www.elsevier.com/locate/jmbbm Available online at www.sciencedirect.com Research Paper Fatigue properties of magnesium alloy AZ91 processed by severe plastic deformation Stanislava Fintova ´ a,b,n , Ludvı ´k Kunz b a Brno University of Technology, CEITEC BUT—Central European Institute of Technology, Technická 3058/10, 616 00 Brno, Czech Republic b Institute of Physics of Materials AS CR, Žižkova 22, 616 62 Brno, Czech Republic article info Article history: Received 24 September 2014 Received in revised form 21 November 2014 Accepted 23 November 2014 Available online 29 November 2014 Keywords: AZ91 magnesium alloy ECAP Fatigue Crack initiation abstract Fatigue properties of cast AZ91 magnesium alloy processed by severe plastic deformation were investigated and compared with the properties of the initial cast state. The severe plastic deformation was carried out by equal channel angular pressing (ECAP). The ECAP treatment resulted in a bimodal structure. The bimodality consists in a coexistence of fine grained areas with higher content of Mg 17 Al 12 particles and areas exhibiting larger grains and lower density of Mg 17 Al 12 particles. Improvement of the basic mechanical properties of AZ91 (yield stress, tensile strength and ductility) by ECAP was significant. Also the improvement of the fatigue life in the low- cycle fatigue region was substantial. However the improvement of the fatigue strength in the high-cycle fatigue region was found to be negligible. The endurance limit based on 10 7 cycles for the cast alloy was 80 MPa and for the alloy processed by ECAP 85 MPa. The cyclic plastic response in both states was qualitatively similar; initial softening was followed by a long cyclic hardening. Fatigue cracks in cast alloy initiate in cyclic slip bands which were formed in areas of solid solution. In the case of severe plastic deformed material with bimodal structure two substantially different mechanisms of crack initiation were observed. Crack initiation in slip bands was a preferred process in the areas with large grains whereas the grain boundaries cracking was a characteristic mechanism in the fine grained regions. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Magnesium alloys are traditionally used materials for appli- cations in many engineering fields mainly in automotive, aerospace and electronic (Gupta and Sharon, 2011). Recently, their importance markedly increased also in medical applica- tions. The large spectrum of their utilization is based on their low weight but still good mechanical properties, which can be influenced in broad range by changes of chemical composi- tion (Gupta and Sharon, 2011; Altwicker et al., 1939; Němcová http://dx.doi.org/10.1016/j.jmbbm.2014.11.019 1751-6161/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author at: Brno University of Technology, CEITEC BUT—Central European Institute of Technology, Technická 3058/10, 616 00 Brno, Czech Republic. Tel.: þ420 532 290 301; fax: þ420 541 218 657. E-mail address: fintova@ipm.cz (S. Fintová). journal of the mechanical behavior of biomedical materials 42(2015)219–228