Microstructural features of plastic deformation under superplasticity conditions for steel Kh18AGN

Conclusions

• 1. Dispersed nitride phase precipitated in steel Kh18AGN as a result of the treatment suggested is a main condition for both obtaining a superfine grain size and also for limiting its growth during superplastic deformation.

• 2. In the second optimum rate range for superplastic deformation the main deformation mechanism is grain boundary sliding accomplished by movement of grain boundary dislocations. Intragranular sliding only occurs in narrow boundary areas under conditions of increased diffusion mass transfer.

• 3. With deformation rate above or below the optimum value relative participation of grain boundary sliding as the main mechanism of superplastic deformation decreases as a result of intragranular sliding. With low deformation rates it is combined with active diffusion creep as a result of which an equiaxed grain shape is retained. With high deformation rates diffusion is difficult and grain boundary processes do not manage to remove internal stresses arising with an increase in grain boundary sliding. This brings the condition of superplastic deformation towards normal hot ductile, with which ductility is lower than with deformation in the second rate range.