Creep Mechanisms in Equiaxed and Lamellar Ti-48Al
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Creep Mechanisms in Equiaxed and Lamellar Ti-48Al G. B. Viswanathan 1, S. Karthikeyan 1, V. K. Vasudevan 2 and M. J. Mills 1 1
Department of Materials Science and Engineering, The Ohio State University, Columbus, OH
43210, 2Department of Materials Science and Engineering, University Of Cincinnati, Cincinnati, OH 45221 Abstract Minimum creep rates as a function of stress have been obtained for Ti-48Al binary alloy with a near gamma and a fully lamellar microstructure. TEM investigation reveals that deformation structures in both microstructures are dominated by jogged screw 1/2[110] dislocations in γ phase. A modified jogged screw model is adopted to predict minimum creep rates where the rate controlling step is assumed to be the non-conservative motion of 1/2[110] unit dislocations. In the case of equiaxed microstructure where the deformation was mostly uniform, the creep rates predicted by this model were in agreement with experimental values. Conversely, deformation in lamellar microstructures were highly inhomogeneous where the density of jogged 1/2[110] unit dislocations were seen in varying proportions depending on the width of the γ laths. The creep rates and stress exponents in these microstructures is explained in terms of active volume fractions of γ laths participating in deformation for a given applied stress. 1.0 Introduction The most promising Ti-48Al (at. %) based alloys can have broad categories of microstructures such as equiaxed, duplex and fully lamellar and understanding of creep behavior of these microstructures is very critical to applications [1,2]. Activation energies similar to those for self diffusion, and stress exponents in the range 4-6, reported in these microstructures [2-5] suggest a creep mechanism involving recovery by dislocation climb processes. However, the absence of subgrain formation during minimum creep regime suggest that the power law behavior is different from that of a pure metal. Accordingly, a model based on the mobility of jogged screw dislocations was previously proposed and tested for equiaxed microstructure [6]. In the case of lamellar microstructures, several explanations such as interlocked boundaries and decreased slip distances due to numerous lath boundaries [2], twinning , dynamic recrystallization [7] and sliding [8] have been suggested to be the cause of superior creep resistance. In this paper, an attempt has been made to understand the creep behavior in lamellar microstructures. Constant load creep experiments were performed at various stress levels to obtain stress exponent values. Based on TEM evidence, it is shown that the deformation in lamellar microstructures is similar to that of equiaxed microstructure. Suitable modifications to the original jogged screw model have been proposed to explain creep in lamellar microstructures. 2.0 Experimental Methods The creep study here is based on an alloy of composition: 47.86 at.%, O-0.116 wt.%, N-0.016 at.%, C0.041 at.%, H-0.076 at.% and the balance titanium. To briefly summarize the experimental procedures, cy
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