Development of Nano-oxide Dispersed Austenitic Stainless Steels by Mechanical Alloying Followed by Conventional Sintering and Spark Plasma Sintering

Abstract

Nano-oxide dispersed austenitic alloys are widely used as structural components in different applications such as heat exchanger parts and pressure vessels in thermal or fusion nuclear power plants. Austenitic stainless steels are the main candidate materials in residual heat removal circuits of pressurised water reactor applications. As nuclear applications require high-temperature strength of the candidate material, improving strength of austenitic stainless steels is a challenge. By dispersing fine oxide particles into austenitic matrix higher strength of the material could be achieved.
The present work aims at synthesis of 70.0Fe-19.0Cr-11.0Ni (alloy A), 69.0Fe-19Cr-11Ni-1.0Y2O3 (alloy B), 69.0Fe-19Cr-11Ni-1.0TiO2 (alloy C) (all in wt %) each synthesized through mechanical alloying and subsequent consolidation by conventional sintering and spark plasma sintering methods. Following this mechano-chemical synthesis and consolidation, extensive effort has been undertaken to characterize the as-milled and consolidated products by X-ray diffraction study, scanning electron microscopy, optical microscopy, energy disperse spectroscopy, followed by evaluation of physical (density and porosity), mechanical (hardness and wear resistance) and chemical (oxidation resistance) properties. The average particle size of alloy A powder decreased from 50.86 ± 6.19 µm to 3.5 ± 1.96 µm with increase in milling time from 0 h to 40 h, and the same phenomenon was observed in mechanical alloying of alloy B and alloy C powders. Samples sintered by spark plasma sintering (SPS) recorded high hardness values (476.0 HV - 724.4 HV) which are nearly 1.5 - 2.0 times the hardness values (268.0 - 464.9 HV) of same alloys consolidated by conventional sintering. Furthermore, wear resistance property of spark plasma sintered alloys, in the range of 3.17×10-7 - 15.62×10-7 mm3/mm, followed similar kind of trend as hardness. The wear rate of SPS alloy C (3.17×10-7 mm3/mm) is 1/5th the wear rate of alloy C sintered by conventional sintering (16.8×10-7 mm3/mm). The rate of oxidation of the present austenitic steel decreased with addition of nano-oxides in general and samples sintered by spark plasma sintering show the lowest rate of oxidation. Alloy C sintered by spark plasma sintering offers the maximum improvement in terms of mechanical and oxidation properties as compared to the other alloys and other sintering technique. Thus, it was concluded that mechanical alloying followed by spark plasma sintering (SPS) is the most promising route for synthesizing oxide dispersed austenitic matrix offering attractive mechanical properties.