Influence of Hydrogen and Number of Particle Variants on Ordinary and Two-Way Shape Memory Effects in Ti–Ni Single Crystals

Abstract

The ordinary and two-way shape memory effects (SMEs) are investigated for [\( \overline{1} \)12] single crystals of Ti–51.3Ni (at.%) alloy aged at 823 K for 1.5 h in free state and under tensile stress of 150 MPa without hydrogen and after saturation by hydrogen. It is established that without hydrogen in [\( \overline{1} \)12] single crystals with one and four variants of Ti₃Ni₄ particles the maximum magnitude of the ordinary SME is 1.9–2.6% under the external stress σ_ext = 250 MPa. Under σ_ext > 250 MPa, crystals are destroyed. The magnitude of the two-way SME caused by the B2–R–B19' MT equal to 1.1% at σ_ext = 0 is observed in [\( \overline{1} \)12] single crystals with one variant of Ti₃Ni₄ particles. The physical reason for the observed two-way SME is the internal compressive stresses oriented along the [\( \overline{1} \)12] directions arising from one variant of Ti₃Ni₄ particles as a result of aging under tensile stress of 150 MPa. It is established that hydrogen does not influence the T_R temperature, reduces the plasticity, and suppresses the two-way SME. The suppression of two-way SME in the [\( \overline{1} \)12] single crystals of the Ti–51.3Ni (at.%) alloy with one variant of Ti₃Ni₄ particles is caused by shielding of stress fields from one variant of Ti₃Ni₄ particles and multiple nucleation of R- and B19' martensite variants under loading with saturation by hydrogen.