TY - JOUR

T1 - Effect of off-stoichiometric compositions on microstructures and phase transformation behavior in Ni-Cu-Pd-Ti-Zr-Hf high entropy shape memory alloys

AU - Piorunek, D.

AU - Oluwabi, O.

AU - Frenzel, J.

AU - Kostka, A.

AU - Maier, H. J.

AU - Somsen, C.

AU - Eggeler, G.

N1 - Funding Information: This study was supported by the German Research Foundation (DFG) through the priority program SPP 2006 “Legierungen mit komplexer Zusammensetzung – Hochentropielegierungen (CCA – HEA)” [grant numbers 313773923 , 388671975 ]. The infrastructural support by the Zentrum für Grenzflächendominierte Höchstleistungswerkstoffe (ZGH) is gratefully acknowledged.

PY - 2021/3/15

Y1 - 2021/3/15

N2 - High entropy shape memory alloys (HE-SMAs) show reversible martensitic phase transformations at elevated temperatures. HE-SMAs were derived from binary NiTi, to which the elements Cu, Pd, Zr and Hf are added. They represent ordered complex solid solutions. Their high temperature phase is of B2 type, where the added elements occupy sites in the Ni-(Cu, Pd) and Ti-sub-lattices (Zr, Hf). In the present study, advanced microstructural and thermal characterization methods were used to study the effects of the additional alloy elements on microstructures and phase transformations. The ratios of Ni-equivalent (Ni, Cu, Pd) and Ti-equivalent (Ti, Zr, Hf) elements in HE-SMAs were varied to establish systems that correspond to stoichiometric, under- and over-stoichiometric binary alloys. It is shown that basic microstructural features of cast and heat-treated HE-SMAs are inherited from the nine binary X–Y subsystems (X: Ni, Cu, Pd; Y: Ti, Zr, Hf). The phase transition temperatures that characterize the martensitic forward and reverse transformations depend on the concentrations of all alloy elements. The data obtained demonstrate how martensite start temperatures are affected by deviations from the composition of an ideal stoichiometric B2 phase. The findings are discussed in the light of previous work on the concentration dependence of SMA transformation temperatures, and directions for the development of new shape memory alloy compositions are proposed.

AB - High entropy shape memory alloys (HE-SMAs) show reversible martensitic phase transformations at elevated temperatures. HE-SMAs were derived from binary NiTi, to which the elements Cu, Pd, Zr and Hf are added. They represent ordered complex solid solutions. Their high temperature phase is of B2 type, where the added elements occupy sites in the Ni-(Cu, Pd) and Ti-sub-lattices (Zr, Hf). In the present study, advanced microstructural and thermal characterization methods were used to study the effects of the additional alloy elements on microstructures and phase transformations. The ratios of Ni-equivalent (Ni, Cu, Pd) and Ti-equivalent (Ti, Zr, Hf) elements in HE-SMAs were varied to establish systems that correspond to stoichiometric, under- and over-stoichiometric binary alloys. It is shown that basic microstructural features of cast and heat-treated HE-SMAs are inherited from the nine binary X–Y subsystems (X: Ni, Cu, Pd; Y: Ti, Zr, Hf). The phase transition temperatures that characterize the martensitic forward and reverse transformations depend on the concentrations of all alloy elements. The data obtained demonstrate how martensite start temperatures are affected by deviations from the composition of an ideal stoichiometric B2 phase. The findings are discussed in the light of previous work on the concentration dependence of SMA transformation temperatures, and directions for the development of new shape memory alloy compositions are proposed.

KW - Alloy design

KW - High entropy alloy

KW - Martensitic transformation

KW - Microstructure

KW - Shape memory alloy

KW - Thermodynamic constitution

UR - http://www.scopus.com/inward/record.url?scp=85097457037&partnerID=8YFLogxK

U2 - 10.1016/j.jallcom.2020.157467

DO - 10.1016/j.jallcom.2020.157467

M3 - Article

AN - SCOPUS:85097457037

VL - 857

JO - Journal of alloys and compounds

JF - Journal of alloys and compounds

SN - 0925-8388

M1 - 157467

ER -