Aging effects on hardness and dynamic compressive behavior of Ti–55Ni (at.%) alloy

Materials for impact applications require a unique combination of high hardness, high dynamic compressive strength, and reasonable toughness (i.e., suitable ductility). This paper presents the aging effects on the microstructural evolution, hardness, and compressive high-strain rate behavior of Ni-rich Ti–55Ni (at.%) alloy. Aging heat-treatments of Ti–55Ni between 400 °C and 1000 °C led to the following precipitation sequence: Ti3Ni4 → T2Ni3 → TiNi3. Among these, Ti3Ni4 and T2Ni3 were found to be metastable phases since they transformed to TiNi3 phase during longer aging times or higher aging temperatures. Initial investigations were focused on precipitation studies to obtain a time–temperature–transformation (T–T–T) diagram for Ti–55Ni. High-strain rate compression tests and hardness studies indicated that significant strengthening by Ti3Ni4 and Ti2Ni3 precipitates was accompanied by very low failure strains, i.e., low ductility. However, precipitation of TiNi3 during high-temperature aging led to a good balance between strength (1400–2000 MPa) and ductility (10–20%).