The variation of work hardening characteristics of Al-5 wt% Mg alloy during phase transition M.A. Mahmoud a,n , M. Sobhy a , A.F. Abd El-Rehim a , R.M. Abdel Rahman b a Physics Department, Faculty of Education, Ain Shams University, P.O. Box 5101, Heliopolis 11771, Roxy, Cairo, Egypt b Modern Academy for Engineering and Technology in Maadi, Basic Science Department, Physics Section, Cairo, Egypt article info Article history: Received 24 March 2010 Received in revised form 15 May 2010 Accepted 17 May 2010 Keywords: Work hardening characteristics Al–Mg alloys Precipitates abstract The aim of this study is to investigate the effect of aging conditions on the stress strain behavior along with microstructure changes of the Al-5 wt% Mg alloy. Following solid solution treatment and aging of specimens at temperatures ranging from 373 to 573 K for various aging times (1/4 to 4 h), stress  strain tests were performed at different testing temperatures (313 343 K). The work hardening parameters (s y , s f , w p and Y) were found to decrease continuously with pre-aging times at all aging and testing temperatures, where the softening parameters (e f and L) oppose this behavior. The variation in stress strain parameters with increasing aging temperatures and aging times was explained on the basis of structural transformations taking place in the Al Mg alloy. A precipitate-dislocation intersections mechanism was assumed as the rate-controlling mechanism for alloy. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Al–Mg alloys are widely used in the automotive and marine industries offshore constructions and in materials subjected to cryogenic conditions. Examples of use are hull plates for ships, body plates for cars, helicopter decks, buildings, containers and tanks for storing or transportation of liquid gases. To stabilize plastic and corrosion properties of Al–Mg alloys, it is important to understand the structural changes that occur during the aging processes [1]. The traditional heat treatment procedure of a precipitation-hardened alloy comprises: (i) an initial solution heat treatment at a temperature at which the precipitating elements are brought into solid solution followed by quenching usually at 273 K, and (ii) a final annealing treatment or aging at a temperature at which precipitates can nucleate, grow and coarsen to some extent. It is well known that these reactions (nucleation, growth and coarsening) can be strongly influenced by the time of aging of the alloy [2]. Experimentally the observed aging sequence in Al–Mg alloys is [3–8]. supersaturated ðaÞ solid solution-G:P: zones-b 00 -bu-b: It has been suggested [8] that G.P. zones that have been formed during the quenching process or after aging the Al–Mg alloy at low temperatures act as nuclei for the precipitates. In contrast, Asano et al. [9] suggested that no evidence has been obtained for G.P. zones acting as nuclei for the precipitates. On the other hand, it has been reported [10–12] that there exist two types of nuclei for the precipitates in the Al–Mg alloy, one with solute-rich clusters G.P. zones and another with vacancy-rich clusters. In general, commercial Al–Mg alloys contain impurities of Si. In particular, Si atoms can react with Mg atoms to form Mg 2 Si particles. Such particles may act as nucleation sites for phase’s formation during aging processes [13]. In the lowest temperature range, the first phase to be formed is b 00 -precipitates [Al 3 Mg, also called G.P. zones II or ordered G.P. zones] just after the appearance of G.P. zones I in the matrix [14]. Differential scanning calorimetry (DSC) generally reveals the existence of G.P. zones and b 00 - precipitates in the range 50–100 1C [2,14,15]. As G.P. zones and b 00 -precipitates are coherent and appear approximately at the same temperature interval [6,16], it is difficult to separate them. At higher temperature, b 0 particles (Al 3 Mg 2 , hexagonal) and b particles (Al 3 Mg 2 , complex fcc) have been formed in the matrix [8,14,17]. Generally, b 0 -precipitates form first, while the b-precipitates appear only in the later stages of aging when the Mg depletion of the matrix is nearly complete [18]. The study of precipitation of the b 0 - and b-precipitates in Al–16 at% Mg by DSC and transmission electron microscopy (TEM) showed that there is no direct transformation from b 00 to b 0 , and that b 0 -precipitates do not form on defects, although an abundant presence of vacancy type defects (mostly dislocation loops) was observed [15]. Al–Mg alloys play an important role in engineering applica- tions, on account of their excellent mechanical and electrical properties as well as corrosion resistance. Research and develop- ment work in the field of Al–Mg alloys, therefore, occupied a Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.05.051 n Corresponding author. E-mail addresses: moustafa_a_mahmoud@hotmail.com (M.A. Mahmoud), afabdelrehim@yahoo.com (A.F. Abd El-Rehim). Physica B 405 (2010) 3616–3623