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Application of a Modified Jogged-Screw Model for Creep of Titanium Aluminides: Evaluation Of The Key Substructural Parameters

Published online by Cambridge University Press:  15 February 2011

Subramanian Karthikeyan
Affiliation:
Dept. of Materials Science and Engineering, The Ohio State University, Columbus, OH-43210.
Junho Moon
Affiliation:
Dept. of Materials Science and Engineering, The Ohio State University, Columbus, OH-43210.
Gopal B. Viswanathan
Affiliation:
Dept. of Materials Science and Engineering, The Ohio State University, Columbus, OH-43210.
Michael J. Mills
Affiliation:
Dept. of Materials Science and Engineering, The Ohio State University, Columbus, OH-43210.
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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.

Type
Research Article
Copyright
Copyright © Materials Research Society 2003

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