The effect of alloying additions on the superplastic properties of Ti-6 pct Al-4 pct V
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I.
INTRODUCTION
THE
aim of the present work was to determine to what extent the superplastic properties of Ti-6 pct AI-4 pct V, first discovered by Lee and Backofen in 1967, ~ could be improved by minor alloying additions These were selected on the basis of their diffusional characteristics in the/3 phase since the strain rates which are predicted by all the theories or mechanisms proposed to describe superplastic flow are, for a given stress and grain size, dependent upon the value(s) of the appropriate diffusion coefficient(s) used in the constitutive equations. Hence, alloying elements which increase the appropriate diffusion coefficients are also expected to increase superplastic deformation rates irrespective of the particular mechanism which is considered to be rate-controlling. The effect may either be masked or enhanced by microstructural changes in the alloy consequent upon the alloying element additions--variations in grain size, volume fraction of the a and /3 phases, tot example. These factors have been investigated in the work of Wert and Paton 2 on enhanced superplasticity in modified Ti-6 pct AI-4 pct V alloys in which alloying additions were made on the basis of their diffusivities in the/3 phase and also their/3 stabilizing characteristics, providing greater/3 volume fractions than Ti-6 pct Al-4 pct V at low superplastic deformation temperatures. The /3 phase of titanium behaves "anomalously" with respect to its diffusional properties. 3 The lattice self diffusion coefficient may be expressed as the sum of two terms, one with a 'normal' value for the activation energy (as predicted from the correlation with melting point) and the other with a value of about half this. Radioactive anTi tracer data4 gives: Dv(t~ ri = [ 1.09 x 10 -4 exp - 2 4 8 . 0 0 0 + 3.58 RT • 10-8 exp
- 129.230] m2 s- t ~ J
[1]
The first 'normal' term dominates the diffusional behavior at high temperatures whereas the second 'anomalous' term J.R. LEADER is Section Leader, Fabrication Division, Combustion Technology and Engineering Centre, Lucas Aerospace Ltd., Burnley BB 11 4BD, United Kingdom. D. F. NEAL is Senior Technical Officer, Research Department, IMI Titanium Ltd., P.O. Box 704, Witton, Birmingham B6 7UR, United Kingdom. C. HAMMOND is Senior Lecturer, Department of Metallurgy, University of Leeds, Leeds LS2 9JT, United Kingdom. Manuscript submitted February 29, 1984. METALLURGICALTRANSACTIONS A
dominates at lower temperatures. Considerable progress has been made recently in understanding this anomalous diffusional behavior? However, the important consequence-in terms of deformation behavior--is that the diffusional characteristics of/3 titanium at relatively low homologous temperatures ( - 0 . 6 T,,) correspond to those of 'normal' metals at much higher values of the homologous temperature ( - 0 . 9 T,,). Hence, diffusional creep mechanisms are of importance in/3-titanium at much lower fractions of the melting temperature than is normally the case. 6 Furthermore, the lattice diffusion coefficients of alloying eleme
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