Abstract
While Zr is frequently added to Al alloys to control grain size with the formation of large (>1 μm) primary precipitates, little research has been conducted on the effect of nanoscale Al3Zr precipitates on Al alloys. By comparing the precipitation and corresponding strength evolution between Al-Cu-Zr alloys with different Zr concentrations, the effects of Zr on Al-Cu precipitation with and without primary Al3Zr precipitates can be observed. In the absence of these large precipitates, all Al3Zr phases can be formed, through high temperature aging treatments, as a dispersion of nanoprecipaites inside the Al grains. In this study, Al-Cu-Zr ternary alloys were produced and heat treated to determine whether an increase in the coarsening resistance of Al-Cu precipitate phases would be observed with a distribution of the more thermally stable Al3Zr nanoprecipitates. Generally, properly aged Al-Cu alloys will coarsen when encountering elevated temperatures higher than ~473 K (~200 °C). Diluted Al-Zr alloys (<0.07 at % Zr) resist coarsening behavior until the significantly higher temperatures of ~673 K (~400 °C), but are comparatively limited in strength because of a limited solubility of Zr in the Al matrix. Hardness testing and transmission electron microscope (TEM) results are discussed, in which it is found that even very small additions of Zr, when properly accounted for during heat treating, produce a finer microstructure and higher strength than in similar Al-Cu binary alloys. No significant change in the thermal stability of strengthening was observed, indicating that the finer precipitate microstructure is resultant from a higher nucleation density, as opposed to a decrease in coarsening behavior.
Highlights
Zirconium is an important alloying addition for aluminum alloys, mainly because of the thermal stability Al3 Zr phase in the Al matrix [1]
It has been demonstrated as a complementary addition to the diluted Al-Sc alloy system for coherent nanoscale precipitation strengthening [5,6,7], and it is used in higher concentrations to form bulk metallic glass [8,9,10]
Experiments were run to determine whether Al3 Zr nanoprecipitates, formed with a custom heat treatment and diluted Zr additions to Al-Cu binary alloys, could increase the thermal stability of the relevant Al-Cu precipitates
Summary
Zirconium is an important alloying addition for aluminum alloys, mainly because of the thermal stability Al3 Zr phase in the Al matrix [1]. The high liquidus temperature range of the Al-Zr phase diagram makes it necessary to reach much higher temperatures with the melting of Zr bearing alloys in order to fully dissolve Al3 Zr precipitates into the liquid This effect is even more pronounced when adding Zr from master alloys with higher comparative Zr contents, as the large Zr precipitates sink and can create regions of the melt with disproportionately high liquidus temperatures. It can be noted that at the Al-Cu solutionizing temperature of 773 K (500 ◦ C), Al3 Zr precipitates have been observed to partially dissolve into the solid solution because of an increased solubility of Zr in Al, indicated in the literature by a decrease in electrical conductivity after undergoing heat treatments at elevated temperatures [5,6] This dissolution behavior is not ideal, but it cannot be reasonably avoided.
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