Nanocrystalline Materials: Fatigue


Sean R. Agnew Department of Materials Science and Engineering, University of Virginia

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The processes of damage accumulation and the resulting fracture of materials under cyclic loading at stress levels below the tensile strength are collectively referred to as fatigue. These phenomena are quite sensitive to materials structure, including crystal structure grain size, character and distribution of grain boundaries, dislocation density and arrangement, internal stress, texture, surface quality, etc. Additional complexity is introduced by a wide variability of testing and service conditions, including environments. Various factors can be of greater or lesser importance for different aspects of fatigue. However, grain size can be regarded as a key structural factor affecting nearly all aspects of fatigue. This is not surprising because the grain is the elementary structural unit of polycrystalline solids. Furthermore, grain size is known to strongly affect all known mechanisms of inelastic deformation, particularly dislocation slip and deformation twinning. Notably, without plastic deformation mechanisms leading to damage accumulation, there would be no fatigue.

The advent of nanocrystalline materials after the early work by Gleiter opened new horizons for the discovery and design of new materials with unusual properties, as well as opportunities for scientific investigation of potentially novel mechanisms heretofore unobserved in classical materials systems. The purpose of the present article is to review the state-of-the-art within both of these contexts, as investigations of the fatigue behavior of nanocrystalline (NC) materials have been motivated by the possibility that NC materials will have enhanced fatigue resistance, as compared to their coarse-grained counterparts, as well as basic a desire to understand the fundamental mechanisms of deformation and fracture in nanoscale.

Before discussing the fatigue behavior of NC materials, it is useful to briefly outline why the properties, in general, and fatigue response, in particular, of NC materials are considered unique.

  1. Nanomaterials are at the “interface” between amorphous and ordinary crystalline solids. Some authors (such as Gleiter) have entertained the notion that their structure might be considered a two-phase composite consisting of small perfect crystalline regions of nanodimensions surrounded by relatively thick glassy-like grain boundaries. Hence NC materials may inherit some properties from both crystals and glasses, while potentially remaining distinct from both of these extremes.

  2. The number of atoms associated with grain boundaries can approach the number of atoms within the grain interior. Another way of stating this is that the volume fraction of grain boundaries can become as large as 50%. Thus the grain boundaries and their specific structure and properties will play a very important role in most, if not all, properties of NC materials.

  3. Finally, a grain size of the order 100 nm corresponds to the structural level obtained within severely deformed metals. Specifically, the defects accommodating plastic deformation (dislocations) self-assemble into the collective structures with a characteristic length-scale in tens of nanometers.

A variety of techniques have been developed in the past decade for manufacturing nanomaterials (see Ref. for a review). Among these, some of the more important are inert gas condensation, electrodeposition, devitrification from an amorphous precursor obtained by rapid solidification or ball milling, and severe plastic deformation (SPD) (see entry on “Nanocrystalline Substances: Synthesis and Properties”). The present article emphasizes the latter approach because severe plastic deformation for grain reduction has the advantage of producing of fully dense, bulk ultrafine grain (UFG) and NC materials with desired purity or target composition. In other words, SPD allows one to produce material suitable for investigation of fatigue behavior using classical testing methods and possessing dimensions large enough for structural applications, where fatigue properties are of interest. Among SPD techniques, the equal channel-angular pressing (ECAP) technology introduced by Segal as a cold (or warm) working technique allows extremely large strains to be imposed on bulk samples without fracture. The technique has proven to be capable of fabricating massive samples with a variety of UFG and nanostructures. As the majority of experimental results concerning fatigue of nanocrystals have been obtained on ECAP materials so far, the present review will primarily be concerned with these materials.

ECAP is performed by passing a billet through two intersecting channels of the same cross section. Severe plastic deformation occurs by simple shear on the plane of intersection between the channels. Because the cross-section geometry does not change during processing, ECA pressing can be repeatedly performed through various routes determined by possible rotations of the billet between subsequent passes. With repeated pressing, the material hardens dramatically so that unusually high strengths can be achieved. Ideally, the resultant strain imposed per ECA pass is controlled by the included angle between channels 2θ solely (given sharp die channel corners), and the cumulative shear strain Γ after N passes is Γ = 2N cot θ. The effective strain is given by εi = 2N cot θ/ √ 3. Thus when the tool angle 2θ = 90°, Γ = 2N and the amount of the imposed strain can be substantially higher than is usually attained in standard cold-working procedures, such as rolling.