The eutectoid decomposition of Ti-X alloys.

Eutectoid decomposition in steels can result in either a lamellar (pearlite) or non-lamellar (bainite) microconstituent. Similarly, in Ti-X alloys, both a lamellar and non-lamellar microconstituent have been observed following eutectoid decomposition. It has been suggested that the lamellar and non-lamellar microconstituents observed in Ti-X alloys are synonymous with pearlite and bainite in steels. This thesis presents a thorough review of all relevant literature on eutectoid decomposition in Ti-X alloys as well as pearlite and bainite in steels. A programme of experimental work has been conducted covering nine Ti-X alloys; a hypoeutectoid, eutectoid and hypereutectoid alloy of each Ti-Co, Ti-Cr and Ti-Mn. Various heat treatments were completed including isothermal, slow cooling and thermomechanical treatments. Only eutectoid and hypereutectoid Ti-Co contained the lamellar microconstituent. All remaining alloys contained a non-lamellar microconstituent. The lamellar microconstituent formed from both plates of alpha(Ti) and allotriomorphs of Ti2Co. Heat treatment at progressively higher undercooling resulted in the lamellar microconstituent being increasingly replaced by a spheroidal microconstituent. The non-lamellar microconstituent consisted of plates of alpha(Ti) separated by beta(Ti) with the intermetallic phase at alpha(Ti); beta(Ti) boundaries. Once the reaction had reached completion no beta(Ti) remained. Definitions for pearlite and bainite have been presented based upon well documented experimental evidence. The Ti-X lamellar and non-lamellar microconstituents have been shown to be synonymous with pearlite and bainite in steels based upon these definitions. Pearlite and bainite have also been shown to form by distinct mechanisms and it is therefore incorrect for them to be considered as products which are in any sense opposites to each other. A new hypothesis has been proposed suggesting that it is not pearlite and bainite that are competing processes in Ti-X alloys, but pearlite and the formation of alpha(Ti) plates. This can be extended to decomposition in any eutectoid system that might be viewed as the competition between pearlite and plate formation