Cu-substitution effect on thermoelectric properties of the TiNi-based shape memory alloys

Abstract

We have studied the effects of Cu substitution on thermoelectric properties of Ti50Ni50-xCux (0 ≤ x ≤ 30 at. %) shape memory alloys by means of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) measurements. From the electrical resistivity and Seebeck coefficient studies, it is found that the Cu substituted TiNi alloys show a metallic nature in the entire temperature range. However, thermal hysteresis behavior was observed in all the TiNi-based alloys near martensitic transition, which confirms the first order phase transition. Transformation starting temperature of B19′ martensite (Ms) is found to be decreased with increase in Cu substitution (x > 5), whereas that of B19 martensite (M′s) increased gradually with Cu content, and the thermal hysteresis behavior becomes weaker upon substitution of Cu. It is also found that the separation between B19 and B19′ phases in the 7.5% Cu doped TiNi alloy is clearly evident in the Seebeck coefficient measurement, which is not seen in the resistivity data. Finally, analysis of thermal conductivity reveals that the anomalous feature in κ at the B19 ↔ B19′ transformation for 7.5, 10, and 15% Cu-substituted TiNi alloys which can be mainly attributed to the electronic contribution, while a large anomalous peak observed at the B19 → B2 transformation in the warming process is due to change in the lattice thermal conductivity. The relative change in thermal conductivity (Δκ/κ) near martensitic transformation is found to be increased with increase in Cu content, reaches a giant value of 200% for 10% Cu-substituted TiNi alloy and then starts to decrease with further Cu substitution.

Damping Characteristics of Ti50Ni50−xCux (x = 0~30 at.%) Shape Memory Alloys at a Low Frequency

“…Figure 1a–c show the tan δ value versus T curves for Ti 50 Ni 50− x Cu x ( x = 0, 5, 7.5 at.%) SMAs, respectively. According to the reported transformation sequence [11,12,13], Figure 1a indicates a B2→B19’ transformation peak appearing at 24 °C with a tan δ value of 0.11 in cooling, and a B19’→B2 transformation peak occurring at 78 °C with a tan δ value of 0.09 in heating. Figure 1b,c show the same transformation peaks as Figure 1a.…”

“…In addition, there are relaxation peaks at about −70 °C and −50 °C in cooling and heating, respectively. The studies of electric resistivity, ρ, and Seebeck coefficient, S , versus T for Ti 50 Ni 50− x Cu x ( x = 0~30 at.%) SMAs demonstrate that the transformation sequence exhibited in x = 7.5 at.% SMA is B2↔B19’ by ρ tests and is B2↔B19↔B19’ with M S ’ − M S = 15 °C by S tests [11]. Here, M S ’and M S are the starting transformation temperatures of B2→B19 and B19→B19’, respectively.…”

“…The addition of Cu into TiNi SMAs improves the SME and PE properties and reduces the temperature hysteresis of the SMAs [9,10]. It has been reported that the transformation sequences of Ti 50 Ni 50− x Cu x ( x = 0~30 at.%) SMAs are B2↔B19’, B2↔B19↔B19’ and B2↔B19, when the Cu-contents are x ≤ 7.5, 10 ≤ x ≤ 15 and 20 ≤ x ≤ 30, respectively [11,12,13]. The damping properties exhibited in TiNiCu SMAs can be examined with a dynamic mechanical analyzer (DMA).…”

Pseudoelasticity response of aged Ti50Ni25Cu25 shape memory ribbon under nanoindentation tests

“…In conventional bulk Ti 50 Ni 50Àx Cu x ternary alloys, the formation of B24B19 transformation is observed if Cu content is higher than 7.5 at. % (x S 7.5) [7,8].…”

Damping Capacities of Ti₅₀Ni_50&minus;<i>_x</i>Cu<i>_x</i> Shape Memory Alloys Measured under Temperature, Strain, and Frequency Sweeps

“…12) Van Humbeeck et al 16) proposed that, for TiNi-based and Cu-based SMAs, the total internal friction, Q ¹1 , can be decomposed into three terms, IF PT , IF I and IF Tr . The IF PT term is associated with phase transformation internal friction and is independent of the cooling/heating rate.…”