Richard D. James

Aerospace Engineering and Mechanics

University of Minnesota

 

A way to search for new materials with “unlikely” combinations of physical properties

 

 

I guess that one cannot live too long in the AEM department without seeing a demonstration of the shape memory effect.  It is remarkable that a rather stiff piece of material (at least from the viewpoint of the measured linearly elastic moduli) can undergo, then completely recover, such large deformations.  But these materials are very rare indeed, as is evidenced by the fact that they were nearly all discovered more or less by accident, and by their very special compositions: Ni­­50.6Ti49.4 , Cu69 Al27.5Ni3.5, Cu68Zn15Al17

            The property of shape-memory is caused by a first order phase transformation.  A great many materials have first order phase transformations, but what makes shape memory materials special is that the phase transformation is highly reversible, even when it is induced by applying a lot of stress.  So, a key scientific question in seeking new shape memory materials is, “What makes phase transformations reversible?”

            If one looks at shape memory materials at the atomic level, one sees that, because of the presence of this big first order phase transformation, the two phases of the material can have quite different properties.  In particular, the lattice parameters (that measure the distances between atoms) of the two phases are quite different.  It is known from basic quantum mechanical studies that all kinds of properties of crystals – most interestingly electromagnetic properties – are very sensitive to the lattice parameters.  This idea was exploited in the discovery of ferromagnetic shape memory materials, which can be made to undergo a large change of shape by applying a magnetic field.  There is increasing evidence that one might be able to have phase transformations between two phases with properties of interest in diverse applications: ferroelectricity/ferromagnetism, solubility nonsolubility of H2 , high band gap/low band gap, insulator/conductor, opaque/transparent, high/low index of refraction, luminescent/nonluminescent.  Such materials would be like shape memory materials, but, instead of the phases being characterized only by different strains, they would have very different electromagnetic properties, with the consequence of truly unprecedented behavior.  Again, the key for applications is being able to go back and forth easily through the transformation: reversibility.

            With graduate student Jerry Zhang, the speaker has recently found what appears to be the most important factor governing the reversibility of phase transformations.  This will be explained in the talk. The idea lends itself to a systematic method of discovering interesting new materials.