The Effect of Milling Time on the Microstructural Characteristics and Strengthening Mechanisms of NiMo-SiC Alloys Prepared via Powder Metallurgy.

Chao Yang,Maxim Avdeev,Gordon J Thorogood,Xingtai Zhou,Ondrej Muránsky,Hanliang Zhu,Hefei Huang

doi:10.3390/ma10040389

Abstract

A new generation of alloys, which rely on a combination of various strengthening mechanisms, has been developed for application in molten salt nuclear reactors. In the current study, a battery of dispersion and precipitation-strengthened (DPS) NiMo-based alloys containing varying amounts of SiC (0.5–2.5 wt %) were prepared from Ni-Mo-SiC powder mixture via a mechanical alloying (MA) route followed by spark plasma sintering (SPS) and rapid cooling. Neutron Powder Diffraction (NPD), Electron Back Scattering Diffraction (EBSD), and Transmission Electron Microscopy (TEM) were employed in the characterization of the microstructural properties of these in-house prepared NiMo-SiC DPS alloys. The study showed that uniformly-dispersed SiC particles provide dispersion strengthening, the precipitation of nano-scale Ni3Si particles provides precipitation strengthening, and the solid-solution of Mo in the Ni matrix provides solid-solution strengthening. It was further shown that the milling time has significant effects on the microstructural characteristics of these alloys. Increased milling time seems to limit the grain growth of the NiMo matrix by producing well-dispersed Mo2C particles during sintering. The amount of grain boundaries greatly increases the Hall–Petch strengthening, resulting in significantly higher strength in the case of 48-h-milled NiMo-SiC DPS alloys compared with the 8-h-milled alloys. However, it was also shown that the total elongation is considerably reduced in the 48-h-milled NiMo-SiC DPS alloy due to high porosity. The porosity is a result of cold welding of the powder mixture during the extended milling process.

Highlights

Molten Salt Reactors (MSRs) have been selected as a candidate for Generation IV nuclear reactors, due to their inherent safety, economical efficiency, and online refuelling capabilities, as well as their capacity to produce hydrogen from seawater using the waste heat of the reactor [1,2]
The structural materials in the primary molten salt loop will be subjected to extreme operating environments, such as high temperatures, strong neutron irradiation and severe corrosion from molten salt coolants [3]
Its disadvantages in high-temperature strength and irradiation resistance limit the development of fully commercial MSRs [5]

Summary

Introduction

Molten Salt Reactors (MSRs) have been selected as a candidate for Generation IV nuclear reactors, due to their inherent safety, economical efficiency, and online refuelling capabilities, as well as their capacity to produce hydrogen from seawater using the waste heat of the reactor [1,2]. Hastelloy-N, developed at Oak Ridge National Laboratory (ORNL), is generally accepted as the most promising candidate material in many MSR designs due to its superior corrosion. Materials 2017, 10, 389 resistance in molten salt at high temperatures [4]. Its disadvantages in high-temperature strength and irradiation resistance limit the development of fully commercial MSRs [5]. In our initial study [6,7], a SiC dispersion-strengthened Ni-matrix (Ni-SiC) composite was successfully prepared via a powder metallurgy (PM) process. SiC particles, which possess outstanding thermal stability at temperatures up to 850 ◦ C, were homogeneously dispersed in the Ni matrix [8]

Methods

Results

Conclusion