Special Features of Functional Properties of Heterophase High-Strength Ni50.2Ti37.3Hf12.5 Polycrystals and Single Crystals

Resumo

Stress-induced B2–B19' martensitic transformations (MTs) in Ni_50.2Ti_37.3Hf_12.5 (at.%) alloy aged at 773 K, 3 h are studied. Single crystals have lower (by 100 K) transformation temperatures (TTs) in comparison with polycrystals. Aging of single crystals results in precipitation of H- and H'-phase particles with sizes up to 15 nm thereby leading to significant strengthening of the B2 phase with σ_cr(B2) = 1540 MPa and manifestation of superelasticity (SE) in a temperature interval of 90 K. Low strength properties of the B2 phase with σ_cr(B2) = 950 MPa in polycrystals are caused by precipitation of only H-phase particles. This leads to the absence of SE. Additional martensite aging of polycrystals enhances the strength properties of the B2 phase by a factor of 1.5 up to 1450 MPa, thereby leading to the appearance of the high-temperature SE in a wide temperature interval of 110 K. This state is characterized by enhanced TTs, reduced temperature intervals of stress-induced MT, and maximal reversible strain value up to 2.0%.