On The Superior High Temperature Hardness of Precipitation Strengthened High Entropy Ni‐Based Alloys

Abstract

The microstructure and high temperature hardness of two face-centered cubic high entropy Ni-based alloys with L1 2 g 0 precipitates have been studied. Both alloys exhibit higher mixing entropy and with the advantages in lower density and lower cost of raw materials than conventional Ni-based superalloys. Their g 0 solvus are above 1 150 C, and the g-g 0 microstructure can be thermodynamically stable after isothermal ageing from 700 to 1 100 C for at least 500 h. By XRD peak deconvolution, positive lattice misfits between g and g 0 have been shown till elevated temperatures. The results from nano-indentation test indicate that their highly alloyed g 0 phase have rendered more significant strengthening, and the underlying mechanism can be attributed to the higher anti-phase boundary energy. Therefore, with minor refractory additions, the bulk hardness of present alloys can surpass that of commercial superalloy from room to high temperature.

Coherent Precipitation and Strengthening in Compositionally Complex Alloys: A Review

“…A further addition of Al or Ti into these LTMs-based HEAs can produce phase transformations [ 105 , 106 , 107 , 108 , 109 , 110 , 111 , 112 , 113 , 114 , 115 , 116 , 117 , 118 , 119 , 120 , 121 , 122 , 123 , 124 , 125 , 126 , 127 , 128 , 129 , 130 , 131 ], and then lead to microstructural diversities due to the strong interactions between Al/Ti and LTMs, resulting in an enhancement of strength. It is primarily attributed to the coherent precipitation of intermetallic phases, such as L1 2 -Ni 3 Al from the FCC matrix [ 125 , 126 ], and B2-NiAl [ 110 , 118 , 119 , 120 , 121 , 122 , 123 , 124 ] and L2 1 -Ni 2 AlTi [ 107 , 108 , 109 ] from the BCC matrix. As example, the (NiCoFeCr) 94 Ti 2 Al 4 (at.%) HEA possesses a special coherent microstructure with fine spherical L1 2 -(Ni 3 (Al,Ti)) nanoprecipitates in the FCC matrix, resulting in a significant strength improvement with a yield strength over 1 GPa [ 125 ].…”

“…As example, the (NiCoFeCr) 94 Ti 2 Al 4 (at.%) HEA possesses a special coherent microstructure with fine spherical L1 2 -(Ni 3 (Al,Ti)) nanoprecipitates in the FCC matrix, resulting in a significant strength improvement with a yield strength over 1 GPa [ 125 ]. A newly-developed kind of high entropy Ni-based alloys, such as Ni 48.6 Al 10.3 Co 17 Cr 7.5 Fe 9.0 Ti 5.8 Ta 0.6 Mo 0.8 W 0.4 (at.%), exhibit a higher high-temperature hardness resulted from the γ′-L1 2 precipitation-strengthening of γ-FCC matrix, where the coherent γ/γ′ microstructure can be thermodynamically stable after aging from 700 °C to 1100 °C for at least 500 h [ 126 ]. It is noted that a small amount of Al addition in HEAs (e.g., Al 0.3 FeCoNiCr [ 115 , 116 , 117 ] and Al 8 Co 17 Cr 17 Cu 8 Fe 17 Ni 33 [ 127 ]) generally renders spherical L1 2 -Ni 3 Al particles precipitated in the FCC matrix, resulting in high strength and good ductility, similar to that in Ni-based superalloys [ 5 , 132 ].…”

The High Temperature Tensile and Creep Behaviors of High Entropy Superalloy

“…Furthermore, since the addition of refractory elements can easily promote the formation of detrimental topological-close packed (TCP) phases, the fraction of TCP was designed to be less than 1 vol% while adjusting the refractory additions. The ingot of HESA was prepared by vacuum-arc-melting process followed by directional-solidification (DS) to produce columnar microstructure; the setup of DS casting furnace has been described in our previous work 15 . The solution-heat-treatment (SHT) was then conducted at 1210 °C for 10 h to homogenize chemical segregations, and followed by a primary aging at 1000 °C for 3 h and a secondary aging at 800 °C for 20 h to grow and refine the morphology of γ′ precipitates.…”

“…For example, the composition of HESA in current study is Ni 47.9 Al 10.2 Co 16.9 Cr 7.4 Fe 8.9 Ti 5.8 Mo 0.9 Nb 1.2 W 0.4 C 0.4 (at%), so the calculated ΔS mix is 1.60 R, which is higher than those of conventional superalloys such as CM247LC (ΔS mix = 1.29 R), Rene′ N5 (ΔS mix = 1.22 R), and CMSX-2 (ΔS mix = 1.14 R). Previous studies have shown that the γ - γ′ microstructure of HESAs can remain stable against topologically-close-packed (TCP) phase formation from 700 to 1100 °C for at least 500 h 15 , and with enhanced γ′ solvus temperatures above 1150 °C 16 . Furthermore, HESAs have shown good resistances against high temperature oxidation and corrosion by forming protective Al 2 O 3 and Cr 2 O 3 , respectively 17 .…”

Structure and high temperature mechanical properties of novel non-equiatomic Fe-(Co, Mn)-Cr-Ni-Al-(Ti) high entropy alloys

“…In the Ni-based superalloys a face-centered cubic (fcc) matrix is strengthened by fine precipitates of a Ni 3 (Al, Ti) phase with an L1 2 structure [21]. Similar kind of microstructure (the fcc matrix with the L1 2 precipitates) can be produced in some HEAs [22][23][24][25][26][27][28]; in Refs. [25,27] such alloys have been referred to as high entropy superalloys (HESAs).…”