Strong Plastic Deformation Induced By Al - 4 Wt. % Of Cu Alloy After Precipitation Phase To Dissolve And Aging Behavior Research

The re-dissolution of the second phase particles during severe plastic deformation (SPD) is a significant micro-phenomenon, which is of significance for developing a new heat-treatment technique of age hardening alloy. In this paper, the re-dissolution of the second phases and the subsequent re-aging behavior in Al-Cu binary alloy during severe plastic deformation (equal channel angualr pressing ECAP and multi-axial compressions MAC) was investigated by hardness measurement、 X-ray diffraction pattern analysis (XRD)、 transmission electron microscopy (TEM)、 differential scanning calorimetry (DSC) and three-dimensional atom probes (3DAP). The characteristics and mechanisms of reprecipitation of the second phase during re-aging after severe plastic deformation was studied. The main results are as follows:(1) The process of nanostructure formation in Al-4wt.%Cu binary alloy bearing θ" precipitates during ECAP and MAC was of a distinct step character. Work hardening was tipical for the first stage, in which the Vickers hardness increased rapidly. In the second stage, work hardening and softening due to the re-dissolution of the θ" phase occurred simultaneously. So we can observe that the hardness in this stage was almost invariable, even decreased during ECAP. Thereafter, in the third stage hardening due to grain size refinement became dominant. The grain size was progressively reduced, causing the hardness to re-increase gradually until it approached a saturated value.(2) Comparing with the ECAP processing with simple shear deformation, the θ" precipitates in the Al-Cu binary alloy dissolved more rapidly as the sample subjected to the MAC processing with partial pure shear deformation. As the deformation continued after the θ" precipitates re-dissoved into Al matrix, dynamic precipitation of θ phase took place.(3) The dissolution of the deformable θ" particles was attributed to the releasing of the strain energy and the subgrain boundary energy accumulated in the particles, as well as the θ’/matrix interface energy during SPD processing. Large volumes of SPD-induced defects attached to boundaries that extend into the matrix could facilitate the atom transfer and accelerated dissolution phenomenon.(4) The deformed microstructures recovered apparently during the subsequent re-aging treatment even at low temperature (≤100℃). The hardness increase was insignificant, even decreased continuously in this process due to the recovery softening.(5) In Al-4wt.%Cu alloy subjected to SPD-induced dissolution of the second phase, aging resulted in the precipitation of G.P. zones and equilibrium Al2Cu (0) phase on grain boundaries, skipping the formation of metastable0’and0" phases and the precipitation rate of0phase accelerated greatly. The formation of G.P. zones was mainly associated with the high vacancy density. Precipitation of equilibrium0phase was observed along the grain boundaries during re-aging. That could attribute to extremely increasing of nucleation sites for0phase at non-equilibrium grain boundaries (GB), as well as a large number of dislocations and vacancies acting as diffusion channel for Cu atoms.