Microstructural Evolution of Nanocrystalline ZrO 2 in a Fe Matrix During High-Temperature Exposure
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INTRODUCTION
UNDERSTANDING the microstructural evolution of a material with time and temperature is an important aspect of materials development since it governs its behavior under service conditions. Several microstructural aspects such as the grain size and its distribution, the stability of precipitates/dispersoids and dislocation structure of a material evolve with time and temperature. The study of such evolution is particularly important when the material is in the nanocrystalline (NC) state, owing to its large grain boundary fraction resulting in higher surface energy compared with its bulk counterparts. Owing to this, the NC materials exist in a ‘far-from-equilibrium’ configuration[1,2] that is always associated with an intrinsic instability, due to which there is a thermodynamically driven force that always tends to take them towards equilibrium. This force becomes much more active when the kinetics
K.G. RAGHAVENDRA, ARUP DASGUPTA, and S. SAROJA are with the Materials Characterization Group, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam 603102, India. Contact e-mail: [email protected] C.N. ATHREYA and V. SUBRAMANYA SARMA are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Madras, Chennai 600036, India. K. JAYASANKAR is with the Advanced Materials Technology Department, CSIR Institute of Minerals & Materials Technology, Bhubaneswar 751013, India. Manuscript submitted September 8, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
favors it, which is why the NC materials lose their significance at elevated temperature operations resulting in grain growth of the materials.[3,4] Continuous efforts are in place to overcome this issue through thermodynamic and kinetic stabilization approaches. The former approach is based on eliminating the driving force by suitable alloy additions that can produce a metastable equilibrium state in the nano-domain. The latter technique is based on reducing the grain growth mobility by pinning and/or drag mechanisms.[5–8] Oxide Dispersion Strengthened (ODS) alloys can be classified as nanostructured materials because they derive their superior properties from the uniformly dispersed ultrafine nanometric oxide such as Y2O3, MgO, La2O3 or ZrO2.[9–14] These alloys have gained widespread attention owing to their superior mechanical properties in hostile environments, which make them attractive candidate material for applications in high-temperature turbine blades, heat exchanger tubing and nuclear core structural materials.[15,16] Though Y2O3 is a widely used dispersoid, use of ZrO2 has recently gained interest.[14] A recent investigation reported that the mechanical properties of ZrO2 dispersed ODS are attractive.[17] In addition, Zr addition to ODS steel to improve the thermal stability of Y2O3 dispersoids is also attracting attention.[18,19] Fe-ZrO2 nanocomposite has attracted attention not only for its structural applications, but also owing to its interesting magnetic properties such as enhanc
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