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Matthew G. Killough, Principal Investigator

Interaction Between Material Defects and Martensitic Microstructure

In a variety of solids, the molecular lattice undergoes a change of shape as the material is cooled past a specific transition temperature. This so called martensitic phase transition typically involves a loss of symmetry in the lattice as the material cools from the high temperature (austinite) phase to the low temperature (martensite) one. Macroscopically, this loss of symmetry means that the martensite has a multiplicity of stress-free strains called variants of martensite. Typically, the material organizes itself into a microstructure involving fine scale laminates of the different variants. In steels, for example, this microstructure gets locked into place and gives the steel its strength. This research focuses on another class of martensitic materials known as shape memory alloys (SMAs). In contrast to steels, SMAs have the ability to change their martensitic microstructure in response to applied forces giving them very unusual and highly nonlinear macroscopic elastic properties.

Although the elastic properties of SMAs are not very well understood, recent experiments have shed some light on this subject. These experiments studied the response of a single crystal of CuAlNi to a program of dead loads. A hysteresis loop was discovered in the plot of the macroscopic deformation as a function of the applied load. The experiments clearly indicate that the kinetics of the microstructure controls the macroscopic response of the system. The conjecture was that an energetic barrier to the nucleation of new microstructural interfaces gives rise to metastable microstructures and causes the hysteresis, and that this nucleation barrier was the result of a competition between bulk elastic energy and surface energy at the interfaces.

In previous work, a model for the kinetics of the martensitic microstructure was developed in addition to an efficient numerical method for simulations using the model. The morphology of the microstructure that developed during the simulations matched the experimental observations very closely. However, the proposed nucleation barrier seemed not to be present in this model although the model took both elastic and surface energies into account. The simulations found no metastable microstructures, and although there was a hysteresis in the simulations, it was qualitatively different from the physical one.

This research is focusing on two projects that grew out of this continuing work. Both projects look at the interaction between microstructure and material defects, but the projects give these two phenomena reciprocal roles. The first project investigates the effects of defects on the evolution of the martensitic microstructure. The conjecture, motivated by analogy with micromagnetics, is that the defects may serve to pin the interfaces in place, stabilizing some of the transient microstructures from the previous idealized simulations. The second project looks at the role of a martensite-like elastic microstructure on the growth of precipitates from a solid matrix. A model has been developed that is appropriate for certain physical systems that couples the elasticity equations to the Cahn Hilliard equation that governs the compositional order parameter.

This information is available in alternative formats upon request by individuals with disabilities. Please send email to alt-format@msi.umn.edu or call 612-624-0528.

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