**2. A short overview of mechanical properties of metallic glasses**

A close relationship exists between properties of materials whose magnitude is usually determined by their bond strength. The choice of a material is the result of several compromises. For instance, the technical appraisal of an alloy will generally be a compromise between mechanical resistance, such as fracture toughness and some other properties such as yield strength, Young's modulus, density etc. A convenient way of bringing out these relationships is by a series of figures or charts, in which one parameter is plotted as a function of another.

plasticity hinders the application of bulk metallic glasses (BMGs) as structural materials, and prevent their usage for instance in load-bearing structures. The need for the development of

**Figure 1.** Ashby map of the damage tolerance of materials (fracture toughness *vs.* yield strength) including oxide glasses, ceramics, polymers, metals, monolithic metallic glasses (Fe-based glasses, Zr-based glasses, Ti-based glass, Pt-based glass) designated by crosses, Pd-based glass designated by a filled star, and ductile-phase-reinforced metallic glass composites designated by circles. (reproduced from Demetriou et al. [2] with permission from copyrighted ©Nature,

In this regard, great improvements in fracture toughness have been made as new alloy compositions were discovered and optimized to extend the fracture toughness limit for BMGs. The occupation of a new region of property space for BMG composites (BMGCs), designated by circles, is shown in the Ashby's map (**Figure 1**) [2]. Contours correspond to values for the

the crack tip because of the high stresses generated by the sharp stress concentration [3]. It is mostly this enhanced plasticity scale in BMGCs, which is at the origin of their superiority over other technical candidate materials. It is argued to be due to the fact that all the shear bands initiated in plastically soft regions with lower yield stress (or lower shear modulus) are prevented in the surrounding regions of higher yield stress or stiffness. This enhancement in both ductility and toughness is analogous to the tempering of plastics by inclusion of rubber particles [4]. As indicated by the arrow, the conjunction of toughness to the strength,

. In this zone area, plastic deformation predominantly occurs at

Metallic Glasses for Triboelectrochemistry Systems http://dx.doi.org/10.5772/intechopen.78233 79

new efficient and resistant materials is thus essential in order to ensure this objective.

plastic-zone radius, K<sup>2</sup>

2011).

c /πσ<sup>2</sup> y

An important relationship is that between fracture toughness and the yield strength, a typical characteristic of the damage tolerance of materials. Values of most engineering materials, including monolithic metallic glasses, and ductile-phase-reinforced metallic glass composites [2] are shown in **Figure 1**.

Generally, ductile metals have very high fracture toughness and fairly low yield strength. Metallic glasses, however, show limited plastic yielding and have toughness-strength (K<sup>c</sup> -σ<sup>y</sup> ) relationships that lie between brittle ceramics and marginally tough materials. The lack of

At the present time, there are available for use in excess of 45,000 different metallic alloys [1]. Albeit, the steels and cast irons make up the largest use on a weight basis, the number of different nonferrous alloys exceed the number of ferrous alloys. The primary nonferrous alloys are those in which the base metal consists of either aluminum, copper, nickel, magnesium,

With the introduction of new metallic alloys and the breakthrough in the production of the so-called glassy metals, what was the best choice several years ago may no longer be so. Over the years, considerable efforts and great progress have been made in the field of materials selection, in particular through the improvement of the specific properties of different sorts of alloys. These growing developments, inter alia, include processes for enhancing their metallurgical, mechanical, physical, chemical, and especially their tribocorrosion properties. Alternatives in the composition have also been formulated to improve the workability (e.g., glass-forming ability) of many metallic glass alloys through, commonly, monitoring the quenching rate. Critical cooling rate and maximum attainable size, known as critical casting thickness, are both direct indicators of glass-forming ability (GFA). The smaller the critical

In order to conduct a meaningful evaluation of a design alloy, all essential data required to fit with the most appropriate material must be disposable. It is the purpose of this chapter to supply as much of this information as possible for commercially available metallic glass materials, and for use in systems where mechanical and chemical surface interactions take place and leading to deterioration effects. The main strategies known to meet these requirements are outlined. Since mechanical wear and corrosion are broad fields, where the interplay between several mechanisms can occur, different approaches to prevent them have been contracted. Some of the most common ways are discussed. Examples from different classes of

cooling rate and/or the larger maximum attainable size, the higher is the GFA.

**2. A short overview of mechanical properties of metallic glasses**

A close relationship exists between properties of materials whose magnitude is usually determined by their bond strength. The choice of a material is the result of several compromises. For instance, the technical appraisal of an alloy will generally be a compromise between mechanical resistance, such as fracture toughness and some other properties such as yield strength, Young's modulus, density etc. A convenient way of bringing out these relationships is by a series of figures or charts, in which one parameter is plotted as a function of another. An important relationship is that between fracture toughness and the yield strength, a typical characteristic of the damage tolerance of materials. Values of most engineering materials, including monolithic metallic glasses, and ductile-phase-reinforced metallic glass composites

Generally, ductile metals have very high fracture toughness and fairly low yield strength. Metallic glasses, however, show limited plastic yielding and have toughness-strength (K<sup>c</sup>

relationships that lie between brittle ceramics and marginally tough materials. The lack of


titanium, zirconium or zinc [1].

78 Metallic Glasses - Properties and Processing

metallic glass material are given.

[2] are shown in **Figure 1**.

**Figure 1.** Ashby map of the damage tolerance of materials (fracture toughness *vs.* yield strength) including oxide glasses, ceramics, polymers, metals, monolithic metallic glasses (Fe-based glasses, Zr-based glasses, Ti-based glass, Pt-based glass) designated by crosses, Pd-based glass designated by a filled star, and ductile-phase-reinforced metallic glass composites designated by circles. (reproduced from Demetriou et al. [2] with permission from copyrighted ©Nature, 2011).

plasticity hinders the application of bulk metallic glasses (BMGs) as structural materials, and prevent their usage for instance in load-bearing structures. The need for the development of new efficient and resistant materials is thus essential in order to ensure this objective.

In this regard, great improvements in fracture toughness have been made as new alloy compositions were discovered and optimized to extend the fracture toughness limit for BMGs. The occupation of a new region of property space for BMG composites (BMGCs), designated by circles, is shown in the Ashby's map (**Figure 1**) [2]. Contours correspond to values for the plastic-zone radius, K<sup>2</sup> c /πσ<sup>2</sup> y . In this zone area, plastic deformation predominantly occurs at the crack tip because of the high stresses generated by the sharp stress concentration [3]. It is mostly this enhanced plasticity scale in BMGCs, which is at the origin of their superiority over other technical candidate materials. It is argued to be due to the fact that all the shear bands initiated in plastically soft regions with lower yield stress (or lower shear modulus) are prevented in the surrounding regions of higher yield stress or stiffness. This enhancement in both ductility and toughness is analogous to the tempering of plastics by inclusion of rubber particles [4]. As indicated by the arrow, the conjunction of toughness to the strength, potentially accessible to metallic glasses extends beyond traditional benchmarks towards levels formerly inaccessible to any material (e.g., Pd-based alloys in [2]). One direct result of the unique microstructure is the high toughness-to-yield strength ratio, mainly accessible to BMGCs. Their strength exceeds that of the strength limit of known crystalline pure metals or alloys and approaches that of engineering ceramics, whereas their toughness is markedly high, among metallic alloys.

for example is fundamental, but rather that they depend on the triboelectrochemical system approach. In particular they are determined by a combination of a number of more fundamental properties of the contacting materials, testing parameters, and test conditions, especially the nature of the environment in which the tests take place. An important aim of research in tribocorrosion field is precisely that of determining the nature of this dependence, so that the triboelectrochemical behavior may be predicted from a knowledge of a system approach and the more fundamental properties of interacting surfaces. Although, this aim has not been achieved yet, a fair progress has been made, and still more work is required to have a good comprehension of just which are the important distinctive features determining the surface

Metallic Glasses for Triboelectrochemistry Systems http://dx.doi.org/10.5772/intechopen.78233 81

Tribocorrosion of two contacting solids in relative motion is, just as friction, a system parameter. A triboelectrochemical system consists of implementing electrochemical techniques to a tribological designed system (i.e., tribometer type, and complete material system). That mechanical designed system is of a great importance since this will enable to simulate as much as possible the entire material system used in the field, and the constraints that have associated with it (e.g., similarity of the wear mechanisms active in the laboratory test and in the field, such as abrasion, adhesion, fatigue, penetration hardness, bending, existence or not

• The choice of a tribometer type with respect to its characteristics (e.g., contact noise, vibra-

• The choice of a body (material specimen under test), and a counter-body (material antagonist, usually a ball, a pin, a plate or a disk), which constitutes the contact system geometry (e.g., sphere-on-flat, sphere-on-sphere, cylinder-on-flat, flat-on-flat…). This choice has to consider the metallurgical and chemical features of the two solid bodies (e.g., inertness, composition of contacting materials, microstructure, surface film composition, etc.);

• The choice of a lubricant or other matter between the two bodies (e.g., composition of the

• The choice of surrounding media and temperature (e.g., relative humidity, vacuum, composition of the corrosive environment, pH, aggressiveness, ionic conductivity, etc.);

• The loading conditions (pressure or force, relative velocity, acceleration, or frequency, a

• And finally, the choice of the type of relative movement (uni- or bidirectional, continuous

The measuring instruments in tribocorrosion tests allow to monitor in real-time and on-line the foregoing system parameters (e.g., contact conditions: normal or tangential force, relative displacement, velocity, etc.). The main focus is to promptly control any variation or change

medium, viscosity, solid particles in suspension, stagnant or stirred, etc.);

interaction behavior in a triboelectrochemical system.

of a third body, erosion, corrosion, their combinations, etc.).

Generally, the tribological configuration involves:

working distance, or number of contacts);

tions, residual stress …);

or reciprocating).

**3.1. Elements of tribocorrosion (instrumentation)**

Some recent significant developments have been made towards the design of this kind of BMGC materials. This was achieved through the successful implementation of effective composite microstructures, which typically combine a strong glassy matrix with ductile crystalline reinforcements that suppress fracture while sustaining high strength. This variety of composite materials can only be obtained through the commitment of a nanocrystallization process [5] or *via* the reinforcement with ceramic particles [6]. Current studies on BMGCs performance are still at the development stage and concern the evaluation of either their mechanical properties or their corrosion resistance, but the perspective is very promising.

In recent years, a wide variety of industries including food, medical and pharmaceutical, aircraft components, electronics, building materials, and automobile industries have been promoting the technological development of newly composite materials including the vitreous-based composites to achieve suitable strength/density, and toughness/stiffness ratios.
