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Many engineering materials call for increasing strength and ductility. Unfortunately, the material properties of high ductility and high strength are usually mutually contradictory. These conflicting requirements have generated interest in transformation induced plasticity or TRIP steels, which mainly consist of a mixture bainite, ferrite and retained austenite. The superior strength and ductility of these types of steels is due to the strain induced transformation of retained austenite to martensite. Intercritical deformation (i.e. deformation in the austenite + ferrite two phase region) has been extensively studied on C-Mn and microalloyed steels, but not on TRIP steels. Intercritical rolling increases the volume fraction and decreases the grain size of ferrite. These characteristics may be beneficial to the ductility of TRIP steels, since the volume fraction of retained austenite may increase with ferrite volume fraction by increasing the level of C segregation to the untransformed austenite. As well, the size of the retained austenite may decrease with decreasing ferrite grain size, thus increasing the stability of retained austenite against strain induced transformation. The aim of this study was to investigate the effect of the intercritical deformation on the characteristics of retained austenite and resulting mechanical properties of a TRIP steel.

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Thesis (M.S. - Hydrology and Water Resources)--University of Arizona, 1983. / Includes bibliographical references (leaves 98-99).

This work was undertaken to study the nature of the deformation mechanisms in polycrystalline zinc and cadmium over a temperature range from 77°K to 300°K. It has been observed that the only non basal slip system which is observed under normal light microscopy is that of second order pyramidal [ll22] <ll23>. At temperature above [formula omitted], the amount of non basal slip is greater in zinc than in cadmium. The amount of twinning, substructure formation and grain boundary migration is comparable in both systems. Negative work hardening beyond the U.T.S. at temperatures above Tн = .4 is associated with recrystallization. In both systems at temperatures below Tн = .26 a region of temperature and strain rate independent linear work hardening occurs. The extent of linear hardening increases with decreasing temperature below Tн = .26. Above Tн = .26, polycrystalline hardening in both systems is parabolic from yield on and the rate of hardening at a given value of strain decreases with increasing temperature. Cadmium single crystals showed a similar trend in that below .26 both [formula omitted] remained constant. However above .26 there was a steady decrease in the shear hardening rates. It was observed that the Cottrell-Stokes law is obeyed only in the linear hardening regions of polycrystals and in Stage II hardening of single crystals below .26. When dynamic recovery occurs [formula omitted] increases with increasing strain. It has been observed that below .26 the linear hardening rate in cadmium decreased with increasing grain size ( constant specimen dimensions) so that [formula omitted] The value of [formula omitted] was shown to correspond to the tensile hardening rate during Stage II single crystal deformation. The tensile hardening rate was used because of the extensive twinning found to be associated with Stage II hardening. The grain size dependence of 0 has been interpreted in terms of a grain size dependence of the extent of [ll22] <1123> slip. It was found that during linear hardening in both zinc and cadmium the difference in flow stress at two different temperatures is a reversible difference implying that the dislocation configurations produced with increasing strain do not vary in nature or extent with temperature. Under such conditions it is possible to formulate a mechanical equation of state. Extensive rate theory measurements have been made in both systems in order to attempt an evaluation of the rate controlling mechanisms both during linear hardening and.during dynamic recovery. The former has tentatively been associated with intersection. Dynamic recovery on the other hand has been linked to the loop annealing observations of Price. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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