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Big Picture

Nano/sub-micron grain structures in metastable austenitic stainless steels. Big Picture. Paulo J. Ferreira, University of Texas at Austin, DMR 0355234.

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Big Picture

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  1. Nano/sub-micron grain structures in metastable austenitic stainless steels Big Picture Paulo J. Ferreira, University of Texas at Austin, DMR 0355234 The roadmap for the next twenty-five years of the stainless steel automotive industry, as laid out by the European Steel Technology Platform (shown as the grey band) is to develop Fe-based alloys that have exceptional strength and ductility. With these novel materials, engineers will be able to design complex structures that can sustain natural environments (such as humid air and corrosive media), be durable and light-weight. In this regard, Stainless Steels (SS) are good candidates due to their excellent corrosion resistance properties, but they lack the combination of high strength and ductility. As we uncover exciting potential applications for SS, an important question arises: Is it possible to develop commercial SS with high strength and high ductility? This is the focus of this work. 12

  2. Nano/sub-micron Stainless Steels Paulo J. Ferreira, University of Texas at Austin, DMR 0355234 1 second 10 seconds 100 seconds Transmission Electron Microscope images of samples annealed at 800C, 900C and 1000C for various annealing times are shown in the adjacent figure. The microstructure of the sample annealed for 1 second at lower annealing temperatures of 800C and 900C show a mixture of nano/sub-micron grains and large grains. At longer annealing durations of 10 and 100 seconds, equiaxed austenite grains are observed for both annealing temperatures. Similar equiaxed grain morphology is seen in samples annealed at the highest temperature of 1000C. Additionally, rapid grain growth is observed for all these annealing temperatures. Furthermore, the microstructure of the 800C, 900C annealed samples taken from different regions reveal the presence of secondary phase precipitates with sizes around 10-20 nm. These precipitates were identified by nano-beam diffraction technique o be chromium nitride (CrN) precipitates. In samples annealed at 1000C, there is a marked reduction in the number of secondary phase precipitates in these samples, as CrN dissolves at higher temperatures. 800ºC 900ºC 1000ºC 14

  3. Nano/sub-micron Stainless Steels with High Strength and Ductility Paulo J. Ferreira, University of Texas at Austin, DMR 0355234 Yield Strength The yield strength for samples annealed at different conditions is shown in the adjacent figure. Samples annealed at 600C show an exceptional strength of ~ 1.5-1.8GPa. However, it should be kept in mind that these samples are primarily comprised of martensite phase, for which a high tensile strength is expected. Samples annealed at higher temperatures contain a larger fraction of austenite, and the yield strengths of samples annealed at 700°-1000° C are reduced. Nonetheless, these samples still exhibit yield strengths that are at least 50% higher when compared with conventional processed AISI 301LN SS (~ 350MPa). Ductility The uniform elongation properties of samples annealed under various annealing conditions is shown in the adjacent figure. The samples annealed at 600C exhibit poor ductility behavior. This is due to the presence of the martensitic phase, which is predominant in these samples. When the samples are annealed at higher temperatures, the extent of uniform elongation depends on the percentage of austenite present in the sample. Such behavior is clearly seen in samples annealed at 700C where the percent elongation changes from ~ 10% to ~ 30% when the annealing duration is increased from 1 to 100 seconds (because austenite content changes from ~ 35% to ~ 95%). Following this argument, the uniform elongation of samples annealed at 800C, 900C and 1000C are similar and also comparable to those of conventional processed AISI 301LN SS (these samples have ~ 95% austenite). . 13

  4. Paulo J. Ferreira, University of Texas at Austin, DMR 0355234 Summary Big Picture - Results In a time-span of four years, the results achieved through the collaborative work between Outokumpu Stainless Oy, Tornio-Finland, the University of Oulu, Oulu-Finland, and the University of Texas, Austin-USA are a significant contribution to the stainless steel automotive industry’s twenty five year vision for developing advanced commercial steels with high strength, ductility. Internships This collaboration facilitated two graduate student from the University of Texas – Austin, USA to take part in two internships at (1) Outokumpu Stainless Oy, Tornio-Finland, and (2) the University of Oulu, Oulu-Finland. Additionally, a graduate student from the University of Oulu, Oulu-Finland interned at the University of Texas – Austin, USA as well. Education The graduate students involved in this work gained a large experience in alloy development, viz. – processing, materials characterization, modeling and analysis. 15

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