Superelasticity and Impact Properties of Two Shape Memory Alloys : Ti₅₀Ni₅₀ and Ti₅₀Ni₄₈Fe₂

Abstract
Superelasticity phenomena and Impact behaviour have been studied on Ti₅₀Ni₅₀ and Ti₅₀Ni₄₈Fe₂ shape memory alloys. Both alloys have been made by powder metallurgy (combustion synthesis). The main objective of the present work was to induce superelastic properties for temperatures ranging from -25°C to 0°C on a TiNi-based alloy by the addition of 2% Fe. It is well known that superelasticity properties are enhanced by the occurence of premartensitic R phase transition. On the other hand, Fe addition decreases the martensitic transformation temperature (Ms) promoting the occurence of intermediate "premartensitic" R phase. For binary TiNi alloys, cold working and partial recovery heat treatment is necessary to obtain such as properties. This paper describes the microstructure, superelasticity and Charpy Impact behaviour of both Ti₅₀Ni₅₀ and Ti₅₀Ni₄₈Fe₂. Results have shown that : Impact energy curves display unexpected trend for testing temperatures ranging from - 196 to + 50°C. In particular, for TiNiFe alloy, typical Impact energy values are : 25J/cm² at -196OC, 4J/cm² at - 25°C (the minimal value obtained) and 20J/cm² at + 20°C. Complementary fractographic examinations have shown a brittle fracture mode for test temperatures ranging from -25 to 0°C. On the other hand, these temperatures correspond to the maximum of superelasticity, that is 1.5 to 2 % of reversible strain measured by loading and unloading during tensile test. These results show a high brittleness probably induced by the intermediate "R" phase. Consequently, the optimization of superelasticity should be detrimental to the ductility/toughness of the TiNi alloys.