No abstract

Recently, NiTiHf-based HTSMAs have been shown to exhibit unique precipitation and mechanical behavior. In this study, a rolled plate of NiTiHf HTSMA was homogenized and heat treated at various times and temperatures and characterized using a barrage of analytical techniques including high-energy synchrotron X-ray diffraction (SR-XRD). Neither homogenization nor any of heat treatments studied significantly affect the austenitic or martensitic transformation temperatures. H-phase was observed to precipitate at heat treating times below 30 minutes and then to subsequently dissolve away for times of 30 minutes and above. The presence of H-phase dramatically increases the material strength by almost a factor of 2, Lastly, an over-aging effect occurs with increasing time due to the disappearance of the H-phase.

The growth of solid phases which are produced by diffusion in bilayers or multilayers of different chemical species is studied by an approach based on an effective diffusivity. In this approach one does not assume a priori any phase layers and interfaces. A numerical procedure to obtain the solutions of the diffusion differential equation is presented and applied to a fictitious example and to a real situation. Good agreement is obtained between the calculated values and the experimental data reported in the literature for the Co/Si system. In the absence of nucleation restrictions and reaction barriers, the model is applicable to any binary diffusion couple with a given initial composition profile, for bulk or thin film configurations.