J. San Juan

Damping behavior during martensitic transformation in shape memory alloys

Abstract The internal friction spectrum of shape memory alloys, which undergo a thermo-elastic martensitic transformation, is composed of three terms: the intrinsic term IF Int , related to the damping properties of each phase, the phase transition term IF PT , due to the martensitic phase transformation itself, and the transitory term IF Tr , linked to the kinetic effects during the phase transition. Therefore, in order to use the martensitic transformation to design high damping materials, it is very useful to understand the microscopic origin of each term, and to obtain it separately. In this work, we present a methodology for the analysis of the three terms of the internal friction spectrum, in light of theoretical models and observed experimental behavior. Based on this analysis, we discuss several conditions for the use of shape memory alloys as high damping materials in practical applications.

Dependence of the martensitic transformation characteristics on concentration in Cu–Al–Ni shape memory alloys

Abstract Up to now there have been very few studies of the evolution of the temperatures and characteristics of the martensitic transformation in Cu–Al–Ni shape memory alloys as a function of the concentration. The comparison of these studies is very difficult because of the different microstructural conditions and thermal treatments of the used alloys. Besides, the influence of the two kinds of martensites, β 3 ′ (18R) and γ 3 ′ (2H), into which the β phase is transformed for a large range of concentrations, has not been taken into account. In this work we have carried out a systematic study of the temperatures and characteristics of the martensitic transformation for a wide range of concentrations of technological interest. In order to improve the reproducibility of results, we have always used β phase single crystals under the same thermal treatment. The results allow us to obtain an empirical relationship between the alloying element concentration and the martensitic transformation temperatures. Furthermore, a concentration–martensite phase map has been established, showing clearly the concentration range where both kinds of martensites, β 3 ′ and γ 3 ′, coexist.

Advanced Shape Memory Alloys Processed by Powder Metallurgy

c) the large orientation dependence of the transformation strain, and d) the grain boundary segregation. In order to suppress the intergranular fracture and to improve the ductility of these alloys, the stress concentration in the grain boundaries must be controlled—either by development of high-textured alloys or development of fine grain alloys. [5] The high texture enables the accommodation of stress among adjacent grains and, as a consequence, the stress concentration in the grain boundaries decreases. Alternatively, the stress concentration decrease is due to the grain-size refinement, which produces smaller stress among adjacent grains. Several techniques are suitable for producing the grain-size refinement: addition of other elements in small quantities, [7] melt spinning, [8‐10] and powder metallurgy. [11,12] Finally, powder metallurgy is a relatively new technique in this area, and no attempts to use it in the development of this kind of alloys had been made, in spite of its promising capabilities. [14,15] Thus, taking into account these facts, a new production process of Cu‐Al‐Ni SMA by powder metallurgy is developed in this work. The first part of the present paper contains a detailed description of each stage of the process, whereas the second one is devoted to the characterization of thermomechanical and fracture properties of the obtained materials. Finally, a comparison of these properties with those of single crystals and polycrystals of the same kind of alloys but obtained by classical methods, is also performed.

Martensite nucleation on dislocations in Cu-Al-Ni shape memory alloys

In the present work, the martensite nucleation on dislocations has been observed. Cu–Al–Ni shape memory alloys have been superelastic cycled inside the transmission electron microscope. The in situ experiences show that the dislocations in β phase can be a nucleation site for γ3′ and β3′ martensites, which at the same time have been characterized by electron diffraction. The martensite plates can nucleate on the dislocations when the stress is applied and retransform to the βL21 phase when the sample is unloaded. The results are discussed in terms of the atomic configuration of the dislocation core, which facilitates the martensite nucleation.In the present work, the martensite nucleation on dislocations has been observed. Cu–Al–Ni shape memory alloys have been superelastic cycled inside the transmission electron microscope. The in situ experiences show that the dislocations in β phase can be a nucleation site for γ3′ and β3′ martensites, which at the same time have been characterized by electron diffraction. The martensite plates can nucleate on the dislocations when the stress is applied and retransform to the βL21 phase when the sample is unloaded. The results are discussed in terms of the atomic configuration of the dislocation core, which facilitates the martensite nucleation.

Ordering temperatures in Cu-Al-Ni shape memory alloys

The ordering temperatures Tc1 (disordered β to B2 order) and Tc2 (B2 to L21 order) have been obtained in Cu–Al–Ni shape memory alloys with different concentrations by electrical resistivity. The dependence of the ordering temperatures on the concentration has been established. Also, a modification of the theoretical calculations has been proposed to predict the ordering temperatures in Cu–Al–Ni ternary alloys. A good agreement between the theoretical ordering temperatures and the experimental results has been found.

Determination of the next-nearest neighbor order in β phase in Cu-Al-Ni shape memory alloys

The metastable β phase undergoes two order–disorder transitions during quenching from high temperature in Cu-based shape memory alloys. First, it undergoes a B2 ordering at nearest neighbors and then a second one at next-nearest neighbors that has not been clearly established. Neutron powder diffraction measurements have been performed at room temperature in a fully ordered alloy (Cu-27.4 Al-3.6 Ni at. %). Rietveld analysis allows to conclude that a L21 order at next-nearest neighbors is present in the ordered phase. The site occupancy of the different positions has been determined.

Precipitation of the stable phases in Cu-Al-Ni shape memory alloys

The Cu-Al-Ni based shape memory alloys are developed as an alternative to the classically Cu-&-Al and Ti- Ni used alloys. The main interest of these alloys horn a technological point of view is their possible use at temperatures near 200°C (l), in advantage over the Cu-&Al and Ti-Ni alloys whose maximum use temperatures are limited to 100°C (1) by several reasons. The main problem that must be solved to guarantee the Ilability of the Cu-Al-Ni alloys at high temperatures is to determine the limit of stability of the overcooled S and martensite metastable phases. In this work we study the precipitation kinetics of the y , pro-eutectoid phase and the y ,+a eutectoid decomposition between 400 “C and 482 o C. It should be noted that the y , phase has been generally named y 2 (2,3) , although we will denominate that phase y , according with the last reviews (4,5). The study of these precipitation kinetics will allows us to obtain the values of the microstructural parameters that control these processes and to estimate the stability of the 8 phase at the using temperatures of these alloys.

Analysis of the internal friction spectra during martensitic transformation by a new temperature rate method

Abstract The internal friction spectra of a martensitic transformation obtained as a function of temperature with T ≠0 , consists of three different contributions: transitory, phase transition, and intrinsic. In order to get a more quantitative information from these internal friction spectra, such as the volume fraction of transformed material, it is necessary to separate the spectra in their different contributions. This paper proposes a new method which allows to analyse the IF spectra. The new procedure is based on the strong dependence of the transitory term on the temperature rate. Thus, the separation of the three different contributions to the IF is completed starting from several IF spectra carried out at different temperature rates. Using this method it is possible to check the validity of the different models proposed in the literature for the explanation of the transitory term. Along with the presentation of the method, its application to a thermoelastic Cu–Al–Ni shape memory alloy is shown.

Quantitative analysis of δ′ precipitation kinetics in Al–Li alloys

The Rietveld method has been applied to X-ray spectra in order to study the precipitated mass fraction of δ′ and δ in Al–Li alloys. The method allows us to obtain quantitatively the δ′ and δ precipitate mass fractions, and their evolution with aging time. Furthermore, this method also gives directly the cell parameter evolution of the matrix phase and indirectly the mean half radius of δ′ precipitates through an appropriate calibration curve. Experimentally, this calibration has been approached by previously studying the evolution of the mean half radius of δ′ by transmission electron microscopy (TEM). Thermoelectric power has also been shown to be a powerful technique to study the microstructural evolution of Al–Li alloys, being sensitive to the different stages of precipitation associated to the δ′ and δ phases. The comparison of the different experimental results allow us to stablish a clear difference between the precipitation kinetics and the hardening kinetics.

Long-term superelastic cycling at nano-scale in Cu-Al-Ni shape memory alloy micropillars

Superelastic behavior at nano-scale has been studied along cycling in Cu-Al-Ni shape memory alloy micropillars. Arrays of square micropillars were produced by focused ion beam milling, on slides of [001] oriented Cu-Al-Ni single crystals. Superelastic behavior of micropillars, due to the stress-induced martensitic transformation, has been studied by nano-compression tests during thousand cycles, and its evolution has been followed along cycling. Each pillar has undergone more than thousand cycles without any detrimental evolution. Moreover, we demonstrate that after thousand cycles they exhibit a perfectly reproducible and completely recoverable superelastic behavior.