Formation of nanosized martensite particles in stainless steels
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I. INTRODUCTION
MARTENSITIC transformations are diffusionless, and they occur during quenching of metals and alloys. Austenitic steels (face-centered cubic (fcc) structure) transform, in part, into ␣ martensite, which has a body-centered cubic (bcc) or body-centered tetragonal (bct) structure.[1,2] A great deal of research has been performed on the crystalline structure of martensite,[1,2] the crystallographic correspondence of martensite and its parent phase,[3] and the effects of martensite on mechanical properties[4] in ferrous materials. As the transformation proceeds in an autocatalytic way, the propagation velocity of the martensitic transformation is thought to be very fast in steels, although only a few attempts have been made to measure the velocity of the martensitic transformation. (The martensitic transformation occurs at a slow rate in some alloys such as In-Tl alloys; however, it normally occurs at a very high speed in most of the metals and alloys, including steels.) Bunshah and Mehl[5] reported a velocity of 1000 m/s in an Fe-Ni alloy by means of electrical resistivity measurements. Takashima et al.[6] reported that the velocity was in a range between 110 and 200 m/s in a stainless steel, measured using an acoustic emission (AE) technique. Thus, it was believed to be too difficult to control the size of small martensite particles in steels. (It is possible, however, to control the percentages of martensite formed in the parent phase on the order of 50 pct, 90 pct, and so on. In this article, we discuss much smaller martensite particles, the size of which are of the order of nanometers.) Recently, Borgenstam, et al.[7] measured Mg-Ms , where Mg and Ms are the temperatures below which martensite can grow if it has already nucleated and below which the formation of martensite starts upon cooling, respectively. They showed that the value was considered to be less than or equal to 8 K. This could suggest that the martensite M. SHIMOJO, Research Associate, K. TAKASHIMA, Associate Professor, and Y. HIGO, Professor, are with the Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, 226-8503, Japan. T. INAMURA, Graduate School Student, is with the Tokyo Institute of Technology, Yokohama, 226-8503, Japan. T.H. MYEONG, formerly Graduate School Student, Tokyo Institute of Technology, is Master with the R & D Division, Absoltech Co., Chun-Buk, 570-749, Korea. Manuscript submitted August 3, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
nucleus can be formed by controlling the temperature precisely. Thus, the aim of this article is to show that the martensite nucleus is formed and observed by a control of the temperature. II. EXPERIMENTAL PROCEDURE The materials employed in this study were types 304 and 316 stainless steel rods, which had been hot extruded. The chemical compositions of these materials are shown in Tables I and II, respectively, which were measured using emission spectrochemical analysis. In our previous article,[8] we described the formation of submicron-sized marten
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