**1. Introduction**

212 Corrosion Resistance

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Nanocrystalline (nc) materials are single or multi-phase polycrystalline solids with a grain size of a few nanometers, typically less than 100 nm. Owing to the very fine grain size, the volume fraction of atoms located at grain boundaries or interfaces increases significantly in nanocrystalline materials [1]. A simple geometrical estimation, where the grains are assumed as spheres or cubes, yields the following values for the volume fraction of the interfaces: 50% for 5 nm grains, 30% for 10 nm grains and about 3% for 100 nm grains [2-5]. These values of interface volume fraction are several orders of magnitude higher than those of conventional microcrystalline materials. Consequently, nanocrystalline materials exhibit properties that are significantly different from and often improved over, their conventional microcrystalline (mc) counterparts. For example, nanocrystalline materials exhibit increased mechanical strength [6-10], enhanced diffusivity [11], improved corrosion resistance (some nanocrystalline materials) [12-20], optical, electrical and magnetic properties [21-24]. Due to their unique properties, nanocrystalline materials have attracted considerable research interests and the field of nanocrystalline materials has now become one of major identifiable activities in materials science and engineering.

Metals exposed to high temperature oxygen-containing environments form oxides. If an oxide scale can form and the oxide is dense and adherent, then this scale can function as a barrier isolating the metal from the external corrosive atmosphere. This oxide scale is called protective oxide scale. On the other hand, if a non-protective oxide scale is formed, oxygen can penetrate through the scale and the oxidation will extend further into the metal substrate, causing a rapid metal recession. Of all oxides, chromia and alumina are two kinds of oxides thermodynamically and kinetically feasible to meet the requirement for resisting high temperature oxidation.

Alloys containing aluminium or chromium can be selectively oxidised to form alumina or chromia. To form an external oxide scale, the concentration of aluminium or chromium should reach a critical value. Nanocrystalline alloys promote this selective oxidation process by reducing the critical value. For example, in the conventional microcrystalline Ni-20Cr-Al alloy system, > 6 wt% Al is required to form a protective Al2O3 scale at 1000°C [25]. If the Al content is lower than 6 wt%, complex oxide mixtures consisting of Cr2O3, NiCr2O4 and internal Al2O3 form, resulting in high reaction rates and poor oxidation resistance. With a nano-crystalline alloy structure, this value can be substantially reduced to 2 wt% Al, when

Oxidation Resistance of Nanocrystalline Alloys 215

nanometer-sized crystals and forms a network between them [1,21]. As grain size reduces,

Various researchers support the view of the two phase model of nanocrystalline metals. For example, extended x-ray-adsorption fine structure (EXAFS) and Mössbauer spectroscopy of ball-milled iron indicated the presence of two phases as characterized by significantly different atomic arrangements. These different atomic arrangements can be attributed to the presence of interfacial region and crystalline region [51]. Similarly, EXFAS investigation of nanocrystalline Fe and Pd indicated a large reduction in the atomic coordination number, supporting the idea of a very disordered structure at the interfaces [52-56]. Positron-lifetime spectroscopy measurements showed a large density of vacancy-like defects in grain boundaries and relatively large free volume at the triple points arising from misorientationinduced atomic instability of these sites [57]. Elastic relaxation of the interfaces occurred with a very different parameter than conventional coarse grain size polycrystalline materials. Modelling of thermoplastic properties and structure demonstrates that two phase model is an appropriate mean to account for the vibrational density of states and excess energy density in terms of grain boundary [58]. Similar to experimental findings, computer simulation of nanocrystalline iron has shown that grain boundary component in

nanocrystalline material is very high and is a strong function of grain size [59,60].

**2.2 Volume fraction of crystalline and intercrystalline regions** 

(*Cic*) of nanocrystalline materials associated with intercrystalline regions,

In discussing the structure of nanocrystalline materials further, the following terminology will be used in the text. Three types of grain contacts are possible in a polycrystalline material. They include, a) contact surfaces, b) contact lines and c) contact points. Surfaces of two grains which contact one another are called as contact interfaces. A contact line may represent a common line for three or more adjacent grains. A contact line of three grains is called triple junction. The boundary of grain is its surfaces. Grain boundaries which are seen in the metallographic slides are the section of interfaces by slide plane. A triple point is a section of triple junction by a plane. A detailed description of the terminology used here can

Mutschele and Kirchheim [62] proposed following relation to evaluate the volume fraction

*C d ic* 3 / 

and grains are considered to be cubes. Later, Palumbo et. al. [2] have shown that equation 1 was not suitable for the calculation of volume associated with triple points and to make it more general (to account for the triple points associated with the intercrystalline component) they proposed following relationship for the calculation of the intercrystalline component ( *ic Vt* ):

> 1 *ic t <sup>d</sup> <sup>V</sup>*

is the average grain boundary thickness and *d* is the average diameter of the grains

*d* 

where, *d* is the maximum diameter of an inscribed sphere. This yields the following relation

3

(1)

(2)

the IC increases and it may even exceed CC.

be found elsewhere [61].

where,

for grain boundary volume fraction ( *gb Vt* ),

the grain size is 60 nm [26]. This promotion effect is also evident for the K38G alloy containing 3.5-4 wt% Al and 16% Cr, which forms external Cr2O3 scale and internal Al2O3 precipitates in the cast form (large grains) but only Al2O3 when in the form of sputtered nano-crystalline structure [27-30].

The unique structure and high grain boundary fraction, enhances diffusion of impurities and alloying elements, and changes materials thermodynamic properties [31-33] which are expected to cause a considerable difference in the resistance of nanocrystalline materials to environmental degradation (oxidation) at high temperatures. For practical application of these nanocrystalline materials, an acceptable level of resistance to environmental degradation is required. However, the effect of the nanocrystalline structure on the hightemperature oxidation resistance has attracted only a limited research attention. Oxidation resistance of nanocrystalline Ni-Cr-Al [34-36], Fe-Cr [37-39], and Zr [40-43] based alloys have been mainly investigated and in most of the cases, oxidation resistance has been reported to improve due to nanocrystalline structure.

The properties of oxides (Al2O3, SiO2 or Cr2O3) formed during oxidation [44,45,46] also depend upon the grain size of the alloy and nanocrystalline structure alters the properties of oxide. For example, more uniform oxide scale with finer grain size and higher Cr or Al content is formed on the nanocrystalline alloys [34-36]. The oxide scales formed from nanocrystalline materials exhibit enhanced plastic deformation (due to fine grain size of formed oxide), which can release the stresses accumulated in the scales, and therefore the scale spallation tendency is reduced. It was reported that cyclic and long-time oxidation resistance was significantly improved by applying nanocrystalline coatings on type 304 stainless steel [47,48], Ni-Cr-Al [27], Co-Cr-Al [49], and Ni-(Co)-Cr-Al [28-30].

In order to investigate the possible differences in oxidation resistance along with any underlying mechanisms, understanding the nanocrystalline structure of a material is essential. This chapter will therefore first describe the structure of nanocrystalline materials, their thermodynamic properties and the possible effects of changes in the material structure (caused by such fine grain size) that may influence the oxidation resistance of a material.
