Age hardening in Cu-2.5 Wt pct Ti
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INTRODUCTION
ADDITIONS of a few percent titanium to copper have been known for some time to make possible age hardening and in fact substantial levels of strength. ~Our original interest in this alloy's combination of reasonable strength and relatively high thermal conductivity, at temperatures up to 500 ~ was for a possible application 2 which we have described elsewhere. 3 The present study thus had the purpose both of evaluating Cu-Ti alloys for that application and of investigating fundamental aspects of hardening phenomena in this alloy system. The precipitation behavior of alloys with Ti contents up to 6 wt pet has been extensively investigated and reviewed several times.4-8 In brief, the decomposition during aging between 300 and 500 ~ is understood to proceed as follows: (i) spinodal decomposition, with compositional fluctuations aligned along (100) directions; (ii) ordering of the Ti-rich regions, which either precedes 4 or coincides6 with (iii) formation of a coherent, metastable phase called/3', which has the stoichiometry CuaTi, a body-centered tetragonal Dla structure4'6'9 and which tends to continue to exhibit rows of particles aligned along (100>; (iv) formation of the equilibrium phase, /3, formerly thought to be Cu3Ti but which is now known ~~ also to have the stoichiometry CunTi and a complex structure, by a discontinuous or cellular reaction. 5'9 Despite the considerable amount of work devoted to these alloys in the last decade, it is not yet clear (for a given alloy and aging temperature) exactly when during aging the transitions from (i) to (iii) in the above sequence occur. It has also been pointed out 8 that the widely-varying properties reported for as-quenched material suggest at least that this microstructure is sensitive to quenching rate, and probably that the spinodal decomposition cannot be suppressed in richer alloys at ordinary quenching rates. 6'7 There has also been considerable interest in the agehardening process in these alloys, partly as a means of testing Cahn's theory ~2 of the hardening during the initial spinodal decomposition. Although some authors have
ANTHONY W. THOMPSON, Professor in the Department of Metallurgy and Materials Science, and JAMES C. WILLIAMS, Professor in the Department of Metallurgy and Materials Science and Dean, Carnegie Institute of Technology, are with Carnegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted August 24, 1983. METALLURGICALTRANSACTIONS A
reached the conclusion that Cu-Ti age hardening does obey the Cahn model, TM it has also been argued that those conclusions are suspect. 8 Moreover, it is now clear that the agehardening behavior is more complex than was originally recognized. 8'~4-~7Since information about the age-hardening mechanism was one goal of the present work, it was considered desirable to obtain complete hardening vs time curves to supplement our earlier report. 14 For our study, an alloy of 2.5 wt pet Ti was chosen. In addition to transmission electron microscopy (TEM) of aged specimens and measurements
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