Secondary precipitation and allotropic transformation of Cobalt-Rich Co-Ti-C alloys using transmission electron microsco
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IN the search for an alternative to the Co-WC hard metal system, recent developments have involved the use of metal carbides other than WC. With Ni or Co alloys as binder phases, carbides of Ti, Ta, and Cr have been used. The role of precipitation of hard carbide particles in Co alloys is thus of considerable importance both for Co-superalloys and for hard metal systems with Co as a binder phase. Despite this, few detailed studies of the influence of composition and heat treatment variables on the precipitation processes have been reported. The aim of the present work was therefore to examine the precipitation behavior in the Co-Ti-C b~btl~lll, LU I.~I[~Ii:I,15LCIILK; LIIK; ~ t ~ U I I U ~ I y IAl~t~llAlti:tLCb
formed during aging, examine their nucleation and growth conditions, and establish their influence on mechanical properties. Two quasibinary compositions from the Co-rich corner of the Co-Ti-C system were chosen for these studies: one, Co-1.25Ti-1.25C at. pct, lying near and one, Co-2.5Ti-2.5C at. pct, lying somewhat above the solubility limit of TiC in Co at the eutectic temperature
BIRGIT E. JACOBSON, formerly with the Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, is now Senior Research Associate, Department of Physics and Measurement Technology, Link6ping University, S-58183 Link0ping, Sweden. Manuscript submitted April 27, 1978.
(Fig. 1). Aging was carried out with both as-quenched, solution treated material and prior deformed material, cold rolled to 30 pct reduction after solution treatment. Electron microscopy was used for the precipitation investigations, and the associated strengthening effects were followed by macro- and microhardness measurements. There exists two allotropic modifications of pure cobalt, a close-packed-hexagonal form, stable at temperatures below 420 ~ and a face-centered-cubic form, stable at temperatures up to the melting point. The fcc ~ HCP-Co phase transformation is a martensitic transformation, consistent with the shear of ( 111 ) f~c-C~n n l n n ~ g in ~ 1 19~ d i r e c t i o n ~ _ T h e t w o lattices thus have the close-packed plane in c o m m o n and are fully coherent across this interface. The crystallographic relationship between the two phases is { 111 ) fcc//{001 ) H C P and (110) f c c / / ( 1 1 0 ) HCP. The transformation is reversible with a hysteresis effect on the transformation temperature, and it is sluggish in nature due to a low driving force. When martensite forms in steel, the chemical driving force at M, is about AG = - 1250 J/mol, but in cobalt it is much smaller: AG = - 13.8 J / m o l on cooling at Me, and AG = - 9 . 2 J / m o l on heating at A 3. The amount of fcc-Co transformed during cooling is thus strongly dependent on the purity of the starting material. Characteristic variations occurred in the matrix constitution of the Co-Ti-C alloys because of this martensitic transformation, completed to various extents during quenching from the
ISSN 0360-2133/80/0711-1167500.75/0 METALLURGICAL TRANSACTIONS A 9
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