Synthesis of submicrometer-grained-ultrahigh-carbon steel containing 10% aluminum by ball-milling of powders
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Synthesis of submicrometer-grained-ultrahigh-carbon steel containing 10% aluminum by ball-milling of powders Eric M. Taleff Department of Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin, Austin, Texas 78712-1085
Mamoru Nagao and Yoshio Ashida Materials Design Section, Materials Research Laboratory, Kobe Steel, Ltd., 1-5-5 Takatukadai, Nishi-ku, Hyogo, Japan
Oleg D. Sherby Department of Materials Science and Engineering, Stanford University, Stanford, California 94305-2205 (Received 11 December 1995; accepted 20 May 1996)
An ultrahigh-carbon (1.25 wt. %) steel alloy containing 10 wt. % aluminum (UHCS–10Al) was processed by a powder metallurgy technique. Gas-atomized powders were subjected to ball-milling in an attritor in order to obtain a submicrometer grain size. Powder material was consolidated by both hot isostatic pressing (HIP) and by hot isopressure extrusion (HIE). Bulk material with submicrometer grain sizes was produced from attrited powders. The chemical composition and microstructure of this material are characterized at each processing step, from atomization through consolidation. Tensile tests show that a high strength results from the submicrometer grain size produced in the bulk material.
I. INTRODUCTION
Ball-mill attritor processing of materials has received attention in recent years as a means for the mechanical alloying of elemental powders and as a means for creating very fine particulate dispersions in dispersion-strengthened alloys.1–6 An exciting result of these studies is the extremely fine grain sizes produced in powders of many different materials upon recrystallization, often on the order of nanometers.5–7 There is, however, some difficulty in retaining such fine grain sizes after consolidation of processed powders into bulk material. This difficulty generally arises from the elevated temperatures and pressures necessary to obtain high density bulk material from powders. Grain coarsening during powder consolidation often leads to much larger grain sizes in the resulting bulk material than in the original powders. This problem can be addressed to a certain extent by utilizing particular two-phase material systems. Ball-mill attrition can produce very fine grain sizes in two-phase alloy powders by introducing a high degree of deformation, which creates grain sizes on the order of hundreds of nanometers. In using a two-phase alloy, grain growth during consolidation can be minimized when the secondary phase helps to prevent grain growth of the primary phase. The effects of second-phase particles in maintaining a fine, stable grain size during warm deformation are well known in the field of superplastic alloys.8 Examples of such materials include many alloys of ultrahigh-carbon steel (UHCS). In superplastic ultrahigh-carbon steel alloys fine ferritic grain sizes are stabilized during superplastic deformation J. Mater. Res., Vol. 11, No. 11, Nov 1996
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