Achieving ultra-high strength of Al-Cu-Li and Al-Zn-Mg-Cu alloys by the combination of High Pressure Torsion and age hardening

Abstract

This report presents an experimental study on two types of heat treatable aluminium alloys to determine the best process for achieving ultra-high-strength materials by means of severe plastic deformation (SPD) and heat treatment. It is well-known that age hardening and grain refinement are two mechanisms that contribute to strengthening. Accordingly, high pressure torsion (HPT), which induces an ultra-fine grain (UFG) structure, combined with artificial ageing were performed on an Al-Cu-Li and an Al-Zn-Mg-Cu alloy. A series of hardness measurements against various ageing times and temperatures were investigated to provide an insight into strengthening. A strong increase in hardness of Al-Cu-Li alloys was achieved through the combination of age hardening and HPT. Specifically, following solution treatment, materials were processed through five turns of HPT at room temperature (RT), followed by low temperature ageing (i.e. T4-HPTAA). For the Al-Cu-Li alloy, a micro-hardness up to ~240 Hv for ageing 110 ^oC/60h with the maximum percentage increase of 8.5% after as-HPT condition was achieved. However, the low temperature ageing after HPT does not improve the hardness of AlZn-Mg-Cu alloy. Besides, a further improvement in the hardness to ~260 Hv for AlCu-Li was accomplished by a pre-ageing 110 ^oC/24h before HPT in combination with a post-HPT ageing process at 110 ^oC for approximately 180h (i.e. T6-HPT-AA). These novel multi-stage processes give rise to an increase in hardness by a factor of 2 as compared to the T4 condition (~120 Hv). Under the same process sequence using similar ageing temperatures, the Al-Zn-Mg-Cu alloy shows post-HPT age-softening. However, a reduced age-softening rate was obtained compared with the sample that had undergone solution treatment followed by HPT and ageing process, which connotes the stability of the material increased due to pre-HPT ageing. As both T4-HPT-AA and T6-HPT-AA processes do not enhance the hardness of the Al-Zn-Mg-Cu alloy, further investigations in microstructural analysis were performed only on Al-Cu-Li alloy. Accordingly, X-ray diffraction (XRD), Transmission electron microscopy (TEM), Differential scanning calorimetry (DSC) and Atom probe tomography (APT) characterisation techniques were conducted on the optimum condition processed Al-Cu-Li alloy in all stages of two processing procedures (T4- HPT-AA and T6-HPT-AA) from solution treatment to final ageing hardening. Aimed at improving the understanding of the combined strengthening effects of HPT and age hardening on the strength of the 3rd generation Al-Cu-Li alloy. Grain size of T4 and T6-processed samples was dramatically refined during HPT from microscale level ~3 µm down to ~ 90 nm and ~ 115 nm, respectively. Such small grain size (UFG structure) was retained after subsequent peak ageing 110 ^oC /60h in T4-HPT-AA and 110 ^oC /180h in T6-HPT-AA conditions. In addition, HPT introduces large amount of dislocations with the highest dislocation density of 3.50×1014 m-2 in T4-HPT condition. No long-range ordered precipitates were observed by both XRD and TEM techniques after HPT and subsequent ageing treatments. Instead, atom probe tomography (APT) provided clear evidence that Cu-Mg co-clusters are homogeneously distributed in the matrix of T4 and T6 processed samples, and they segregate strongly to the grain boundaries (GBs) during HPT. Further ageing treatment after HPT leads to the segregation of clusters at the dislocations. Finally, a strengthening model that incorporates dislocation hardening, grain boundary hardening, solid solution strengthening and a new short-range order cluster strengthening mechanisms was used to predict the yield strength of the Al-Cu-Li alloy. In this work, for the first time, a new cluster strengthening model is addressed to provide detailed explanations for each individual type of cluster strengthening mechanism, i.e. clusters in matrix, at grain boundaries and dislocations. The predicted strength from the model demonstrates that the combined effect of all three types of Cu-Mg clusters is the dominant mechanism for the high strength of the Al-Cu-Li alloy.

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ORCID for Nong Gao: orcid.org/0000-0002-7430-0319

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