Microstructural Design for Improving Ductility of An Initially Brittle Refractory High Entropy Alloy

Abstract

Typically, refractory high-entropy alloys (RHEAs), comprising a two-phase ordered B2 + BCC microstructure, exhibit extraordinarily high yield strengths, but poor ductility at room temperature, limiting their engineering application. The poor ductility is attributed to the continuous matrix being the ordered B2 phase in these alloys. This paper presents a novel approach to microstructural engineering of RHEAs to form an “inverted” BCC + B2 microstructure with discrete B2 precipitates dispersed within a continuous BCC matrix, resulting in improved room temperature compressive ductility, while maintaining high yield strength at both room and elevated 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.…”

“…Thus, a weave-like microstructure of BCC and B2/L2 1 always occurred in these BCC-based HEAs, since it is sensitive to the Al/Ti content, such as the AlFeCoNiCr HEA, leading to a serious brittleness [ 114 ]. Massive efforts have been done to research for the cuboidal or spherical B2/L2 1 precipitation in various systems through adjusting both Al and transition metals [ 107 , 108 , 109 , 118 , 119 , 120 , 121 , 122 , 123 , 124 , 128 , 129 ]. It is fascinating that spherical or cuboidal B2/L2 1 nanoprecipitates are coherently-existed not only in Al/Ti-LTM HEAs (e.g., Fe 34 Cr 34 Ni 14 Al 14 Co 4 at.% [ 121 ]), but also in refractory HEAs consisted of Al and early transition metals [ 111 , 128 , 129 ].…”

“…Massive efforts have been done to research for the cuboidal or spherical B2/L2 1 precipitation in various systems through adjusting both Al and transition metals [ 107 , 108 , 109 , 118 , 119 , 120 , 121 , 122 , 123 , 124 , 128 , 129 ]. It is fascinating that spherical or cuboidal B2/L2 1 nanoprecipitates are coherently-existed not only in Al/Ti-LTM HEAs (e.g., Fe 34 Cr 34 Ni 14 Al 14 Co 4 at.% [ 121 ]), but also in refractory HEAs consisted of Al and early transition metals [ 111 , 128 , 129 ]. For instance, the coherent precipitation of spherical B2 nanoparticles in BCC matrix improves the room- temperature compressive ductility on a large extent of refractory Al 0.5 NbTa 0.8 Ti 1.5 V 0.2 Zr HEA, while maintains high yield strength at both room and elevated temperatures [ 129 ].…”

High Entropy Alloys: Ready to Set Sail?

“…In another study [17], it was shown that the addition of Ti and Al to single phase FCC CoCrFeNi HEAs leads to precipitation hardening effects due to presence of ordered FCC precipitates in a random FCC matrix, and these contribute to a strengthening increment between 0.3 and 0.4 GPa, which is significantly larger than counterpart contributions from strain hardening, grain boundary hardening, and solid solution strengthening. Lately, the concept of utilizing such spinodally-induced strengthening and order hardening effects in HEAs has given rise to a new generation of modulated, nano-phase structured, BCC-refractory HEAs that mimic super alloy type microstructures [26,27]. showing the dislocation motion across an A2/B2 interface.…”

Hexagonal Closed-Packed Precipitation Enhancement in a NbTiHfZr Refractory High-Entropy Alloy

“…Dirras et al [17] reported the tensile yield strength of the as-cast TiZrHfTaNb HEA is 800-840 MPa, but the fracture strain is only~4%. Accordingly, intensive efforts have been devoted to improve the ductility of bcc HEAs at room temperature [18][19][20][21][22]. Recently, Huang et al enhanced the comprehensive properties of brittle TaHfZrTi RHEAs through generating a dual-phase structure [22].…”