https://material.karlov.mff.cuni.cz/people/janecek/studenti/FyzMetNano/Estrin_AM61_2013_782.pdf

Extreme grain refinement by severe plastic deformation: A wealth of challenging science

ZnO tetrapod materials for functional applications

Inspired by the gradient structures of biological materials, researchers have explored compositional and structural gradients for about 40 years as an approach to enhance the properties of engineering materials, including metals and metallic alloys. The synthesis of various gradient nanostructured materials, such as gradient nanograined, nanolaminated nd nanotwinned metals and alloys, has provided new opportunities to understand gradient-related mechanical behaviour. These emerging gradient materials often exhibit unprecedented mechanical properties, such as strength–ductility synergy, extraordinary strain hardening, enhanced fracture and fatigue resistance, and remarkable resistance to wear and corrosion, which are not found in materials with homogeneous or random microstructures. This Review critically assesses the state of the art in the field of gradient nanostructured metallic materials, covering topics ranging from the fabrication and characterization of mechanical properties to underlying deformation mechanisms. We discuss various deformation behaviours induced by structural gradients, including stress and strain gradients, the accumulation and interaction of new dislocation structures, and unique interfacial behaviour, as well as providing insight into future directions for the development of gradient structured materials. Gradient nanostructured metals and alloys are an emerging class of materials that exhibit a combination of excellent mechanical properties that are not possessed by their homogeneous counterparts. This Review assesses the fabrication, characterization and deformation behaviour of these materials, as well as the challenges and future directions of the field.

Mechanical properties and deformation mechanisms of gradient nanostructured metals and alloys

A new catalyst-free method has been developed to grow self-assembled GaN wires on c-plane sapphire substrates by metal-organic vapour phase epitaxy. This approach, based on in situ deposition of thin SiNx layer (~2 nm), enables epitaxial growth of c-oriented wires with 200-1500 nm diameters and a large length/diameter ratio (>100) on c-plane sapphire substrate. The detailed study of the growth mechanisms shows that a combination of key parameters is necessary to obtain vertical growth. In particular, the duration of SiNx deposition prior to the wire growth is critical for controlling the epitaxy with the substrate. The GaN seed nucleation time determines the mean-size diameter and structural quality and a high Si-dopant concentration promotes the vertical growth. Such GaN wires exhibit UV-light emission centered at about 350 nm and a weak yellow band (~550 nm) at low temperature. This approach may be viewed as a fast and reproducible technique to grow GaN wires by MOVPE. Compared to other techniques, it allows studying quite systematically the influence of the growth parameters without being dependent on time consuming ex situ surface preparations like surface patterning used in selective area growth (SAG). The growth of heterostructures, as longitudinal n-u and n-p wires (using Si and Mg dopants) as well as a core-shell InGaN/GaN MQW using the wires as templates has been demonstrated. The growth occurs on the non-polar m-plane facets of the wire and not on c-plane as it is the case on 2D materials with the same sapphire substrate. It gives different piezoelectric contributions to the wires optical properties, which have been studied by cathodo- and photo-luminescence. The fundamental building blocks of wire-based blue LED were demonstrated during this thesis. The realisation of an efficient device still requires a deeper understanding and optimization of the parameters controlling the material growth and an optimization of the electrical contacts.

https://tel.archives-ouvertes.fr/tel-00524190/document

Self-assembled growth of catalyst-free GaN wires by metal–organic vapour phase epitaxy

Precipitation phenomena in an ultrafine-grained Al alloy

The mechanical properties and microstructural features of aluminum 5083 (AI5083) weldments processed by gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) are investigated. Weldments processed by both methods are mechanically softer than the parent material AI5083, and could be potential sites for plastic localization. It is revealed that AI5083 weldments processed by GTAW are mechanical more reliable than those by GMAW. The former bears higher strength, more ductility, and no apparent microstructure defects. Perceivable porosity in weldments by GMAW is found, which could account for the distinct mechanical properties between weldments processed by GTAW and GMAW. It is suggested that caution should be exercised when using GMAW for AI5083 in the high-speed-train industry where such light weight metal is broadly used.

Microstructure and mechanical properties of aluminum 5083 weldments by gas tungsten arc and gas metal arc welding

Fatigue tests with rotary bending (52.5 Hz) and ultrasonic cycling (20 kHz) methods were performed, respectively on two high strength steels so as to investigate the effects of inclusion size and location on the fatigue behavior from low cycle fatigue to very-high-cycle fatigue (VHCF) regimes. Under rotary bending test, four fracture patterns were observed with respect to different locations of inclusion on fracture surface. The mean crack growth rate within FGA (fine granular area of crack origin) was estimated to give the value with the magnitude of 10(-13) m/cycle for the specimens with the fracture pattern of qFGA+surface fisheyeq. Under ultrasonic cycling test, the size effect of inclusions on VHCF was investigated with two groups of specimens. One is with the inclusion size between 20 and 60 mu m and the other is with the inclusion size between 10 and 30 mu m. The results show that large inclusions lowered the proportion of net FGA at given stress levels and led to the shortening of VHCF life. The degradation of VHCF strength caused by the increase of inclusion size is ascribed to the decrease of the critical stress of FGA formation for large inclusions.

Effects of inclusion size and location on very-high-cycle fatigue behavior for high strength steels

Effects of strength level and loading frequency on very-high-cycle fatigue behavior for a bearing steel

This paper studies the formation mechanism of fine granular area (FGA) in high-strength steels and predicts the threshold value of its formation. Fatigue experiments are carried out by using rotating bending and ultrasonic fatigue testing machines on a high carbon chromium steel (GCr15) with three different heat treatments. The results show that the range of stress intensity factor at the periphery of FGA (Delta K(FGA)) keeps constant with an average value of 5.2 MPa m(1/2), which is close to the traditional crack growth threshold (Delta K(th,0)), 5.0 MPa m(1/2). A theoretical model based on the plastic zone at crack tip is proposed to predict the value of Delta K(FGA) and the predictions are in good agreement with experimental data. (C) 2011 Elsevier B.V. All rights reserved.

Jijia Xie

Papers

Microstructure and mechanical properties of aluminum 5083 weldments by gas tungsten arc and gas metal arc welding

Effects of inclusion size and location on very-high-cycle fatigue behavior for high strength steels

Effects of strength level and loading frequency on very-high-cycle fatigue behavior for a bearing steel

Prediction of threshold value for FGA formation

Effects of inclusion size and stress ratio on fatigue strength for high-strength steels with fish-eye mode failure