On the deformation morphology of bulk metallic glasses underneath a Vickers indentation

16th International Symposium on Metastable, Amorphous and Nanostructured Materials, Beijing, PEOPLES R CHINA, JUL 05-09, 2009International audienceThe techniques commonly used for observing the deformation mechanisms underneath a Vickers indentation in metallic glasses (chemical etching, bonded interface) induce artefacts such as cracks or semi-circular shear-bands. We propose an alternative technique based on the propagation of indentation corner cracks through a pre-existing imprint, which is possible in metallic glasses such as iron-based compositions. With this procedure, only radial shear-bands are observed. Comparisons between the chemical etching or the bonded interface techniques and the new technique are made. (C) 2010 Elsevier B.V. All rights reserved

A method for measuring the contact area in instrumented indentation testing by tip scanning probe microscopy imaging

The determination of the contact area is a key step to derive mechanical properties such as hardness or an elastic modulus by instrumented indentation testing. Two families of procedures are dedicated to extracting this area: on the one hand, post mortem measurements that require residual imprint imaging, and on the other hand, direct methods that only rely on the load vs. the penetration depth curve. With the development of built-in scanning probe microscopy imaging capabilities such as atomic force microscopy and indentation tip scanning probe microscopy, last generation indentation devices have made systematic residual imprint imaging much faster and more reliable. In this paper, a new post mortem method is introduced and further compared to three existing classical direct methods by means of a numerical and experimental benchmark covering a large range of materials. It is shown that the new method systematically leads to lower error levels regardless of the type of material. Pros and cons of the new method vs. direct methods are also discussed, demonstrating its efficiency in easily extracting mechanical properties with an enhanced confidence

Materials characterisation part II: tip geometry of the Vickers indenter for microindentation tests

This is the second of two papers by the authors associated with materials characterisation methods based on hardness testing. It is important to have knowledge of the tip geometry of the indenter employed in the hardness test as this affects the correctness of the value of contact area parameter used to determine the mechanical properties. In this paper, outcomes of a study concerned with the tip geometry of the Vickers microindenter are presented. Results from experiment are compared with results from published works and the most current accepted analytical models. A new non-contact methodology based on a residual imprint imaging process is developed and further compared with other methods using experimental and numerical analyses over a wide range of material properties. For confirmation, an assessment was undertaken using numerical dimensional analysis which permitted a large range of materials to be explored. It is shown that the proposed method is more accurate compared with other methods regardless of the mechanical properties of the material. The outcomes demonstrate that measuring contact area with the new method enhanced the overall relative error in the resulting mechanical properties including hardness and Young’s modulus of elasticity. It is also shown that the value of the contact area using actual indenter geometry obtained from experimental load-displacement analysis or FEM numerical analysis is more accurate than the value obtained from the assumption of perfect indenter geometry and hence can be used for materials with low strain hardening property. © 2017 Springer-Verlag Londo

Indentation of bulk metallic glasses: Relationships between shear bands observed around the prints and hardness

International audienceIndentation studies are conducted on two different bulk metallic glasses and for a variety of indenters: spherical, conical and pyramidal. Shear bands are observed around the prints for only some types of indenters and reasons for this are sought. Analytical models of hardness accounting or not for the pressure dependence of amorphous alloys and finite element analyses are used. It is shown that these bands are observed only when we leave the elastoplastic regime of indentation in favour of a fully plastic one. Important conclusions are drawn about hardness values. In metallic glasses, compared with, for example, crystalline alloys, because of their tremendous yield strains, determining the true hardness of a particular amorphous alloy is not straightforward; it depends on the correct choice of indenter. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Crystalline defects in bulk metallic glasses: consequences on fracture toughness determination and ductility

International audienc

Hardness, toughness, brittleness and cracking systems in an iron-biased bulk metallic glass by indentation

International audienceThe mechanical behaviour of an FeCoCrMoCBY bulk metallic glass is investigated by means of indentation testing and ultrasonic echography. The elastic moduli (Young's modulus of 225 GPa, Poisson's ratio of 0.337), hardness (13.5 GPa), indentation toughness (2.2 MPa root m) and indentation brittleness (6 mu m(-1/2)) are determined. Unlike most metallic glasses, this iron-based alloy exhibits cracking features above a critical load. The cracking systems are observed by means of stepwise polishing and consist in very shallow Palmquist (or radial) cracks, which is quite uncommon in other brittle materials such as glasses and ceramics. (C) 2008 Elsevier Ltd. All rights reserved

Fatigue pre-cracking and toughness of the Zr55Cu30Al10Ni5 bulk metallic glass for two oxygen levels

International audienceSamples of a Zr-based bulk metallic glass with a small (1000 appm) or very small (less than 300 appm) oxygen content are pre-cracked by fatigue and tested for toughness evaluation. It is shown that oxygen trapped in oxide dendrites eases the initiation of a straight crack and embrittles the glass even at such low concentrations. In contrast, when oxygen is dissolved in the glass, fatigue crack initiation becomes difficult and it is not possible to get a crack passing straight through the glass. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Pressure sensitive flow and constraint factor in amorphous materials below glass transition

International audienceThe constraint factor, C (given by the hardness-yield strength ratio H/Y in the fully plastic regime of indentation), in metallic glasses, is greater than three, a reflection of the sensitivity of their plastic flow to pressure. Furthermore, C increases with increasing temperature. In this work, we examine if this is true in amorphous polymers as well, through experiments on amorphous poly(methyl methacrylate) (PMMA). Uniaxial compression as well as spherical indentation tests were conducted in the 248-348 K range to construct H/Y versus indentation strain plots at each temperature and obtain the C-values. Results show that C increases with temperature in PMMA as well. Good correlation between the loss factors, measured using a dynamic mechanical analyzer, and C, suggest that the enhanced sensitivity to pressure is possibly due to beta-relaxation. We offer possible mechanistic reasons for the observed trends in amorphous materials in terms of relaxation processes