Materials Science and Engineering, 56 (1982) 97 - 115 97 Creep Response of Strip-cast Al-lwt.%Mn-lwt.%Mg Alloy to Thermomechanical Treatment w. BLUM, R. KREUTZER and H. MULLER Institut fiir Werkstoffwissenschaften L UniversiM't Erlangen-Niirnberg, Martensstrasse 5, D 8520 Erlangen (F.R.G.) G. G. SAHA Aerial Delivery Research and Development Establishment, Agra (Ind&) P. RAMO RAO* Department o f Metallurgical Engineering, Banaras Hindu University, Varanasi (India) (Received December 28, 1981) SUMMARY Strip -cast A l- 1 w t. %Mn- 1 w t. %Mg was sub - ]ected to thermomechanical treatment (TMT) in order to combine a dislocation substructure of submicron initial subgrain size Lo with phase structure hardening, manganese either being precipitated or not. The influence of TMT on the mechanical properties was investigated between room temperature and 573 K by means of tests at a constant elonga- tion rate and creep tests. The microstructure was investigated using transmission electron microscopy. TMT enhances the room tem- perature yield strength Oy according to Oy = o ° + kL~(' where o ° and k are constants. At elevated temperatures the maximum strength is reached under most conditions after a small amount of strain (a few per cent). This is due to the microstructural changes that accom- pany deformation. The subgrain size changes during deformation from Lo towards a steady state value L s, which in material which has no precipitated manganese is inversely propor- tional to stress. Under conditions where Ls ~ Lo, substructural hardening is prominent. However, for L s >> Lo, there is virtually no hardening effect. Under intermediate condi- tions there is an initial strain range in which the specimen work hardens while the sub- structure coarsens. This is interpreted in terms of glide associated with recovery of the dislocation structure. *Present address: Defence Metallurgical Research Laboratory, Hyderabad 500258, India. 1. INTRODUCTION A fine subgrain structure introduced through thermomechanical treatment (TMT) can improve the strength properties of metals and alloys [ 1 ], A striking method employed in recent attempts to improve the properties of high strength aluminium alloys is to combine TMT and alloying additions to optimize mechanical properties and corrosion resistance [2]. Significant improvements in room temperature strength and stress corrosion cracking resistance have thus been achieved [3 - 8]. However, in so far as creep resistance is concerned, a pre-induced dis- location substructure via TMT in several aluminium alloys (e.g. those of the 2000 and 7000 series, as well as RR58, which are particle strengthened) was found to be either ineffective or deleterious [9 - 12]. In the present A1-Mn-Mg alloy which has as much as 1.2% Mn in solid solution in the strip-cast condition, on the basis of the published transformation behaviour [13 - 25] the presence of manganese was expected to retard drastically all diffusion-controlled reactions such as recrystallization and precipi- tation. With manganese in solid solution the recovery-annealing (RA) temperatures in the alloy could easily be higher than the intended creep test temperatures, namely around 475 K, and the substructure could be stabi- lized through solute pinning. The choice of this alloy was thus principally dictated by our quest for a satisfactory answer to the problem of preserving a stable submicron subgrain structure which then would give rise to significant creep strengthening, a problem 0025-5416/82/0000-0000/$02.75 © Elsevier Sequoia/Printed in The Netherlands