Grain boundaries may influence high temperature strength in many ways: as barriers to dislocation motion, as sources or sinks of vacan¬cies and dislocations, and by grain boundary shearing. In all of these considerations the type of boundary must be taken into account. Experi-mental results on the influence of grain size on creep of aluminum sug-gest why coarse grain materials are often stronger in their steady state creep resistance over fine grain materials. As-recrystallised fine grain aluminum is weak because it usually contains high angle random boundaries and as such can easily deform by boundary shearing; in addition, such boundaries may be good sources or sinks of dislocations and vacancies. On the other hand, coarse grain aluminum is strong because it develops subgrains during creep; the subgrain boundaries, which consist of dis¬locations, are good barriers to dislocations, and these boundaries do not exhibit shearing. The strength of the subgrain boundary formed dur-ing creep of coarse grain aluminum appears to be strongly dependent on purity; addition of 250 atom parts per million iron increases the steady state creep resistance of coarse grain aluminum by one thousand times at 400° C.
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