Microstructural alterations in bearing steels under rolling contact fatigue

The formation of microstructural alterations in bearing steels under rolling contact fatigue (RCF) is systematically studied. A literature review summarizes current understanding in this field, leading to the key to the formation of these microstructural features being carbon redistribution as a consequence of cyclic rolling contact. In this context, a novel theory is postulated to describe the migration of carbon caused by gliding dislocations. The theory combines the Cottrell atmosphere theory with the Orowan equation and is capable of quantifying the dislocation-assisted carbon flux. Based on the proposed theory, models are suggested for different types of microstructural alterations formed in rolling contact fatigued bearings – dark etching regions (DERs), white etching bands (WEBs) and white etching areas (WEAs). Very good agreement is obtained between the predications made by the models and the experimental data from both this research and the literature. Moreover, the models consider the effects of contact pressure, temperature, rotational speed and number of cycles, and thus can be applied for universal RCF testing conditions. The reproduced microstructural features are also characterized using advanced characterization techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT), with the observation validating the postulated formation mechanisms. It is demonstrated that DERs, WEBs and WEAs follow the same principle during formation – strain induced carbon redistribution. This is the first time that these microstructural alterations are quantitatively described using a unified theory. The achievements obtained from this research can be far reaching. It not only leads to great progress in understanding the phenomenology of RCF in bearing steels, but also can be further extended to other scenarios with similar phenomena such as severe plastic deformation and hydrogen embrittlement.SK

The relationship between 100Cr6 steelmaking, inclusion microstructure and rolling contact fatigue performance

A processing-microstructure-performance approach is followed to study three bearing steel samples manufactured from the most frequently used continuous casting routes. The inclusion microstructures of the samples were altered by varying the metallurgy and hot working conditions. Inclusion size distribution information is obtained, showing the steel-making route that results in the highest cleanliness. 3D analysis of inclusion morphologies using electrolytic extraction indicates the irregularities on the surface to be favourable sites for crack nucleation under RCF. Flat-washer and ball-on-rod tests were conducted to study the rolling contact fatigue life of the steels, with the results from the flat-washer testing method being more representative for bearing life. This research suggests that early fatigue of bearings is governed by silicate fragmentation and late fatigue by TiN inclusions

DeepSeek LLM: Scaling Open-Source Language Models with Longtermism

The rapid development of open-source large language models (LLMs) has been truly remarkable. However, the scaling law described in previous literature presents varying conclusions, which casts a dark cloud over scaling LLMs. We delve into the study of scaling laws and present our distinctive findings that facilitate scaling of large scale models in two commonly used open-source configurations, 7B and 67B. Guided by the scaling laws, we introduce DeepSeek LLM, a project dedicated to advancing open-source language models with a long-term perspective. To support the pre-training phase, we have developed a dataset that currently consists of 2 trillion tokens and is continuously expanding. We further conduct supervised fine-tuning (SFT) and Direct Preference Optimization (DPO) on DeepSeek LLM Base models, resulting in the creation of DeepSeek Chat models. Our evaluation results demonstrate that DeepSeek LLM 67B surpasses LLaMA-2 70B on various benchmarks, particularly in the domains of code, mathematics, and reasoning. Furthermore, open-ended evaluations reveal that DeepSeek LLM 67B Chat exhibits superior performance compared to GPT-3.5

Approaches to model structural and contact fatigue

Failures caused by structural and contact fatigue of materials are a key issue in modern industry. In order to ensure the safe use of mechanical components, a variety of fatigue modeling approaches have been developed. The first part of this article focuses on the modeling of traditional fatigue; by approaching fatigue problems from a fracture point of view, fatigue life models for crack initiation and propagation are introduced; also, fatigue life modeling based on damage accumulation parameters are presented. The second part of this article reviews contact fatigue modeling approaches; the description of the complex stress state under contact fatigue is given first, followed by a review of plasticity-based material behavior models for contact fatigue; then a number of contact fatigue life modeling approaches are presented and compared; finally, the modeling of microstructural alterations under contact fatigue is introduced. The advantages and limitations of the introduced fatigue models are discussed as well.</p

A unified theory for microstructural alterations in bearing steels under rolling contact fatigue

Three major types of microstructural alterations occurring under rolling contact fatigue, white etching areas (WEAs), dark etching regions (DERs) and white etching bands (WEBs), are modelled under a unified approach: dislocation-assisted carbon migration theory. Following our previous work on DERs and WEBs, a novel model is proposed to describe dislocation cell formation in WEAs. The proposed model yields predictions of WEAs appearance, agreeing with experimental observations. Bearing life can be estimated by the WEA appearance model. The three microstructural alterations models are combined and, for the first time, it becomes possible to predict the occurrence and the formation progress of WEAs, DERs and WEBs with a unified theory. Microstructural alteration maps are plotted as a function of number of cycles, temperature, contact pressure and stress cycle frequency. The models are validated by the experimental results reported over the last 50 years

Rolling Contact Fatigue-Related Microstructural Alterations in Bearing Steels: A Brief Review

Bearings are vital components that are widely used in modern machinery. Although usually manufactured with high-strength steels, bearings still suffer from rolling contact fatigue where unique microstructural alterations take place beneath the contact surface as a result of the complex stress state. Studying these microstructural alterations is a hot research topic with many efforts in recent decades. In this respect, the key information regarding four major types of microstructural alterations, white etching areas/white etching cracks, dark etching regions, white etching bands and light etching regions is reviewed regarding the phenomenology and formation mechanisms. Then, classical and state-of-the-art models are established to predict their formation and are summarised and evaluated. Based on the current research progress, several key questions and paradoxes for each type of microstructural alteration are raised, suggesting possible research directions in this field

Rolling Contact Fatigue-Related Microstructural Alterations in Bearing Steels: A Brief Review

Bearings are vital components that are widely used in modern machinery. Although usually manufactured with high-strength steels, bearings still suffer from rolling contact fatigue where unique microstructural alterations take place beneath the contact surface as a result of the complex stress state. Studying these microstructural alterations is a hot research topic with many efforts in recent decades. In this respect, the key information regarding four major types of microstructural alterations, white etching areas/white etching cracks, dark etching regions, white etching bands and light etching regions is reviewed regarding the phenomenology and formation mechanisms. Then, classical and state-of-the-art models are established to predict their formation and are summarised and evaluated. Based on the current research progress, several key questions and paradoxes for each type of microstructural alteration are raised, suggesting possible research directions in this field

Widely Tunable Distributed Bragg Reflectors Integrated into Nanowire Waveguides

Periodic structures with dimensions on the order of the wavelength of light can tailor and improve the performance of optical components, and they can enable the creation of devices with new functionalities. For example, distributed Bragg reflectors (DBRs), which are created by periodic modulations in a structure’s dielectric medium, are essential in dielectric mirrors, vertical cavity surface emitting lasers, fiber Bragg gratings, and single-frequency laser diodes. This work introduces nanoscale DBRs integrated directly into gallium nitride (GaN) nanowire waveguides. Photonic band gaps that are tunable across the visible spectrum are demonstrated by precisely controlling the grating’s parameters. Numerical simulations indicate that in-wire DBRs have significantly larger reflection coefficients in comparison with the nanowire’s end facet. By comparing the measured spectra with the simulated spectra, the index of refraction of the GaN nanowire waveguides was extracted to facilitate the design of photonic coupling structures that are sensitive to phase-matching conditions. This work indicates the potential to design nanowire-based devices with improved performance for optical resonators and optical routing