98 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII Influence of the corrosive environment on the Portevin–Le Chatelier plastic instability phenomenon in Al–1Mg and Al–3Mg model alloys Aleksandra Towarek * , Joanna Zdunek, Jarosław Mizera, Anna Dobkowska, Rafał Molak Materials Science and Engineering Faculty, Warsaw University of Technology, Warsaw, * aleksandra.towarek@wimpw.edu.pl The aim of this work was to indicate the influence of changes on the surface of materials caused by the corrosive medium (3.5% NaCl solution) on the in- tensity and character of the Portevin–Le Chatelier effect in model aluminium alloys. For this purpose, two Al alloys, containing 1% and 3% of magnesium, were subjected to tensile testing in the as-cast state and after the exposition in NaCl solution for various time. Several electrochemical measurements were held to determine materials’ corrosive behaviour and microscopic observations to evaluate the surface character of the samples. High corrosion resistance of the materials resulted in a very slight alteration of their surface development, which didn’t lead into any significant variations in the plastic instability phenomenon PLC. Key words: Portevin–Le Chatelier effect, aluminium alloys, plastic instability, corrosion. Inżynieria Materiałowa 3 (211) (2016) 98÷103 DOI 10.15199/28.2016.3.1 © Copyright SIGMA-NOT MATERIALS ENGINEERING 1. INTRODUCTION Portevin–Le Chatelier effect (PLC effect) is a well-known phe- nomenon occurring in many aluminium alloys during deformation [1], manifesting itself in a form of characteristic serrations on the stress–strain curve, caused by rapid changes of force in small exten- sions. It can lead to inhomogeneous deformations in materials mi- crostructure, resulting in the deterioration of its mechanical proper- ties [2]. Apart from the most commonly analysed structural factors, like grain size, precipitations or texture, there are some extrinsic ones which can also strongly influence materials vulnerability to PLC effect [3]. Abduluyahed in his research [4, 5] compared serrated flow in 316 and 316L austenitic steels during tensile testing in air and in vacuum. He reported a considerable decrease in serration frequen- cy in vacuum conditions, which can be subjected to the fact that there is no oxygen layer on the surface, which could crack during deformation and work as a stress concentration. Temperature was considered as a factor influencing PLC effect, by Yilmaz [6], who performed tensile testing of 1020 low-carbon steel in room tem- perature and in increased temperature ranging from 50 to 85°C, simultaneously registering potential variations on the sample sur- faces. According to his results, in the tested steel which didn’t ex- hibit jerky flow in the room temperature, it occurred while heating and kept on intensifying until a certain value, when it disappeared again. There was also a noticeable relationship between the PLC effect in enhanced temperature and the values of potential on the surface of samples. In the same publication [6] the author describes the difference in serrations character depending on the roughness of surface, using polished and unpolished samples. For the unpolished samples the PLC effect was much more intensive, the curves less regular and the strain propagation twice slower. The same behav- iour was observed by Abbadi [7] in 5000 aluminium alloy. Jerky flow did not occur in this material when the samples were properly polished before testing, but it was present when the surface of the samples was rough and unpolished. In both cases the explanation of this phenomenon is similar and analogical to that in materials exhibiting a brittle oxygen layer on the surface. All the irregularities on the rough surface work as tension concentrators, increasing the intensity of serrations. Zdunek in her work [3] related the character of PLC oscillations with the strain rate during deformation, describing three types of the oscillations possible. Type A oscillations are observed at high strain rates and have a form of sudden, irregular changes of stress with low amplitude. Type B oscillations occur with medium strain rates, have irregular stress amplitude, but a cyclic character. Type C static oscillations are characterized by regular, cyclical stress varia- tions with high amplitude and are observed at low strain rates only. Corrosion resistance of aluminium alloys depends mostly on their ability to develop a passive layer on the surface. It forms easily in atmospheric conditions, but in many cases has low homogeneity and thickness or poor cohesion to the surface. This results in the pit- ting corrosion of the material, when subjected to the environment containing aggressive ions, halides in particular [8]. Moreover, al- loying elements, such as magnesium, used to enhance mechanical properties of material, may deteriorate its corrosive performance. This is due to the fact that the formed intermetallic phases have dif- ferent structure and electrochemical activity then the matrix and can become the centers of corrosion or lead to the further inhomogene- ity of material [9]. Presence of magnesium can also cause the occur- rence of stress cracking corrosion in aluminium alloys, because the precipitations have a tendency to form on the grain boundaries and have a strongly anodic character. This may in result influence me- chanical properties and elongation of material during tensile testing [10]. 2. EXPERIMENTAL 2.1. Materials Two model aluminium alloys were used in the experiments. Al– 1Mg alloy, containing app. 1 wt % of magnesium and Al–3Mg with app. 3 wt % of magnesium. Full composition of the materials is shown in the Table 1. Presence of silicon in the alloys is a result of fabrication method and they have a form of primary, insoluble precipitations. The materials were obtained with semi-continuous casting and extrusion, in the form of rods with 20 mm diameter, and subjected to subsequent annealing. Both alloys are dual phased with equiaxial microstructure and distinctive grain boundaries. Preced-