Microstructures and Stabilization Mechanisms of Nanocrystalline Iron-Chromium Alloys with Hafnium Addition
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INTRODUCTION
NANOCRYSTALLINE materials, with grain sizes less than 100 nm, have attracted extensive attention due to their unique physical, chemical, electrical, and mechanical properties.[1–6] These enhanced properties are intrinsically related to large grain boundary area per unit volume in nanocrystalline materials. Nevertheless, a large grain boundary area per unit volume makes nanocrystalline materials thermally unstable and susceptible to grain coarsening to reduce the total grain boundary energy. The low thermal stability of nanocrystalline materials is a roadblock to many applications, especially at elevated temperatures. It has been reported that substantial grain growth can occur even at room temperature in nanocrystalline Cu,[7] Al,[8] and Ag.[9] The understanding of the thermal stability mechanisms in nanocrystalline materials is fundamentally needed to design thermally stable nanocrystalline materials for applications at elevated temperatures. In the last decade, significant improvements in the thermal stability of nanocrystalline materials have been reported.[10–17] Most of them are multicomponent systems, either alloys or containing second-phases and/or impurities. Mechanisms that can stabilize nanocrystalline materials include solute drag, second-phaseparticle pinning (also called Zener pinning), lowering WEIZONG XU and MOSTAFA SABER, Postdoc Scholars, LULU LI, Graduate Student, CARL C. KOCH, YUNTIAN ZHU, and RONALD O. SCATTERGOOD, Professors, are with the Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695. Contact e-mail: [email protected] Manuscript submitted October 2, 2014. Article published online June 2, 2015 4394—VOLUME 46A, SEPTEMBER 2015
grain boundary energy by segregation of solute atoms (thermodynamic stabilization), chemical ordering, and porosity.[13] At high temperatures, Zener pinning or thermodynamic stabilization is believed to play a dominant role in grain stabilization. The former is a kinetic stabilization by second-phase particles that can be described by the Zener equation.[18] This mechanism has been found to contribute to the superior stability of nanocrystalline Al and Fe-Al alloys,[15,16] in which the small size and large volume fraction of particles produce pinning. The thermodynamic stabilization mechanism, on the other hand, utilizes the grain boundary segregation of non-equilibrium solute atoms to produce a minimum in the Gibbs excess free energy of grain boundaries, and consequently suppresses grain coarsening.[19–21] This mechanism was found responsible for the high thermal stability of nanocrystalline Ti-W or Al-Pb alloys, in which no precipitation of second-phase particle was observed.[19,20] As a matter of fact, in many alloy systems such as Fe-Zr,[11,12] Fe-Cr-Zr,[10,22] NiMn,[23] Co-P,[24] or Ni-P,[25,26] both second-phase precipitation and grain boundary segregation could occur and compete for solute atoms from the matrix. In a recent work, our group reported that Hf could help stabilize nanocrystalline Fe-Cr a
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