Evolution of intermetallics, dispersoids and elevated-temperature properties at various Fe contents in Al-Mn-Mg 3004 alloys

Nowadays, great interests are rising on aluminum alloys for the applications at elevated temperature, driven by the automotive and aerospace industries requiring high strength, light weight and low cost engineering materials. As one of the most promising candidates, Al-Mn-Mg 3004 alloys have been found to possess considerably high mechanical properties and creep resistance at elevated temperature resulted from the precipitation of a large number of thermally stable dispersoids during heat treatment. In present work, the effect of Fe contents on the evolution of microstructure as well as high temperature properties of 3004 alloys has been investigated. Results show that the dominant intermetallic changes from α-Al(MnFe)Si at 0.1 wt. % Fe to Al6(MnFe) at both 0.3 and 0.6 wt. % Fe. In the Fe range of 0.1 to 0.6 wt. % studied, a significant improvement on mechanical properties at elevated temperature has been observed due to the precipitation of dispersoids, and the best combination of yield strength and creep resistance at 573K (300°C) is obtained in the 0.3% Fe alloy with finest size and highest volume fraction of dispersoids. The superior properties obtained at 573K (300°C) makes 3004 alloys more promising for high temperature applications. The relationship between the Fe content and the dispersoid precipitation as well as the materials properties has been discussed

Effect of Metastable Mg2Si and Dislocations on α-Al(MnFe)Si Dispersoid Formation in Al-Mn-Mg 3xxx Alloys

The effect of metastable Mg2Si and dislocations on the formation of α-Al(MnFe)Si dispersoids in Al-Mn-Mg 3xxx alloys were studied by a close examination of the dispersoid precipitation process using the quench technique and TEM observation. Special attention was paid to the nucleation mechanisms. Mg plays an important role in promoting the formation of α-Al(MnFe)Si dispersoids. The number density and volume fraction of the dispersoids in the Mg-containing alloy are much higher than those in the control alloy without Mg, resulting in a strong dispersoid strengthening effect. During the heating process in the Mg-containing alloy, metastable Mg2Si precipitated and dissolved, leaving local Si-rich areas on pervious metastable Mg2Si, which provide favorable nucleation sites for α-Al(MnFe)Si dispersoids. It was found that β′-Mg2Si precipitates were more effective at the promotion of the dispersoid nucleation than β″-Mg2Si. In the deformed sample, the dislocations become the preferable sites for the α-Al(MnFe)Si dispersoid nucleation. By reducing dispersoid-free zones, the dispersoid distribution became more uniform compared to the non-deformed sample. The dispersoid nucleation mechanisms based on both metastable Mg2Si and dislocations are proposed and discussed

Improving the Elevated-Temperature Properties by Two-Step Heat Treatments in Al-Mn-Mg 3004 Alloys

In the present work, two-step heat treatments with preheating at different temperatures (175 °C, 250 °C, and 330 °C) as the first step followed by the peak precipitation treatment (375 °C/48 h) as the second step were performed in Al-Mn-Mg 3004 alloys to study their effects on the formation of dispersoids and the evolution of the elevated-temperature strength and creep resistance. During the two-step heat treatments, the microhardness is gradually increased with increasing time to a plateau after 24 hours when first treated at 250 °C and 330 °C, while there is a minor decrease with time when first treated at 175 °C. Results show that both the yield strength (YS) and creep resistance at 300 °C reach the peak values after the two-step treatment of 250 °C/24 h + 375 °C/48 h. The formation of dispersoids is greatly related to the type and size of pre-existing Mg2Si precipitated during the preheating treatments. It was found that coarse rodlike β′-Mg2Si strongly promotes the nucleation of dispersoids, while fine needle like β″-Mg2Si has less influence. Under optimized two-step heat treatment and modified alloying elements, the YS at 300 °C can reach as high as 97 MPa with the minimum creep rate of 2.2 × 10−9 s−1 at 300 °C in Al-Mn-Mg 3004 alloys, enabling them as one of the most promising candidates in lightweight aluminum alloys for elevated-temperature applications