Ni-Ti-Nb ALLOYS IN STRUCTURAL ENGINEERING

The addition of a third component (Nb) to a binary alloy (Ni-Ti) affects and widen the thermal hysteresis and make these ternary alloys useful for prestressing applications in civil engineering. This topic is a key aspect in this research. Ni-Ti-Nb alloys were first reported in 1986 (Melton et al. 1986; Melton et al. 1989) and the main aspect to be highlighted is they are easier to handle and store owing to their larger temperature hysteresis. This allows the material to be prestrained at a low temperature at martensite state, be safely transported at ambient temperatures, be activated at a higher temperature with reverse transformation to austenite and retain high values of recovery stresses at ambient temperatures (fig. 2.14c-d).

(Cladera, Weber, et al. 2014) analyzed the distinct ranges of generation of recovery stresses for two different alloys as binary Ni-Ti and ternary Ni-Ti-Nb. It is schematically depicted in figures 2.19, 2.20 and 2.10. Figures 2.19a and 2.20a show a stress-strain path for a narrow hysteresis alloy (i.e. Ni–Ti) and figures 2.19b and 2.20b a wide hysteresis alloy (i.e. Ni–Ti–

Nb). In these cases, the process starts with a pre-treatment with loading and the material being twinned martensite, evolving with an almost horizontal plateau, in which the detwinning process takes place and unloading with a prestrain maintained. For Ni–Ti alloys, in case of impeded strains, the recovery stress increases during heating but decreases during subsequent cooling (fig. 2.19a and 2.20a).

The recovery stress during heating may exceed the detwinning stress value (s) because the energy (or stress) needed to detwin the martensite is significantly lower than the energy (temperature) needed to carry out the reverse transformation. However, when cooling, the recovery stress may drop down almost to zero because of forward transformation. To avoid any loss of recovery stress, the Ms temperature is often chosen to be below the ambient temperature (situation not represented in fig. 2.19, see fig. 2.14c-d). In this case, the SMA is typically cooled down below the Mf temperature for prestraining and stored at a temperature below As, by liquid nitrogen if needed. Throughout the service life, the SMA then remains in the high-temperature austenitic phase where it keeps the high recovery stress (Melton et al. 1986). For Ni–Ti–Nb alloys (fig. 2.20b), which have a larger hysteresis (fig. 2.14d), after the same treatment the prestraining is still done at cooled conditions, but storage can be at ambient temperature, if it is below As (Uchida et al. 2011).

State of the art

Figure 2.19. Schematic stress–strain diagrams of prestraining and generation of recovery stresses during the activation of the reverse transformation in a constrained SMA: a) Narrow hysteresis alloy, i.e., Ni–Ti; b)

Wide hysteresis alloy, i.e., Ni–Ti–Nb. From (Cladera, Weber, et al. 2014)

Figure 2.20. Schematic thermomechanical paths during the activation and cooling of a constrained SMA: a) narrow hysteresis alloy, i.e., Ni–Ti; b) wide hysteresis alloy, i.e., Ni–Ti–Nb. From (Cladera, Weber, et al.

2014)

Figure 2.20 shows the evolution of recovery stresses and temperature during heating and cooling of the two above mentioned alloys. During the heating step (path 1), the elastic stress in the SMA first decreases due to the suppressed thermal expansion. Once the stress-temperature path crosses the As boundary, a reverse transformation to the austenite phase occurs (path 2). This transformation activates the SME, which generates tensile stresses in the SMA, because the contraction is inhibited, until the complete transformation of the alloy into austenite. However, some of the stress is lost due to thermal expansion. The thermal expansion effect is recovered by thermal contraction during subsequent cooling (Lee et al.

2013). The slope of the stress variation due to pure thermal expansion/contraction varies

Chapter 2

depending on the alloy and this slope can be obtained from Eqs. 2.23 to 2.26. The elongation of the sample due to the internal force is:

Δ𝑙_𝐹 = 𝐹𝑙

𝐸𝐴 (2.23)

where

F is the force applied to the sample, l is the length of the sample,

E is the elastic modulus of the sample, and A is the area of the cross-section of the sample.

The elongation due to the thermal expansion is:

Δ𝑙_𝑇 = 𝛼_𝑇 Δ𝑇 𝑙 (2.24)

where

T is the temperature increment of the sample,

 is the Coefficient of Thermal Expansion (CTE).

Since  = F / A, and taking into account that the element is constrained, the elongation due to the thermal expansion must equal the elongation due to the internal force:

Δ𝑙_𝑇 = Δ𝑙_𝐹 (2.25)

A relationship (slope) of stresses due to thermal increment is obtained:

σ = 𝛼_𝑇 𝐸 Δ𝑇 (2.26)

Thus, for an austenite phase Ni–Ti alloy, considering E = 60 GPa and  = 11x10^-6/ºC, the slope would be  E = 0.66 MPa/ºC.

The behavior during cooling back to ambient temperature (AT) depends mainly on the width of the hysteresis (path 3). For a Ni–Ti SMA with a narrow hysteresis, the recovery stress drops down due to forward transformation (figs. 2.19a and 2.20a). For a wide hysteresis Ni–

Ti–Nb alloy, the austenite remains stable as long as the AT is above the Ms boundary, maintaining a high recovery stress (figs. 2.19b and 2.20b). An alloy with AT between Ms

and As and a narrow temperature hysteresis (fig. 2.20a, i.e., Ni–Ti) is not appropriate for acting as permanent prestressing because the recovery stresses decreases considerably after heating and cooling back to the AT. The recovery stresses would even become negligible if

State of the art

the martensite finish temperature, Mf, was higher than the AT, a situation that is not shown in fig. 2.20a.

As stated before, in the absence of heating or cooling, the SMA is at AT and from civil engineering point of view, this temperature defines the phase in which the alloy should be stable in its structural applications. (Janke et al. 2005) proposed that, for external applications in outdoor structures, the ambient temperature can be assumed to be situated between -20 ºC in winter and 60 ºC under intense solar radiation in summer. For this reason, high hysteresis SMAs are required for structural applications involving the SME.

2.5 ACTIVE STRENGTHENING OF REINFORCED CONCRETE