| dc.description.abstract | Among Al alloys, 2219 Al alloy possesses highest strength at elevated temperatures. However, the application of this alloy is also restricted to a maximum temperature of 150°C, above which, the strengthening precipitates coarsen rapidly resulting in a steep loss in strength. In the present investigation, an attempt has been made to improve the elevated as well as the room temperature properties of commercial 2219 alloy by the addition of small amounts of Sc & Mg, Sc & Zr, and Nb & Zr, and these are designated as 2219ScMg, 2219ScZr and 2219NbZr alloys, respectively.
All the three alloys were cast in the form of strips in a water cooled copper mould using suction casting technique with a cooling rate of 102 to 103 K/s. The as-cast strips of 2219ScMg alloys were naturally aged and cold rolled by following three different routes (a) cold rolling, (b) homogenization and cold rolling and (c) hot rolling and cold rolling.
A significant improvement in strength has been achieved by all the three wrought processing routes with greater than 140 MPa increase in 0.2% proof stress at room temperature and greater than 110 MPa increase in 0.2% proof stress at 200°C as compared to 2219-T851 alloy having 0.2% proof stress of 345 MPa at room temperature and 205 MPa at 200°C. Hardness values, measured at room temperature after exposure at 200°C, remain stable up to 1000 h.
Microstructural analysis of 2219ScMg alloy reveals that Al3Sc or Al3(Sc,Zr) dispersoids form during casting and GP zones form on {100} and {111} plane during natural ageing. Subsequently, rolling introduces higher dislocation densities in the matrix. All these microstructural features contribute to the improvement of the room temperature strength of the alloy. On exposure at 200°C, GP zones transform to mainly θ′ and a few Ω precipitates. A finer, homogeneous distribution of θ′ and Ωprecipitates yields higher strength. Sc and Mg atoms are segregated at the θ′/matrix interface, which gives rise to slower growth kinetics of θ′ precipitates. As a result, the alloy exhibits better thermal stability at 200°C. For 2219ScZr and 2219NbZr alloys, the processing of the cast strip involves a two stage ageing procedure. This includes first stage ageing at 375°C for 2219ScZr alloy and at 400°C for 2219NbZr alloy. This is followed by solution treatment at 535°C for 30 minutes and second stage ageing at 200°C for both the alloys.
For 2219ScZr alloy, tensile tests performed at room temperature, 200°C and 250°C show 0.2% proof stress of 456 ± 22 MPa, 295 ± 20 MPa and 227 ± 2 MPa respectively. The alloy is found to be thermally stable at 200°C. It is found that the addition of Sc and Zr results in the formation of Al3(Sc,Zr) precipitates during ageing at 375°C. These precipitates are fully coherent with the matrix and have a significant precipitation hardening effect. They also stimulate the nucleation of θ′′ and θ′precipitates during ageing at 200°C making them finer, homogeneously distributed and thermally stable. Therefore, the strength of the alloy at ambient and elevated temperature is improved.
For 2219NbZr alloy, the tensile tests show that 0.2% proof stress is 409 ± 10 MPa at room temperature and 252 ± 22 MPa at 200°C. Microstructural observations reveal that the increase in strength is mainly due to the high volume fraction of Al3Zr precipitates, which form during ageing at 400°C, and due to the formation of θ′′ and θ′precipitates during ageing at 200°C. It is observed that Al3Zr precipitates facilitate the nucleation of θ′′ and θ′ precipitates making them finer, homogeneously distributed and thermally stable, as in the case of 2219ScZr alloy. | en_US |
