I Grain boundary and its role i deformation (1 Impede slip(as obstacles)at R.T. and low temperature 2 Coordinate the deformation in different grains 3 Enhance(assist) the plastic deformation at high temperature through grain boundary sliding There exists an Equicohesive Temperature at which grain Ob(g b)=ob(grain T≈0.5T◆ Ⅰ.Grain boundary and its role in deformation ① Impede slip (as obstacles) at R.T. and low temperature. ② Coordinate the deformation in different grains. ③ Enhance (assist) the plastic deformation at high temperature through grain boundary sliding.  b graing.b. Tet T There exists an Equicohesive Temperature at which  b(g.b.) = b(grain) Tet  0.5Tm