Sinter-hardening with concurrent improved plasticity in iron alloys induced by spark plasma sintering
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We report on the occurrence of sinter-hardening with concurrent improved plasticity in fine-grained Fe79.3Mo4.5P8.1C6.75B1.35 bulk alloys fabricated by spark plasma sintering (SPS) of metallic glass composite powder. When the sintering temperature is higher than the austenite transformation temperature, the as-fabricated bulk alloys are composed of expected wattle martensite plus Fe3P, Fe7C3, and Fe3Mo3C. Meanwhile, the martensite-containing bulk alloys exhibit increased hardness, fracture strength as well as concurrent improved plasticity. The fracture stress and strain of the martensite-containing bulk alloys are as high as 2573 MPa and 8.6%, respectively. The formation of the martensite microstructure is attributed to that high sintering temperature leads to the austenitization transformation and consequently formed austenite partially transforms into martensite under rapid cooling rate provided by SPS system. The results obtained provide insight into fabrication of iron alloys with good mechanical property by powder metallurgy.
I. INTRODUCTION
Iron and its alloys are characterized by low cost, excellent mechanical properties, and large-scale production, and have been the most widely used structural materials. However, there is still a great demand for improving their mechanical properties. Using an appropriate method to fabricate bulk iron alloys with new structure and good combined mechanical properties has become an important research subject. As we all know, grain refinement is one of the most methods to improve mechanical properties. According to Hall–Patch relation, the strength of materials can be increased gradually by decreasing coarse grain to fine grain, ultrafine grain, and nanograin range.1,2 Also, fabrication of different microstructure morphologies and distributions, such as bimodalgrained or multimodal-grained structure3–5 and composite structure,6–9 is an effective route to improve mechanical properties. In addition, improving mechanical properties can be achieved by obtaining some special phases. As a cost-effective route producing sintered parts, sinterhardening can significantly improve mechanical properties by obtaining martensite microstructure resulted from rapid cooling from sintering temperature without the use of a traditional austenitization, oil quench, and tempering cycle.10 In sinter-hardening, mechanical properties, or martensite content in iron alloys is mainly controlled by hardenability (carbon and alloy contents) and cooling rate.11 Meanwhile, increased martensite content induces
II. EXPERIMENTAL PROCEDURE
High purity (as-received powders had a purity of 99.9 wt.%) elemental powders of Fe, Mo, C, and B and industrial Fe–P intermediate compound powder consisting of 77.3 wt.% Fe and 21 wt.% P were used.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.77 J. Mater. Res., Vol. 29, No. 8, Apr 28, 2014
http://journals.cambridge.org
generally improved hardness, yield, and fracture strength but decreased plasticity.12,13 Therefore, whether
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