*** 9713731 Hemker Fully lamellar two phase TiAl based intermetallic alloys offer a very attractive mix of mechanical properties and are considered to be strong candidates for replacing nickel base superalloys in several structural applications involving temperatures of up to 900' C. At these temperatures, the creep performance of these alloys is of primary concern. Unfortunately, our understanding of the processes that control high temperature deformation in many advanced materials, including TiAl, is currently rather limited. The underlying creep mechanisms in these advanced alloys are often quite different from that in pure metals; the influence of steady-state creep is much smaller than it is in pure metals, and transient deformation processes (i.e.. primary and tertiary creep) have been found to dominate the creep behavior. In these cases, the Dorn description of power-law creep is no longer valid and attempts to characterize the creep behavior with activation energies and stress exponents, derived from minimum creep rates, have met with very limited success. This has profound consequences for the prediction of creep performance, because the FEM codes used for creep analysis require the input of creep laws that characterize the creep behavior of the material. Wherever possible it is desirable to have these laws based on the physical deformation mechanisms. The widely referenced Dorn description of power-law creep is based on diffusion assisted climb in recovery processes that lead to steady state creep. However, as is shown in the PI's RIA related research, in most intermetallic alloys, including TiAl , the diffusion-assisted recovery processes which lead to steady state creep in pure metals are replaced by a gradual evolution of the deformation microstructure. For this reason, the Dorn equation cannot be used to model creep in this set of alloys and it is necessary to develop an alternative set of mechanism-based creep relations for TiAl based lamellar alloys. The primary goal of this work will be to derive a fundamental set of creep laws that are based on observations of microstructural evolution as a function of creep strain. This will require a close integration of mechanics and materials and will involve work at three specific length scales: i) the microscopic deformation mechanisms will be identified and characterized by TEM observations of fully lamellar polycrystalline specimens that have been crept to various amounts of creep strain, ii) the mesoscopic effects of grain size, lamellar spacing, and lamellar orientation will be separated and characterized with single crystal and microsample creep tests, and iii) the macroscopic creep behavior of these alloys will be modeled with constitutive relations that are based on the micro-and mesoscopic measurements. The PI's experience with creep testing, TEM, and TiAl has been teamed with the co-PI's expertise in developing continuum models of multiphase materials in order to assure a bridge between the mechanics and materials issues in this study.***
