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 σext = 250 MPa. Under σext > 250 MPa, crystals are destroyed. The magnitude of the two-way SME caused by the B2–R–B19' MT equal to 1.1% at σext = 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 TR 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.