DYNAMIC RESTORATION MECHANISMS IN HOT TORSION OF AL-5MG AND AL

Al99.9 and Al-5Mg were deformed in torsion at strain rates 10(-3)/s le ss than or equal to epsilon less than or equal to 4/s in the temperatu re range 473 K less than or equal to T less than or equal to 873 K. In addition to dynamic recovery (DRV), geometric dynamic recrystallisati on (DRX) is found to be a restoration mechanism in both materials, bec ause the large angle grain boundaries become serrated as the subgrain structure forms as a result of DRV. A simple model predicts that geome tric DRX by recombination of opposite serrated boundaries starts when epsilon = 0.013x(t)(-1)(d(0)/b)(sigma/G) (epsilon: strain, sigma: stre ss, d(0): initial grain size, x(t): serration amplitude, b: length of Burgers vector, G: shear modulus). The steady state DRX grain size is inversely proportional to sigma/G and thus proportional to the subgrai n size. The model explains quantitatively the extension of the observe d epsilon-T-field of geometric DRX. There are strong indications that in Al-Mg geometric DRX is enhanced by pinching off of unstable serrati ons.