Microstructure and Mechanical Behavior of Precipitation-hardening Multi-principal Element Alloys Fabricated by Additive Manufacturing

Abstract

Modern engineering has long been in demand for high-performance structural materials for harsh working conditions. The idea of multi-principal-element alloy (MPEA) provides a new way for alloy design. Introducing the L1₂ precipitate into face-centered cubic (FCC) MPEAs mainly composed of Fe, Co, Cr, and Ni elements via alloying of Ti and Al has been verified to be an effective strengthening strategy. However, the fabrication of cast alloys mostly involves multiple passes of mechanical treatments and heat treatments to attain a tailored microstructure, making the production routes rather laborious, especially for complex geometries. Compared with the conventional fabrication routes, additive manufacturing (AM) provides a high degree of geometrical freedom in the design and production of metal components with complex structures, and superiority in production efficiency and cost. The high energy input and high cooling rate of the AM methods can suppress phase transition and intermetallic formation and can also lead to a fine microstructure. Moreover, the thermal strength induced by the layer-by-layer fabrication could introduce a large number of inherent dislocations, which could also strengthen the restulting AMed MPEAs. However, introducing precipitates into the AMed MPEAs requires annealing treatment, during which the columnar grains in the AMed MPEAs may undergo recrystallization and transform into equiaxed grains, and the cellular structure boundaries composed of dislocations may also be eliminated. Except for the L1₂ phase, the addition of Ti and Al may lead to the precipitation of other phases, such as the B2 phase and the L2₁ phase. Precipitation may also take place during the fabrication due to the heating and cooling cycle. Therefore, the incorporation of precipitation hardening and additive manufacturing is a promising but challenging task.

In this study, the (FeCoNi)₈₆Ti₇Al₇ (at. %) and Co₄₂Cr₂₀Ni₃₀Ti₄Al₄ (at. %) MPEAs were fabricated by laser powder bed fusion (L-PBF). Annealing was conducted on both MPEAs, and the microstructure evolution, mechanical behavior, and deformation mechanisms were systematically investigated.

Firstly, we adopted the chemical composition of the reported casting (FeCoNi)₈₆Ti₇Al₇ MPEA with excellent mechanical properties. The as-built (FeCoNi)₈₆Ti₇Al₇ exhibits a good combination of ultimate tensile strength (UTS) of 1085 MPa and tensile elongation (TE) of 30.5%. It is evidenced that the joint effects of the hetero-deformation induced hardening from grains with heterogeneous geometrically necessary dislocations densities, in-situ formed L2₁ phase, and the precipitation hardening from the in-situ formed nano L1₂ phase were responsible for the strength.

Secondly, to attain the ideal post-heat-treatment process, the precipitate type, morphology, and distribution of the (FeCoNi)₈₆Ti₇Al₇ MPEA at 873 K, 973 K, and 1073 K were systematically investigated. After annealing at 873 K and 973 K, alternately distributed FCC lamella and BCC (body-centered cubic) lamella precipitated from the matrix. The FCC structure lamellar precipitates are composed of disordered FCC phase and L1₂ phase, and the BCC structure lamellar precipitates are composed of disordered BCC phase, B2 phase, and L2₁ phase. After annealing at 1073 K, precipitates turned to homogeneously distributed L1₂ phase and L2₁ phase. The effect of annealing temperature on the precipitation behavior was discussed from the thermodynamic and kinetic points of view. The hardness tests and tensile tests revealed that the lamellar precipitates are harmful to the ductility of the alloy. Based on the investigation on the precipitation at 973 K to 1073 K, the L-PBFed (FeCoNi)₈₆Ti₇Al₇ MPEA was annealed at 1053 K for 2 h. While introducing a high volume fraction of the L1₂ phase, the cellular structure boundaries composed of dislocations were also reserved. The as-annealed alloy shows a yield strength (YS) of 1136 MPa, UTS up to 1573 MPa, and a total TE of 16.1%. The contribution of different strengthening mechanisms and the deformation behavior were quantitatively evaluated and discussed.

Further, we developed a Co₄₂Cr₂₀Ni₃₀Ti₄Al₄ (in at. %) MPEA assisted by the CALPHAD (CALculation of PHAse Diagrams) technique. The alloy exhibits a superiority of mechanical properties over a wide temperature ranging from 77 K to 873 K via L-PBF and post-heat treatment. The as-annealed alloy achieves an excellent UTS of 1586 MPa and TE of 22.7% at 298 K, UTS of 1944 MPa and TE of 22.6% at 77 K, and UTS of 1147 MPa and TE of 9.1% at 873 K. The excellent mechanical properties stem from the microstructures composed of partially recrystallized grains and heterogeneously precipitated L1₂ phase due to the concurrence of recrystallization and precipitation. The grain boundary hardening, precipitation hardening, and dislocation hardening contribute to the high YS at 298 K and 77 K. Interactions of nano-spaced stacking faults (SFs) including SFs networks, Lomer-Cottrell locks (L-C locks), and anti-phase boundaries (APBs) induced by the shearing of L1₂ phase are responsible for the high strain hardening rate and plasticity at 77 K.

Our work provides new insight into the incorporation of precipitation hardening and additive manufacturing technology, paving the avenue for the development of high-performance structural materials.

Date of Award	4 Sept 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yuntian ZHU (Supervisor), Chih-Ching HUANG (Supervisor) & Yong Liu (External Supervisor)