An experimental study of cyclic deformation of extruded AZ61A magnesium alloy

Abstract

Thin-walled tubular specimens were employed to study the cyclic deformation of extruded AZ61A magnesium alloy. Experiments were conducted under fully reversed strain-controlled tension–compression, torsion, and combined axial–torsion in ambient air. Mechanical twinning was found to significantly influence the inelastic deformation of the material. Cyclic hardening was observed at all the strain amplitudes under investigation. For tension–compression at strain amplitudes higher than 0.5%, the stress–strain hysteresis loop was asymmetric with a positive mean stress. This was associated with mechanical twinning in the compression phase and detwinning in the subsequent tension phase. Under cyclic torsion, the stress–strain hysteresis loops were symmetric although mechanical twinning was observed at high shear strain amplitudes. When the material was subjected to combined axial–torsion loading, the alternative occurrence of twinning and detwinning processes under axial stress resulted in asymmetric shear stress–shear strain hysteresis loops. Nonproportional hardening was not observed due to limited number of slip systems and the formation of mechanical twins. Microstructures after the stabilization of cyclic deformation were observed and the dominant mechanisms governing cyclic deformation were discussed. Existing cyclic plasticity models were discussed in light of their capabilities for modeling the observed cyclic deformation of the magnesium alloy.

Introduction

With excellent properties such as low density, high specific strength, high damping capacity, and good recyclability, magnesium alloys have been increasingly applied to structural components in the automotive and aerospace industries to reduce fuel consumption and greenhouse gas emissions (Eliezer et al., 1998). The structural components in the transportation vehicles unavoidably experience cyclic loading, which results in fatigue failure. An understanding of cyclic deformation and fatigue is critical to the design and durability evaluation of engineering components.

Magnesium alloys have a hexagonal close packed (HCP) structure with very limited number of slip systems distributed asymmetrically over the crystallographic reference sphere. In order to satisfy the von Mises criterion which requires five independent deformation systems for an arbitrary homogeneous straining, various primary and secondary slip and twinning mechanisms have to be activated simultaneously (Graff et al., 2007). The relative prevalence of deformation mechanisms depends strongly upon the crystal orientation, temperature, and loading mode. The dominant slip system of magnesium at room temperature is slip in the close packed direction $〈11\overline{2}0〉$ or 〈a〉 on the basal plane (Roberts, 1960). The critical resolved shear stress (CRSS) of basal slip in pure magnesium is approximately 0.5 MPa (Burke and Hibbard, 1952, Kelly and Hosford, 1968). When the crystal is unfavorably oriented for basal slip, non-basal slip systems, such as prismatic 〈a〉 slip (Hauser et al., 1956, Reed-Hill and Robertson, 1957b, Yoshinaga and Horiuchi, 1963, Couret and Caillard, 1985, Koike and Ohyyama, 2005), pyramidal 〈a〉 slip (Burke and Hibbard, 1952), and pyramidal 〈c + a〉 slip (Reed-Hill and Robertson, 1958, Tegart, 1964, Stohr and Poirier, 1972, Obara et al., 1973, Ando and Tonda, 2000) can be activated. The CRSSs for non-basal slips are two orders of magnitude higher than that of the basal slip (Kelly and Hosford, 1968). In magnesium, two common twin modes, $\{10\overline{1}2\}〈10\overline{1}\overline{1}〉$ and $\{10\overline{1}1\}〈10\overline{1}2〉$, are observed, and $\{10\overline{1}2\}〈10\overline{1}\overline{1}〉$ is the most commonly and easily activated twin (Roberts, 1960, Partridge, 1967, Kelly and Hosford, 1968, Yoo, 1981). Since the c/a ratio of magnesium is smaller than the ideal hard-sphere value of $\sqrt{3}$, the $\{10\overline{1}2\}$ twin is a “tension” twin: its activation is associated with extension parallel to the c-axis of the crystal, while the $\{10\overline{1}1\}$ twin is a “compression” twin (Yoo, 1981). $\{10\overline{1}2\}$ “secondary” twinning or “retwinning” can occur in $\{10\overline{1}1\}$ primary twin, which rotates the basal plane into a position favorable for glide (Barnett et al., 2008). Due to the polar nature of twinning and the strong interaction between dislocation slip and mechanical twins, magnesium and its alloys show unique cyclic deformation behavior (Staroselsky and Anand, 2003).

Cyclic experiments on single crystals of HCP metals for various crystallographic orientations are sophisticated, and little has been done (Armstrong and Horne, 1963, Partridge, 1965a, Partridge, 1965b, Partridge, 1965c, Stevenson and Vander Sande, 1974, Kwadjo and Brown, 1978). Work on cyclic deformation of magnesium single crystals has been limited to crystals oriented for single slip on the basal plane and with a limited range of strain amplitudes (Stevenson and Vander Sande, 1974, Kwadjo and Brown, 1978, Ando et al., 2006). At a low plastic strain amplitude, the cyclic deformation of magnesium is essentially caused by single slip, and the hardening rate and saturation stress are low. At high plastic strain amplitudes, the cyclic plasticity is due to duplex basal slip, resulting in a higher saturation stress. The $\{10\overline{1}2\}$ twinning was found to form only during the compressive half cycle and minor detwinning was observed (Stevenson and Vander Sande, 1974). Also, very limited work has been conducted on fatigue and fracture of magnesium single crystals. Kwadjo and Brown (1978) and Partridge (1965b) observed persistent slip bands and slip band extrusion in fatigued magnesium. Partridge (1965c) and Reed-Hill and Robertson, 1957a, Reed-Hill and Robertson, 1957c observed fatigue cracks in mechanical twinning. When the crystals are unfavorably oriented for slip and twinning, cleavage fracture takes place on four different planes: $\{0002\}\text{,}\{10\overline{1}1\}\text{,}\{10\overline{1}2\}$, and $\{10\overline{1}0\}$ (Schmid and Boas, 1935).

Extensive studies have been conducted on fatigue properties of magnesium alloys under constant-amplitude loading, as being reviewed by Ogarevic and Stephens (1990) and Potzies and Kainer (2004). Magnesium alloys are usually classified into casting and wrought (rolled, extruded, and forged) alloys. Casting magnesium alloys have defects such as casting pores and inclusions, whereas wrought alloys are casting defect-free. Therefore, wrought magnesium alloys exhibit superior fatigue properties and are appropriate for the study of the intrinsic fatigue mechanisms of magnesium alloys (Agnew and Duygulu, 2005, Bettles and Gibson, 2005, Lou et al., 2007). However, most of the earlier studies were on the fatigue resistance of cast magnesium alloys and the fatigue data on the wrought magnesium alloys were limited. In the past several years, significant work was been done on the cyclic deformation and fatigue of wrought magnesium alloys, such as AZ31, ZK60, AM30, and AM50. AZ31 and ZK60 are among the most common wrought magnesium alloys and AM30 and AM50 were newly developed wrought magnesium alloys with improved ductility and formability (Chen et al., 2007, Begum et al., 2008).

Extruded and rolled magnesium alloys have strong basal plane textures. The c-axis of most grains is perpendicular to the direction of extrusion and parallel to the sheet normal. Although there are differences in chemical composition and thermo-mechanical processing, these wrought magnesium alloys exhibit similar unusual plastic deformation characteristics (Hasegawa et al., 2007, Lou et al., 2007, Lee et al., 2008, Wu et al., 2008b, Lin and Chen, 2008, Fan et al., 2009). The materials show a pronounced difference in yield stress between tension and compression. Under strain-controlled tension–compression at a relatively large strain amplitude, the hysteresis loops exhibit an asymmetry with a positive mean stress, significant Bauschinger effect, and strong asymmetry in flow curves. During the uniaxial tensile deformation starting from the undeformed state, the hardening curve is a normal concave-down shape. Unusual concave-up or sigmoidal-shaped hardening curves are observed during in-plane compression or tension following compression. The evolution of microstructures of wrought magnesium alloys during cyclic deformation was observed by Brown et al. (2007) and Wu et al. (2008a) via in situ neutron diffraction and by Yin et al. (2008a) via in situ electron backscattered diffraction (EBSD). These observations revealed that the unique inflected flow curves were caused by the activation of twinning in compression and detwinning in the subsequent tension. The unique orientation relationship between the parent grains and the twin grains favors detwinning during the subsequent loading reversal. Wu et al., 2008a, Wu et al., 2008b observed that more twinning occurred as the loading cycle increased but saturated after a certain number of cycles. However, a small volume fraction of residual twins gradually increased with increasing loading cycles, which may dictate the low-cycle fatigue behavior of the magnesium alloy.

Under fully reversed strain-controlled tension–compression, it was found that the asymmetry of stress–strain hysteresis loop, the plastic strain amplitude, the mean stress, and the stress amplitude increased with increasing total strain amplitude (Lin and Chen, 2008). Begum et al. (2009b) studied the effect of strain ratio and strain rate on cyclic deformation characteristics and fatigue behavior of an extruded AZ31 alloy. With an increase in strain ratio, a stronger cyclic hardening rate, more asymmetric stress–strain hysteresis loop, smaller stress amplitude, lower mean stress, and higher initial plastic strain amplitude were observed due to increased compressive stress. Under strain-controlled cyclic loading, the asymmetry of the stress–strain hysteresis loops persisted until fatigue failure; while under stress-controlled cyclic loading, the asymmetric stress–strain hysteresis loops only existed in the initial cycles and disappeared gradually with the increase in loading cycles (Yin et al., 2008a, Hasegawa et al., 2007). Wu et al. (2008a) compared the stress–strain response of ZK60 loaded at a strain amplitude of 1.2% for specimens taken along different material directions. The stress–strain hysteresis loops are asymmetric between the compression and tension when loaded in the extruded direction but symmetric when loaded in the transverse direction. Matsuzuki and Horibe (2009) noticed that there was little difference in cyclic deformation behavior between extruded AZ31 and annealed AZ31 alloys. Bentachfine and Pluvinage (1996) examined the biaxial fatigue behavior of a magnesium–lithium alloy under axial–torsion nonproportional loading. It was found that the life duration depended on the phase angle between axial and torsion loading. The twinning density depended on the phase angle and displayed a minimum at an angle of 90°. To the best of authors’ knowledge, no one else has conducted multiaxial cyclic deformation experiments on magnesium alloys.

Wrought AZ61A magnesium alloy is one of the most common commercial magnesium alloys, but very limited work has been done on its fatigue properties (Shih et al., 2002, Chamos et al., 2008, Sajuri et al., 2006, Zeng et al., 2009, Li et al., 2010a, Li et al., 2010b). Experimental studies of the cyclic deformation of the material were even much less. The current work was dedicated to an experimental investigation of cyclic plasticity of extruded AZ61A alloy using tubular specimens subjected to strain-controlled tension–compression, cyclic torsion, and both proportional and nonproportional axial–torsion loading.