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I.

INTRODUCTION

IN the development of microalloying technology, much attention has been paid to the investigation of the combined effect of several microalloying elements forming a multicomponent precipitate. Thermodynamic structures for the multicomponent phase diagram Fe-EMi-EX~ of dilute iron alloys containing any number of solutes have been developed by a number of authors for quite general cases [1-6] (here, M is the substitutional transition metal Ti, Nb, and V; X is the interstitial element C and N). Many experimental studies on complex carbonitride precipitation during various thermomechanical treatments t3-1~ have been carded out in the past decade. It has been reported tT] that in (TixNbl-x) ( C y N I - y ) precipitates, internal composition gradients exist and specific patterns of composition are correlated with different morphologies and alloy compositions. However, theoretical understanding of the kinetic process for this kind of compound precipitates is limited. While multicomponent diffusion theory was developed in the late 1940s and the 1 9 5 0 S , [1Ll2'13] very few applications to more than three components have been reported. This is due, in part, to the complications of the theoretical treatment and the difficulty of finding the "ideal" practical case to which the theory can be directly applied. However, we will demonstrate that subject to some reasonable simplifications and machine computation, the multicomponent diffusion theory may be applied to the kinetics of complex carbonitride precipitation in microalloyed steels.. The objectives of this study are to obtain complete experimental kinetic information about the multicomponent carbonitride precipitates in austenite alloyed with titanium and niobium and comparative theoretical predictions with the inclusion of both thermodynamic and HEILONG Z O U and J.S. K I R K A L D Y are with the Department of Materials Science and Engineering and the Institute for Materials Research, respectively, McMaster University, Hamilton, ON, Canada L8S 4M1. Manuscript submitted June 6, 1990. METALLURGICAL TRANSACTIONS A

kinetic effects. In the present study, a model has been developed to describe the kinetic processes of precipitation pertaining to carbonitride in the initial growth stages (before coarsening). The present experimental work has been carded out on the characteristics and the growth kinetics of (TixNbl_x) (CyNl-y) precipitates via transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). II.

EXPERIMENTAL PROCEDURES

The general experimental procedures are described elsewhere, t141 We mention here only some modifications. The chemical compositions of the alloys studied are given in Table I. The encapsulated specimens were given a solution treatment at 1390 ~ for 2 hours and quenched rapidly in agitated iced brine. They were then reencapsulated and up-quenched to 1000 ~ or 1100 ~ and held for various times. However, for aging times less than 10 minutes, specimens ranging from 0.5 to 2 mm in thickness were up-quenc