject previous hypotheses on deformation mechanisms. The experimental work will be complemented with modeling of the mechanical response using crystal plasticity and finite element method modeling (FEM). The nanoscale dimensions of the gamma require a combination of dislocation and FEM analysis, and the crystal plasticity method appears to be an efficient means to achieve these objectives. The motion of dislocations and their resistance to flow is accounted for in the crystal plasticity component of a user-defined material subroutine; thus, different types of dislocation interactions and velocity laws can be incorporated. Another important component of the user subroutine is the accounting of geometrically necessary dislocations (GND) through strain gradient plasticity. These dislocations can account for the observed interface dislocation networks, and preliminary analysis shows that the strain gradient effect is in direct agreement with a number of observations related to a "rafting" microstructure observed in these alloys.

The major intellectual merit of the proposal is a combination of novel experimental techniques being developed to probe internal stresses at the nanoscale level, and finite element modeling studies that include crystal plasticity and evolution of geometrically necessary dislocations. Prediction can be made about strain rates as well as changes in internal energy that drives the kinetics of rafting. The broader impact of the program will be in the following areas. At the scientific level, the methodology will form a framework to understand nanophase structures, where constrained deformation requires the incorporation of geometrically necessary dislocations, and where microstructural stability can be an important issue. At the industrial level, the collaboration with Research Applications Inc. will aid insertion of the modeling methodology into the turbine industry. In addition, active interaction with Cannon Muskegon (alloy developer) and Pratt & Whitney (aircraft engine manufacturer) will directly benefit both material users and suppliers. Finally, at the educational level, both graduate and undergraduate students in the program will interact directly with industry and the national laboratory and prepare them for careers in science and technology. The infrastructure program in place in the state will foster nanomaterials research and collaboration among the universities and the national laboratories.

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