Effects of Microstructure and Deformation Conditions on the Hot Formability of Ni–Ti–Hf Shape Memory Alloys

Abstract

Ingots of Ni-Ti-Hf shape memory alloys were prepared by vacuum arc re-melting. Isothermal hot compression tests were conducted at temperatures ranging from 700 to 1000 degrees C and at strain rates from 10(-2) s(-1) to 1.0 s(-1). A decrease in the Ni content below 50.2 at.% significantly deteriorated the hot workability due to the formation of a brittle second phase. Also, the low Ni content alloy showed poor workability when the temperature exceeded 900 degrees C. Additional compression tests were conducted under various conditions to clarify the effects of the chemical composition, solidification anisotropy, and the strain rate.

Production, Mechanical and Functional Properties of Long-Length TiNiHf Rods with High-Temperature Shape Memory Effect

“…TiNiHf alloys seem to be more attractive for wide practical application because they perform required mechanical and functional properties in combination with relatively low cost and greater stability of the operational characteristics as compared to TiNiZr alloys [ 15 ]. Therefore, to date, numerous studies of TiNiHf alloys revealed the main features of the formation of their structural state and properties [ 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 ]. Ternary TiNiHf alloys are usually divided into two groups, one with increased Ni content and one with reduced Ni content, similarly to binary TiNi SMAs.…”

“…Ternary TiNiHf alloys are usually divided into two groups, one with increased Ni content and one with reduced Ni content, similarly to binary TiNi SMAs. According to studies [ 16 , 17 , 18 , 19 , 20 ], alloys with low Ni content (less than 49.5 at.%) have insufficient technological plasticity due to the precipitation of the (Ti, Hf) 2 Ni-type phase. An increase in the Ni content leads to a decrease in the characteristic temperatures of martensitic transformations; therefore, in order to obtain a high-temperature state in the alloy, the content of Hf also should be increased [ 16 , 21 ].…”

“…The melting processes of TiNi-based SMAs from pure charge components are often associated with some difficulties, such as an increased concentration of gas impurities due to the high reactivity of pure titanium. For TiNiHf alloys, an increase in the concentration of gas impurities, in particular oxygen, is especially critical because of an increase in the concentration of embrittling (Ti, Hf) 2 Ni-type excess phases with a high concentration of Ti and Hf, as well as HfO 2 in alloys with a high concentration of Hf and Ni [ 16 , 17 , 18 , 19 , 20 ]. Therefore, the application of finished Ti-Ni billets as a charge component for melting TiNiHf alloys may decrease the concentration of gas impurities due to the absence of pure titanium during the melting process.…”