Direct measurement of the effect of cold rolling on β phase precipitation kinetics in 5xxx series aluminum alloys

Abstract

11The aluminum magnesium alloy system has notable attributes, such as a high strength to weight 12 ratio and weldability, but it can become susceptible to sensitization at relatively low 13 temperatures, which can lead to stress corrosion cracking. It is well established that growth of the 14 secondary β phase is temperature driven, however there is little understanding about the role of 15 prior strain in the β phase nucleation and growth process. Understanding the effect of cold rolling 16 will also bring new insight into better themomechanical treatments leading to a more appropriate 17 temper to improve sensitization resistance. In this study cold rolled AA5456-H116 is observed 18 during in situ transmission electron microscopy heating experiments. The results of this study 19 show the impact of an increased dislocation density, due to cold rolling, on β phase precipitation 20 as well as the effect of misorientation on growth and kinetics. The effect of additional 21 dislocations is also observed to show an increase in precipitate density and a lowering of 22 nucleation temperature attributed to Mg pipe diffusion. 23 24 29 marine, automotive, and aircraft industries as well as food handling and chemical industries [1-30 4]. As global warming plays a larger part in material selection, further development on Al-Mg 31 usage has been conducted to improve fuel consumption and emissions, as well as increase 32 recycling potential of final automotive components [1][2][3][4]. Use of this material in service, 33 however, has been undermined by stress corrosion cracking [5] resulting from sensitization at 34 moderate and even low temperatures [6]. Growth of a deleterious secondary phase, known as the 35 β phase (Al 3 Mg 2 ), during sensitization of Al-Mg alloys has been a problem for in-service naval 36 vessels for many years [7]. Specifically, while exposed to harsh in-service environments, such as 37 sea water, a galvanic couple is formed between the aluminum matrix and the β phase 38 precipitates[8].This ultimately leads to a preferential dissolving of these precipitates that result in 39 intergranular cracking [4,[8][9][10]. But it is possible for crack propagation to occur from non-40 continuous beta precipitates as well [11]. 42It is well known that the formation of β phase is temperature driven [12][13][14] but the nucleation 43 and growth mechanisms are not fully understood. It is generally agreed upon that during 44 exposure to elevated temperatures (50-300°C) the Mg segregation begins from the supersaturated 45 solution (>3%Mg or higher[1]),resulting in the formation of a final, stable β equilibrium phase[5, 46 differential calorimetric analysis, indicating that a direct transformation from its metastable 48 precipitate form, β', was not required. The majority of studies [16][17][18] have shown β phase 49 precipitation during long term growth periods at elevated temperatures and have observed the 50 formation of this secondary phase occurs at the grain boundaries (GBs), sub-grains, and defects 51...

Transmission Electron Microscopy Study of Microstructure Evolution of Al-Mg-Sc Alloy after Annealing at 120°

“…4), which are the most detrimental for the corrosion resistance of the material -microstructure with boundaries decorated by this type of particles is the most susceptible to the exfoliation corrosion. As the precipitation of the Al 3 Mg 2 phase was reported to be dependent on the nature of the grain boundaries in the material [5,6,8,9], the distribution of the misorientation angles higher than 5 • was calculated from the EBSD orientation maps and plotted in Fig. 5.…”

“…The β-phase can form either continuously or discontinuously along the (sub)grain boundaries or near the pre-existing Mn-rich particles. The influence of the boundary character on the β-phase formation is not yet fully understood [8]; however, the structure with elongated (sub)grains is more prone to the β-phase precipitation [9].…”

“…Aluminium alloys for sheet production are commonly prepared by direct-chill casting which is followed by several steps of rolling and annealing. Such procedure is energy and material consuming and produces microstructure with elongated grains which is unfavorable in the terms of β-phase formation and exfoliation corrosion [8]. The elongated structure could be partially removed by recrystallization treatment; however, such is not desired as the elevated temperatures would beside the recrystallization lead also to the coarsening of Al 3 (Sc,Zr) particles, which would transform to the stable state and thus loose their strengthening effect.…”

Effect of Er on Microstructure and Mechanical Properties of 5052 Aluminum Alloy with Big Width-To-Thickness Ratio

“…5XXX series aluminum alloys are widely used in aerospace, automobile and ship due to their low density, high specific strength, good corrosion resistance and easy processing [16][17][18][19]. The 5052 aluminum alloy is one of the most typical and commonly used aluminum alloys in the 5XXX series only containing Mg as a strengthening element.…”

The Evolution of Microstructure during Annealing of Al-Mg-Sc-Zr Alloy Deformed by Equal Channel Angular Pressing

“…Their strength may be increased either by addition of suitable alloying elements such as scandium and zirconium [3] or by decreasing the grain size by severe plastic deformation [4]. The formation of β-phase Al 3 Mg 2 particles, which promote the exfoliation corrosion [5] may be suppressed by the use of casting procedures alternative to direct-chill (DC) casting and rolling, which produce layered and pancake grain structure [6].…”