Francisco C. Lovey

Metastable effects on martensitic transformation in SMA

The applicability of shape memory alloys (SMA) for dampers in civil engineering requires guaranteed behavior with well controlled or irrelevant changes after several years of inactivity and/or under the action of daily or seasonal temperature effects. The long time analysis of the aging temperature effects on a NiTi alloy shows similar behavior to other SMA but with higher time constants at temperatures near 373 K: 1.9 days at 410 K and near 100 days at 363 K with an activation energy value near 12500 K (RT=104 kJ mol−1). At the present state of the art, the measurements show that the change of transformation temperatures under the action of the external temperature (or ‘room temperature’ change) is close to 15 K. The direct use of the as furnished material can suffer relevant changes of working properties in devices under the action of direct sunlight or high room temperature working conditions.

METASTABLE EFFECTS ON MARTENSITIC TRANSFORMATION IN SMA Part 4. Thermomechanical properties of CuAlBe and NiTi observations for dampers in family houses

AbstractThe behavior of shape memory alloys (SMA) allows their use as a passivensmart material. In particular, the existence of a hysteretic cycle in thendomain of the elementary coordinates strain-stress-temperature (σ, ε, T) suggests its application for dampingnin mechanical and/or in civil engineering. We are working in the applicationnof SMA as dampers for earthquakes in small houses as family homes. For dampersninstalled in the inner porticos of the house, the suggested SMA is the CuAlBenand, eventually, the NiTi. At room temperature the used SMA wires inducesnforces situated between 2–3kN/wire. The properties related with thendamping applications for CuAlBe and NiTi, i.e., the SMA creep and the self-heatingnwill be presented, together with some other minor stress and temperature effectsnon NiTi modifying the hysteretic behavior.

Pseudoelastic cycling in Cu–14.3Al–4.1Ni (wt.%) single crystals

Abstract Pseudoelastic β1↔β′1 and β1↔β′1+γ′1 cycling in Cu–14.3Al–4.1Ni (wt.%) single crystals were performed. For the β1↔β′1 cycling no evolution of the stress-strain curves was observed up to N=2700 in a specimen oriented close to the [100] direction. For the β1↔β′1+γ′1 cycling, made in a sample with orientation near [110], the martensitic transformation changes with the number of cycles, showing a clear inhibition of the γ′1 martensite as the cycling progresses. The disappearance of the 2H martensite is discussed in terms of the introduction of defects during cycling.

Metastable effects on martensitic transformation in SMA: Part IX. Static aging for morphing by temperature and stress

The NiTi SMA was studied by temperature (mainly at 373xa0K) and by stress–temperature aging. The temperature induces monotonic changes in Rs transformation temperature near 10–20xa0K/year. The simultaneous stress/temperature aging produced strong changes in the stress–strain cycles and in the necessary stress to continue transforming with an increase of maximal stress near 45xa0MPa/month at 373xa0K.

Damping via Cu-Zn-Al shape memory alloys (SMA): the action of diffusive effects on the macroscopic description

The thermoelastic martensitic transformation is the origin of the peculiar properties of Cu-based SMA. One of the potential uses consists on taking advantage of their hysteresis cycle to smooth the oscillations induced by wind or quakes. The experimental study at mesoscale level of the Cu-based alloys and in preliminary level of TiNi allows the development of a one-dimensional model that describes their response to the external thermodynamic forces. The evolution associated to diffusion effects (phase coexistence and temperature-time on parent phase) is also included. The results furnished by the model establish the appropriate guarantee of the alloy properties for series of working cycles separated by long times in austenite phase as the scarce events (as in earthquakes). Using a simple case, the simulation determines the boundaries of the suitable fluctuation zone related to the diffusive effects (summer, winter and coexistence effects). The results establish a way to guarantee the damper behavior for several years.

Shape memory alloys: Local and global transformations by high resolution thermal analysis

The use of high resolution automatised equipment (HRTA, conduction calorimetry, stress-strain-temperature) allows to study, locally and/or globally, the martensitic transformations of the alloys with memory (Cu-Zn-Al). The observations allow to establish the phenomenology of the effects of the cycling on the crystalline structure. Particularly, the formation of defects, the interactions between the interphases and the precipitates that change the relative stability of the phases, the effects of the nucleation and the growth. The experimental research of the area of coexistence of the two phases allows to set a phenomenological model coherent with the experimental observations.ZusammenfassungUnter Anwendung einer hochauflösenden automatisierten Apparatur (HRTA, Leitfähigkeitskalorimetrie, Zug-Dehnungstemperatur) können sowohl lokal als auch global die martensitischen Umwandlungen von Legierungen mit Erinnerungsvermögen (Cu-Zn-Al) untersucht werden. Die Beobachtungen gestatten die Feststellung der Phänomenologie des Einflusses von periodischen Schwankungen auf die Kristallstruktur, insbesondere Fehlstellenbildung, Wechselwirkungen zwischen Zwischen-phasen und Fällungen, die die relative Stabilität der Phasen verändern sowie der Einfluß von Keimbildung und Keimwachstum. Die experimentelle Ergründung der Gebiete mit zwei koexistenten Phasen erlaubt die Schaffung eines phänomenologischen Modelles, welches mit den experimentellen Beobachtungen übereinstimmt.

A short review on the interaction of precipitates and martensitic transitions in CuZnAl shape memory alloys

The main effects of γ non-equilibrium nanoprecipitates in CuZnAl shape memory alloys are briefly reviewed. Aspects related to the nucleation and growth of precipitates are commented on and their effect on stress induced martensitic transitions is analyzed. Results concerning the relationship between the size of precipitates and the hysteresis of the stress induced β-18R transition are studied. The improvement of the two-way shape-memory effect after the introduction of precipitates is shortly commented on. The 18R–6R transition is also analyzed and recent findings on the optimization of the mechanical reversibility associated to the 18R–6R transformation in a matrix with a high density of nano precipitates are reviewed.

Correction to: Civil Engineering Applications: Specific Properties of NiTi Thick Wires and Their Damping Capabilities, A Review

We regret that a tribute, which was submitted with the original manuscript by the authors, in honor of Prof. Shuichi Miyazaki was unfortunately omitted during the publication process.

Remarks on the Particular Behavior in Martensitic Phase Transition in Cu-Based and Ni–Ti Shape Memory Alloys

Many macroscopic behaviors of the martensitic transformations are difficult to explain in the frame of the classical first-order phase transformations, without including the role of point and crystallographic defects (dislocations, stacking faults, interfaces, precipitates). A few major examples are outlined in the present study. First, the elementary reason for thermoelasticity and pseudoelasticity in single crystals of Cu–Zn–Al (β-18R transformation) arises from the interaction of a growing martensite plate with the existing dislocations in the material. Secondly, in Cu–Al–Ni, the twinned hexagonal (γ′) martensite produces dislocations inhibiting this transformation and favoring the appearance of 18R in subsequent transformation cycles. Thirdly, single crystals of Cu–Al–Be visualize, via enhanced stress, a transformation primarily to 18R, a structural distortion of the 18R structure, and an additional transformation to another martensitic phase (i.e., 6R) with an increased strain. A dynamic behavior in Ni–Ti is also analyzed, where defects alter the pseudoelastic behavior after cycling.

Civil Engineering Applications: Specific Properties of NiTi Thick Wires and Their Damping Capabilities, A Review

This study describes two investigations: first, the applicability of NiTi wires in the damping of oscillations induced by wind, rain, or traffic in cable-stayed bridges; and second, the characteristic properties of NiTi, i.e., the effects of wire diameter and particularly the effects of summer and winter temperatures and strain-aging actions on the hysteretic behavior. NiTi wires are mainly of interest because of their high number of available working cycles, reliable results, long service lifetime, and ease in obtaining sets of similar wires from the manufacturer.