R. Portier

Transformation and ageing behaviour of melt-spun Ni–Mn–Ga shape memory alloys

Abstract The effects of rapid quenching on the transformation behaviour of three melt-spun ribbons of Ni–Mn–Ga alloys has been studied by X-ray and electron diffraction, calorimetry, thermomechanical and magnetisation tests. One step martensitic transformation (MT) has been detected for each ribbon accompanied by the formation of modulated martensitic crystal structures with 7- or 10-layer periods, which differ from those observed for single crystals of the same compositions. The MT temperatures can be raised or lowered by ageing in the parent phase, depending on composition. The ageing also causes a significant increase of both Curie temperature and magnetization in all alloys. These effects indicate the occurrence of some atomic rearrangement processes leading to a more uniform element distribution and, probably, change of atomic order. The activation energies for these processes have been estimated to be between 0.2 and 0.7 eV.

Interface mobility in Ni49.8Ti42.2Hf8 shape memory alloy

Abstract The ternary Ni49.8Ti42.2Hf8 shape memory alloy shows mechanical peculiarities compared with NiTi alloy. These are linked to (i) the distortion of the habit plane of the ternary alloy and (ii) the observed (001) mechanical twinning, which helps to accommodate plastic deformation in Ni49.8Ti42.2Hf8 and not in NiTi alloy. The distortion of the habit plane limits its mobility, evidence of which being brought here by internal friction. The (001) mechanical twinning causes the appearance of a very thin microstructure in martensite laths deformed by tension. A model is proposed as a possible mechanism giving birth to this supplementary mechanical twinning: repeated slips by a/2 on (001) planes would create translative (001) micro-twins coherent with the observed microstructure. These micro-twins may be produced by the dissociation of dislocations with the Burgers vector a.

Long Period Superstructures in Cu3+xPd

Electron microscopy and electron diffraction results on the one-dimensional long period superstructures (1D-LPS) in CU3+xPd are presented. The observations strongly support the models (like the ANNNI model) in which the profile of the modulating function smoothens with rising temperature. Near the order-disorder transition temperature, the low PD content LPS have an incommensurate character.

Crystallographic structures and phase transformations in ZrPd

Abstract Phase transformations and crystal structures in the equiatomic compound ZrPd were examined using in-situ heating and cooling in a transmission electron microscope (selected-area diffraction and high resolution imaging) and Rietveld refinement of high resolution neutron powder diffraction data. Three phases were characterized: a high temperature cubic B2-type phase (Pm3m; Z = 1; a = 0.335 97(3) nm at 800°C); an orthorhombic CrB-type phase formed by martensitic transformation from the B2 phase (Cmcm; Z = 4; a = 0.333 19(3) nm, b = 1.030 1(1) nm and c = 0.441 11(4) nm at 400°C) and a monoclinic variant of the CrB-type phase at room temperature (assumed space group Cm; Z = 8; a = 0.666 11(6) nm, b = 0.874 99(7) nm, c = 0.542 35(6) nm; β = 108.21(1)°). The monoclinic ⇌ orthorhombic phase transformation was found to occur reversibly around 200°C.

Crystallographic description of coincidence‐site lattice interfaces in homogeneous crystals

Coincidence-site lattice interfaces (CSLI) are frequently observed in crystals where a rigid framework remains invariant on both sides of the interface. They also seem to minimize the interface energy, for example, in metals where, empirically, the greater the density of the coincidence-site lattice the more stable the grain boundary becomes. Group-theory considerations allow the determination of all the possible interface operations which leave a given sublattice invariant. A classification of these CSLI with respect to the number of equivalent sublattices they leave invariant is a guide for the prediction of the most stable type of interfaces with respect to the sublattice considered. Examples from different types of crystals illustrate the method, which also applies for translation boundaries, twins and grain boundaries.

Characterization of two different types of long‐period ordered alloys by high‐resolution electron microscopy

High-resolution electron microscopy can be used to differentiate between two structural models of long-period ordered alloys. These models differ in the nature of the disorder as seen in the stacking irregularities which can have planar boundaries as exemplified by Ag3Mg or wavy boundaries as in AuCu II. In the composition range 22-27 at. % Mg, Ag3Mg is built up with a regular arrangement of two kinds of structural layers, 1 or 2 LI 2 cells in thickness. Some stacking disorder exists, but this alloy can be locally described using a space-group notation and has the character of being an infinitely adaptative structure. High-resolution images have been used to show the perfect planarity of the boundaries in Ag3Mg, thus demonstrating the way in which disorder is accommodated in this alloy.

Phases AnBnO3n+2 des syste`mes La2Ti2O7-CaTiO3, Nd2Ti2O7-CaTiO3 et Ca2Nb2O7-CaTiO3. II. Nouvelles caracte´ristiques cristallographiques et interpre´tation du maclage re´ve´le´par microscopiee´lectronique

Abstract The simultaneous use of X-ray diffraction and electron microscopy techniques enables us to explain the structural characteristic features of A n B n O 3 n +2 phases in the systems: La 2 Ti 2 O 7 -CaTiO 3 , Nd 2 Ti 2 O 7 -CaTiO 3 , and Ca 2 Nb 2 O 7 -CaTiO 3 ; and the systematic twinning of the observed crystals. The latter seems to be correlated with the making, during the growth, of “defect sheets,” whose symmetry element induces the orientation in position of the two twin crystals.

Ni49.8Ti42.2Hf8 shape memory alloy strips production by the twin roll casting technique

Abstract The twin roll casting technique has been used for the first time to produce semi-finished Ni49.8Ti42.2Hf8 shape memory alloy thin strips. Some cracks appear on these new products but can be removed by optimization of the operational parameters. The aspect, dimensions and thermodynamical behavior of the strips are studied and interpreted according to the operational parameters and the resulting microstructure, taking into account the complex heat and mass flows during the process.

An icosahedral phase in annealed austenitic stainless steel

Abstract Austenitic 316L stainless steel alloys annealed at 550°C for 100 h present a few ferrite precipitates surrounded by a new interfacial phase, here called the I-phase, that develops at the level of the austenite/ferrite interface. The I-phase presents the typical patterns of an icosahedral quasicrystal with a primitive hypercubic lattice of parameter A = 0.63 nm. The marked orientation relationships between the I-phase and the ferrite precipitates strongly suggest that this phase results from a decomposition of the ferrite and not of the austenite. The I-phase is metastable and transforms eventually after annealing at 700°C to the stable crystalline σ-phase.

Etude des phases ordonnées à longue période irrationelle. Les alliages AuCu II et AuCu–Zn

It is shown, by electron diffraction and high-resolution microscopy, that AuCu II is a typical example of an irrational long-period alloy, contrary to Ag3Mg, for instance, which is a typical rational long-period alloy. Antiphase boundaries are not planar (as is the case for Ag3Mg) but rather fluctuating around a mean position, as shown by high-resolution images; this structure is well described by Jehanno & Perios model [J. Phys. (1964), 25, 966-974], which always involves some disorder localized in the boundaries themselves. The long period is always the result of an average of domains of different lengths, thus giving a statistical meaning to the irrationality. This structural behaviour is not confined to alloys with large values of the mean length of domains, as is demonstrated by the study of AuCu-Zn.