• PDF / 3,042,351 Bytes
• 10 Pages / 590.28 x 785 pts Page_size
• 84 Downloads / 232 Views

DOWNLOAD

REPORT

---

I. I N T R O D U C T I O N

EVER since the pioneering works of Ham and Cairns t~j and of Schmatz, t2] Fe-Mn-A1-C austenitic alloys, being attracted as strategic element-free potential substitutes for the traditional Fe-Ni-Cr austenitic stainless steels, t3'4] have been touted by several investigators t5'6'71 as a new class of structural steels possessing the age-hardening property. Age hardening in Fe-Mn-AI-C austenitic alloys was previously reported by James [5] and Kayak E6}to occur in the course of isomorphous precipitation of K carbide of (Fe,Mn)3A1Cx chemical formula with ordered L'I 2 crystal structure (filled-AuCu3) from the supersaturated austenitic phase. Following these reports, an in-depth aging investigation was carried out by Krivonogov et al. Is] They studied conventionally prepared Fe-28 wt pct Mn9 wt pct A1-0.9 wt pct C alloy by means of magnetic measurements, hardness tests, transmission electron microscopy (TEM), and X-ray diffractometry. In that work, they reported the observation of a modulated structure that formed in the initial decomposition stage during aging at 723 to 923 K. The modulation was in the (100)v direction. They also examined the overall sequence of the decomposition process, but, unfortunately, the fine details of interpretation of their experimental results were lacking. For this reason, Storchak and Drachinskaya [91 subsequently re-examined the structural changes of the same alloy during aging at 723 to 923 K by rotating crystal X-ray diffraction of a single crystal specimen, in which K W A N 11. H A N is Assistant Professor, Department of Metallurgical Engineering, Y e u n g n a m University, G y o n g s a n , K y o n g b u k , Korea. W O O N G K. C H O O is Professor, Department of Materials Science and Engineering, Korea Advanced Institute o f Science and Technology, P.O. Box 131, Seoul, Korea. Manuscript submitted May 6, 1985. METALLURGICAL TRANSACTIONS A

they observed the sideband phenomenon in the initial stage of aging, supporting previous Krivonogov et a l . ' s TEM observation of the {100} modulated structure. Storchak and Drachinskaya attributed the sideband phenomenon to the modulated structure consisting of carbon-rich and carbon-poor zones. In the next stage, a coherent metastable phase was observed to replace the modulated structure.* But the equilibrium K carbide precipitation *There has been some disagreement in designation of the coherent metastable phase: Krivonogov e t a l . {8] used notation K0 or sometimes ordered K in designating all the coherent precipitates, regardless of the solute-rich zone and the metastable phase. Storchak and Drachinskaya tg} later denoted it as coherent metastable K phase. In order to avoid some confusion, a unified designation seems necessary. We hereby propose a designation, ~', for the coherent metastable p h a ~ which forms prior to precipitation of K carbide.

was observed to occur via heterogeneous reaction at temperatures above 923 K, and brittle fl-Mn phase also precipitated after prolonged aging. {s'9] Storchak and Drach