A dispersion of precipitates introduces a threshold stress which opposes deformation. The effect of precipitate shape on dispersion strengthening at elevated temperature has been examined. Three kinds of Mod.9Cr-1 Mo steels without Nb(C.N), with spherical Nb(C.N) and with complex Nb(C.N)+V(N.C) "V-wing" were designed to havethe same microstructures except for the shapeof precipitates. Thethreshold stresses were measuredby tensile and creep tests at 600'C. Two sorts of effects have been observed. The first is that complex precipitates restrain dislocation from climbing even in a condition in which dislocation climbs over a spherical precipitate. Mobile dislocations are considerably trapped at the concavity of the V-wings during deformation. The second effect is that the probability of dislocation trapping at V-wing is larger than that at spherical Nb(C. N). Thesetrapping effects play important roles in the strengthening effect of precipitate.

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