Effect of porosity and tension–compression asymmetry on the Bauschinger effect in porous sintered steels X. Deng, G.B. Piotrowski, J.J. Williams, N. Chawla * Department of Chemical and Materials Engineering, Fulton School of Engineering, Arizona State University, P.O. Box 876006, Tempe, AZ 85287-6006, USA Available online 1 August 2005 Abstract The Bauschinger effect is a common phenomenon in metallic materials. In this paper, the Bauschinger effect in Fe–0.85Mo–2Ni powder metallurgy (PM) steel was investigated for different porosities and as a function of loading sequence (compression–tension versus tension– compression). Both the porosity and loading sequence had a significant effect on the magnitude and asymmetry of Bauschinger effect. Compression followed by tensile loading lead to a higher Bauschinger effect than tension followed by compression. This asymmetry of Bauschinger effect was more significant for higher porosity. Crack formation and propagation, observed in this study, were the main factors influencing the asymmetry in Bauschinger effect. Finite element analysis, based on the actual microstructure of the steels, yielded good agreement with the experimental stress-strain behavior. FEM showed that both the Bauschinger effect in the steel matrix and porosity contribute to the global Bauschinger effect of the PM steels. q 2005 Elsevier Ltd. All rights reserved. Keywords: Bauschinger effect; Tension; Compression; Powder metallurgy; Steel Sintering 1. Introduction The Bauschinger effect is defined as the phenomenon, where the strength of a material on forward loading (e.g. in tension) is reduced upon reverse loading (e.g. in compression). An important example of the practical importance of the Bauschinger effect is in sheet metal forming or rolling. While forming or rolling may be done in one direction, the stress in service may be applied in the reverse direction. From a fundamental point of view, knowledge of the Bauschinger effect is a necessary prerequisite to understanding and developing models to understand the cyclic fatigue behavior of metallic materials and composites [1]. In monolithic metals, the Bauschinger effect has been attributed to long-range internal stresses caused by dislocation pileup. Upon reverse loading, dislocation motion is aided by the internal stress or ‘back-stress’ from pile-up [2–8]. Orowan [8] proposed that in particle-hardened materials, the barriers ahead of the dislocations are denser and stronger than the barriers behind them, so reverse deformation is easier. In heterogeneous matrials, such as ceramic particle reinforced metal matrix composites, the Bauschinger effect is influenced by the nature of the reinforcement and residual stresses [9–14]. In addition, in composites, the nature and magnitude of the initial loading direction has a significant influence on the degree of Bauschinger effect. A more significant Bauschinger effect was observed under compression–tension loading sequence than for tension–compression sequence. This asymmetry in the Bauschinger effect was attributed to thermal residual stresses from processing [9,10,14]. In sintered steels, the origin of the Bauschinger effect is quite different from that in monolithic metals or composites. This can be attributed to the heterogeneous microstructure caused by inherent porosity. A large number of studies have focused on the mechanical behavior of porous metals [15–31]. In particular, these have focused on tensile behavior [15–18], strength differential in tension and compression [19], fatigue behavior [20,21], crack propagation and fracture [22–24], and analytical modeling [25–31]. Very few studies have focused on the Bauschinger effect [32,33]. The porosity appears to affect the Bauschinger effect in two ways. First, the pores will experience large volume expansion in tension and shrinkage in compression. Thus, pore growth is much International Journal of Fatigue 27 (2005) 1233–1243 www.elsevier.com/locate/ijfatigue 0142-1123/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2005.06.041 * Corresponding author. Tel.: C1 480 9652402; fax: C1 480 9650037. E-mail address: nchawla@asu.edu (N. Chawla).