Papers by Author: Simon P. Ringer

Abstract: This article presents an inverse analysis method based on an instrumented indention to extract materials properties from multilayer material systems. In this case, a 12-layers system comprising of two alternate materials is considered. Each layer is 1 μm thick. The material properties selected for the layers are within the range of common commercial aluminium alloys. The yield stress and strain hardening exponent of the two layers were identified based on a power law type equation to define the stress-strain relationship. A 2D axis-symmetric indenter having 70.3 half angle was used, which is representative of a Berkovich or a Vickers indenter. The use of finite element analyses was substituted with a fast and equally accurate approach for the iterative optimization procedure. Thus, the computation time was considerably reduced. The robustness is tested using pseudo-experimental results, in terms of indentation curve and imprint on the material, with added random noises of 2.5%, 5.0%, 7.5% and 10.0%. The proposed approach provides a good estimate of the sought material properties. It is envisaged that this approach can become of assistance in the evaluation of the material properties for multilayer coatings and small devices.

Abstract: This study investigates the effect of N diffusion on a Nb-microalloyed steel made by twin roll casting at 525o C in a KNO3 salt bath. Nitriding up to 4 h increases the yield strength of the steel by ~50% with only a small drop in ductility, while 6 hours of nitriding causes brittle fracture. The improved mechanical performance after 4 hours of nitriding is thought to be a combined effect of solid solution strengthening of N diffusion and dispersion strengthening from extremely fine Nb-rich precipitates. Coarse features along grain boundaries consistently observed in steel nitrided for 6 hours are considered to be responsible for brittle fracture in samples nitrided for longer.

Abstract: In recent years, the pursuit of higher strength metals and alloys has led researchers to nanometer scale grain refinement. New nanocrystalline engineering techniques have successfully increased properties for a wide range of materials. Here we report a nanocrystalline 7075 alloy processed by high-pressure torsion that exhibits ultra-high strength and features a hierarchical solute architecture. The new hierarchy of solute architecture was discovered through high-resolution characterisation using novel techniques we have developed in atom probe tomography. These new techniques – nanotexture and fine scale solute cluster measurements, are the focus of this paper. Our results indicate that nanometer-scale engineering of solid solutions could offer a pathway towards a new generation of super-strong alloys that hold promise for creating entirely new regimes of property-performance space.

Abstract: The proposed work makes use of an inverse analysis approach to identify the mechanical properties of a material sample which can be described by means of the elastic modulus, yield stress and strain-hardening parameter. The particularity of the proposed work is the exploitation of the indentation curve as well as the imprint left by the indenter at the end of the test as input information to the inverse analysis. The numerical simulations required by the analysis are carried out by means of the finite element software Abaqus. This paper describes initial work carried out to validate the robustness of the inverse procedure for stainless steel samples using computer-generated data.

Abstract: A novel solid state and surfactant-free method has been developed for synthesis of polyaniline superstructure composed of dendritic nanofibers. The good crystallinity of the product is confirmed by XRD, while FT-IR analyses indicate the formation of highly doped emeraldine salt of polyaniline product. Microscopic observations show that the product is in the form of polyaniline dendrites with a less-than-40 nm fiber diameter. The unique growth mechanism of the superstructure in solid-state reaction has been discussed.

Abstract: The contribution of nano-scale particles observed using Atom Probe Tomography in an increase of yield strength of conventional and advanced HSLA steels was studied. The advanced HSLA steel showed higher yield strength than conventional HSLA steel. There were two types of carbides, which primarily contribute to an increase in yield strength of conventional HSLA steel: (i) coarse TiC with average size of 25±5nm and (ii) fine TiC with average radius of 3±1.2nm. The presence of two types of carbides was found in the microstructure of advanced HSLA steel: (i) nano-scale Ti0.98Mo0.02C0.6 carbides with average radius of 2.2±0.5nm, and (ii) C19Cr7Mo24 particles with an average radius of 1.5±0.3nm. The contribution of precipitation hardening in the yield strength of advanced HSLA steel due to the nano-scale particles was 174MPa, while this value in the conventional HSLA steel was 128MPa.

Abstract: The microstructures of Al-0.2Sc and Al-0.2Sc-0.12Zr alloys have been investigated.The results show that Al3Sc and Al3(Sc1-xZrx) dispersoids exist in as-rolled Al-0.2Sc and Al-0.2Sc-0.12Zr alloy respectively, and their orientation is (001)α║(001)dispersoid, [010]α║[010]dispersoid.The Al3Sc particles in as-rolled Al-0.2Sc were precipitated from hot rolling process, while the larger Al3(Sc1-xZrx) particles in as-rolled Al-0.2Sc-0.12Zr from the solidification, and the small particles also from hot rolling process. There is segregation of Sc and Zr in the Al3(Sc1-xZrx) dispersoid, and Sc is rich in the outside shell while Zr rich in the core of the particles. It is believed that the grain and subgrain boundaries can be pinned by the Al3Sc particles when annealed after cold-rolling, and this may lead to restricting the recrystallization of the Sc-containing alloys.And it’s not until the dissolution of Al3Sc that the recrystallization can happen in this kind of alloys.

Abstract: The present paper outlines the effect of small addition of Sc,Ag and Mg on the ageing characteristics and microstructural evolution in some microalloyed Al alloys,such as Al-Cu-Mg, Al-Ag and Al-Li alloys.Small quantities of these microalloying elements can modify the ageing characteristics of these alloys,and significant changes have been found in their microstructural evolution after small microalloying element additions. The observed effects are explained in terms of the atomic behaviour during the early ageing stage,and one of the key factors governing this effect is Sc/Vacancy mechanism.

Abstract: During age-hardening of certain Al-Zn-Mg-Cu alloys, a 90% hardness increase can occur with 75 seconds. The clustering and precipitation of solute element species during this early rapid hardening (RH) period has been investigated through atom probe tomography, transmission electron microscopy, and Vickers hardness measurements. This study has focussed on the effect of copper by analysing three alloys; Al-2.0Zn-1.8Mg-0.7Cu, Al-2.0Zn-1.7Mg-0.2Cu and Al-1.9Zn-1.7Mg (at.%). The early RH reaction in these alloys accounts for up to 70% of the total hardening (peak hardness minus as-quenched hardness) and takes place during the first 60 seconds of ageing. We report preferred solute-solute interactions in the as-quenched materials. This quenched-in nanostructure acts as a template for subsequent solute clustering, the nature of which we have correlated with ageing.

Abstract: The trace addition of Sn (0.01 at.%) to an Al-1.7Cu (at.%) alloy has been investigated using atom probe tomography (APT), transmission electron microscopy (TEM) and high resolution transmission electron microscope (HRTEM). We have studied samples in the as-quenched (AQ) condition and following ageing at both 160 and 200 °C for very short ageing times so as to better understand the early stages of the decomposition processes. Our data reveal independent Cu-Cu and Sn-Sn clusters in the AQ condition, though we did not observe Cu-Sn clustering. We observed for the first time that some of these initial Cu-clusters develop into GP zones during subsequent ageing at temperatures as high as 200 °C. The Sn atom clustering results in precipitation of independent 􀀂- Sn particles after aging for 30 sec. The GP zones consequently undergo reversion and this liberates Cu atoms which seem to participate in a cluster-assisted heterogeneous nucleation of the 􀀃􀀂 phase at the interface of the 􀀂-Sn. For ageing at 200 °C, this process is complete within 180 sec.