Cryogenic Deformation Behaviour of Aluminium Alloy 6061-T6

Abstract

Aluminium alloy 6061-T6 (AA6061-T6) shows a promising potential for cryogenic structural applications. This alloy exhibits remarkable monotonic tensile properties at low temperatures. However, there is a limited number of studies on the cryogenic deformation behaviour. In this study, both monotonic and cyclic loading were conducted, and various microstructure characterisation techniques were performed to understand influence of cryogenic temperatures on microstructure evolution and deformation behaviour of this alloy. At cryogenic temperatures, the aluminium alloy exhibited superior mechanical properties over those at room temperature. Yield stress, UTS and elongation at failure increased by 18%, 33%, and 53% at 77 K compared to those at room temperature. Such increase in mechanical properties was attributed by the stronger resistance to dislocation movement due to the reduced thermal assistance. Work hardening rate also increased as dynamic recovery was suppressed at lower temperatures. As a result, a high density of dislocations was evenly distributed within grain interior and led to a homogeneous deformation. The test temperature appeared to have a significant influence on fatigue performance; maximum stress response increased by 23% at 108 K with respect to those at room temperature. During cyclic loading, a high number of dislocations was generated to accommodate prescribed strain because of the resistance to dislocation movement including the pinning of dislocations by β’’ precipitates which are known to be sheared at room temperature. Thus, the alloy exhibited an enhanced cyclic hardening behaviour without a noticeable cyclic softening phase. Fatigue life improved by 143% at 108 K with respect to that at room temperature as the homogeneous deformation prohibited localised slip activity and delayed formation of slip bands which act as crack initiation sites. Moreover, the initiation and propagation of secondary cracks at 108 K retarded the propagation of main crack to improve fatigue life. Graphical Abstract