Al-Cu Creep Resistant Alloys

Dr. Jovid Rakhmonov and Dr. Christoph Kenel, in collaboration with Dr. Amit Shyam, ORNL

Al-Cu alloys gain significant strength from nanosized, metastable θ' (Al2Cu) precipitates forming during aging heat treatments. However, upon exposure above 200 C, these precipitates coarsen rapidly and eventually transform into the undesirable, equilibrium ? phase, resulting in a significant loss in alloy strength. Recent studies have shown that θ' precipitates become resistant to coarsening and to θ- transformation, during prolonged exposure at 300-350 C, when the alloy contains microadditions of transition metals, such as Zr and Mn [1, 2]. A series of high-temperature-resistant Al-Cu-Mn-Zr alloys have been developed at ORNL, with Mn and Zr segregating to θ' interfaces (Figs. 1 and 2), thus enhancing the stability of this phase through the combination of thermodynamic and kinetic factors [3]. Moreover, increasing Cu concentration beyond its maximum solubility in α-Al contributes positively to the hot-tearing resistance of these Al-Cu-Mn-Zr alloys [4]. A combination of enhanced high-temperature strength and hot-tearing resistance makes these alloys promising candidates for high-temperature powertrain components, such as cylinder heads.

In this work, we study the role of intradendritic θ' and L12 (Al3Zr) precipitates, as well as grain-boundary θ particles, on the creep resistance at 300 C of a series Al-Cu-Mn-Zr alloys, deformed either in tension or in compression. This study also sheds light on the stability of the θ' phase under combined elevated temperature and applied stress.

Funding support

  1. Oak Ridge National Laboratory

Related Publications

  1. J.D. Poplawsky, B.K. Milligan, L.F. Allard, D. Shin, P. Shower, M.F. Chisholm, A. Shyam, The synergistic role of Mn and Zr/Ti in producing ?'/L12 co-precipitates in Al-Cu alloys, Acta Mater. 194 (2020) 577-586.
  2. J. Rakhmonov, K. Liu, L. Pan, F. Breton, X.G. Chen, Enhanced mechanical properties of high-temperature-resistant Al-Cu cast alloy by microalloying with Mg, J. Alloys Compd. 827 (2020) 154305.
  3. S. Bahl, L. Xiong, L.F. Allard, R.A. Michi, J.D. Poplawsky, A.C. Chuang, D. Singh, T.R. Watkins, D. Shin, J.A. Haynes, A. Shyam, Aging behavior and strengthening mechanisms of coarsening resistant metastable ?' precipitates in an Al-Cu alloy, Mater Design 198 (2021) 109378.
  4. A.S. Sabau, B.K. Milligan, S. Mirmiran, C. Glaspie, A. Shyam, J.A. Haynes, A.F. Rodriguez, J. Gonzalez Villarreal, J. Talamantes, Grain Refinement Effect on the Hot-Tearing Resistance of Higher-Temperature Al-Cu-Mn-Zr Alloys, Metals 10(4) (2020).

Fig. 1 (from Ref. [1]). (a) A schematic of the viewing direction for the 2D contour plots along with an APT atom map of the Al-Cu-Mn-Zr alloy exposed to 300 C for 2100 h containing a ?' precipitate with Al and Cu ions displayed as blue and orange dots, respectively, and a 10 at.% Cu iso- concentration surface. (b) An extracted ROI from the dataset shown in (a) with the axes redefined such that the precipitate is viewed in its cross section in the [110] direction. (c) 2D contour plots of a precipitate cross section for an Al-Cu-Mn-Zr alloy exposed to 350 C for 2000 h. The coherent and semi- coherent interfaces are marked by solid and dotted lines in the Cu profiles, respectively.

Fig. 2 (from Ref. [1]). A schematic showing the segregation behavior of Mn (yellow) and Zr/Ti (purple) to at the interface between Al matrix (blue) and ?' precipitate (orange) in Al-Cu-Mn-Zr alloys. For the as-aged state, only Mn is effectively diffusing and segregating to the semi-coherent interface. After an extended 300 C heat treatment, Mn covers the entire precipitate, while Zr/Ti only segregates to the coherent interface. After an extended 350 C heat treatment, Al3(ZrxTi1-x) begins to form on the coherent interface as Mn infiltrates the precipitate core with higher Mn content closer to the interface.