Out-of-plane magnetic patterning on austenitic stainless steels using plasma nitriding

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.A correlation between the grain orientation and the out-of-plane magnetic properties of nitrogen-enriched polycrystalline austenitic stainless steel surface is performed. Due to the competition between the magnetocrystalline anisotropy, the exchange and dipolar interactions, and the residual stresses induced by nitriding, the resulting effective magnetic easy-axis can lay along unusual directions. It is also demonstrated that, by choosing an appropriate stainless steel texturing, arrays of ferromagnetic structures with out-of-plane magnetization, embedded in a paramagnetic matrix, can be produced by local plasma nitriding through shadow masks

On the nitrogen-induced lattice expansion of a non-stainless austenitic steel, Invar 36®, under triode plasma nitriding

Chromium, as a strong nitride-forming element, is widely regarded to be an “essential” ingredient for the formation of a nitrogen-expanded lattice in thermochemical nitrogen diffusion treatments of austenitic (stainless) steels. In this article, a proprietary “chrome-free” austenitic iron-nickel alloy, Invar® 36 (Fe-36Ni, in wt pct), is characterized after triode plasma nitriding (TPN) treatments at 400 °C to 450 °C and compared with a “stainless” austenitic counterpart RA 330® (Fe-19Cr-35Ni, in wt pct) treated under equivalent nitriding conditions. Cr does indeed appear to play a pivotal role in colossal nitrogen supersaturation (and hence anisotropic lattice expansion and superior surface hardening) of austenitic steel under low-temperature (≤ 450 °C) nitrogen diffusion. Nevertheless, this work reveals that nitrogen-induced lattice expansion occurs below the nitride-containing surface layer in Invar 36 alloy after TPN treatment, implying that Cr is not a necessity for the nitrogen-interstitial induced lattice expansion phenomenon to occur, also suggesting another type of γN

Surface evolution during low temperature plasma assisted nitriding of austenitic stainless steel

Plasma-assisted nitriding is an attractive surface treatment for metallurgical surface modification to improve wear, hardness and fatigue resistance of austenitic stainless steels. However, this technique requires low temperature processing in order to avoid chromium nitride precipitation and hence the degradation of corrosion properties. This paper presents a low temperature high rate plasma nitriding process and will emphasis on the consequences of nitrogen incorporation on the metallographic and crystallographic properties of the sample surface

High and low cycle fatigue behavior of linear friction welded Ti-6Al-4V

Linear friction welded Ti-6Al-4V was investigated in fatigue at various stress amplitudes ranging from the high cycle fatigue (HCF) to the low cycle fatigue (LCF) regime. The base material was composed of hot-rolled Ti-6Al-4V plate that presented a strong crystallographic texture. The welds were characterized in terms of microstructure using electron backscatter diffraction and hardness measurements. The microstructural gradients across the weld zone and thermomechanically affected zone of the linear friction welds are discussed in terms of the crystallographic texture, grain shape and hardness levels, relative to the parent material. The location of crack nucleation under fatigue loading was analyzed relative to the local microstructural features and hardness gradients. Though crack nucleation was not observed within the weld or thermomechanically affected zones, its occurrence within the base material in LCF appears to be affected by the welding process. In particular, by performing high resolution digital image correlation during LCF, the crack nucleation site was related to the local accumulation of plastic deformation in the vicinity of the linear friction weld.Peer reviewed: YesNRC publication: Ye

IN-PLANE and out-of-plane deformation at the SUB-GRAIN scale in polycrystalline materials assessed by confocal microscopy

International audienceHigh-resolution digital image correlation (HR-DIC) techniques have become essential in material mechanics to assess strain measurements at the scale of the elementary mechanisms responsible of the deformation in polycrystalline materials. The purpose of this study is to demonstrate the use of laser scanning confocal microscopy (LSCM) coupled with DIC techniques to deepen knowledge on the deformation process of polycrystalline nickel-based superalloy at room temperature. The LSCM technique is capable of detecting both in-plane and out-of-plane strain localization within slip bands at the sub-grain level. The LSCM observations are consistent with previous in-situ scanning electron microscopy (SEM) studies: The onset of crystal plasticity occurs primarily near Σ3 twin boundaries with bulk locations in the elastic domain (macroscopic stress as low as 80% of the 0.2 % offset yield strength (Y.S.0.2%)). This intense irreversible strain localization occurs with either a high Schmid factor (μ > 0.43) or a significant elastic modulus difference between the pair of twins (ΔΕ > 100 GPa). In the plastic deformation domain, transgranular slip activity following slip systems with the highest Schmid factor is mostly responsible for the deformation at the grain level, thus leading to strain percolation. The simultaneous in-plane and out-of-plane deformation assessment via the HR-LSCM-DIC technique was found to be essential for the identification of active slip systems. Finally, the HR-LSCM-DIC technique enabled the quantification of the real glide amplitude involved in the three-dimensional shearing process at the grain level that solely in-plane measurements cannot provide

High-Throughput High-Resolution Digital Image Correlation Measurements by Multi-Beam SEM Imaging

International audienceBackground Recent improvements in spatial resolution and measurement sensitivity for high-resolution digital image cor-relation (HR-DIC) now provide an avenue for the quantitative measurement of deformation events and capturing the physicalnature of deformation mechanisms. However, HR-DIC measurements require significant time due to scanning electron imageacquisition; such a limitation prevents the widespread use of HR-DIC for material characterization.Objective Apply a novel SEM acquisition technology to enhance HR-DIC measurements for high throughput applications.Methods Multi-beam SEM technology is employed to image an entire gauge length at once at high resolution and at nearly ahundredfold acceleration of typical HR-DIC image acquisition, even when automated stage movement and image acquisitionare employed. These images were fed into a discontinuity-tolerant HR-DIC software to determine slip localization inducedby non-metallic inclusions and grain structure.Results Slip localization was able to be analyzed to an unprecedented level, with over 210,000 slip bands able to be investi-gated, with the most intense slip localizing near and parallel to twin boundaries and in the vicinity of non-metallic inclusionclusters. Additionally, secondary slip activation and grain boundary shearing by intense dislocation pileups are observed toreduce slip amplitude near and parallel to twin boundaries.Conclusions By performing HR-DIC in conjunction with a multi-beam SEM, high-throughput measurements of large field-of-view, high-resolution images were able to be performed in a timely manner. These measurements provided an immensenumber of slip events for statistical analysis to be performed on to relate to microstructural features