G.A. López

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.

Micro pulling down growth of very thin shape memory alloys single crystals

Shape memory alloys (SMAs) have attracted much attention in the last decades due to their thermo-mechanical properties such as superelasticity and shape memory effect. Among the different families ...

Ordered vacancy distribution in 2/1 mullite: a superspace model

A mullite single crystal with composition Al4.84Si1.16O9.58 (2) exhibiting sharp satellite reflections was investigated by means of X-ray diffraction. For the refinement of a superspace model in the superspace group Pbam(α0½)0ss different scale factors for main and satellite reflections were used in order to describe an ordered mullite structure embedded in a disordered polymorph. The ordered fraction of the mullite sample exhibits a completely ordered vacancy distribution and can be described as a block structure of vacancy blocks (VBs) that alternate with vacancy-free blocks (VFBs) along a and c. The incommensurate nature of mullite originates from a modulation of the block size, which depends on the composition. The displacive modulation is analyzed with respect to the vacancy distribution and a possible Al/Si ordering scheme is derived, although the measurement itself is not sensitive to the Al/Si distribution. An idealized, commensurate approximation for 2/1 mullite is also presented. Comparison of the ordered superspace model with different preceding models reconciles many key investigations of the last decades with partly contradicting conclusions, where mullite was usually treated as either ordered or disordered instead of considering simultaneously different states of order.

Crystal structure determination of a ternary Cu(In,Sn)2 intermetallic phase by electron diffraction

Small grains of an intermetallic phase with an approximate composition Cu(In,Sn)2 were observed in a metal matrix composite obtained from powders of a Cu–Al–Ni shape-memory alloy and an In–Sn matrix alloy. Samples of this composite were prepared for transmission electron microscopy and the crystal structure of the intermetallic phase was carefully investigated by applying electron diffraction techniques (microdiffraction, convergent-beam electron diffraction and precession), based on the analysis of the symmetry and the relative positions of reflections in the zero- and high-order Laue zones. It was found that the intermetallic phase has a body-centred tetragonal unit cell with lattice parameters a = 0.70 (3) nm and c = 0.56 (2) nm. Its crystal symmetry can be described by the I4/mcm (No. 140) space group.

Mullite – towards a unified superspace model

Mullite Al4+2xSi2−2xO10−x x ( = vacancy) is a well known material in the ceramics industry with a broad range of applications, but from a fundamental point of view its crystal structure is not completely understood. The dependence of the modulation wave vector q = (α 0 γ) on the composition lacks an explanation and it is not known why for vacancy concentrations x > 0.5 a lowering of the symmetry is observed [1]. A recent superspace model (SSM) described in Pbam(α01⁄2)0ss results in a brick pattern of alternating vacancy blocks and vacancy-free blocks [2]. Assuming that, independent of the composition, the brick pattern is maintained and intermeshing of vacancy blocks is avoided by the structure, the relation α = (1−x)/2 is derived from the SSM. It allows to predict the vacancy distribution of any mullite with x ≤ 0.5 including the cases of the prominent 3/2 mullite (x = 0.25), 2/1 mullite (x = 0.4) and 5/2 mullite (x = 0.5). Models derived from the SSM are depicted in the Figure below. The limit results from one of the atomic domains which has a negative length for x > 0.5, i.e. a new SSM is required. This border coincides with an observed change of the symmetry from orthorhombic to monoclinic because γ then deviates from 1⁄2. As the deviation is very small, the nature of the modulation seems to be the same for the complete solid solution range and the dependence of q on the composition is a result from the block pattern that determines the vacancy distribution. The SSM can be easily adapted to monoclinic symmetry with superspace group P21/a(α0γ)0s to describe a monoclinic brick pattern of alternating vacancy blocks and vacancy-free blocks. These predictions are strongly supported by HRTEM observations and image simulations [1]. However, the suggested vacancy ordering scheme is expected to be only present when higher order satellite reflections are present in the reciprocal space. In many samples this is not the case and first order satellite reflections appear as maxima in a characteristic diffuse scattering pattern [3], i.e. that the long-range vacancy ordering is not present and vacancies are rather midrange ordered. Nevertheless the dependency of q on x is maintained independent of the diffuseness, which indicates that the disordered vacancy distribution is strongly related to the structure of the highly ordered SSM. In [2] it is also shown that a disordered SSM of [3] and the ordered SSM are indeed strongly related. It means that although mullite samples usually exhibit a lower ordering degree than described by the model, it establishes a basic vacancy ordering pattern from which structural models that take diffuse scattering into account must originate. This work is supported by the Basque Government (No. IT-779-13 & predoctoral grant) and FEDER (MAT2015-66441-P). A travel grant by the Master and Doctoral School (UPV/EHU) is highly appreciated.