Transformation entropy change and precursor phenomena in Ni-rich Ti-Ni shape memory alloys
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Many issues concerning the transformation behaviors in the Ni-rich Ti–Ni system remain unresolved, such as the isothermal nature of the B199-martensitic and R-phase transformations and the precursor phenomena in the B2-parent phase. To clarify the origins of these behaviors, we investigated the transformation latent heat, specific heat, and superelastic behaviors of several Ni-rich Ti–Ni alloys in terms of the entropy change. An anomalous, very wide hump in the specific heat was detected for the B2-parent phase, which can likely be attributed to the precursor phenomenon in the B2-parent phase. In the critical region where the anomalous hump intersects the B199-martensitic transformation, some evidences of the R-phase transformation were observed, such as a tweed-like microstructure and a specific heat peak with first-ordertransformation characteristics. These findings suggest a strong relationship between the R phase and the precursor state in the B2-parent phase.
I. INTRODUCTION
Despite Ti–Ni shape memory alloys (SMAs) likely being the most studied SMAs,1 the origins of the intriguing martensitic transformation (MT) behaviors that appear in their Ni-rich portion remain under discussion. Regarding the thermal MT from the B2-parent phase to the B199-martensite phase, which is herein called B2/B199 MT, it has been reported that the B2/B199 MT starting temperature (TMs) decreases with increasing Ni content and suddenly disappears at approximately Ti–51.4 Ni (at.%).2 Some anomalous properties associated with this behavior, such as a frequency-dependent storage modulus3 and history-dependent (non-ergodic) strain,4 have been observed and explained by the concept named “strain glass”.3 Very recently, using the conversion of intensive variables from stress (r) to composition (x) in the Clausius– Clapeyron relation, the present authors revealed that the sudden drop of TMs with increasing Ni composition originates from the decrease in the entropy change DSB2–B199 (5 SB2 SB199 where SB2 and SB199 mean the entropy of the B2 and B199 phases, respectively) with decreasing temperature.5 This conversion also suggested that the isothermal nature of the B2/B199 MTs6,7 should arise in relation to the dramatic increase in the transformation stress hysteresis (rhys), as confirmed by the superelastic behaviors at low temperatures for Ti–51.8 Ni.8 Contributing Editor: Yang-T. Cheng a) Address all correspondence to this author. e-mail: [email protected] b) Present Address: RIKEN, Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan. DOI: 10.1557/jmr.2017.377
In the Ni-rich Ti–Ni section, another kind of MT, known as the R-phase (B2/R) transformation, has to be considered to take place as well as the B2/B199 MT. The B2/R transformation is typically observed in Ti–Ni alloys annealed at low temperatures9 but is believed not to take place in quenched alloys. However, Ji et al. recently reported that the B2/R transformation can be introduced by isothermal holding just above the B2/B199 MT equilibrium temperature (T0) in a quenched
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