Effect of pre-rolling temperature on microstructures, tensile properties and fracture behaviors of Al-5.9Zn-1.9Mg alloy during thermomechanical treatment

Abstract

The microstructures, mechanical properties, and fracture behaviors of an Al-5.9Zn-1.9Mg alloy subjected to thermomechanical treatment across different pre-rolling temperatures have been exhaustively investigated in present work. The pre-deformation temperature exerts a modest influence on grain morphology, while it profoundly impacts the dislocation configurations and precipitation behaviors. Elevating the rolling temperature from ambient to 170 ℃ results in a reduction in dislocation density within grains accompanied by a notable enhancement in their distributional uniformity. While advancing the temperature to 320 ℃ prompts the premature formation of precipitates during deformation, which diminishes the precipitation during the subsequent ageing. Tensile results reveal that the thermomechanical treatment incorporating pre-rolling at 170 ℃ confers a substantial strengthening effect on the alloy on the basis of both grain boundary strengthening and dislocation strengthening stemmed from pre-deformation along with the precipitation strengthening generated by ageing. Furthermore, the microstructure exhibits a relatively scarce presence of inhomogeneous features such as dislocation pile-ups and micro shear bands, contributing favorably to enhance the ductility of the alloy that presents the mixture of cleavage fracture and dimple-induced failure.

1. Introduction

Al-Zn-Mg-(Cu) alloy has been widely used in aerospace and other industrial fields due to its desirable comprehensive properties including high strength, toughness and failure resistance. As a kind of ageing-strengthening alloy, its main ageing precipitation sequence is determined as supersaturated solid solution (SSSS) → GP-zone → η′ → η-MgZn2 [1−3]. Conventional peak ageing, i.e. ageing after solution quenching, mainly depends on precipitation strengthening to improve mechanical properties of the alloy. It is usually considered that the main matrix precipitates (MPs) are metastable η′, while the grain boundary precipitates (GBPs) are equilibrium phase η with larger size [4 −7], which are contributed from the larger coarsening rate of GBPs due to the distortional strain energy [8, 9]. Evidently, both the nucleation and growth of the precipitates are close related to the microscopic defects such as dislocations and internal boundaries. Therefore, the strength of the alloy can be regulated by increasing the plastic deformation before ageing to affect the precipitation of precipitates [10 −13], and the thermomechanical treatment (TMT) has been developed based on this strategy [14−18].

The enhancement of mechanical properties of the alloy by processing route related to pre deformation and its strengthening effects has been widely confirmed. LI et al [19] reported that compared to conventional peak ageing, 7075 alloy can achieve an increase in ultimate tensile strength (UTS) of 139 MPa after applied to solution treatment, room temperature rolling with thickness reduction of 80% before artificial ageing. CHEN et al [20] suggested that the UTS, yield strength (YS) and elongation to failure (Ef) of 665 MPa, 628 MPa and 11.62% can be obtained for 7A99 alloy by cold-rolling with thickness reduction of 10% prior to solid solution at 480 ℃ for 1 h and ageing at 130 ℃ for 12 h. Similarly, LIU et al [21] revealed that the maximum UTS, YS and Ef values of 571 MPa, 513 MPa and 13.6% can be achieved for 7050 alloy processed by TMT with different deformation routes. However, in some cases, pre-deformation before ageing may also have a negative influence on strength of the alloy. CHEON et al [22] reported that compared to T6 treatment, the mechanical properties of Al-Zn-Mg-Cu-Sc-Zr-Mn alloy that applied to pre-rolling with thickness reduction of 30% before artificial ageing were improved, while both the strength and elongation were decreased by pre-rolling up to 50%. WU et al [23] found that there was ~30 MPa decrease in the UTS of Al-8.26Zn-1.95Mg-1.89Cu-0.08Sc-0.17Zr (wt.% ) alloy that was pre-deformed by 10% cold rolling before T74 treatment. HAN et al [24]