Stable crack growth geometries as a strategy to circumvent FIB artefacts in small scale fracture testing

Die vorliegende Arbeit befasst sich mit stabilen Risswachstumsgeometrien, die durch in-situ-Mikrobiegeexperimente untersucht werden. Es werden mehrere Experimente an unterschiedlichen Geometrien in einem Rasterelektronenmikroskop (REM) durchgeführt, um Brucheigenschaften verschiedener Materialsysteme zu ermitteln. Im ersten Teil der Experimente werden Brückenkerben verwendet, um die Auswirkung von Kerbartefakten aus mit dem fokussierten Ionenstrahl (FIB) gefrästen Kerben auf Bruchexperimente zu verstehen, die mit einzelnen Kragarmgeometrien durchgeführt werden. Zweitens wird die Flüssigmetall-Ionenquelle (Ga+) des FIB durch eine Gasfeld-Ionenquelle (Ne+) ersetzt, um die Rolle der Ionenspezies bei Bruchexperimenten zu bestimmen. Einkristallines Silizium wird als Testmaterial für beide Versuchsreihen verwendet. Im letzten Kapitel wird ein Versuchsaufbau für Experimente zum stabilen Risswachstum unter Verwendung einer neuen Geometrie vorgestellt. Nach ersten Finite-Elemente-Methode (FEM)-Berechnungen wird die Geometrie an einer Hartstoffschicht auf Silizium als Modellmaterial validiert. Anschließend werden umfangreiche FEM-Berechnungen durchgeführt, um Leitlinien für den erfolgreichen Einsatz der Geometrie für Experimente zum stabilen Risswachstum zu erstellen. Die Ergebnisse zeigen, dass Proben mit tiefen Kerben und dünnen Materialbrücken ein Rissstillstand beobachtet wird. Bei flachen Kerben mit ähnlichen Materialbrücken wurde der Rissstopp experimentell nicht beobachtet, aber die Bruchzähigkeit für Silizium lag im selben Bereich wie bei tiefen Kerben (1,1 ± 0,1 MPa m0,5). Dickere Brücken führen zu einer geometrieabhängigen scheinbaren Bruchzähigkeit, die etwa 50 % höher ist als der erwartete Wert für einkristallines Silizium. Der Wechsel der Kerb-Ionenspezies von Gallium zu Neon erzeugt schärfere Kerben, aber die Silizium-Neon-Wechselwirkungen erzeugen eine ioneninduzierte Schadensschicht, die die Bildung neonhaltiger Blasen an der Kerbfront fördert. Diese Schädigungsschicht erhöht die scheinbare Bruchzähigkeit einer scharfen Kerbe. Durch thermische Behandlung wird das eingeschlossene Neon aus der beschädigten Schicht freigesetzt, wodurch eine scharfe Kerbfront entsteht. In diesem Fall entspricht die gemessene Bruchzähigkeit den für Galliumkerben gemessenen Werten. Diese Ergebnisse belegen die Notwendigkeit einer eingehenden Analyse der Ionenwirkung bei der Prüfung von Mikro-Cantilevern mit Edelgasionen wie Ne+. Die Eignung der neuen einseitig auskragenden Delaminationsgeometrie für Experimente zum stabilen Risswachstum wird nachgewiesen. Die Rissantriebskraft nimmt mit der Rissausdehnung in der Geometrie ab, was ein katastrophales Versagen verhindert. Infolgedessen bildet sich ein natürlicher Riss aus der FIB-Kerbe, und der endgültige Bruch erfolgt, nachdem der Riss über den Bereich der FIB-Fräskerbe hinausgewachsen ist. Dadurch wird der Einfluss von FIB-induzierten Artefakten wie Eigenspannungen aufgrund von Ionenimplantation, endlichen Kerbradien und anderen kristallinen Defekten auf die Bruchzähigkeit von Materialien, die auf der Mikroebene getestet werden, reduziert. Darüber hinaus zeigt die neue Geometrie Anzeichen von Rissablenkung, wenn ein natürlicher Riss von der FIB-beeinflussten Zone weg wächst. Die Delamination der Grenzflächen erfolgt auf stabile Weise, und die Grenzflächenzähigkeit liegt zwischen 3-7 J/m2. Finite-Elemente-Berechnungen zeigen, dass die Bildung von Rissen an der Grenzfläche in der neuen Geometrie eine kurze Auskragung, eine kurze Risslänge, eine dicke Schicht und einen großen Winkel zwischen der Schicht und dem Substrat erfordert. Ein hohes Elastizitätsmodul des Films verhindert auch die Rissverzweigung in den Film. Nach der Rissentstehung an der Grenzfläche setzt sich die Delamination entlang der Grenzfläche fort, da die Rissantriebskraft abnimmt. Bei sehr langen Rissen wird die Auskragung nachgiebiger, und es kann zum Bruch der Dünnschicht kommen

How to avoid FIB-milling artefacts in micro fracture? A new geometry for interface fracture

Focused ion beam (FIB) based small-scale fracture studies have been well established in recent years despite the ongoing discussion of possible artefacts caused by FIB milling. Stable crack growth geometries—where the FIB-prepared notch stably propagates through the sample—have the potential to ameliorate some of the FIB-based challenges. In this work, we propose a new sample geometry for testing interface toughness at the micron scale which results in intrinsically stable crack growth. This geometry is straightforward to fabricate using established FIB-based methods and testing setups. We prove the stability of crack growth by finite element modelling and by experimentally applying the approach on a hard coating–silicon interface. We observe that even with small imperfections, the FIB-milled notch propagates towards the interface and the natural crack stably grows along the interface

A geometry for quantitative analysis of interface fracture at the micron scale

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Recrystallization mechanisms and microstructure development in emerging metallic materials: A review

This review is devoted to the understanding of the recrystallization mechanisms and its role in the control of the microstructure in emerging metallic materials. Recrystallization is a very pervasive transformation phenomenon that is considered to be very important in efficient microstructure designs. Currently, there is hardly any work which has attempted to present a concise and systematic review of the recrystallization in emerging materials with a view to reconcile its manifestations with trends established from recrystallization studies in traditional alloys. This review aims to address this by first reviewing the fundamental and nascent recrystallization mechanism concepts and then analyzing their forms in emerging metallic materials, such as high strength steels, Ti- and Mg-based alloys, as well as high-entropy and shape-memory alloys. The reviews on these systems show that the classic recrystallization concepts are still relevant for explaining the recrystallization behavior and by extension to the microstructure development in the materials. However, in some instances, structural factors exclusive to these materials influenced the driving force and recrystallization behavior yielding outcomes sufficiently distinct from that observed in traditional alloys. Basically, deformation processing and material factors such as stress accumulation, inhomogeneous strain distribution, stored energy, available slip systems, phase composition, microstructural variability, initial grain size, texture, stacking fault and lattice distortion energies, strain path, deformation temperature, and solute clustering and diffusion rates were at play in determining the recrystallization mechanisms and kinetics in these emerging metallic materials. Keywords: Recrystallization mechanisms, Microstructure, Deformation processing, Stored energy, Emerging metallic material

Critical evaluation of seismic activities in Africa and curtailment policies – a review

Abstract The present paper discusses the seismicity of Africa and the need for the implementation of vibration control strategies in Africa, erroneously considered as aseismic. The review catalogues information on the seismicity of Africa, attesting that virtually every region in the African continent has come under the threat of some form of seismic event. The magnitudes and intensities of these seismic activities has resulted in devastations, including: loss of lives, building and civil structures collapse, displacements of people, economic losses, psychological traumatization, and grave fear. Evidences available show that most of the devastations are accentuated by tremor induced collapse of buildings and civil structures. It is thus imperative for substantive, simply implementable, and sustainable proactive measures for controlling the threats of seismic events be discussed within the African context. Current mitigation strategies in Africa include the establishment of seismic codes which govern the design of civil structures. The challenge to the implementation and the effectiveness of the existing African seismic codes was briefly discussed, and new measures which can reduce seismic risks were also highlighted The submissions of the paper is envisaged will create awareness on the budding seismic activities in Africa and awaken relevant authorities on the need for timely and practicable mitigation strategies to be in place to avert the attendant catastrophes associated with seismic occurrences

Enhancing plastic deformability of Mg and its alloys—A review of traditional and nascent developments

Mg and its alloys have continued to attract interest for several structural and super-sensitive applications because of their light weight and good combination of engineering properties. However for some of these applications, high plastic deformability is required to achieve desired component shapes and configurations; unfortunately, Mg and its alloys have low formability. Scientifically, the plastic behaviour of Mg and its alloys ranks among the most complex and difficult to reconcile in metallic material systems. But basically, the HCP crystal structure coupled with low stacking fault energies (SFE) are largely linked to the poor ductility exhibited by Mg alloys. These innate material characteristics have regrettably limited wide spread applicability of Mg and its alloys. Several research efforts aimed at exploring processing strategies to make these alloys more amenable for high formability – mediated engineering use have been reported and still ongoing. This paper reviews the structural metallurgy of Mg alloys and its influence on mechanical behaviour, specifically, plasticity characteristics. It also concisely presents various processing routes (Alloying, Traditional Forming and Severe Plastic Deformation (SPD)) which have been explored to enhance plastic deformability in Mg and its alloys. Grain refinement and homogenising of phases, reducing CRSS between slip modes, twinning suppression to activate non-basal slip, and weakening and randomisation of the basal texture were observed as the formability enhancing strategies explored in the reviewed processes. While identifying the limitations of these strategies, further areas to be explored for enhancing plasticity of Mg alloys are highlighted. Keywords: Mg alloys, HCP crystal structure, Basal texture, Severe plastic deformation, Formabilit

Reconciling viability and cost-effective shape memory alloy options – A review of copper and iron based shape memory metallic systems

Shape memory alloys (SMAs) are group of alloys that display anthropomorphic characteristics. These alloys recover their pre-deformed morphology when heated above their transition temperatures after being deformed in their lower temperature phase (martensitic phase). This unique material behavior is explored in industrial and technological applications where capacity for strain recovery is a key design parameter. Copper and iron based SMAs are largely viewed as potential cost effective substitute to Ni–Ti SMAs judging from their promising shape memory properties, damping capacity and other functional properties. Despite their outstanding potentials, the susceptibility of copper based SMAS to phase stabilization, transition hysteresis, aging and brittleness creates doubt on the possibility of transiting from the realm of potential to functional long term use in engineering applications. On the other hand the low percentage shape recovery in the Fe based SMAs also creates a gap between the theory and potential use of these alloys. This paper takes a critical look at the science of shape memory phenomena as applicable to copper and iron based SMA systems. It also covers the limitations of these systems, the effect of processing parameters on these alloys, proposed solutions to limitations associated with this group of shape memory alloys and thoughts for future consideration

How to avoid FIB-milling artefacts in micro fracture? A new geometry for interface fracture

Focused ion beam (FIB) based small-scale fracture studies have been well established in recent years despite the ongoing discussion of possible artefacts caused by FIB milling. Stable crack growth geometries—where the FIB-prepared notch stably propagates through the sample—have the potential to ameliorate some of the FIB-based challenges. In this work, we propose a new sample geometry for testing interface toughness at the micron scale which results in intrinsically stable crack growth. This geometry is straightforward to fabricate using established FIB-based methods and testing setups. We prove the stability of crack growth by finite element modelling and by experimentally applying the approach on a hard coating–silicon interface. We observe that even with small imperfections, the FIB-milled notch propagates towards the interface and the natural crack stably grows along the interface

Martensite aging phenomena in Cu-based alloys: Effects on structural transformation, mechanical and shape memory properties: A critical review

Cu-based shape memory alloys have been hyped as the ‘heir’ and pragmatic substitute to NiTi alloys for shape memory applications. Considerations from relatively low materials cost, processing ease, and modest shape memory properties, have been advanced as reasons justifying this projection. However, structural transformation induced phase stabilization - referred to as martensite ageing, has been reported to be a huge scourge constraining the thermo-responsiveness of these alloys, and limiting their service reliability. Studies on the mechanisms and effects of martensite ageing in Cu-based shape memory alloys (SMAs) have been reported in bits and patches, or encapsulated in broad ranged topical issues on the system. A comprehensive and exclusive review of martensite ageing in Cu-based SMAs has been lacking – thus the need for the present work. This review covers the general mechanisms of martensite ageing and its effects on the transformation behaviour, mechanical properties, shape memory functionality, and considers the implications on commercial utilization of the Cu-based SMAs. Specifically, Cu-Al-Mn, Cu-Al-Be, Cu-Al-Ni, Cu-Zn-Al, and Cu-Zn-Sn alloys were studied. The observations indicated that factors such as alloy composition, phase and microstructural parameters, and processing conditions, significantly dictate the mechanism and propensity to martensite stabilization, and also the extent to which the mechanical and shape memory characteristics are altered

Phase characterisation and mechanical behaviour of Fe–B modified Cu–Zn–Al shape memory alloys

The microstructures, phase characteristics and mechanical behaviour of Cu–Zn–Al alloys modified with Fe, B, and Fe–B mixed micro-alloying additions has been investigated. Cu–Zn–Al alloys were produced by casting with and without the addition of the microelements (Fe, B and Fe–B). The alloys were subjected to a homogenisation – cold rolling – annealing treatment schedule, before the alloys were machined to specifications for tensile test, fracture toughness, and hardness measurement. Optical, scanning electron microscopy and X-ray diffraction analysis were utilised for microstructural and phase characterisation of the alloys. A distinct difference in grain morphology was observed in the alloys produced – the unmodified alloy had predominantly needle-like lath martensite structure with sharp grain edges while significantly larger transverse grain size and curve edged/near elliptical grain shape was observed for the modified Cu–Zn–Al alloys. Cu–Zn with fcc structure was the predominant phase identified in the alloys while Cu–Al with bcc structure was the secondary phase observed. The hardness of the unmodified Cu–Zn–Al alloy was higher than that of the modified alloys with reductions in hardness ranging between 32.4 and 51.5%. However, the tensile strength was significantly lower than that of the modified alloy grades (28.37–52.74% increase in tensile strength was achieved with the addition of micro-alloying elements). Similarly, the percent elongation and fracture toughness (10–23% increase) of the modified alloy was higher than that of the unmodified alloy grade. The modified alloy compositions mostly exhibited fracture features indicative of a fibrous micro-mechanism to crack initiation and propagation, characterised by the prevalence of dimpled rupture