S. Fintová, L. Kunz: Cyclic plastic behavior and fatigue life of AZ91 alloy in as-cast and ultrafine-grained state Materials Engineering - Materiálové inžinierstvo 21 (2014) 109-115 109 CYCLIC PLASTIC BEHAVIOR AND FATGUE LIFE OF AZ91 ALLOY IN AS-CAST AND ULTRAFINE- GRAINED STATE Stanislava Fintová 1, 2, 3,* , Ludvík Kunz 2 1 CEITEC – Central European Institute of Technology, Brno University of Technology, Technická 3058/10, 61600 Brno, Czech Republic 2 Institute of Physics of Materials Academy of Sciences of the Czech Republic, v. v. i., Žižkova 22, 616 62 Brno, Czech Republic 3 University of Žilina, Faculty of Mechanical Engineering, Univerzitná 1, 010 26 Žilina, Slovak Republic * corresponding author: tel: +420 541 212 301, e-mail: fintova@ipm.cz. Resume Fatigue properties of magnesium alloy AZ91 in as-cast and in ultrafine-grained state prepared by equal channel angular pressing were investigated. The fatigue strength in the low-cycle fatigue region was found to be substantially improved by the severe plastic deformation, whereas the improvement in the high-cycle fatigue region is negligible. The cyclic plastic response in both states is qualitatively similar; short initial softening is followed by a long cyclic hardening. The observed fatigue behavior was discussed in terms of specific microstructural features of both states and on the basis of cyclic slip localization and fatigue crack initiation. Available online: http://fstroj.uniza.sk/journal-mi/PDF/2014/16-2014.pdf Article info Article history: Received 22 December 2013 Accepted 23 February 2014 Online 15 September 2014 Keywords: AZ91; ECAP; Bimodal structure; Fatigue; Plastic deformation; Crack initiation. ISSN 1335-0803 (print version) ISSN 1338-6174 (online version) 1. Introduction AZ91 magnesium alloy, the most popular magnesium alloy from AZ group, exhibits perfect castability and good mechanical properties combined with good corrosion resistance (for high purity version of the alloy) and low production costs. Because this alloy has been used in industry since more than eighty years [1] its mechanical and corrosion behaviour has been thoroughly investigated in the past and the results can be found in plenty of papers, e.g. [2 - 6] Beyond the favourable properties the alloy has also some disadvantages consisting mainly in its poor formability and limited ductility at room temperature. This is a natural consequence of its hexagonal close packed (hcp) structure with restricted number of available slip systems and presence of an intermetallic phase Mg 17 Al 12 [2 - 4, 7 - 10]. Within the last some years the alloy, primarily intended for castings, has been investigated with the aim to answer the question if severe plastic deformation (SPD) can influence its tensile strength,  UTS, yield stress,  y , and, particularly, improve the ductility due to the grain refinement. One of the most popular SPD techniques is equal channel angular pressing (ECAP). In the case of magnesium alloys the ECAP treatment has to be done at elevated temperatures by reason of activation of more slip systems in the hcp structure. Otherwise undesirable cracks appear in the processed material. It has been shown that the ultimate strength and ductility of AZ91 alloy can be significantly improved by ECAP treatment. The improvement of  UTS  y and ductility of AZ91 is more significant when compared to the other AZ magnesium alloys. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ or send a letter to Creative Commons, 444 Castro Street, Suite 900, Mountain View, California, 94041, USA. This copy of the article was downloaded from http://www.mateng.sk , online version of Materials Engineering - Materiálové inžinierstvo (MEMI) journal, ISSN 1335-0803 (print version), ISSN 1338-6174 (online version). Online version of the journal is supported by www.websupport.sk .