Influence of Processing Method on the Grain Boundary Character Distribution and Network Connectivity Page: 4 of 14
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controlling the properties. A demonstration that boundary properties depend on misorientation
has been presented in these proceedings by Bedrossian et al.  who have studied the
susceptibility of individual grain boundaries to corrosion by coupling automated EBSD with
atomic force microscopy (AFM). The AFM observations have identified the sites of localized
attack observed in the EBSD as random grain boundaries. The deepest attack occurred at certain
triple junctions composed of three random grain boundaries and no attack was observed at the E3
boundaries. A number of observations of this type have revealed a correlation between
misorientation and localized corrosion processes . These findings, coupled with our recent
experimental results, suggest that increasing the special fraction is a necessary, but insufficient
condition to assure property improvements that depend on intergranular processes. This is
because the GBCD is a scalar quantity and does not contain details of the connectivity of the
random grain boundary network. In order to improve properties, it appears imperative that the
random grain boundary network be disrupted, which is accompanied by an increase in the
Research in the area of grain boundary engineering has concentrated on two primary
thermomechanical-processing paths to improve the GBCD, strain annealing and strain
recrystallization. Strain annealing typically involves low levels of strain (on the order of 3 to
7%) followed by long annealing times. In the case of high-purity Ni studied by Thomson and
Randle , strains of 6% followed by anneals for 168 hours were shown to modestly increase
the special fraction. An additional benefit of the strain annealing lies in the "fine-tuning" of the
misorientations to smaller deviations from the exact CSL description. Multiple, or repeated
strain annealing treatments on ultrahigh-purity Ni-16Cr-9Fe were performed by Was et al. 
to improve creep and cracking resistance. The improvement in the GBCD resulted from a two or
three step deformation and annealing treatment. Tensile straining of 2 to 5% was followed by
annealing at 890 C to 940 C for 1 to 20 hours. This process essentially doubled the number
fraction of special boundaries from 16 - 20% in the solution annealed material to 26 - 43% in the
specially processed case. It should be noted that the authors reported by number fraction and
excluded coherent twin boundaries in their analysis, thus understating the total special fraction
relative to other values in the literature reported by length fraction.
A fundamentally different approach to grain boundary engineering was taken by Palumbo
and co-workers . Their approach uses a sequential strain-recrystallization process to modify
the grain boundary network. Typically, low to moderate levels of strain, in the range of 5 to 30%
are followed by short anneals, on the order of 2 - 10 minutes . The annealing temperatures
and times are chosen to allow recrystallization without subsequent excessive grain growth. This
sequential processing is effective in producing a fine grain size, reducing texture, and increasing
the special fraction. Palumbo et al. have reported significant improvements in properties
including corrosion resistance, stress corrosion cracking, weldability, creep, and total elongation
to failure [2-13, 16-18]. The present authors have studied strain-recrystallization in oxygen-free
electronic (ofe) Cu and several Ni-based alloys. Successful sequential thermomechanical
processing induces multiple twinning, that is the formation of twin-related variants or E9 and
E27 boundaries [30,31].
The substantial improvements in the properties are most intriguing due to the potential
transferability of the processing protocol to industrial settings. Typical metalworking practices
start with ingot casting, generally followed by extensive forming steps used to reduce the ingot to
usable forms such as bars, tubes, plates, or sheets. The thermomechanical processing sequences
generally involve large deformation steps; with a minimum number of anneals to reduce the
length of time, and thus overall cost of the forming process. Although this practice has been
successful in producing quality products at affordable cost, recent work of Palumbo et al. [2-13,
16-18] indicates that significant improvements in many properties are possible with
modifications to the processing schedules. This investigation focuses on evaluating the influence
of processing method and annealing temperature on the resultant GBCD and connectivity of the
random grain boundary network.
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Kumar, M & King, W.E. Influence of Processing Method on the Grain Boundary Character Distribution and Network Connectivity, article, December 20, 1999; California. (digital.library.unt.edu/ark:/67531/metadc742066/m1/4/: accessed October 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.