Processing of Aluminum Alloys for Elevated Temperatures

The main goal of the development of aluminum alloys for temperatures >200 °C was to replace high-strength aerospace materials for elevated temperatures based on iron or titanium. The disadvantages of the latter include a relatively high specific weight and high cost. A wide variety of alloy systems employing rapid solidification, mechanical alloying, or a combination of both techniques, followed by hot consolidation, were investigated. Alloying elements with low solubility and low diffusion rates in aluminum, such as Ni, Fe, Co, V, Mo, W, Ti, Zr, and Ce, were used in the development of alloys for elevated temperatures.

Normally, the processing temperatures for aluminum alloys are >400 °C, which is higher than planned service temperatures. Processing includes the removal of hydrates from the surface of rapidly solidified powders, hot forging, and hot extrusion. In many alloys, the partial coarsening of microstructure takes place during hot processing, resulting in deterioration of mechanical properties. As it has already been mentioned, for a number of aluminum alloys, especially those which do not contain magnesium, hydrites can be removed at T < 300 °C (Ref 31). Because cold sintering guarantees the retention of metastable microstructures of rapidly solidified powders of aluminum alloys (Ref 26, 34, 35, 36), the method can be used for investigating the relationship between microstructure and properties, starting from the microstructure of an as-atomized powder.

The thermal stability of cold sintered samples can be monitored indirectly by measuring the room temperature yield stress, (Ty, after annealing at various temperatures. The dependence of iry on annealing temperature (1 h anneal) for the cold sintered (curve 1) and hot extruded at 371 °C (curve 2) Al-6.2Ni-5.9Fe (wt%) alloy is shown in Fig. 11. It can be seen that up to —300 °C, there is no significant decrease in Cy of the cold sintered samples, and that &y of the cold sintered samples is ---30% higher than that of the hot extruded samples. Pronounced coarsening of intermetallic precipitates is observed at temperatures >350 °C. The effect of annealing time at 300 °C on the room temperature yield stress, Gy, of the cold sintered Al-6.2Ni-5.9Fe alloy is shown in Fig. 12. There is almost no drop in (Ty after long (up to 300 h) exposures, which is consistent with the thermally stable microstructure observed in TEM (Ref 18). Other rapidly solidified aluminum alloy powders for elevated temperatures (Al-Ni-Co, Al-Fe-Ce), as well as mechanically alloyed rapidly solidified Al-Fe-Ce alloys, were successfully consolidated to full density employing cold sintering.

Fig. 11 Room temperature yield stress in compression as a function of annealing temperature (1 h exposure)

for the cold sintered (P = 3 GPa) and hot extruded Al-6.2Ni-5.9Fe alloy (air-atomized powder)

S 600

S 600

c_X- T

>

10 100 Annealinig time, h

10 100 Annealinig time, h

1000

Fig. 12 Room temperature yield stress in compression as a function of annealing time at 300 °C for cold sintered Al-6.2Ni-5.9Fe alloy

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