High Strength Nanocrystallized Multilayered Structure Obtained by SMAT and Co-Rolling

Abstract

Abstract:In the present study, a method is presented combining surface nanocrystalline treatment (SMAT) and the co-rolling process. The aim of this duplex treatment is the development of a 316L stainless steel semi-massive multilayered bulk structure with improved yield and ultimate tensile strengths, while conserving an acceptable elongation to failure by optimizing the volume fraction and distribution of the nano-grains in the laminate. To characterize this composite structure, tensile tests as well as sharp nanoindentation tests were carried out to follow the local hardness evolution through the cross-section of the laminate. Furthermore, transmission electron microscope (TEM) observations were carried out to determine the correlation between the microstructure, the local hardness and the mechanical response of the structure.

Effect of interfacial oxidation occurring during the duplex process combining surface nanocrystallisation and co-rolling

“…Twins observed in Fig. 5 c) are the result of the SMAT and are considered to be at the origin of a grain refinement in materials with low stacking fault energies (SFE), as discussed in [1,3,5]. These twins were eliminated by the thermomechanical treatment of laminate T SMAT-650 , and only a low density of annealing twins is visible.…”

“…In accordance with the well known Hall-Petch relationship, the high strength of the co-rolled laminate T SMAT-550 can be explained by an increased volume fraction of nano-and ultrafine grains. As shown in several publications, nanocrystalline layers can sustain an important part of the load and greatly increase the mechanical strength of the material [3,5]. Therefore, raising the volume fraction of nano-grain boundaries in the material leads to a strength increase of the whole structure.…”

“…The recently developed SMAT technique can induce grain refinement down to the nanometer scale in the top surface layer of metals and alloys such as iron, titanium, copper or stainless steel [1][2][3][4]. Unfortunately the affected depth remains limited (≈ 250 µm for 316L stainless steel), which leads to a rapid saturation of the yield strength of SMATed bulk materials [5]. To overcome this problem, a novel process has been developed to increase the volume fraction of nano-grains.…”

“…This duplex process combines the SMAT with a subsequent co-rolling process in order to produce multilayered bulk structures with improved yield and ultimate tensile strengths, while conserving an acceptable elongation to failure. The duplex process is described in detail in [5,6]. However, during the heat treatment before co-rolling, the impurities deposited on the surface of SMATed samples cause bonding imperfections due to interfacial oxidation during the process.…”

“…In the present work, nanocristallised 316L stainless steel multilayer structures were produced by the duplex process as described in [5,6]. Two different rolling temperatures, 550°C and 650°C, were imposed in order to quantify the effect of the heat treatment on the interface quality.…”