3. Accommodation Mechanisms for Grain Boundary Sliding
  Some simultaneous accommodation mechanisms must operate to avoid the types of overlaps between sliding grains as shown in Fig. 1, and between a sliding grain and a blocking grain in front of the sliding grain. Many authors have suggested that openings (or voids) also should be avoided to maintain a coherency between grains (e.g. Crossman and Ashby, 1975; Edward and Ashby, 1979). However, complete coherency at grain boundaries is not a rigid requirement in grain boundary sliding (Langdon, 1970). Also grain boundary microcracking can occur even in crystal plastic deformation without a failure (Peach and Spiers, 1996). Therefore we include grain boundary openings in accommodation mechanisms, unless they are connected to make a large-scale fracture (Ree, 1988, 1994). Accommodation mechanisms suggested in the literature of metallurgy and materials science are elastic distortion, dislocation movement and diffusion. The details of grain boundary openings will be explained later.

Elastic distortion can accommodate grain boundary sliding by deforming sliding and blocking grains elastically. With this accommodation mechanism the sliding displacement must be small relative to the length of the sliding surface, and it may be recoverable when the stress is removed (Raj and Ashby, 1971).

Dislocation movement can accommodate grain boundary sliding in such ways as by forming a localized deformation zone adjacent to the triple junction within a blocking grain ("triple-point fold", Fig. 2a) or at ledges of the grain boundary between sliding grains (Gifkins, 1976; Etheridge and Wilkie, 1979; Langdon and Vastava, 1982; Zeuch, 1984; Drury and Humphreys, 1986; Hashimoto et al., 1986), by deforming a whole blocking grain with slip and twinning (Fig. 2b, Crossman and Ashby, 1975), or by climb and glide within the grain boundary mantle (Fig. 2c, Gifkins, 1976).

Fig. 2. Accommodation mechanisms of grain boundary sliding. (a) Localized deformation adjacent to the triple junction within a blocking grain (grain 3). (b) Intragranular plastic deformation of a whole grain. Dotted lines represent deformation bands. (c) Dislocation glide and climb in the grain mantle (after Gifkins 1976). Dotted lines represent boundaries between grain mantle and core. Movements of dislocations in the mantle are schematically shown. (d) Diffusion along grain boundaries around the triple junction (after Gifkins 1976).

Diffusional accommodation occurs by transport of material either along the grain boundaries or through the lattices of grains (Fig. 2d, Raj and Ashby, 1971; Ashby and Verrall, 1973). Grain boundary sliding accommodated by this mechanism is considered as a normal part of diffusional creep, since diffusional creep should be accompanied by grain boundary sliding to maintain coherency between deforming grains (Raj and Ashby, 1971; Ashby and Verrall, 1973; Langdon, 1975; Gifkins, 1976; Speight, 1976; Langdon and Vastava, 1982). However, it will be somewhat arbitrary, like a chicken-and-egg problem, to say that one mechanism accommodates another mechanism in plastic deformation where several coupled deformation mechanisms operate (Jessell personal communication, 1991). To solve this problem, the accommodation can be considered in three ways. First, by comparing the contribution to the total strain, it can be said that the mechanism contributing less strain is the accommodation mechanism. Secondly, if two mechanisms contribute almost the same strain to the total strain, we may treat each deformation mechanism in a time sequence and a mechanism coming later is the accommodation mechanism. Lastly, if the contribution of each mechanism to the total strain is about the same and if the two mechanisms initiate at the same time, then the two mechanisms are mutually accommodating.
 
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