Enhancing the fatigue property of rolled AZ31 magnesium alloy by controlling {10-12} twinning-detwinning characteristics
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Sung Hyuk Park and Chong Soo Leeb) Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea (Received 22 September 2009; accepted 21 December 2009)
An improvement of the fatigue resistance of rolled AZ31 magnesium alloy was attempted by reducing the tensile mean stress developed during fatigue deformation, which was achievable by tailoring the {10-12} twinning-detwinning characteristics of the material through the precompression process. The modification of the {10-12} twinningdetwinning characteristics made it possible to control the plastic deformation mechanisms activated during fatigue deformation so that the imposed tensile strain could be fully accommodated by detwinning alone, which led to a significant reduction of tensile flow stress, finally resulting in the reduction of mean stress. I. INTRODUCTION
Wrought magnesium (Mg) alloys are known to have an intense basal texture with most c-axes of the hexagonal close-packed (HCP) unit cells perpendicular to the process direction so that their plastic deformation along the principal direction of processing (i.e., the extrusion direction, any direction on the rolling plane, etc.) is significantly contributed to by the {10-12} twinning under compression but not under tension. This causes a considerable deformation anisotropy between tension and compression.1–10 The {10-12} twinning induces the lattice rotation of 86.3 in the twinned region and this reoriented lattice is favored for detwinning under subsequent load reversal. Hence, tensile deformation following compressive deformation can proceed with a significantly reduced flow stress as compared to tensile deformation alone.10–13 The fatigue deformation of wrought Mg alloys has also been reported to follow this mechanism, that is, the alternation of the {10-12} twinning and detwinning during each cycle.12–16 However, although the twins formed during compressive loading are almost completely recovered by detwinning, the detwinning starts from an early stage of deformation, even from compressive unloading (the significant internal tensile residual stress generated in the material during the twinning can drive the detwinning event even under compressive unloading, that is, no tensile reloading is required for Address all correspondence to these authors. a) e-mail: [email protected]; [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2010.0094 784
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J. Mater. Res., Vol. 25, No. 4, Apr 2010 Downloaded: 13 Mar 2015
detwinning16) so that it is exhausted at the early stage of tensile loading compared to the twinning under compressive loading. This requires the activation of slip mechanisms to accommodate the remainder of the tensile strain to be imposed, causing a significant increase in tensile flow stress.15,16 For this reason, stress-strain behavior of wrought Mg alloys during fatigue deformation becomes asymmetric between tension and compression, and a remarkable tensile mean stress, which is general
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