RESUMO
Additive manufacturing of aluminum alloys is largely dominated by a near-eutectic Al-Si compositions, which are highly weldable, but have mechanical properties that are not competitive with conventional wrought Al alloys. In addition, there is a need for new Al alloys with improved high temperature properties and thermal stability for applications in the automotive and aerospace fields. In this work, we considered laser powder bed fusion additive manufacturing of two alloys in the Al-Ce-Mg system, designed as near-eutectic (Al-11Ce-7Mg) and hyper-eutectic (Al-15Ce-9Mg) compositions with respect to the binary L â Al + Al11Ce eutectic reaction. The addition of magnesium is used to promote solid solution strengthening. A custom laser scan pattern was used to reduce the formation of keyhole porosity, which was caused by excessive vaporization due to the high vapor pressure of magnesium. The microstructure and tensile mechanical properties of the alloys were characterized in the as-fabricated condition and following hot isostatic pressing. The two alloys exhibit significant variations in solidification structure morphology. These variations in non-equilibrium solidification structure were rationalized using a combination of thermodynamic and thermal modeling. Both alloys showed higher yield strength than AM Al-10Si-Mg for temperatures up to 350 °C and better strength retention at elevated temperatures than additively manufactured Scalmaloy.
RESUMO
The analytical limitations in atom probe tomography such as resolving a desired set of atomic planes, for solving complex materials science problems, have been overcome by employing a well-developed unique and reproducible crystallographic technique, involving synergetic coupling of orientation microscopy with atom probe tomography. The crystallographic information in atom probe reconstructions has been utilized to determine the solute site occupancies in Ni-Al-Cr based superalloys accurately. The structural information in atom probe reveals that both Al and Cr occupy the same sub-lattice within the L12-ordered γ' precipitates to form Ni3(Al,Cr) precipitates in a Ni-14Al-7Cr (at%) alloy. Interestingly, the addition of Co, which is a solid solution strengthener, to a Ni-14Al-7Cr alloy results in the partial reversal of Al site occupancy within γ' precipitates to form (Ni,Al)3(Al,Cr,Co) precipitates. This unique evidence of reversal of Al site occupancy, resulting from the introduction of other solutes within the ordered structures, gives insights into the relative energetics of different sub-lattice sites when occupied by different solutes.
RESUMO
The determination of atomic scale structural and compositional information using atom probe tomography is currently limited to elemental solids and dilute alloys. In the present article, a unique coupling of orientation microscopy and atom probe tomography successfully facilitates the crystallographic study of non-dilute alloy systems, with high evaporation fields. This reproducible methodology affords a new perspective to the conventional atom probe tomography of ordered precipitate strengthened superalloys. The high accuracy in crystallographic site-specific sample preparation results in high spatial resolution in APT, which has been demonstrated in Co-base superalloys. The practical applications of this technique can be extended to accurately characterize the nature of buried order/disorder interfaces at the atomic scale, as well as the site occupancies associated with different solute atoms in multi-component superalloys.
