, ▁gra in ▁growth ▁is ▁the ▁increase ▁in ▁size ▁of ▁gra ins ▁( c ry st all ites ) ▁in ▁a ▁material ▁at ▁high ▁temperature . ▁This ▁occurs ▁when ▁recovery ▁and ▁rec ry st all isation ▁are ▁complete ▁and ▁further ▁reduction ▁in ▁the ▁internal ▁energy ▁can ▁only ▁be ▁achieved ▁by ▁reducing ▁the ▁total ▁area ▁of ▁gra in ▁boundary . ▁The ▁term ▁is ▁commonly ▁used ▁in ▁met all ur gy ▁but ▁is ▁also ▁used ▁in ▁reference ▁to ▁cer am ics ▁and ▁min er als . ▁ ▁Import ance ▁of ▁gra in ▁growth ▁Most ▁materials ▁exhib it ▁the ▁Hall – P etch ▁effect ▁at ▁room - temper ature ▁and ▁so ▁display ▁a ▁higher ▁yield ▁stress ▁when ▁the ▁gra in ▁size ▁is ▁reduced . ▁At ▁high ▁temper atures ▁the ▁opposite ▁is ▁true ▁since ▁the ▁open , ▁dis ordered ▁nature ▁of ▁gra in ▁boundaries ▁means ▁that ▁vac an cies ▁can ▁diff use ▁more ▁rapidly ▁down ▁boundaries ▁leading ▁to ▁more ▁rapid ▁Co ble ▁cre ep . ▁Since ▁boundaries ▁are ▁regions ▁of ▁high ▁energy ▁they ▁make ▁excellent ▁sites ▁for ▁the ▁nucle ation ▁of ▁precip it ates ▁and ▁other ▁second - ph ases ▁e . g . ▁M g – Si – C u ▁phases ▁in ▁some ▁al umin ium ▁allo ys ▁or ▁mart ens ite ▁plat lets ▁in ▁steel . ▁Depending ▁on ▁the ▁second ▁phase ▁in ▁question ▁this ▁may ▁have ▁positive ▁or ▁negative ▁effects . ▁ ▁R ules ▁of ▁gra in ▁growth ▁Gra in ▁growth ▁has ▁long ▁been ▁studied ▁primarily ▁by ▁the ▁exam ination ▁of ▁section ed , ▁pol