Pope, David P
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Publication Strain-Rate Dependence of the Brittle-to-Ductile Transition Temperature in TiAl(2000-11-27) Khantha, Mahadevan; Vitek, Vaclav; Pope, David PThe brittle-to-ductile transition (BDT) and the strain-rate dependence of the brittle-to-ductile transition temperature (BDTT) have been recently investigated in single crystals of TiAl [1]. It was found that the activation energy associated with the BDTT is 1.4 eV when the slip is dominated by ordinary dislocations and 4.9 eV when it is dominated by superdislocations. Despite this difference in the activation energies, the BDTT, while varying with the strain-rate, remains in the same temperature range, viz., between 516-750C and 635-685C for ordinary and superdislocations, respectively. In this paper, we examine how the activation energy of the BDTT can vary with the type of dislocation activity and explain why it can attain values which are clearly much larger than the activation energy for dislocation motion. We describe a strain-rate dependent mechanism of cooperative dislocation generation in loaded solids above a critical temperature and use it to explain the characteristics of the BDT in TiAl. We show that the activation energy associated with the BDTT is a composite value determined by two or more inter-dependent thermally activated processes and its magnitude can be much larger than the activation energy for dislocation motion in certain materials. The predictions of the model are in good agreement with observations in TiAl.Publication Using Web-Based Technology in Laboratory Instruction to Reduce Costs(2002-09-03) Powell, Rita M; Anderson, Helen; Van der Spiegel, Jan; Pope, David PThe authors report the results of a project to reemphasize high quality, hands-on laboratory courses in the engineering curriculum while reducing their costs through the application of web-based teaching tools. The project resulted in substantial gains in productivity of faculty and staff, increased utilization of laboratory space, cost reductions in equipment, and improved quality of learning for our students.Publication Dislocation Screening and the Brittle-to-Ductile Transition: A Kosterlitz-Thouless Type Instability(1994-08-01) Khantha, Mahadevan; Pope, David P; Vitek, VaclavWe propose a new model for the brittle-to-ductile transition based on the Kosterlitz-Thouless concept of dislocation screening. In this model, thermal fluctuations assisted by the applied stress drive the spontaneous generation of dislocations and the instability occurs well below the melting temperature. In the limit of zero stress, our model reduces to the Kosterlitz-Thouless theory of the melting transition, and, in the opposite limit of zero temperature, we obtain the Rice-Thomson result for the brittle-to-ductile transition.Publication Dislocation/Twin/Interface Interactions during Deformation of PST TiAl Single Crystals, an AFM Study(2002-12-02) Chen, Yali; Pope, David P; Vitek, VaclavPST TiAl crystals oriented such that the deformation axis lies in the (111) interfacial planes have been deformed in compression. This deformation produces so-called "channeled flow" in which the strain perpendicular to the (111) interfaces is zero, while the other two strains are equal and opposite in sign. Thus the sample simply shortens axially and spreads laterally in the channels defined by the (111) interfacial planes. We have examined the fine structure of deformation bands on the free surface of these deformed samples using AFM to see how the deformation processes interact with the boundaries. By measuring the offset angle at the surface we have been able to show that not only is the macroscopic displacement vector parallel to the lamellar boundaries, but the total shear vector in each layer is also parallel to the lamellar boundaries. However these deformation bands have very different characters, requiring complex deformation processes at the boundaries in order to satisfy this requirement. Some consist of either just super dislocations or just ordinary dislocations with Burgers vectors lying in the interface. But others consist of a special combination of twinning and ordinary dislocations in fixed ratio, such that the net shear vector also lies in the boundary, even though the individual twinning and dislocation shear directions are inclined to it. This results in deformation that is homogeneous and completely 'channeled' inside each lamella with no shear vector perpendicular to the lamellar boundaries. We have also shown that the cooperative twinning and slip is homogeneous on the nano-scale, i.e., the twinning and slip occurs in the same volume of material.Publication Abnormal deformation behavior in Polysynthetically-twinned TiAl crystals with A and N orientations ---- an AFM study(2004-11-29) Chen, Yali; Pope, David PPolysynthetically-twinned TiAl crystals were deformed by compression with loading axis parallel and perpendicular to the lamellar interfaces. The deformation structures on the free surfaces were scanned using a dimension AFM with scan directions parallel and perpendicular to the lamellar interfaces. Abnormal deformation behaviors were observed to occur in both orientations. When the compression axis is parallel to the lamellar interfaces, the gamma and alpha lamellae deform primarily by shear in planes inclined with the lamellar interface, while the shear vectors lie in the interface. However, in-plane shear, shear in slip planes parallel to the lamellar interfaces, also occurs along the lamellar interfaces. When the loading axis is perpendicular to the lamellar interface, in-plane shear was found to be dominant at the beginning stage of plastic deformation and contributes more to the macroscopic strain. These behaviors are controversial to the Schmid’s Law since the applied resolved shear stress for these deformation systems is zero. The abnormal phenomenon was explained by the large coherency stresses along the lamellar interfaces.Publication Rationalization of the plastic flow behavior of Polysynthetically-twinned (PST) TiAl crystals based on slip mode observation using AFM and Schmid's law(2004-11-29) Chen, Yali; Pope, David PPST TiAl samples of different orientations were prepared and deformed by compression at room temperature. The deformation structures on the free surfaces were scanned using an AFM. It was found that when the angle between the lamellar interfaces and the loading axis is between 20 degree and 80 degree, PST samples deform primarily by shear in slip planes parallel to the lamellar interfaces. When the angle is below 20 degree, both the gamma phase and the alpha 2 phase deform by shear in slip planes inclined with the lamellar interfaces, but the shear vectors lie in the interface. When the angle is close to 90 degree, complex deformation behavior occurs. Shear in planes parallel to the lamellar interfaces contributes more to the overall strain in the directions perpendicular to the loading axis and the out-of-plane shear contributes to the strain in the compression direction. The characteristic U-shape curve of the yield stress versus the angle between the loading axis and the lamellar interfaces can be explained quite well using different C.R.S.S. for the three different deformation modes.