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A Study of the Role of Grain-Boundary Engineering
in Promoting High-Cycle Fatigue Resistance

Grain Boundary Engineering:

    It is commonly accepted that the properties of polycrystalline materials are largely controlled by their structure and the interfaces that they contain, particularly by grain boundaries. Grain boundary engineering is a new concept in materials science and engineering, first proposed by Prof. T. Watanabe in the early 1980s. It is an approach designed to control the properties of materials by controlling the grain boundary character distribution (GBCD), mainly by promoting a high proportion of so-called special grain boundaries in them.

    There are two important characteristics that may govern the microtexture of bulk materials: grain boundary plan orientation and the proportion and distribution of special grain boundaries. Polycrystals tend to cleave on specific planes on which cracks grow easily. So it is advantageous to deflect growing cracks by frequent changes in grain boundary plan orientation. It is also applicable to make a high proportion and contiguity of special grain boundaries to improve the properties of materials. Different from random grain boundaries, special grain boundaries are characterized by particular misorientation and extensive areas of good fit (¡°Special¡± grain boundaries are described by a ¡°sigma number¡± ¡Æ (1<¡Æ<29), which is defined as the reciprocal of the fraction of lattice points in the boundaries that coincide between the two adjoining grains on the basis of the coincident site lattice (CSL) model.). Thus, there is low distortion of atomic bonds and relatively little free volume for special grain boundaries and consequently low boundary energy.

    In light of this, the notion of ¡°grain-boundary engineering¡± has been suggested as a viable means to improve the mechanical properties such as strength, ductility, creep, and corrosion resistance through a systematic modification of grain boundary distribution. Recently, sequential thermomechanical processing such as mechanical deformation by cold rolling followed by mid- or high- temperature annealing has been adopted to optimize microstructures, specifically by increasing the fraction of special grain boundaries and breaking-up the interconnected random grain boundary network.

    Grain boundary engineering is a promising approach to improving the bulk properties of materials and has been studied extensively after computer assisted EBSD (Electron Backscatter Diffraction) was developed over the period of 1982 to 1984. As a new characterization technique to investigate micro-mechanisms of grain boundary engineering in improving the properties of materials, EBSD is more effective than traditional methods such as x-ray or neutron diffraction in bulk texture measurements, and is not time-consuming in comparison with TEM (Transmission Electron Microscopy) in local grain boundary distribution analysis.

Grain Boundary Engineering on a Nickel-base Superalloy (ME3)
    To vary the grain-boundary character distribution, the effects of several thermomechanical processing parameters were first evaluated, including pre-strain (by cold rolling), annealing time and temperature. Based on earlier grain-boundary engineering processing of Inconel 600, cold rolling was used to vary the pre-strain from 5 to 20%, followed by annealing at temperatures from 1000  to 1170º§³ with annealing times of 15 to 45 min; in addition, air cooling instead of water or oil quenching was adopted to avoid quench cracking.

    Based on a series of preliminary multi-parametric optimization tests, the following processing sequence was adopted to promote a high fraction of special grain boundaries:
  • as-received plates were electro-discharge machined into 35 x 30 x 15 mm sections
  • sections were solutionized at 1175º§³ for 1-2 hr, followed by an air cool to room temperature to dissolve the ¦Ã¡ä precipitates
  •  microstructures were then grain-boundary engineered using four cycles of strain and high-temperature annealing of the single-phase alloy, specifically involving cycles of cold rolling (10% reduction in thickness per cycle) followed by a 30 min anneal at 1150º§³ in an air furnace
  • finally, a duplex aging treatment (4 hr at 843º§³, followed by 8 hr at 760º§³) was carried out to re-precipitate the ¦Ã¡ä as a bimodal distribution of cuboidal precipitates.
    Additionally, to compare with the as-received and grain-boundary engineered microstructures, some as-received sections from the fine-grain (GE) heat were grain-coarsened by heat-treating for 3.5 hr at 1175º§³; this heat treatment led to little or no change in the grain-boundary character distribution compared to the as-received material.

    The EBSD results on the grain-boundary character distribution and texture of the as-received and grain-boundary engineered microstructures are given in Figure 1; both the fine-grain (GE) and coarse-grain (NASA) as-received and grain-boundary engineered structures are shown. In this figure, the random-grain boundary network is enhanced in black and the special grain boundaries are in color, red representing ¦² 3 (twin) boundaries and yellow other special boundaries.
    For both alloys, the (number) fraction of special grain boundaries, fN, in the as-received condition constitutes no more than 29% of the total number of boundaries. After grain-boundary engineering, however, this ¡°special fraction¡± by number was increased to 41-42% . With respect to grain size, the coarse-grain alloy remained effectively unchanged after the grain-boundary engineering processing, whereas the initially fine-grain alloy showed grain coarsening from an average of 1.3 to 13 µm. Additional EBSD scans verified that these microstructures were isotropic regardless of the plane of observation. Specifically, grain-boundary engineering resulted in an increase in the number fraction of special boundaries, from 0.29 to 0.42 and from 0.28 to 0.41 in the fine- and coarse-grain alloys, respectively. Additionally, the length fractions (fL) of special boundaries were correspondingly increased in the two alloys from 0.36 to 0.57 and 0.38 to 0.56. The disproportionate increase in the length fraction in comparison with the number fraction (fN) is entirely due to the increased frequency of lower energy special boundaries like annealing twins (¦² 3).



Figure 1 Grain-boundary character distribution and texture pole figures, derived using EBSD for the ME3 alloy in the (a) as-received fine-grain (GE), (b) as-received coarse-grain (NASA), (c) grain-boundary engineered (fine-grain), (d) grain-boundary engineered (coarse-grain) conditions (for the plane perpendicular to the rolling direction). Random boundaries are shown as black lines and special boundaries are in color (red: ¦² 3 (twin), yellow: other special boundaries). The {001}, {011}, and {111} pole figures, showing the rolling (RD), transverse (TD), and normal (ND) directions, were also generated from the EBSD data.