Analysis of a Rapidly Solidified High-Phosphorus Austenitic Steel Containing an Amorphous Phase
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B.H.
Kear,
B.C.
Giessen,
and M. Cohen,
343
editors
ANALYSIS OF A RAPIDLY SOLIDIFIED HIGH-PHOSPHORUS AUSTENITIC STEEL CONTAINING AN AMORPHOUS PHASE T. F. KELLY, G. B. OLSON, AND J. B. VANDER SANDE MIT, Cambridge, MA ABSTRACT Rapid solidification of a high-phosphorus austenitic steel produces a fine cellular solidification structure containing an amorphous phase at the cell walls. The amorphous phase, which is stable to \,500* C, is enriched in phosphorus and chromium, but contains significantly less phosphorus than conventional glass-forming alloys. Hot consolidation of powders produces a chemically-uniform metastable austenite which can be effectively precipitation hardened by phospho-carbides. INTRODUCTION
The combined effects of chemical uniformity, extended solid solutility, and refinement of inclusions permit a greater tolerance for impurities in rapidly solidified materials. Phosphorus, which is a well-known embrittling impurity in high-strength steels, has long been beneficially employed in ferrous powder metallurgy. Phosphorus has also been found to provide effective strengthening of austenitic stainless steels via (M,P) 2 3 C6 phospho-carbide precipitation, where normal carbide precipitation is too heterogeneous to allow significant strengthening [1]. This strengthening mechanism may be of particular value for the high-strength metastable austenitic steels known as TRIP steels [2], which achieve high ductility and toughness through a deformation-induced martensitic transformation, but which normally require a difficult thermomechanical treatment to achieve their high strengths. While a precipitation-hardenable TRIP steel would be a desirable alternative, the choice of hardening phases is limited by the requirement for a moderate alloy carbon content (ýx'.3 wt.%C) to ensure a high work hardening effect from the deformation-induced martensite. A preliminary study of phospho-carbide strengthened TRIP steel achieved a good combination of austenite strength and transformation behavior, but fracture ductility and fabricability were limited by the severe phosphorus segregation associated with conventional ingot metallurgy [3]. In the first phase of an investigation of the effect of rapid solidification processing on this alloy, some unusual solidification structures were observed, which we report here. MATERIALS AND PROCESSES Rapidly solidified powders of an alloy of the composition in Table I were produced by the Pratt and Whitney centrifugal atomization process, After sizing of the powders, electron-transparent specimens were prepared from composite TABLE I
Alloy Composition wt.% at.%
Fe 72.7 71.3
Cr 16.1 17.0
Ni 10.3 9.6
Si 0.15 0.29
P 0.42 0.74
C 0.30 1.37
344 foils of powder in electrodeposited nickel by a combination of Jet electropolishing and ion-beam milling. Specimens were observed in a Vacuum Generators HB-5 scanning transmission electron microscope (STEM) permitting fine-scale x-ray fluorescence microanalysis. SOLIDIFICATION STRUCTURES As observed in a parallel study of a high-sulfur a
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