Efficient Exploration of New Multi-Principle Element Alloys with Exceptional Mechanical Properties using Laser Additive Manufacturing

  Copyright: IEHK Fig. 1

Multi-principle element alloys (MPEAs) are a new class of alloy which are based on at least four elements, each with high concentrations around 5 – 35 at%. When they were first synthesized, they were expected to contain high amounts of intermetallics from the different constituents. However, mostly simple crystal structures were obtained, which was attributed to the high-entropy effect. The high configurational entropy MPEAs leads to the stabilization of simple crystal structures like fcc or bcc, which is why they are also known as high-entropy alloys (HEAs) or compositionally complex alloys (CCAs) when multi-phase microstructure are purposefully introduced. By tailoring the different elements inside the alloy, never seen before combinations of functional and mechanical properties are possible. For example, mechanical properties can be improved by combining effects like substitutional and interstitial hardening, dual-phase microstructures and precipitates, activation of TRIP or TWIP to surpass conventional materials. Up until now however, materials development in MPEAs could not surpass the mechanical properties of steels.

Therefore, a methodology combining thermodynamical modeling with high-throughput screening via additive manufacturing (AM) is introduced to find promising MPEAs in the System Al-C-Co-Cr-Fe-Mn-Ni. The goal is to evaluate the effects of Al and C on the mechanical properties by exploiting different hardening mechanisms. First, the system is screened with equilibrium phase calculations using a custom CALPHAD database. After manufacturing interesting candidates using different AM techniques, properties are first evaluated using fast methods like hardness, X-ray diffraction and microscopy. In the next step, interesting compositions are more thoroughly characterized using different SEM-methods, high resolution methods (TEM, APT) and tensile testing to evaluate their deformation behavior.

First results using this approach are shown in Fig. 1, where Al and C were used in the equiatomic Co-Fe-Mn-Ni system for substitutional and interstitial hardening, respectively. Surprisingly, besides improvements in strength from the hardening mechanisms, the ultimate elongation was improved as well, which is usually not seen in conventional alloys. With a look at the deformation microstructure, the activation of TWIP was observed. Therefore, different hardening and deformation mechanisms can be effectively combined in MPEAs to improve their mechanical properties.

Fig. 1: Steps involved using the proposed methodology. (a) Laser powderbed fusion process used to synthesize samples with elemental powder blends in the Al-C-Co-Fe-Mn-Ni system with rapid (top) and deep screening (bottom) samples. (b) Obtained results from tensile testing in equiatomic Co-Fe-Mn-Ni with different Al and C additions. Higher strength and elongation were observed with 0.6 wt% C, which was shown to be caused by TWIP with (c) deformation twinning (blue) in fractured samples by EBSD.

This work is part of the priority programme (Schwerpunkprogramm) SPP2006 “CCA – HEA” of the German Research Foundation (Deutsche Forschungsgemeinschaft DFG).

References:

[1] C. Haase, F. Tang, M.B. Wilms, A. Weisheit, B. Hallstedt, Combining thermodynamic modeling and 3D printing of elemental powder blends for high-throughput investigation of high-entropy alloys – Towards rapid alloy screening and design, Mater. Sci. Eng. A. 688 (2017) 180–189. doi:10.1016/j.msea.2017.01.099

[2] Z. Li, D. Raabe, Strong and Ductile Non-equiatomic High-Entropy Alloys: Design, Processing, Microstructure, and Mechanical Properties, Jom. 69 (2017) 2099–2106. doi:10.1007/s11837-017-2540-2