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Nanostructured materials are potential candidates for future structural materials in advanced nuclear reactors. La-doped nanocrystalline 304 austenitic stainless steel (NC304-La) is an advanced nanostructured steel, in which excellent small grain size of ~45 nm is achieved and stabilized by the doped La element. We carried out irradiation using 3.3 MeV Fe ions to 108 dpa at room temperature and 500 °C, and then characterized the microstructural change in NC304-La by transmission electron microscopy, scanning transmission electron microscopy and atom probe tomography. We studied the detailed microstructural evolution and elemental behaviors in irradiated NC304-La, focusing on the effects of La element on the stability of nanograins and nanoparticles in NC304-La.

It is critical to obtain nanocrystalline materials in large chunks rather than only in thin film or rod shapes and nanostructured bainitic steels are good examples of bulk nano materials. This article aims to investigate the hardenability of nanobain steel to estimate the biggest cross section that can be successfully austempered to form nanocrystalline bainite. Isothermal heat treatments were conducted at different temperatures using salt bath furnaces, the quench severity factors of which were determined at bainitic transformation temperature ranges. It was found that, the proper chemical composition selection increased the hardenability and shifted the upper C-curve of the Time–Transformation–Temperature diagram to longer heat treatment times. Accordingly, large chunks of nanostructured bainite with almost 330, 340 and 390 mm diameters could be austempered to obtain nanobain successfully in salt bath furnaces of 300, 250 and 210 °C, respectively. The nanocrystalline bainite was composed of nanoscale bainitic subunits of 30–70 nm thicknesses depending on transformation temperature. [ABSTRACT FROM AUTHOR]

Abstract: Nanocrystalline steel was developed by severe cold-rolling and subsequent annealing of low carbon steel with an original microstructure of lath martensite. X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicated that steel with an average grain size of −20 to −300nm can be obtained. The tensile tests revealed that the steel with grain size of −20nm possess a strength of 2520MPa and poor ductility at ambient temperature. [Copyright &y& Elsevier]

In this study, microstructural evolution and mechanical properties of Fe–Mn–Al–C steel were investigated with the variation of shear strain imposed by high-pressure torsion (HPT). Two different initial grain sized steels were used: (1) fine grained (FG: ≈10 μm) and (2) coarse grained (CG: ≈100 μm) steels, and the amount of strain was varied by increasing the number of revolution (R) of HPT. At the maximum R (10R) of HPT, FG and CG microstructures were refined to the average grain size of 20.7 ± 5.2 nm and 51.0 ± 11.0 nm, respectively. With increasing R, ultimate tensile strength (UTS) of both FG and CG steels increased, and however, FG 10R showed lower UTS (Δσ ≈ 350 MPa) than that of FG 5R, indicating a softening phenomenon of inverse Hall-Petch (IH–P) relation. TEM observation of the finest grained FG 10R revealed the absence of deformation twins and the formation of numerous tilt/twist nanocrystalline boundaries, which might explain the softening behavior in this regime. [ABSTRACT FROM AUTHOR]

Helium accumulation in advanced nuclear systems would accelerate the failure of structural materials. Nanostructured materials have the potential to mitigate helium effects by providing more nucleation sites for He bubbles. La-doped nanocrystalline 304 austenitic stainless steel (NC304-La) possesses ultra-fine and stable nano-grains ~45 nm in diameter and high-density fine nano-precipitates. In this work, we implanted NC304-La with helium ions, then annealed at different temperatures after implantation. We found that combining the key features of nanocrystalline steels and oxide dispersion-strengthened steels, NC304-La showed excellent bubble swelling resistance. We characterized the interaction between He bubbles and the two features in NC304-La: grain boundaries and nano-precipitates. Through detailed study for the heat-treatment related behaviors of He bubbles in this multi-sink system, the mechanisms of He bubble coarsening resistance in NC304-La were proposed. [Display omitted] [ABSTRACT FROM AUTHOR]

We fabricate ultrastrong gradient nanocrystalline (NC) 316L stainless steel with an extremely fined grain size of 5 nm using the Ultrasonic Strain Engineering Technology (USET) at ambient temperature. We evaluate the strain rate sensitivity (SRS), an indicator of the rate controlling mechanism in the plastic deformation of metals and alloys, for the ultrastrong gradient NC 316L stainless steel by strain rate jump nanoindentation and micropillar compression test. The significant decrease of the SRS for the NC 316L stainless steel with a grain size of 5 nm is attributed to the phase transformation from the austenite face center cubic (FCC) to the martensite body center cubic (BCC) structures during the grain refinement process. In addition, the grain boundary activities in NC 316L stainless steel is largely suppressed by the embodied nanosized intermetallic phases and metal precipitates. We propose an in-situ grain structure stability mechanism to achieve such extremely fined NC stainless steel via USET at ambient temperature. We further adopt and validate a rate-dependent constitutive model for the NC 316L stainless steel. The work provides an efficient way to produce ultrastrong gradient NC 316L stainless steel that can be widely used in the light-weighting and mechanical strengthening of aerospace and transportation engineering. • Fabricated the extremely fined nanocrystalline steel with average grain size of 5 nm. • Identified the ultrastrong nanocrystalline steel with yield strength of 2.1 GPa. • Quantified the strain rate sensitivity of the gradient nanocrystalline steel. • Established the rate constitutive modeling of the gradient nanocrystalline steel. [ABSTRACT FROM AUTHOR]