Room Temperature Recovery of Cryogenically Deformed Aluminium Alloys

Abstract

Increased formability of aluminium alloys has been demonstrated via cryogenic deformation. In previous studies, the microstructures of samples deformed at low temperatures were analysed after reheating to room temperature (RT) and storage. However, after heating the dislocation structure and density of the deformed material do not reflect the cryogenic situation. In this work, we investigate the evolution of flow stress during recovery in Al-Mg and Al-Mg-Si alloys. We examine the RT recovery behaviour of samples pre-strained at 77 K to different strain levels, and evaluate the structural stability upon subsequent deformation. We also study microstructural evolution via in-situ synchrotron X-ray diffraction, starting from initial conditions at cryogenic temperatures to long-term RT-recovery. Recovery of cryogenically deformed samples at RT results in reduction of the flow stress, in dependence on RT storage. The recovery process can be divided into three distinct sections, each based on a different mechanism characterized by either the arranging or the annihilation of dislocations. Subsequent further straining at room temperature after cryogenic forming also generates plastic instabilities and premature fracture due to unfavourable hardening and recovery assisted softening interplay.

Ageing Behaviour of Al–Mg–Si Alloys After Cryogenic and Room Temperature Deformation

“…At 20% strain, a strength level has been already reached which exceeds the ultimate tensile strength achieved at RT. Therefore, in the case of direct subsequent deformation, early failure occurs ( Figure 5), as reported in a previous study [22]. After the heat treatment, these samples have a similar ultimate elongation compared to the RT samples and no damage is done to the material due to the cryogenic deformation results.…”

“…Figure 1 shows that not only are the initial values elevated, but also the artificial ageing curves up to 30 min are shifted to greater values, due to LN 2 pre-straining. Two potentially overlapping effects can be assumed: (i) The dislocation densities are greater at the beginning of the heat treatment and remain so, to a certain extent, for up to 30 min at 185 • C, despite an increased recovery rate [22]; (ii) The ageing behaviour changes due to cluster formation at dislocations [15,16,[18][19][20] and is even more accelerated due to greater dislocation density.…”

“…Obviously, the pre-deformation is too low to result in noticeable differences between RT and LN 2 . Please note that the tensile samples were stored at RT for two weeks prior to the paint bake treatment, which has been shown to already reduce increased dislocation density created via deformation in cryogenic conditions [22]. This can explain the small difference between RT and LN 2 yield strength before heat treatment, despite the increased strain hardening at low temperature pre-deformation.…”

“…The stronger the applied pre-deformation, the more pronounced this plateau is and the longer the heat treatment is necessary to restore an original flow onset. This is the result of simultaneous processes during further deformation: the competition between strain hardening and softening due to dynamic recovery [22]. The flow onset is scarcely discussed in previous publications on RT pre-deformation, as most studies do not show tensile test curves.…”

“…When the measurements were conducted at cryogenic temperatures (LN 2 ), the reduced section of the tensile testing sample, as well as the extensometer, were kept in a Dewar filled with liquid nitrogen. Previous measurements with this setup are explained in detail by Gruber et al [22]. After pre-straining and two weeks of RT storage, the samples were artificially aged for different durations.…”