Application of a Modified Jogged-Screw Model for Creep of Titanium Aluminides: Evaluation Of The Key Substructural Param
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Application of a Modified Jogged-Screw Model for Creep of Titanium Aluminides: Evaluation Of The Key Substructural Parameters Subramanian Karthikeyan, Junho Moon, Gopal B. Viswanathan and Michael J. Mills Dept. of Materials Science and Engineering, The Ohio State University, Columbus, OH-43210. ABSTRACT A modification of the jogged-screw model has been adopted recently by the authors to explain observations of 1/2[110]-type jogged-screw dislocations in equiaxed Ti-48Al under creep conditions. The aim of this study has been to verify and validate the parameters and functional dependencies that have been assumed in this previous work. The original solution has been reformulated to take into account the finite length of the moving jog. This is a better approximation of the tall jog. The substructural model parameters have been further investigated in light of the Finite Length Moving Line (FLML) source approximation. The original model assumes that the critical jog height (beyond which the jog is not dragged) is inversely proportional to the applied stress. By accounting for the fact that there are three competing mechanisms (jog dragging, dipole dragging, dipole bypass) possible, we can arrive at a modified critical jog height. The critical jog height was found to be more strongly stress dependent than assumed previously. The original model assumes the jog spacing to be invariant over the stress range. However, dynamic simulation using a line tension model has shown that the jog spacing is inversely proportional to the applied stress. This has also been confirmed by TEM measurements of jog spacings over a range of stresses. Taylor's expression assumed previously to provide the dependence of dislocation density on the applied stress, has now been confirmed by actual dislocation density measurements. Combining all of these parameters and dependencies, derived both from experiment and theory, leads to an excellent prediction of creep rates and stress exponents. The further application of this model to other materials, and the important role of atomistic and dislocation dynamics simulations in its continued development is also discussed. INTRODUCTION High specific stiffness, good oxidation resistance up to 900¡C, and high temperature strength (~600 MPa at 600¡C) make γ-TiAl alloys promising as potential high temperature structural materials [1]. In this regard, the understanding of creep behavior in these materials is of utmost importance. Despite the significant body of work done in the area of creep deformation, the present understanding of high temperature creep mechanisms is very limited [2]. Clearly a difficulty is that creep is influenced by numerous microstructural parameters including grain size, colony size, lamellar spacing and distribution [3], and many of the investigations in γ-TiAl have been made on different structures. Thus, a clearer understanding of creep mechanisms in the equiaxed microstructure is imperative for understanding the more complicated scenario in the fully lamellar "engineeri
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