High-temperature phase characterization of AlCrFeNiTi compositionally complex alloys

In this work, alloys composed of the 5 base-elements Al, Cr, Fe, Ni and Ti and elements in minor amounts were produced by arc melting and powder metallurgy. The alloys’ phase distribution was analyzed via high energy X-ray diffraction (HEXRD), neutron diffraction and electron backscatter diffraction (EBSD). The two studied compositions have low concentrations of the elements Al and Ti, medium concentrations of the elements Cr and Fe and high amounts of Ni (chemical composition in at.-%: Al10Cr20Fe20Ni40Ti5, Al10Cr20Fe20Ni35Ti10). The goal then was to analyze the microstructure of these compositions at elevated temperatures in dependence of their chemical composition and the production method. Analysis of both compositions show the presence of B2-phases, L1$_2$-phases, Full Heusler-phases (H_L21), C14_Laves-phase and σ-phase. The combination of dilatometric tests with microstructure analysis via HEXRD and neutron diffraction gives the possibility of observing changes in phase amounts in dependence of temperature. A quantification of phase amounts in the compositions is supported by thermodynamic calculations of stable phases, applying the calculation-of-phase-diagrams (CALPHAD) method within the software Thermocalc. Due to its detrimental influence on ductility the existence of σ-phase is undesired in nickel-chromium-iron alloys and hence σ-phase precipitation with an associated dissolution of the phases B2 and L1$_2$ at elevated temperatures was studied in detail

Lattice strain during compressive loading of AlCrFeNiTi multi-principal element alloys

In this work, multi-principal element alloys (MPEAs) with the five base elements Al, Cr, Fe, Ni and Ti plus elements in minor amounts were produced by powder metallurgy and their microstructure and elastic behavior were analyzed via light and scanning electron microscopy, electron backscatter diffraction (EBSD)and synchrotron X-ray diffraction. The two studied compositions are an MPEA with Al, Cr, Fe, Ni and Ti in equimolar ratio as well as a similar composition with a concentration of Ti reduced to 10 mol%. The goal is to analyze the microstructural behavior of these compositions during macroscopic loading in dependence of chemical composition and phases present. Analysis via synchrotron X-ray diffraction predicts the presence of body-centered cubic phases, Full Heusler-phases and C14_Laves-phases in both compositions, MPEA5and MPEA_Ti10. Synchrotron X-ray diffraction offers the possibility to monitor the deformation of these phases during macroscopic loading of specimens. Thermodynamic calculations of stable phases predicted a microstructure of MPEA5 consisting of body-centered cubic and Full Heusler-phases at room temperature. Further calculation and X ray diffraction experiments showed the stabilization of minor amounts of C14_Laves-phase (Fe2Ti) at room temperature with a decreasing amount of Ti. MPEA5 showed the development of long and un-branched cracks during compressive testing, which resulted in a remarkable decrease in lattice-dependent elastic moduli. MPEA_Ti10 exhibited branched cracks during compression tests. Also, the lattice-dependent elastic moduli of MPEA_Ti10 did not change notably during the compression tests. In both compositions, the Full Heusler-phase showed the lowest lattice-dependent elastic moduli, hence taking the largest share of the overall deformation among all phases present in the materials under macroscopic loading

Additive Manufacturing of CrFeNiTi Multi-Principal Element Alloys

High entropy alloys (HEAs) and their closely related variants, called multi-principal element alloys (MPEAs), are the topic of a rather new area of research, and so far, the gathered knowledge is incomplete. This is especially true when it comes to material libraries, as the fabrication of HEA and MPEA samples with a wide variation in chemical compositions is challenging in itself. Additive manufacturing technologies are, to date, seen as possibly the best option to quickly fabricate HEA and MPEA samples, offering both the melting metallurgical and solid-state sintering approach. Within this study, CrFeNiTi MPEA samples were fabricated via laser powder-bed fusion (PBF-LB) and solid-state sintering of mechanically alloyed powder feedstock. The main emphasis is on the PBF-LB process, while solid-state sintering serves as benchmark. Within a volumetric energy density (VED) window of 50 J/mm3 to 83 J/mm3, dense samples with large defect-free sections and an average micro-hardness of 965 HV0.1 were fabricated. Clear correlations between the local chemical alloy composition and the related micro-hardness were recorded, with the main factor being the evaporation of titanium at higher VED settings through a reduction in the C14_Laves phase fraction