Abstract: The available experimental results have been summarized concerning the effect of grain size on minimum creep rate.There are two types of creep rate-grain size relations.One is that there is a criti- cal grain size above which creep rate is independent of grain size,below which creep rate increases with the decrease of grain size.The other is that there is an intermediate grain size at which creep resistance is optimum.The first relation usually occurs at higher temperatures(>0.5 T_m),and intermediate stress ranges,while the second relation at interme- diate temperature ranges(0.4-0.5 T_m)and higher stresses.For the two types of creep rate-grain size relations,the increase of the creep rates with the de- crease of grain size for small grain sizes is all due to grain boundary sliding.For large grain sizes,a dis- location climb mechanism is dominant in creep deformation for the first relation,while a Hall-Perch grain boundary strengthening effect is believed to play an important role by dislocation glide mechanism for the second relation.

Key words: grain size, creep