Martensitic transition and magnetoresistance in a Cu-Al-Mn shape memory alloy. Influence of aging

We have studied the effect of ageing within the miscibility gap on the electric, magnetic and thermodynamic properties of a non-stoichiometric Heusler Cu-Al-Mn shape-memory alloy, which undergoes a martensitic transition from a $bcc$-based ($\beta$-phase) towards a close-packed structure ($M$-phase). Negative magnetoresistance which shows an almost linear dependence on the square of magnetization with different slopes in the $M$- and $\beta$-phases, was observed. This magnetoresistive effect has been associated with the existence of Mn-rich clusters with the Cu$_2$AlMn-structure. The effect of an applied magnetic field on the martensitic transition has also been studied. The entropy change between the $\beta$- and $M$-phases shows negligible dependence on the magnetic field but it decreases significantly with annealing time within the miscibility gap. Such a decrease is due to the increasing amount of Cu$_2$MnAl-rich domains that do not transform martensitically.Comment: 9 pages, 9 figures, accepted for publication in PR

RELATIONSHIP BETWEEN SURFACE MARTENSITE, THIN FOIL AND BULK MARTENSITE

The possibility that surface martensite can serve as nucleus for bulk martensite is investigated by transmission electron and light microscopy in ternary Cu-Zn-Al alloys. For this reason the relationship of the variants and structures between surface martensite, thin foil and bulk martensite is studied. It is known that the surface martensite can serve as nucleus of bulk martensite in samples with the surface close to a (011)ß pole. The necessity to compensate the shape deformation induces, in some cases, additional variants which were not present on the surface. In samples with the surface far away from the (011)ß pole it is found that the surface martensite can grow only in the thinnest region of the foil

Detailed Study of Stacking Faults in Martensite

Relaxations around non-basal plane stacking defects in Cu-Zn-Al martensite have been analyzed by transmission electron microscopy, in order to determine the fault planes and corresponding displacement vectors. Following the results obtained for the 2 H phase, a model was proposed for the kind of non-basal faults present in the 18R monocrystalline martensite. Four kinds were found, each with a different relaxation

The Influence of Structural Defects on the Martensitic Transformation

Martensitic single interface transformations, induced by temperature or applied stresses in high resolution apparatus, show an intrinsic thermoelasticity or pseudoelasticity. This phenomenon arises from the interaction of the growing martensite with the preexisting dislocations in the parent phase. The intrinsic thermoelasticity has a great influence on the hysteretic behavior of the transformation, as it prevents the growth of a unique martensite plate. A further increase of the thermodynamically driving force favors the nucleation of new plates. Simulation of the hysteresis loops, when several plates are present, can be performed, in a more realistic way, by considering the various microscopic physical events taking place during the transformation : nucleation of the phases, interaction with the dislocations and stacking faults, number of martensite plates, hysteresis of each plate in single interface condition and interaction between the plates. The interaction of the martensitic transformation with small precipitates also produces an important influence on the hysteretic behavior : the hysteresis becomes larger but decreases gradually with transformation cycling. This behavior can be explained by considering the mechanisms of plastic accommodation of the precipitate in the martensite

DIFFRACTION EFFECTS IN β Cu-Zn AND β-Cu-Zn-Al SURFACE MARTENSITE TRANSFORMATION AND MICROSTRUCTURE

No abstract availabl

Fatigue in Cu-Zn-Al single crystals during pseudoelastic cycling: In situ observations by SEM and optical microscopy

Previous studies have shown that pseudoelastic cycling of Cu-Zn-Al single crystals leads to surface and bulk defects with consequences on the mechanical behavior and fracture properties of these alloys. Recently it has been shown that the number of cycles to nucleate surface defects depends on the composition of the alloy. In this work we have carried out in-situ tensile cycling experiments under direct observation by scanning electron and optical microscopy. The attention was focused on single crystals with an electron concentration of 1.43. We found strong correlation between the dynamics of the transformation-retransformation cycles and the surface and bulk defects, which form aligned with the trace of the habit plane and the basal plane of the induced martensite. In addition, the orientation of the tensile axis was found to be an important parameter concerning the morphology of the surface defects. Transmission electron microscopy studies have been performed to complete the characterization of defects

PROBLEMS ASSOCIATED TO NUCLEATION AND GROWTH IN THE Cu-Zn-Al SMA

We have studied the characteristics of single-interface single-variant martensitic transformations in Cu-Zn-Al shape memory alloys with a high resolution thermal device (temperature resolution 0.003 K, optical resolution up to 1 µm) Two observations are presented : the appearence of an intrinsic thermoelasticity and the possibility to get two β-variants from a martensite single-crystal. The first phenomena is a phase growth problem (the interaction between interface and dislocations), whilst the second depends on the nucleation of the β phase and permits the breakdown of the SME

EFFECT OF THERMAL CYCLING ON THE MARTENSITIC TRANSFORMATION OF βCu-Zn-Al CONTAINING γ PRECIPITATES

We present a calorimetric and TEM study of the thermal cycling effects on the martensitic transformation in [MATH] Cu-Zn-Al single crystals containing different distributions of [MATH]-phase precipitates. The calorimetric results show a high reproducibility of the transformation during cycling (up to 200 cycles) for dense distributions of coherent precipitates, and a shift towards higher temperatures (mainly the direct transformation) for a distribution of precipitates of bigger size. Arrays of dislocations similar to those formed in precipitate-free specimens were observed in this study, although in a small quantity. Moreover, complex dislocation networks in the matrix surrounding the precipitates were also observed. The effects of both kinds of dislocation arrays on the martensitic transformation are discussed

Recoverable Effects Related to Pseudoelastic Cycling in Cu-Zn-Al Single Crystals

Pseudoelastic cycling of Cu-Zn-Al single crystals has been shown to induce microstructural changes in the material. The mechanical response is also altered. However, some of the observed changes in the mechanical behavior are at least partially recovered at temperatures at which point defects are mobile. In this work results are presented in order to analyze the different contributions associated to the changes of the mechanical behavior during pseudoelastic cycling and in a subsequent ageing stage in the β-phase condition for Cu-Zn-Al single crystals. The magnitudes and kinetics of the recoverable changes are analyzed and compared with the other known phenomena that take place at these temperatures, i.e. the martensite stabilization and the beta phase recovery. Results show that the evolution is not simply related to these phenomena. Additional contributions, like point defects generation during cycling and the effect of dislocation array on the beta phase recovery, need to be considered